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Visual Temporal Processing in Dyslexia and the Magnocellular DeficitTheory: The Need for Speed?

Gregor M. T. McLeanMacquarie University

Geoffrey W. StuartLa Trobe University

Veronika Coltheart and Anne CastlesMacquarie University

A controversial question in reading research is whether dyslexia is associated with impairments in themagnocellular system and, if so, how these low-level visual impairments might affect reading acquisition.This study used a novel chromatic flicker perception task to specifically explore temporal aspects of magnocellular functioning in 40 children with dyslexia and 42 age-matched controls (aged 711). Therelationship between magnocellular temporal resolution and higher-level aspects of visual temporalprocessing including inspection time, single and dual-target (attentional blink) RSVP performance,go/no-go reaction time, and rapid naming was also assessed. The Dyslexia group exhibited significant

deficits in magnocellular temporal resolution compared with controls, but the two groups did not differin parvocellular temporal resolution. Despite the significant group differences, associations betweenmagnocellular temporal resolution and reading ability were relatively weak, and links between low-leveltemporal resolution and reading ability did not appear specific to the magnocellular system. Factoranalyses revealed that a collective Perceptual Speed factor, involving both low-level and higher-levelvisual temporal processing measures, accounted for unique variance in reading ability independently of phonological processing, rapid naming, and general ability.

Keywords: magnocellular pathway, temporal processing, dyslexia

The proposed causes of the complex disorder that is develop-mental dyslexia are many and varied, often reflecting the disciplinebackground of the researchers concerned. Cognitive psychologistsand linguists have been prominent in identifying basic languageand phonological deficits in dyslexia (e.g., Snowling, 2002). Neu-roscientists and vision scientists, on the other hand, have impli-cated more subtle sensory impairments, with a dominant hypoth-esis being that individuals with dyslexia have a specific deficit inthe magnocellular physiological pathway of the visual system(e.g., Lovegrove, Martin, & Slaghuis, 1986; Stein & Talcott,1999). Even within discipline areas, the range of hypotheses isextensive: the psychological research literature contains evidencethat the basis of developmental dyslexia may lie in deficits inphonological awareness (e.g., Snowling, 2002), rapid naming (e.g.,Wolf & Bowers, 1999), short-term memory (e.g., Ramus et al.,2003), and visual attention (e.g., Valdois, Bosse, & Tainturier,

2004; Vidyasagar & Pammer, 2010).To some degree, this variability may reflect heterogeneitywithin the dyslexic population itself: dyslexia is not a unitary

syndrome and is unlikely to have a unitary cause (Boder, 1973;Castles & Coltheart, 1993; Wolf & Bowers, 1999). However, it isalso possible that at least some of these disparate hypotheses canbe reconciled, with poor performance on apparently differenttasks, and across different research domains, being shown toreflect a single underlying impairment.

In the present study, we attempted to effect a reconciliation of this kind. Specifically, we examined evidence for a reconceptual-ization of the controversial magnocellular deficit theory of dys-lexia, in which we propose that a subset of children with readingimpairments have a deficit in that aspect of magnocellular func-tioning associated with fast rates of processing, and not with thataspect of magnocellular functioning associated with threshold con-trast sensitivity to low spatial frequencies varying at slower rates.We then examined whether this magnocellular temporal deficitmight reflect a more pervasive visual speed of processing impair-

ment in dyslexia, which underlies poor performance, not only onsensory input tasks, but also on higher-level speeded processingtasks which have been found to be impaired in dyslexia, such asgo/no-go reaction time, rapid automatized naming, inspectiontime, and attentional dwell time. Finally, we examined what as-pects of reading a visual temporal processing impairment mightaffect, and the degree to which it appears to be associated with, orindependent from, the phonological processing deficits that havebeen most strongly associated with developmental reading disor-ders (Snowling, 2002).

As noted above, the phonological deficit hypothesis is currentlythe most prominent theory of reading impairment (Snowling,

This article was published Online First August 8, 2011.Gregor M. T. McLean, Veronika Coltheart, and Anne Castles, Macqua-

rie Centre for Cognitive Science, Macquarie University, Sydney, Australia;Geoffrey W. Stuart, School of Psychological Science, La Trobe University,Melbourne, Australia.

Correspondence concerning this article shouldbe addressed to Gregor M. T.McLean, Defence Science and Technology Organisation, Fishermans Bend,VIC 3207, Australia. E-mail: [email protected]

Journal of Experimental Psychology: 2011 American Psychological AssociationHuman Perception and Performance2011, Vol. 37, No. 6, 19571975

0096-1523/11/$12.00 DOI: 10.1037/a0024668

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2002). This hypothesis argues that poor readers have an inferiorknowledge of speech sounds and their relationship with letters,thus leading to impaired reading development. These deficits areassociated with difficulties in decoding unfamiliar, or nonsensewords, but impairments are also evident on language based pho-nological processing measures that assess both the explicit ability

to manipulate phonological representations (Phoneme Deletion,Muter, Hulme, Snowling, & Taylor, 1998) as well as the implicitunderstanding of phonological structure (Nonword Repetition,Gathercole, Willis, Baddeley, & Emslie, 1994). However, deficitsin nonword reading and phonological language processes, whilefrequently observed, do not explain all aspects of reading impair-ment, nor are they seen in all poor readers (Castles & Coltheart,1993), and this has led researchers to explore alternative, nonpho-nological impairments such as deficits in the magnocellular path-way of the visual system.

The Magnocellular Deficit Theory of Dyslexia

The magnocellular neural pathway is one of two major parallelpathways in the visual system, 1 the other being the parvocellularpathway. These originate from the midget and parasol ganglioncells in the retina and project through the lateral geniculate nucleus(LGN) via the optic radiations, to the visual and parietal cortices(Merrigan & Maunsell, 1993; Shapley & Perry, 1986). Afterextensive primate and human research, it is widely accepted thatmagnocellular and parvocellular neurons are tuned to respondoptimally to different temporal and spatial frequencies (De Mon-asterio & Gouras, 1975; Derrington, Krauskopf, & Lennie, 1984).The magnocellular system, while insensitive to isoluminantchanges in color polarity, responds optimally to low contrast andlow spatial frequencies (i.e., coarse patterns) (Kaplan, Lee, &Shapely, 1990; Lee, Pokorny, Smith, Martin, & Valberg, 1990).

The magnocellular system also has a very high temporal resolutionand is sensitive to extremely rapid changes in visual input (Nowak & Bullier, 1997; Schiller, Logothetis, & Charles, 1990a, 1990b).The parvocellular system conversely has relatively lower temporalresolution but is sensitive to changes in color and has superiorsensitivity to high spatial frequencies (i.e., fine detail) (Kaplan etal., 1990; Kaplan & Shapley, 1986).

The magnocellular deficit theory of dyslexia posits that a pro-portion of individuals with dyslexia possess some form of deficitin the magnocellular pathway, yet normal functioning in the par-vocellular pathway, and that this deficit plays a causal role in theirreading impairment (see Stein & Talcott, 1999 for a more exten-sive review). Evidence in support of a magnocellular deficit indyslexia has come from studies comparing reading-impaired andcontrol groups on their performance on psychophysical tests de-signed specifically to tap magnocellular functioning. However,while numerous such studies have been carried out, the causal roleplayed by magnocellular deficits in dyslexia, and even the exis-tence of such deficits, remains a controversial issue.

For example, there have been numerous studies assessing con-trast sensitivity to low and high spatial frequencies, with severalreporting that poor readers exhibit inferior sensitivity tomagnocellular-specific low spatial frequency stimuli (e.g., Borst-ing et al., 1996; Demb, Boynton, Best, & Heeger, 1998; Evans,Drasdo, & Richards, 1994; Felmingham & Jakobson, 1995; Martin& Lovegrove, 1984; Martin & Lovegrove, 1987; Ridder, Borsting,

Cooper, McNeel, & Huang, 1997; Slaghuis & Ryan, 1999). How-ever, various issues have been raised in relation to these findings.First, some theorists have questioned whether the spatial frequen-cies used in many of the studies were low enough to specificallytap the magnocellular system, without input from the parvocellularsystem (Skottun, 2000a, 2000b). Second, it has been noted that

many of the studies reporting a magnocellular deficit in dyslexiahave actually found inferior contrast sensitivity to parvocellular aswell as magnocellular specific stimuli in their participants, sug-gesting a more general deficit (Stuart, McAnally & Castles, 2001).In fact, it has been noted that the number of studies reportingfindings in line with a specific magnocellular impairment areoutnumbered by studies reporting either equal deficits across bothmagnocellular and parvocellular stimuli, or even greater deficits insensitivity to parvocellular stimuli (Skottun, 2000a, 2000b). Fi-nally, supporting the hypothesis of a more general deficit, Stuart etal. (2001) and Roach, Edwards and Hogben (2004) have demon-strated that the weaknesses in processing both magnocellular andparvocellular stimuli shown by poor readers can be accuratelysimulated by failures of attention, which are particularly commonin this population (e.g., Willcutt & Pennington, 2000).

