Perception & Psychophysics1973, Vol. 14, No.1, 51·59

Switching attention between the ears to monitor tones *NIGEL HARVEY and ANNE M. TREISMAN

Department ofExperimental Psychology, Oxford University, uxford, England

Alternating dichotic presentation of tones was compared with monaural presentation in two different perceptualtasks. When Ss were asked to detect higher frequency target tones in lists of six background tones, they missed less than5%. However, when the presentation was fast (lO/sec) and the tones alternated dichotically, Ss reported "hearing" onlythree or four tones. In two further experiments, Ss were asked to judge the number of tones or to report their spatialand temporal positions; these tasks were much less accurately performed, for the dichotic lists, than the targetdetection task. The differing effects of switching rate on these tasks are discussed in relation to the types of processingthey require.

Treisman (1971) reported and reviewed experimentsin which Ss were asked to recall a list of auditory speechitems. Performance was worse when the items within alist were presented alternately to the two ears than whenthey were presented sequentially to one or both. Furtherexamination of the limitation involved requiresinvestigation of its functional site, generality, and modeof operation. A precise identification of the changes intask demands associated with appearance of the effectwould also be desirable.

Functional Site of the Limitation

In Treisman's (1971) experiment, there was aninteraction between rate and type of presentation butnot between list length and type of presentation. Thissuggests that the limifation arises mainly duringperceptual analysis rather than during storage or retrievalof the items. In the present experiment, this was testedby looking for the effect on a monitoring rather than arecall task.

Generality of the Limitation

The limitation may occur only when speech items areused as stimuli. The present experiment tested this byemploying pure tones rather than auditory speech itemsin a paradigm otherwise similar to Treisman's (1971). Ifthe perception of auditory speech and pure tonesdepend on the same mechanisms, the result may help toidentify the level of perceptual analysis at which thelimitation operates. Presence of the effect with bothtypes of stimuli would indicate that it arises prior tospeech recognition, whereas its presence only withspeech signals would suggest that it occurs during speechrecognition. Experiments by Lawson (1966) andTreisman and Riley (1969) lend some support to thelatter possibility. They found that pure tones andchanges of a speaker's voice were detected equally wellin the attended and unattended channels. However,

*This research was supported by the Medical ResearchCouncil.

51

Moray and Fee (1970) cast some doubt on Lawson's(1966) finding, in that they did find impaired detectionof tones embedded in the unattended speech messagewhen transients at the beginning and end of the toneswere removed.

Some limitation to attention has also been found withsimultaneous tones. Moray (1970) and Moray andFitter! found limits in Ss' ability to monitor in paralleltwo sequences of tones for increments in eitherfrequency or intensity. These limits may have been duemainly to frequency rather than to dichotic separationof the tone sequences, but Moray and Fitter also foundthat dichotic separation increased the difficulty of theirfrequency-separated tones relative to binauralpresentation. However, there may be factors other thanthe attention division requirement which affectperformance in both experimental and controlcondi tions when stimuli are presented simultaneously.Firstly, binaural presentation may benefit frominteraural summation of information, and so may not bean appropriate control when attention-sharingdecrements are being sought. Secondly, there may becross-channel inhibition with simultaneous dichoticpresentation, as suggested by Sorkin, Pastore, andPohlman (1971). They found a small decrement withwo-channel