Additional evidence in favor of a magnocellular deficit in dys-lexia has come from studies using coherent motion tasks, in whichparticipants must detect a percentage of dots moving in a givendirection among distractor dots moving randomly. Such tasks tapheavily into the motion (MT) areas of the visual cortex whichpredominately receive magnocellular rather than parvocellular in-put and, once again, individuals with dyslexia have been found todisplay impaired performance compared to controls (e.g., Conlon,Sanders, & Wright, 2009; Conlon, Sanders, & Zapart, 2004; Cor-nelissen, Hansen, Hutton, Evangelinou, & Stein, 1998; Cornelis-sen, Richardson, Mason, Fowler, & Stein, 1995; Slaghuis & Ryan,1999; Talcott, Hansen, Assuko, & Stein, 2000; Talcott, Hansen,Willis-Owen, McKinnell, & Stein, 1998; Talcott, Witton et al.,2000; Witton et al., 1998; Wright & Conlon, 2009). However thevalidity of the coherent motion task as a specific measure of magnocellular processing has been questioned, with recent studiessuggesting that MT areas also receive significant parvocellular andkoniocellular inputs (Nassi, Lyon, & Callaway, 2006; Sincich,Park, Wohlgemuth, & Horton, 2004). Consistent with this possi-bility, it has been shown that coherent motion perception can occurwith isoluminant color stimuli to which the magnocellular systemis insensitive (Edwards et al., 2004; Skottun & Skoyles, 2008).These findings suggest that coherent motion is not a pure measureof magnocellular processing and, as such, it is possible that poorreaders inferior performance on these tasks could also be attrib-

utable to general deficits related to task difficulty or inattention.A final, and more direct, line of evidence in support of amagnocellular deficit theory of dyslexia has come from physio-logical postmortem studies of the brains of individuals with dys-lexia. These studies have reported that the magnocellular layers of the LGN in individuals with dyslexia are disordered and more than20% smaller than in the brains of normal readers (Galaburda &Livingstone, 1993; Livingstone, Rosen, Drislane, & Galaburda,

1 A third, koniocellular pathway of the visual system has also recentlybeen discovered between the magnocellular and parvocellular layers andconsists of extremely small cells (Hendry & Reid, 2000).

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1991). It is worth noting, however, that these results came from asmall sample (only five patients), and the findings have yet to bereplicated.

A Revised Magnocellular Deficit Theory

The above findings raise significant questions about the mag-nocellular deficit theory of dyslexia, at least in its original form.Proponents of the theory have responded by referring to therelatively mild nature of the sensory deficits attributed to themagnocellular system (Stein, Talcott, & Walsh, 2000, p. 210).However, another possibility is that the findings concerning thistheory have been mixed because some, but not all, aspects of magnocellular processing are impaired in some poor readers.

As mentioned above, there are two features of the magnocellularsystem that are clearly distinguishable from each other: its superiorcontrast sensitivity for low spatial frequencies and its superiortemporal resolution. These functional aspects of the magnocel-lular system are also neurologically distinguishable. The magno-cellular pathways contrast sensitivity to low spatial frequencies islinked with the large size of magnocells within the system andtheir larger receptive fields (Kaplan et al., 1990). Conversely, thesuperior resolution of the magnocellular system at above-thresholdcontrasts is linked to the observed shorter refractory periods andfaster modulation rates of these neurons (Derrington & Lennie,1984; Ikeda & Wright, 1975). Stein and Talcott (1999) attributedthis to faster membrane kinetics in magnocellular neurons. Theheavier myelination of magnocellular neurons accounts for theirfaster conduction speed which may also contribute to the effi-ciency of processing (Nowak & Bullier, 1997; Schmolesky et al.,1998). If these features of magnocellular processing are neurolog-ically distinguishable, it seems possible that some form of impair-ment or delayed development might lead to damage only to a

particular aspect of magnocellular functioning. That is, magnocel-lular contrast sensitivity and magnocellular temporal resolutionmight not always both be impaired in individuals with dyslexia,but rather may be selectively compromised. In fact, researchassessing both functions in poor readers have reported only min-imal correlations between the two (Evans, Drasdo, & Richards,1993; Skottun, 2000a), with research exploring contrast sensitivityat varying temporal frequencies typically reporting larger deficitsat higher temporal frequencies (Ben-Yehudah, Sackett, Malchi-Ginzberg, & Ahissar, 2001; Cornelissen, 1993; Martin & Loveg-rove, 1987, 1988; see Skottun, 2000a for a review).

Other research suggests that the temporal capacities of themagnocellular system seem the most likely candidates for selectiveimpairment in dyslexia. First, a deficit in processing the coarsepatterns and low-spatial frequencies to which the magnocellularsystem is most sensitive to does not seem intuitively linked to adeficit in processing text, which has a very high spatial frequency.Second, while the notion of a temporal, rather than a spatial,frequency deficit represents a new approach to conceptualizing themagnocellular deficit hypothesis, it is far from a novel concept indyslexia research more broadly. Numerous studies have reportedvisual temporal processing related deficits in dyslexia, rangingfrom low-level double flash detection (e.g., Van Ingelghem et al.,2001) to higher-level deficits in rapid naming of familiar items(e.g., Wolf & Bowers, 1999), and even delayed responses to roadsigns in driving simulators (Sigmundsson, 2005, see Farmer &

Klein, 1995 for a review). It is possible, therefore, that visualtemporal processing deficits in dyslexia extend even to low-leveltemporal resolution deficits within the magnocellular visual sys-tem and that many of the higher-level visual temporal processingdeficits previously reported in dyslexia may be attributable to thislow-level impairment in magnocellular temporal resolution. This

was the hypothesis explored in the present study.

Magnocellular Temporal Resolution in Dyslexia

Many studies have explored magnocellular functioning in dys-lexia, but few have specifically examined temporal aspects of themagnocellular system while controlling for parvocellular function-ing. An exception is the study by Sperling, Lu, Manis, and Se-idenberg (2003), who used a novel phantom-contour paradigmoriginally developed by Rogers-Ramachandran and Ramachan-dran (1998). Here, flicker-defined contours were used to assessmagnocellular as well as parvocellular processing in children withdyslexia. The tasks used stimuli consisting of a field of black andwhite dots that reverse polarity in counterphase to create identifi-able pictures (i.e., black dots representing the figure, and thewhite dots representing the ground). Importantly, when theblack/white dots are replaced with isoluminant red/green dots (towhich the faster magnocellular system is insensitive), the phantomcontours are no longer visible at the same rapid flicker frequency:the color sensitive parvocellular system is capable of extractingcontours based on surface color in the isoluminant red/green dotcondition, yet the isoluminant nature of the stimuli prevents themagnocellular system from detecting contours at higher flickerfrequencies. Conversely, the superior temporal resolution of themagnocellular system is able to extract the black/white contoursbased on differences in dot luminance, despite the slower parvo-cellular system no longer being capable of discerning the counter-

phase of the dots (Quaid & Flanagan, 2005; Sperling et al., 2003).Thus, an important advantage of this task is that, not only does itassess magnocellular temporal resolution, but it also provides asuitable control for parvocellular temporal resolution.

Consistent with a specific magnocellular temporal deficit, Sper-ling et al. (2003) reported that skilled readers were on average ableto identify the phantom contours in the magnocellular (black/ white) condition faster than individuals with dyslexia, yet nosignificant differences were evident between groups in the parvo-cellular (isoluminant red/green) condition. These findings werealso replicated in a later study examining adults with dyslexia(Sperling, Zhong-Lin, Manis, & Seidenberg, 2006). Of furtherinterest, magnocellular temporal resolution was more closely as-sociated with orthographic reading skills (irregular word readingand orthographic choice) than with phonological skills (nonwordreading and phoneme deletion), especially in children. These find-ings provide preliminary evidence suggesting that temporal as-pects of magnocellular function may be associated with readingability independently of the phonological impairments typicallyfound in poor readers.

However, some questions concerning the interpretation of thesefindings remain. First, in both studies the order in which theparvocellular and magnocellular conditions were completed byparticipants was not counterbalanced, with the magnocellular con-dition always being conducted first. As such it is possible that thedyslexia groups inferior performance in the magnocellular condi-

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tion may have been attributable to delayed learning effects ratherthan a specific magnocellular impairment. Such delayed learningeffects have been reported in previous research with dyslexicsamples, especially when completing complex and attention-demanding psychophysical tasks (e.g., Badcock, Hogben, &Fletcher, 2007). Second, Skottun and Skoyles (2006) note that the

reported differences in temporal resolution between magnocellularand parvocellular conditions (approximately 15 Hz and 7 Hzrespectively) are considerably larger than those typically found inprimate research, which estimate the optimal temporal frequenciesof magnocellular and parvocellular neurons to be 7.94 Hz and 6.76Hz, respectively (Levitt, Schumer, Sherman, Spear, & Movshon,2001). This brings into question the degree to which the phantomcontour task can be considered a pure measure of magnocellularfunctioning.

A New Measure of Magnocellular TemporalResolution

In this study, we examined magnocellular temporal resolution indyslexia using a novel chromatic flicker perception task. Based onresearch by Schiller and Logothetis (1990), the task involvesviewing an isoluminant red and green light emitting diode (LED)in which the rate of red/green flicker is gradually increased untilthe red/green LED appears to fuse into a single orange color. Thisflicker frequency is recorded as a measure of parvocellular temporal resolution as it reflects the point at which this system is nolonger capable of discerning the rapid red/green color change (Lee,Martin, & Valberg, 1989; Lee et al., 1990). At this frequency,however, the perception of orange flicker persists, as the superiortemporal resolution of the magnocellular system is still capable of detecting the transient nature of the stimuli (Lee, Martin, & Val-

berg, 1988; Lee et al., 1990). If the flicker frequency is furtherincreased, eventually the LED appears to fuse into a solid orangelight, with this frequency representing the threshold of magnocel-lular temporal resolution 2 (Lee et al., 1990). While these twoconditions allow for magnocellular temporal resolution to be as-sessed in the context of a specific parvocellular control, the chro-matic flicker task can also be used to assess the fastest absolutemagnocellular threshold. This is achieved by flickering both redand green LEDs simultaneously to create a higher-contrast (black/ orange) flicker condition in which a superior flicker threshold isobtainable (Lee et al., 1990).

The chromatic flicker perception task has some significant ad-vantages over the phantom contour measure (Sperling et al., 2003;Sperling et al., 2006). In pilot work, we found that the averagedifferences between magnocellular (average 31.2 Hz) and parvo-cellular temporal thresholds (average 20.5 Hz) were much closer toprevious estimates of magnocellular and parvocellular temporallatencies than was the case for the phantom contour task. Forexample, Levitt et al. (2001) found cut-off temporal frequenciesfor magnocellular and parvocellular functioning of 31.62 Hz and21.88 Hz, respectively. Furthermore, the task is brief and hasminimal attentional load, making it ideal for use in samples of young children and individuals with dyslexia. This measure alsohas very similar demands for both the magnocellular and parvo-cellular conditions, and the order of these conditions can be easilycounterbalanced.

In the present study, we directly examined magnocellular andparvocellular temporal resolution in dyslexia using the chromaticflicker perception task. However, we also sought to test the hy-pothesis that compromised magnocellular temporal resolution, asmeasured by this task, may be associated with some of the higher-level visual temporal processing impairments outside of the broad

magnocellular system that have also been reported in dyslexia. Tothis end, we examined the performance of children with dyslexiaand controls on a range of other measures of visual temporalprocessing including go/no-go reaction time, inspection time, rapidnaming, and single and dual-target (attentional blink) search withina rapid serial visual presentation (RSVP) of stimuli. We outline therationale for the inclusion of these measures in the section below.

Higher-Level Visual Temporal ProcessingImpairments in Dyslexia

Rapid automatized naming. While a variety of speededvisual processing measures have been explored in relation to

dyslexia, perhaps the most widely recognized is the Rapid Autom-atized Naming (RAN) task. This task, developed by Denckla andRudel (1976), involves naming a grid of stimuli (typically letters,digits, colors, or shapes) as quickly as possible. Performance onRAN tasks is not only correlated with reading ability in bothskilled readers and individuals with dyslexia, but is also a signif-icant predictor of reading skills in young children (Felton, Naylor,& Wood, 1990; Lervg & Hulme, 2009; Manis, Seidenberg, &Doi, 1999; Neuhaus, Foorman, Francis, & Carlson, 2001; Wolf &Bowers, 1999).

The link between rapid naming and reading ability also appearsto be independent of phonological processing skills with Wolf andBowers (1999) identifying three subtypes of impairment within alarge sample of individuals with dyslexia: those with a phonolog-ical impairment, those with a rapid naming impairment, and thosewith an impairment in both skills, or a double deficit . Whileperformance on higher-level RAN measures is unlikely to bereduced entirely to a low-level magnocellular deficit, these find-ings add further support to the notion that, at least to some degree,a visual temporal processing deficit may be associated with read-ing performance independently of phonological processing skills.

Reaction time. Individuals with dyslexia have also beenreported to exhibit delays on a variety of reaction time measures(Nicolson & Fawcett, 1994; Sigmundsson, 2005; Spring, 1971).While poor readers typically show no significant deficits in simplereaction time measures (Paulesu et al., 2001), when the task requires the identification of a stimulus, such as in go/no-go, or

choice reaction time measures, impairments become evident (Ni-colson & Fawcett, 1994). These findings suggest that individualswith dyslexia do not exhibit a motor response deficit but instead adeficit in stimulus identification and classification which may beinfluenced by inferior magnocellular temporal resolution. More-over, these deficits in go/no-go reaction time have typically been

2 The term magnocellular temporal resolution refers here to the temporalcharacteristics of the magnocellular division of the peripheral retino-geniculo-striate pathway, independent of its contribution to anterior corti-cal dorsal stream projections, rather than the temporal characteristics of thedorsal striatofugal where pathway as a whole.



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assessed on measures of orthographic reading skills, phonologicalreading skills, and reading fluency as well as on a measure of general reading ability (tests described below). Participants wereselected for the Dyslexia group based on performance in thebottom 15% on any of the four reading measures, but above the30th percentile on a measure of nonverbal reasoning (Ravens

Matrices). Reflecting the heterogeneous nature of dyslexia, somechildren exhibited significant difficulties in some aspects of theirreading, but appeared relatively unimpaired in others. Participantsfor the Control group were selected based on performance abovethe 30th percentile on all four reading tests. 3

A total of 40 children (25 boys; 15 girls) with a significantreading impairment (Dyslexia group) were included in the analysisas well as 42 children (11 boys; 31 girls) with normally developingreading skills (Control group 4 ). The data of an additional eightparticipants (two boys; six girls) with reading skills that failed tomeet either Dyslexia or Control group selection criteria (unclassi-fied participant) were also included in correlation and factor anal-yses.

Materials

Reading assessments. The Word Reading subtest of theWechsler Individual Achievement Test II (WIAT-II) was includedas a general measure of reading ability, as it assessed single wordreading as well as letter/letter-sound identification and rhymeknowledge (The Psychological Corporation, 2002).

The Wheldall Assessment of Reading Passages (WARP) wasused as a measure of reading fluency (Wheldall, 1996). This testcontains three 200-word short stories. Participants were instructedto read the passages as quickly and accurately as possible. Scoreswere recorded as the number of words read correctly within oneminute, averaged across each of the three passages.

A modified version of the Castles and Coltheart (1993) test of irregular and nonword reading was included to independentlyassess orthographic and phonological reading processes. This testconsisted of 30 irregular words (e.g., yacht ) to test orthographicskills and 30 nonwords (e.g., gop ) to test phonological readingskills. The words were presented one at a time for reading aloud,and accuracy was scored. The items were presented in an inter-mixed order so that the children could not guess whether an itemwas a real word or a nonword.

Phonological processing skills. Three language-based mea-sures of phonological processing skill were included from theComprehensive Test of Phonological Processing (CTOPP; (Wag-ner, Torgesen, & Rashotte, 1999). Explicit ability to perceive andmanipulate phonemes was assessed by the Phoneme Deletionsubtest, requiring participants to say the remaining word afterremoving a particular phoneme (e.g., say bold without saying /b/, the correct response being old ). Phonological memory andmore implicit knowledge of phonological structure was assessedby the Nonword Repetition subtest, which requires participants torepeat a series of spoken nonwords increasing in complexity fromone to seven syllables. Short-term memory was assessed using theMemory for Digits subtest: a forward digit span task ranging fromtwo to eight digits.

Low-level visual temporal processing. Magnocellular andparvocellular visual temporal processing thresholds were assessedusing a chromatic flicker perception task. The apparatus consisted

of a plastic capsule (9 cm 16 cm 25 cm) containing atwo-color (red/green) LED. A rubber viewing mask was installedat the open end of the capsule to minimize head movementthroughout the experiment and control for lighting conditions. Thebrightness of the red and green elements of the LED were held atphotopic levels and adjusted to a luminance of 113.4 cd/m 2 (Tek-

tronix Lumacolour photometer with J18 Luminance Head) using avariable resistor with a diffuser used to ensure even light emission.The rate of oscillations in the LED from red to green was con-trolled by the experimenter using an attached laptop computer, andflicker frequencies were adjusted using a step size of 1 ms.

At flicker rates of around 15 to 25 Hz participants could nolonger perceive the rapid changes in color and instead perceivedthe red/green LED fused into an orange color. The flicker fre-quency at which red and green color differentials were no longerdiscernable was recorded as a parvocellular temporal threshold. Atthis point, the rate of flicker was further increased until participantsperceived the flickering LED to fuse into a solid orange light. Themagnocellular temporal threshold for isoluminant color flicker was

recorded at the flicker frequency at which participants could nolonger detect any flicker and typically occurred at ranges from 25Hz to 35Hz. Flicker frequency was assessed using both ascendingand descending trials with parvocellular and magnocellular thresh-olds calculated based on median performance across four reversalswith the order of parvocellular and magnocellular flicker condi-tions counterbalanced.

An additional high-contrast flicker threshold condition was alsoincluded, where both red and green LEDs were flickered on andoff simultaneously to produce a higher contrast orange flickeringlight. While the magnocellular isoluminant condition provides anassessment of the magnocellular temporal advantage over theparvocellular system in processing the same stimuli, the highercontrast created by an orange/black flickering LED provided anassessment of the fastest absolute flicker threshold obtainable inthe magnocellular system. High-contrast flicker temporal thresh-olds were assessed using the same four ascending and descendingtrials used in the magnocellular and parvocellular conditions.

High-level visual temporal processing. All high-level visualtemporal processing tasks were completed on the same 17-inch,75-Hz CRT monitor, except for the rapid naming measures. Thesurrounding environment was kept dimly lit to ensure all stimuliwere clearly visible, and for all computer tasks children wereseated approximately 60 cm from the monitor.

Rapid naming. Rapid naming was assessed using the stan-dardized color and object RAN tasks from the CTOPP. In each of these tasks, participants were required to name aloud as quickly aspossible a grid of 36 items (4 9) presented on a single page. Inthe color RAN condition, stimuli consisted of patches of color

3 Standardized scores are not available for the reading fluency measure,and percentile ranks were created from the sample of readers tested ingovernment and independent schools. Means and standard deviations werecreated for each of the grades assessed (2, 3, 4, 5) with at least 14participants in each grade.

4 The Independent school used to recruit participants for the Controlgroup was an all-girls school, and while each group still maintains a largemix of boys and girls, the gender imbalance between the Control andDyslexia groups is worth noting.



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(red, blue, green, yellow, brown, and black) and in the object RANcondition stimuli consisted of familiar pictures (pencil, boat, star,key, chair, and fish). Both object RAN and color RAN tasksinclude two trials (a total of 72 items for each condition) withperformance scored as time taken to complete both trials. Scoreswere then converted into standardized percentile ranks.

Inspection time. To make the inspection time task appropri-ate for children, the stimulus consisted of a cartoon alien imagewith two antennas (straight lines) of differing length, similar to adesign used by Anderson, Reid, and Nelson (2001). The partici-pants were asked to identify the longer of the two lines, respondingvia a button box. To reduce the influence of left/right confusiondifficulties found in some poor readers (Miles, 1983), buttonscorresponding to the left and right antennae were located on theleft and right sides of the button box, respectively. FollowingBurns and Nettelbeck (2005), the long antennae measured 27 mmand the short antennae measured 22 mm, with the differenceequating to a visual angle of approximately 1.15 . The longerantennae occurred on the left and right equiprobably with eachpresentation.

As shown in Figure 1 each trial began with a small fixation crosspresented for 1000 ms, followed by a random blank presentationbetween 250 ms and 1000 ms, followed by the antennae stimuli, andfinally by a 37 mm lighting flash backward mask presented for 300ms. The initial antennae display time was 266.6 ms (20 screen-frames) after which a staircase algorithm reduced the display time by13.33 ms (1 screen frame) after three correct responses, and increasedthe display time by 13.33 ms after each incorrect response (Burns &Nettelbeck, 2005; Wetherill & Levitt, 1965). Participants continueduntil 10 reversals had occurred, with inspection time thresholds cal-culated as the average of the last eight reversals; resulting in athreshold estimate of approximately 79%.

Before the threshold estimation process, participants completed

a series of 10 practice trials where display times ranged from 253.3to 306.6 ms. The estimation process was not commenced untilparticipants had responded correctly to at least nine of 10 practicetrials. All met this pretest criterion, although a small number of children required a second series of 10 practice trials.

Go/no-go reaction time. Reaction time was assessed using avisual go/no-go reaction time task. To make the task child-friendly, it was modeled on the start of a race, with childreninstructed to respond as rapidly as possible to a traffic lightchanging from red to green, but to give no response when the lightchanged from red to yellow. Prior to each trial, participants were

instructed to place the index finger of their preferred hand on theresponse button. The traffic light was initially illuminated red andfollowing a random interval between 1 and 4 seconds, the trafficlight would either change to green or to yellow.

The task consisted of 10 practice trials followed by 50 experi-mental trials (30 response trials, 20 nonresponse trials) with theintertrial interval controlled by participants. To minimize the in-fluence of outlier reaction times, median response times to thepresentation of the green light were used in the analyses. Responseinhibition was also recorded as the total number of false-alarmresponses (button pressed in response to the yellow light), althoughsuch responses were infrequent for both Dyslexia and Controlgroups (average false alarms were less than 10% in each group).

Rapid serial visual presentation (RSVP) and the attentionalblink. Attentional blink duration and individual RSVP targetidentification were assessed using the same paradigm as describedby McLean et al. (2010) which reported on the attentional blink phenomenon in more detail in a subset of the present participants.The tasks comprised separate single-target and dual-target (atten-tional blink) conditions, in which either one or two targets werepresented in an RSVP stream of distractors.

In the dual-target condition, participants were instructed to identifytwo targets, where the first target was one of four arrows (left, right,up, and down), and the second target was one of six shapes (square,cross, plus, triangle, diamond, and circle) with all targets measuringapproximately 1.5 of visual angle. A diagram of the dual-targetRSVP task is illustrated in Figure 2. Each trial began with a fixation

point presented for 500 ms followed by a fore-period series of two,four, or six distractor items, each displayed for approximately 26.6 msand separated by a blank ISI of 80 ms. Distractor stimuli consisted of an arbitrary superimposition of target stimuli scattered randomlyacross a notional square of approximately 1.5 1.5. The secondtarget followed the initial target after either one (SOA 200 ms),three (SOA 400 ms), five (SOA 600 ms), or seven distractors(SOA 800 ms). 5 Participants responded via a button box to mini-mize verbal demands and were instructed to identify the two targets inthe same order as they were presented. The single-target conditionwas identical to the dual-target condition, except that a distractor waspresented in place of the first target.

The dual-target and single-target conditions were presented in

separate blocks of trials, with the order of the two blocks randomizedacross participants. Both conditions included a series of 10 practiceitems before the experimental trials began. In total, there were 40single-target condition trials and 120 dual-target condition trials. Par-ticipants were provided a short break every 20 trials, and the dual-target condition was conducted over two separate blocks of 60 trialswith other tasks completed in between to ensure children remainedattentive. A pilot study was conducted on a small sample of childrento ensure both the stimuli and task difficulty was appropriate for

5 Because of the refresh rate of the monitor, actual lags were approxi-mately 213.3 ms, 426.6 ms, 639.8 ms, and 854.1 ms.

Figure 1. Schematic diagram of stimulus presentation sequence for theinspection time task (actual stimuli were presented in color).



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young children and that participants demonstrated a significant atten-tional blink effect.

Additional control measures. Nonverbal reasoning ability wasassessed using Ravens Colored Progressive Matrices (Raven, 1956).

A continuous performance task was also included to measuresustained attention and response inhibition. This task was also used inMcLean et al. (2010) and was modeled on similar continuous perfor-mance measures used in previous research (Halperin, Sharma, Green-blatt, & Schwartz, 1991; Stuart, McAnally, McKay, Johnston, &Castles, 2006). The task required participants to observe a serialpresentation stream of shapes (enlarged versions of the same nonal-phabetic stimuli used in the attentional blink paradigm) in which eachof the shapes was easily identifiable and presented for a single frame(approximately 13.3 ms), with an inter-stimulus interval of 1.6 sec-onds (stimuli were not masked). Participants were instructed to pressa single button only when a triangle was presented; however, as inStuart et al.s (2006) study, participants were also instructed not torespond to the triangle if a square was presented two shapes prior.This condition aimed to introduce a response inhibition aspect to thetask and to minimize ceiling effects, particularly in the normal read-ers.

The task required approximately 6 minutes to complete andconsisted of 160 stimuli presented in a pseudorandom order in-

cluding 20 target triangle stimuli, 15 of which required a responseand five of which required no response. To maintain consistencyacross participants, all children completed the task after approxi-mately 60 minutes of testing. Continuous performance was calcu-lated as percentage error and included both misses (when thetriangle was missed) and false positives (failure to inhibit re-sponses to a triangle despite preceding information).

Results

The performance of the Dyslexia and Control groups on themeasures of reading ability, phonological language skill, and ad-ditional control measures are displayed in Table 1. Significant

differences were evident between groups across each of the fourreading measures, with the Dyslexia group performing at least 1.5standard deviations below the mean on all reading tests. TheDyslexia group also exhibited phonological processing deficitscompared with controls, with significant group differences evidenton phoneme deletion, nonword repetition, and forward digit span

tasks. The Dyslexia group was significantly impaired on the con-tinuous performance task, however no significant differences wereevident in nonverbal reasoning performance.

Low-Level Visual Temporal Processing:The Chromatic Flicker Perception Task

To assess task reliability, Cronbachs Alpha was examined forthe ascending and descending trials across each of the three con-ditions of the flicker perception task, with analyses indicatingparvocellular ( .896), magnocellular ( .918), and high-contrast conditions ( .898) were each very reliable measures.Median temporal thresholds for the 90 participants were calculatedfor all conditions and, as expected, parvocellular temporal thresh-olds were the lowest (21.064 Hz), followed by isoluminant mag-nocellular temporal thresholds (30.196 Hz), and high-contrast tem-poral thresholds (38.797 Hz).

Differences between Dyslexia and Control groups were thenexamined across all three conditions. In the magnocellular condi-tion, the Dyslexia group exhibited significantly lower temporalthresholds (29.480 Hz) compared with the Control group (30.838Hz), t (80) 2.332, p .022, d .512. Conversely, no significantdifferences were evident between Dyslexia (20.632 Hz) and Con-trol groups (21.226 Hz) in the parvocellular condition, t (80) 1.039, p .05, d .229; however, Dyslexia temporal thresholdsin the parvocellular condition were somewhat lower then Controls.No significant differences were evident between Dyslexia (38.695

Hz) and Control groups (39.055 Hz) in the high-contrast flicker

Figure 2. Schematic diagram of stimulus presentation sequence for theattentional blink task. Actual stimuli were gray and displayed on a black

background. In the single-target condition, the presentation sequence wasidentical except the first target was replaced with a distractor.

Table 1Summary of Dyslexia and Control Group Performance on Reading Ability, Phonological Skills, and AdditionalControl Measures

Measure

Dyslexia(n 40)

Control(n 42)

Mean SD Mean SD

Chronological age (years) 9.5 1.4 9.6 1.3Reading

WIAT word reading (STD) 82.3 9.9 108.1 9.4WARP reading fluency (PR) 9.3 10.2 63.8 23.3Irregular words (PR) 10.6 10.9 66.5 21.1Nonwords (PR) 13.2 14.4 71.1 23.3

Phonological skillsPhoneme deletion (PR) 38.5 28.8 70.6 22.0Nonword repetition (PR) 48.8 21.5 59.1 21.1Forward digit span (PR) 37.2 26.2 49.3 28.3

Additional control measuresNonverbal reasoning (Ravens PR) 63.0 22.7 72.5 22.6Continuous performance task (PE) 65.6 34.3 37.7 28.3

Note . PR percentile rank; PE percentage error; STD standardscore. p .05. p .01.



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condition, t (80) 0.524, p .05, d .210. Closer inspectionrevealed that this may have been a consequence of insufficienttemporal resolution at higher flicker rates. While the 1-ms resolu-tion provided adequate temporal resolution in the isoluminantmagnocellular and parvocellular conditions (performance had arange of 12 and 17 steps, respectively), the faster flicker rates

required in the high-contrast condition resulted in an observedrange of only six steps. This drastic reduction in variance (over twothirds of the 90 participants performance varied across only twosteps) reduces the likelihood of significant differences being de-tected despite Dyslexia group thresholds being somewhat lower.

Associations with specific reading ability measures. Giventhat the Dyslexia group exhibited temporal thresholds significantlylower than the Control group in the magnocellular condition,Pearson correlations between reading ability and performance onthe flicker perception task were calculated to explore whetherassociations were stronger for particular aspects of reading ability(see Table 2). All 90 participants were included in correlationanalyses and, while the majority of the sample consisted of sepa-

rate control and impaired reading groups, performance on each of these measures was found to be normally distr ibuted(KolmogorovSmirnov one sample test, all p values .1). Con-sistent with the group differences shown above, magnocellulartemporal thresholds were correlated significantly with readingability. However, the associations were relatively equal acrosseach of the three specific reading measures. This was confirmed bya series of Williams t tests (Williams, 1959), which found nosignificant differences in correlation strength (all p values .1).

It can be seen from Table 2 that there were also significantcorrelations between parvocellular temporal thresholds and somemeasures of reading ability, particularly performance on the WIATsubtest and WARP reading fluency task. Given these significantcorrelations, semipartial correlations between magnocellular tem-poral thresholds and reading ability controlling for parvocellulartemporal thresholds were also calculated (bold section of Table 2).These analyses revealed a weak, but significant, association withirregular word reading as well as nonsignificant trends with bothnonword reading and reading fluency. Williams t tests indicatedthat the correlations did not differ significantly in strength (all pvalues .1).

Prevalence of specific magnocellular deficits in the sample.The group analyses outlined above provide evidence for a mag-nocellular deficit in dyslexia at the group level, but the magnitudeof the deficit was relatively small and the correlations shown inTable 2 indicate that magnocellular functioning accounted for nomore than 9% of the variance in any reading measure (less than 5%when controlling for parvocellular temporal thresholds). Giventhis, and the heterogeneous nature of reading difficulties, we wenton to examine how many individual participants within the sampleexhibited a deficit in magnocellular temporal thresholds but nor-mal parvocellular temporal thresholds. Percentile ranks for mag-nocellular and parvocellular temporal thresholds were createdbased on the means and standard deviations of Control groupperformance, 6 and a similar criterion as had been used to select theinitial Dyslexia and Control groups was then applied to magno-cellular and parvocellular temporal thresholds. That is, participantswere identified as having a specific magnocellular deficit based onperformance in the bottom 15th percentile on magnocellular tem-

poral thresholds but above the 30th percentile on parvocellulartemporal thresholds.

As illustrated in Figure 3, despite the small magnitude of thegroup effect, a relatively large proportion participants in the Dys-lexia group exhibited delayed magnocellular temporal thresholdsin the bottom 15th percentile (42.5% of dyslexic sample, compared

with only 14% of unimpaired readers). However, only four of these poor readers also exhibited parvocellular temporal thresholdsabove the 30th percentile. That is, only 10% of the Dyslexia groupexhibited a specific magnocellular deficit. Moreover, an additionaltwo unimpaired readers also met the criteria for a specific mag-nocellular deficit (one Control group participant and one unclas-sified participant).

High-Level Visual Temporal Processing

As shown in Table 3, the Dyslexia group also exhibited signif-icant deficits on the majority of the higher-level visual temporalprocessing measures.

Rapid naming. Rapid naming performance was examinedusing standardized percentile ranks from the CTOPP, with theDyslexia group exhibiting significant deficits in both color RAN,t (80) 3.272, p .002, d .723, and object RAN, t (80) 5.114, p .001, d 1.130. Significant group differences werealso evident in raw reaction times on the RAN task, with theDyslexia group (Object RAN: 79.2 seconds, Color RAN: 74.2seconds) responding on average 18.2 seconds slower for objectRAN and 13.5 seconds slower for color RAN compared with theControl group (Object RAN: 61.0 ms, Color RAN: 60.7 ms). Asignificant Group/Condition interaction effect was also evidentbetween Object and Color RAN conditions with the Dyslexiagroup exhibiting a significantly larger deficit in the Object RANcondition, F (80, 1) 4.868, p .05, p

2 .057.Go-no/go reaction time. In the go/no-go reaction time task,

the Dyslexia group responded on average 42 ms slower than theControl group, t (80) 2.204, p .030, d .485. Despitesignificant delays in reaction time, no significant group differenceswere evident in the number of false-alarm responses ( p .5) or inthe participants reaction time standard deviations across trials( p .6).

Inspection time. The Dyslexia group had slightly slowerinspection times than the Control group, but this difference failedto reach significance, t (80) 1.493, p .05, d .327. However,similar to previous findings exploring inspection time in dyslexia(Kranzler, 1994; Whyte et al., 1985), the inspection time thresh-olds were found to be positively skewed (KolmogorovSmirnov

Z 1.81, p .05), and five participants recorded extremely slowinspection times (more than 120 ms). Consequently, participantswhose inspection time thresholds exceeded 120 ms were all givenan equal value of 120 ms; however, group comparisons of the newinspection time thresholds again did not reveal any significantdifferences between Dyslexia and Control groups, t (80) 1.203, p .05, d .265.

6 Standard deviations were based on Control group performance underthe proviso that a higher proportion of individuals with dyslexia mayexhibit some form of magnocellular deficit as hypothesized by the mag-nocellular deficit hypothesis (Chase & Stein, 2003).



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RSVP measures. Performance on the RSVP measures wasanalyzed separately for single and dual-target identification. Sim-ilar to the findings from a subset of these data (McLean et al., inpress), analyses of single-target performance indicated that theDyslexia group exhibited significant deficits compared with theControl group, t (80) 3.308, p .001, d .727. Similar deficitswere also evident for both T1 performance, t (80) 3.585, p .001, d .786, and T2 performance, t (80) 3.787, p .001, d .834, in the dual-target condition.

Attentional blink differences between the groups were exploredby examining mean second target accuracy at each of the lags (200ms, 400 ms, 600 ms, and 800 ms) 7 when the first target had beencorrectly identified (T2 T1). Two participants from the Dyslexiagroup were removed from further analyses as their first targetidentification was below 50%, dramatically reducing the numberof usable trials per lag and as such the reliability of T2 T1 scores.

Mean percent correct T2 T1 performance as a function of Groupand Lag is illustrated in Figure 4. The 2 (Group: Dyslexia vs.Control) 4 (Lag: 200 ms, 400 ms, 600 ms, and 800 ms) ANOVArevealed a significant main effect for Lag, F (3,78) 26.252, p.001, p

2 .252, consistent with an attentional blink effect. Asignificant main effect for Group was also evident, F (1, 78) 11.650, p .001, p

2 .130, with the Dyslexia group exhibitinginferior performance across all lags. However, the Group-Laginteraction effect was not significant ( F 1), indicating no dif-ferential attentional blink between the Dyslexia and Controlgroups. The significant main effect for Group is consistent with thegeneral deficit in RSVP target identification in the Dyslexia group,as reported in Table 3.

For subsequent correlation and factor analyses, a single measureof attentional blink magnitude was calculated using a similartechnique to that used in previous research, where a difference

score is calculated between T2 T1 percent correct at lags of 800 msand 200 ms (Arnell, Howe, Joanisse, & Klein, 2006). Consistentwith the above analyses (See Table 3), no significant group dif-ferences were evident on this measure ( p .05).

Associations Between Low-Level and High-LevelVisual Temporal Processing

The findings above provide evidence for both low-level andhigh-level visual temporal processing impairments in developmen-tal dyslexia. In light of these findings, a series of exploratorycorrelation and factor analyses were conducted to determine theextent to which low-level delays in magnocellular temporal reso-lution were associated with higher-level temporal processing def-

icits.The Pearson correlation matrix displayed in Table 4 provides

preliminary evidence for a link between low-level magnocellulartemporal thresholds and higher-level visual temporal processingmeasures, with significant correlations evident between magnocel-

7 A similar ANOVA was also completed examining T1 performance ex-cluding the two participants who exhibited T1 performance below 50%. Thisanalysis revealed a significant effect main of Lag, F (3, 78) 12.364, p .001, p

2 .137, and a significant main effect for Group, F (1, 78) 11.716, p .001, p

2 .131, with no significant Group-Lag interaction effect ( F 1).More detailed analyses are available in McLean et al. (2010).

Figure 3. Scatter plot illustrating correlation between magnocellular tem-poral thresholds and parvocellular temporal thresholds for the Dyslexiagroup, the Control group, and the unclassified participants. Scores repre-sent percentile ranks calculated based on the mean and standard deviationof the Control group. Cut-offs represent the 15th percentile for the mag-nocellular condition and 30th percentile for the parvocellular condition.

Table 2Correlation Matrix Exploring Reading Ability and Performance on the Chromatic Flicker Perception Task

WIAT wordreading

Irregularwords Nonwords

WARP readingfluency

Magnocellulartemporal threshold

Parvocellulartemporal threshold

WIAT word reading

Irregular words .825 Nonwords .752 .751 WARP reading fluency .792 .788 .732 Magnocellular temporal threshold .296 .257 .238 .278 Parvocellular temporal threshold .263 .132 .144 .210 .603 High contrast temporal threshold .125 .105 .082 .070 .474 .267Specific magnocellular temporal threshold .179 .226 .191 .194

Note . Bold section indicates semi-partial correlations controlling for parvocellular temporal thresholds. p .05, p .01.



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lular temporal thresholds and the majority of the higher-levelmeasures. Specifically, magnocellular temporal resolution corre-lated significantly with inspection time, go/no-go reaction time,and single-target RSVP target identification, but no associationswere evident with attentional blink magnitude or rapid naming(although significant correlations were evident with Object RAN

raw scores). Moreover, each of these correlations remained signif-icant when controlling for mediating factors 8 including nonverbalreasoning and continuous performance (all p values .05).

To further explore these intercorrelations, a principal compo-nents analysis with varimax rotation was conducted including eachof these seven temporal processing measures. This analysis re-sulted in a three-factor solution and these factors were labeledPerceptual Speed , Verbal Fluency , and Attentional Selection (seeTable 5). The key finding from this three-factor solution is thatlow-level magnocellular temporal thresholds appear to be associ-ated with particular aspects of higher-level temporal processing(namely go/no-go reaction time, inspection time, and single-targetRSVP target identification), yet both rapid naming ability andattentional blink magnitude form isolated factors. These threefactors also appeared to hold regardless of the level of readingability, with a similar variable distribution evident in a principalcomponents analysis of the skilled reader group.

Subsequent factor analyses explored these three temporal pro-cessing factors in relation to other cognitive skills associated withreading development. Table 6 shows the findings from a principalcomponents analysis (varimax rotation) including the previousseven temporal processing measures, the three language-basedphonological processing measures (phoneme deletion, nonwordrepetition, and forward digit span), and the two additional controlmeasures (continuous performance task, and nonverbal reasoning).

This analysis resulted in a five-factor solution, and the factorswere labeled Phonological Short-Term Memory , and g (General

Performance) in addition to the original Perceptual Speed , VerbalFluency , and Attentional Selection factors, which remained largelyunchanged. Of particular interest, the three temporal processing

factors, including Perceptual Speed , were each independent of phonological processing. Notable also is that performance on thephoneme deletion task was more closely tied to the g factor than tothe Phonological Short-Term Memory factor. It is likely this is atleast partially attributable to the complexity of phoneme deletiontasks compared with the relatively simple forward digit span andnonword repetition tasks. The continuous performance task wasalso weakly associated with Perceptual Speed in addition to the gfactor, and this is likely to be attributable to the continued vigi-lance required to remain focused during the inspection time, reac-tion time, and RSVP tasks.

Magnocellular temporal thresholds also partially loaded on thePhonological Short-Term Memory factor. A closer examination of the Pearson correlations indicated that this finding was primarilyattributable to correlations between magnocellular temporalthresholds and forward digit span ( r .377, p .001), rather thanother latent variables within the Phonological Short-Term Memoryfactor. Subsequent factor analyses excluding forward digit spanresulted in magnocellular temporal thresholds falling solely withinthe Perceptual Speed factor. The association between magnocel-lular temporal resolution and forward digit span was highly robust,however, with semipartial correlations controlling for age, contin-

uous performance, nonverbal reasoning, as well as parvocellulartemporal thresholds remaining significant across the entire sample(r .220, p .044), and particularly strong in the Control group(r .491, p .004, see Figure 5).

Predicting specific reading abilities. To explore the rela-tionship between the derived factors and particular aspects of

8 Parvocellular temporal thresholds were not included in these semipar-tial correlation analyses as the parvocellular flicker condition was designedas a specific psychometric control for the magnocellular flicker conditionwhereas the higher-level visual temporal processing measures assessedmore general aspects of temporal processing.

Figure 4. Mean percentage of correct T2 T1 identification as a functionof temporal lag between first and second targets. Unbroken line representsaverage Control group performance, and broken line represents averageDyslexia group performance. Error bars represent standard error.

Table 3Summary of Dyslexia and Control Group Performance on Higher-Level Temporal Processing Measures

Measure

Dyslexia(n 40)

Control(n 42)

Mean SD Mean SD

Rapid namingRapid object naming (PR) 27.6 25.0 55.6 24.7Rapid color naming (PR) 28.7 24.4 47.1 26.2

Go/no-go reaction timeMedian reaction time (ms) 525.5 106.2 481.1 79.0Total false alarms 1.9 1.7 1.4 1.3

Inspection timeInspection time threshold (ms) 63.6 40.8 53.7 20.8

RSVP performanceSingle-target performance (PC) 81.5 13.4 89.8 9.0T1 performance (PC) 81.8 14.6 90.8 7.3T2 performance (PC) 68.1 15.1 79.2 11.1Attentional blink magnitude 20.9 10.6 19.9 9.0

Note. PR percentile rank; PC percent correct; T1 first target;T2 second target. Attentional blink magnitude T2 T1 percent correctat 800 ms minus percent correct at 200 ms. p .05. p .01.



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reading skill, Pearson correlations were examined between each of the five orthogonal factors described above and the three specificmeasures of reading ability (see Table 7). As the factor analysiswas orthogonal, the variance predicted in reading performance byany one factor represents the unique variance explained indepen-

dent of the influence of the other factors. Consistent with previousresearch, Verbal Fluency , Phonological Short Term Memory , andg factors all accounted for a significant independent proportion of variance in reading ability, yet no significant correlations wereevident between the Attentional Selection factor and reading abil-ity. Of note is that the Perceptual Speed factor also accounted forsignificant variance in reading ability, independent of rapid nam-ing, phonological processing, and general performance skills. Thatis, the Perceptual Speed factor represents a unique predictor of variance in reading ability.

It should be noted that the correlations with reading ability wererelatively weak (correlations with reading fluency only trended tosignificance, p .08). In addition, the correlation strength did notvary across different aspects of reading skill. This was confirmedby subsequent Williams t tests which failed to find any significantdifferences in correlation strength (all p values .3). Furthermore,associations between Perceptual Speed and reading ability did notappear constant across the range of reading ability, and while aprincipal components analysis of the Control group produced asomewhat similar five-factor solution, no significant correlationswere evident between the Perceptual Speed factor and readingability.

Discussion

By specifically focusing on temporal aspects of magnocellularfunctioning rather than contrast sensitivity to low spatial frequen-cies, this study took a novel approach to exploring the magnocel-lular deficit theory of dyslexia. The findings indicated that the new

chromatic flicker perception task designed for this purpose pro-vides a reliable assessment of magnocellular temporal resolutionthat also incorporates a careful control for parvocellular temporalresolution. The results from this task provided evidence for aspecific magnocellular temporal deficit in dyslexia, although therelationship with reading ability was weak. In addition, low-levelmagnocellular temporal resolution was found to be associated withperceptual speed components of higher-level temporal processingmeasures. This factor was found to account for significant variancein general reading ability, independently of phonological process-ing, rapid naming, and general performance skills.

Evaluating the Chromatic Flicker Perception Test

The data from the current study indicate that the chromaticflicker perception task is an accurate and reliable measure of magnocellular temporal resolution and provides an excellent con-trol measure of parvocellular temporal resolution. Consistent withprevious human and primate research comparing magnocellularand parvocellular systems, all 90 participants exhibited a lowertemporal resolution in the parvocellular condition than in themagnocellular condition. Similar findings have been reported us-ing a phantom contours measure (Sperling et al., 2003; Sperling etal., 2006), but the large difference in temporal latencies betweenmagnocellular and parvocellular conditions reported with that task have led to questions concerning its validity (Skottun & Skoyles,2006). In contrast, the temporal thresholds recorded using thechromatic flicker perception task were extremely similar to thosetypically found in primate research: Levitt et al. (2001) foundcut-off temporal frequencies of 31.62 Hz for the magnocellulardivision and 21.88 Hz for the parvocellular division of the periph-eral retino-geniculo-striate pathway, which are very close to themagnocellular (30.838 Hz) and parvocellular thresholds (21.226Hz) found in the Control group. Furthermore, the high reliabilityand simplicity of the task makes it an especially valuable tool foruse with children and special populations compared to more com-plex contrast sensitivity, coherent motion, and phantom contourmeasures.

The main reason for using chromatic flicker is that it allowed aspecific focus on the temporal resolution of the magnocellular

Table 4Correlation Matrix Exploring Low-Level and High-Level Temporal Processing Measures

Attentional blink magnitude

Rapid objectnaming

Rapid colornaming

Inspectiontime

Go/no-goreaction time

Single-targetperformance (RSVP)

Attentional blink magnitude

Rapid object naming .031 Rapid color naming .108 .689 Inspection time .066 .151 .142 Go/no-go reaction time .056 .222 .200 .336 Single-target performance (RSVP) .032 .231 .230 .424 .240 Magnocellular temporal threshold .042 .137 .011 .269 .252 .380

p .05. p .01.

Table 5Principal Components Analysis of Low-Level and High-LevelTemporal Processing Measures

n 90

Factor

Perceptualspeed

Attentionalselection

Verbalfluency

Attentional blink magnitude .909Rapid object naming .897Rapid color naming .907Inspection time .746Go/no-go reaction time .620Single-target performance (RSVP) .709Magnocellular temporal threshold .696Percentage of variance explained 15.94% 25.00% 27.81%

Note. Scores below .35 not shown.



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pathway rather than on more general magnocellular functioning, asis the case for contrast sensitivity and particularly coherent motionmeasures. This was done in the hope that the results might clarifysome of the mixed findings of previous research.

Magnocellular Temporal Resolution in Dyslexia

Poor readers were found to exhibit a significant deficit inmagnocellular temporal thresholds but exhibited no significantdeficit in parvocellular temporal thresholds. The overall deficit inisoluminant (but not chromatic) flicker in the individuals with

dyslexia was further confirmed by the semipartial correlations,controlling for parvocellular temporal thresholds, between magno-

cellular temporal thresholds and reading ability. These semipartialcorrelations revealed a significant association between magnocel-lular thresholds and irregular word reading, and weaker correla-tions in the same direction for nonword reading and readingfluency. The significant relationship with irregular word reading isconsistent with previous findings from phantom contour researchlinking magnocellular temporal processing with orthographic read-ing skills (Sperling et al., 2003; Sperling et al., 2006). However, itis important to note that we found no significant differences incorrelation strength across the reading measures, suggesting that

Figure 5. Scatter plot illustrating semipartial correlation between mag-nocellular temporal thresholds and forward digit span in the Control group,controlling for continuous performance, nonverbal reasoning, and parvo-cellular temporal thresholds (standardized scores displayed).

Table 6Principal Components Analysis of Low-Level and High-Level Temporal Processing Tasks in Addition to Phonological Processing and Additional Control Measures

n 90

Factor

Attentionalselection

Verbalfluency

Perceptualspeed

Phonologicalshort-term memory g factor

Attentional blink magnitude .894Rapid object naming .863Rapid color naming .909Inspection time .726Go/no-go reaction time .782Single-target performance (RSVP) .566Magnocellular temporal threshold .537 .487Forward digit span .832Nonword repetition .799Phoneme deletion .409 .568Ravens matrices .850Continuous performance .372 .705Percentage of variance explained 9.86% 14.90% 15.57% 16.26% 14.32%

Note. Scores below .35 not shown.

Table 7Pearson Correlation Matrix of Five Orthogonal FactorsPredicting Specific Aspects of Reading Performance

Factor

Reading ability

Irregularword

readingNonwordreading

Readingfluency

Perceptual speedMagnocellular temporal threshold, Go/

no-go reaction time, Inspection time,Single-target performance (RSVP),Continuous performance .222 .214 .194

Phonological short-term memoryForward digit span, nonword repetition,phoneme deletion, magnocellulartemporal threshold .229 .231 .302

g factorNonverbal reasoning, phoneme deletion,

continuousperformance .362 .468 .245

Verbal fluencyRapid object naming, rapid color

naming .411 .412 .466Attentional selection

Attentional blink magnitude .034 .109 .043

p .05. p .01.



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associations are not likely specific to orthographic or phonologicalreading skills.

This pattern of findings provides evidence of a specific magno-cellular impairment in dyslexia, however, a number of cautionarycaveats must be raised. First, no significant differences were foundbetween groups in the high-contrast flicker condition, although the

Dyslexia group did exhibit lower temporal thresholds. The lack of difference here is inconsistent with a specific magnocellular def-icit. One possibility is that the specific deficit in the isoluminantmagnocellular condition may reflect a compromised connectionbetween midget ganglion and magnocells rather than a specificdeficit in the magnocellular system itself, given there is no changein color polarity in the high-contrast condition. Another possibil-ity, however, is that group differences failed to reach significancebecause of insufficient temporal resolution at the higher flickerfrequencies involved in the high-contrast condition. Flicker fre-quency was adjusted in 1 ms steps for all conditions, yet partici-pants exhibited less than half the range in task performance in thehigh-contrast condition and nearly two thirds of all participantsperformance varied across only two steps. This possibility could betested by manipulating flicker frequency in step sizes smaller than1 ms.

Second, the strength of the association between magnocellularfunctioning and reading ability was relatively weak. While corre-lations between magnocellular temporal thresholds and readingability were statistically significant, they accounted for no morethan 9% of variance in any of the reading measures. Similarly,despite semipartial correlations (controlling for parvocellular tem-poral thresholds) remaining significant with irregular word read-ing, less than 5% of variance was explained. This reflects a farsmaller proportion of explained variance than was the case forlanguage-based measures of phonological processing as well asrapid naming tasks, which explained as much as 35% and 29% of

variance in reading measures, respectively.Finally, a number of poor readers also exhibited lower parvo-

cellular temporal thresholds, suggesting that their impairmentswere not specific to the magnocellular system. Indeed a specificmagnocellular deficit did not appear a common feature in themajority of the poor readers recruited for this study. While manyindividuals with dyslexia (42.5%) did show a significant deficit inmagnocellular temporal resolution (performance below the 15thpercentile), only six participants also showed unimpaired parvo-cellular temporal thresholds (performance above the 30th percen-tile), two of whom were unimpaired readers. Thus, only 10% of theDyslexia group exhibited a specific magnocellular impairment.This finding not only indicates that a specific magnocellular im-pairment is not evident in the majority of poor readers, it alsosuggests that a magnocellular deficit in isolation is unlikely tocause dyslexia, given that some highly skilled readers also exhib-ited such an impairment.

Considering that both ascending and descending trials were usedin all conditions however, it does seem likely that the 42.5% of poor readers who exhibited magnocellular temporal thresholds inthe bottom 15th percentile were actually exhibiting a flicker per-ception deficit rather than simply experiencing difficulties with thegeneral demands of the task. The findings provide only modestevidence for a specific magnocellular deficit, but they do suggestthat a significant proportion of individuals with dyslexia do exhibita more general low-level temporal processing impairment. While

parvocellular and magnocellular flicker conditions were highlycorrelated (see Figure 3), it may simply be that the superiorconduction velocity and temporal resolution characteristics of themagnocellular division of the peripheral retino-geniculo-striatepathway make the isoluminant flicker perception task the moresensitive test of subtle differences in low-level temporal resolution.

Indeed, the findings from the current study indicated temporalprocessing deficits are not only reflected in low-level measures of flicker perception but also across higher-level measures of visualtemporal processing outside of the broad magnocellular system.Note that, in contrast to some theorists (e.g., Pammer & Vidyasa-gar, 2005), it is not our view that higher level temporal processingabilities are necessarily associated with extrastriate cortical areasthat form part of the dorsal where pathway. There is a substantialprojection from the magnocellular system to the ventral whatstream (Nassi & Callaway, 2009) that may support rapid objectand scene recognition (Mace, Thorpe, & Fabre-Thorpe, 2005).

Associations With Higher-Level Visual Temporal

Processing in DyslexiaAn additional aim of this study was to examine the link between

low-level deficits in magnocellular temporal resolution and poorperformance on higher-level visual temporal processing measures.Consistent with previous research, poor readers did exhibit deficitsacross a number of higher-level visual temporal processing mea-sures compared to controls: they showed delays on both object andcolor rapid naming tasks, as well as deficits in go/no-go reactiontime and single RSVP target identification (there was some evi-dence for delayed inspection time thresholds in individuals withdyslexia, but group differences failed to reach significance). In adual-target search task, however, poor readers exhibited inferiorperformance at all temporal lags (no significant Group-Lag inter-

action), indicating no difference in the depth or duration of theattentional blink between groups (Badcock et al., 2008; McLean etal., 2010).

Correlation analyses indicated links between low-level magno-cellular temporal resolution and the majority of the higher-levelvisual temporal processing measures we included, although norelationship was evident with either attentional blink magnitude orrapid naming. Furthermore, these associations remained signifi-cant after controlling for potential mediating factors such as non-verbal reasoning and sustained attention. These relationships werefurther supported in an exploratory factor analysis of all sevenlow- and high-level visual temporal processing measures, whichrevealed a clear three-factor solution with factors labeled VerbalFluency , Attentional Selection , and Perceptual Speed . These threefactors appeared to apply across the full range of reading ability,with a similar three factor solution evident when only participantsfrom the Control group were included.

A more detailed inspection of the processes underlying perfor-mance on the latent variables within each factor suggested thethree factors may indeed be tapping somewhat different mecha-nisms. Within the Perceptual Speed factor, single-target RSVPperformance, inspection time, and flicker perception in the mag-nocellular condition all involve the low-level perception of astimulus or stimulus change. While go/no-go reaction time (alsoincluded in the Perceptual Speed factor) involves a speeded motorresponse, the perceptual identification/recognition of each stimulus



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ability (Carver, 1990; de Jonge & de Jong, 1996; Hage & Stroud,1959). It may be that these higher-cognitive functions, affectinggeneral learning ability, mediate the observed relationship betweenperceptual speed and reading ability. While this hypothesis istentative, it highlights the need for research exploring mediatedcausal pathways, rather than exploring potential direct links be-

tween magnocellular function and reading.Another potentially mediating factor of interest is the role of age. Associations between perceptual speed and age have beenreported (Anderson et al., 2001; Salthouse, 1996), and indeedsignificant correlations between age and the perceptual speedfactor were evident in the current data. Given the small agerange within the sample (7 to 11, SD: 1.4), exploring develop-mental changes in magnocellular temporal resolution and per-ceptual speed was outside the scope of this cross-sectionalstudy. However, it would be of interest to carry out longitudinalresearch to explore whether the inferior perceptual speed seenin some children with reading difficulties is an ongoing deficit,or merely a delay in development. Such a longitudinal studymay also provide some clues as to the relationship betweenperceptual speed and reading ability, as different aspects of reading skill follow different developmental trajectories (Ma-nis, Seidenberg, Doi, McBride-Chang, & Petersen, 1996;Snowling, 2001).

Conclusion

We conclude that a specific deficit in magnocellular temporalresolution is not a feature of the majority of children with devel-opmental dyslexia. However, our findings do indicate that somepoor readers exhibit a more pervasive deficit in low-level percep-tual speed, and that this perceptual speed factor is a significant andunique predictor of reading ability, independent of phonological

processing, rapid naming, and other general performance skills.The mechanism by which perceptual speed might influence read-ing acquisition, if indeed it plays a causal role at all, requiresfurther exploration.

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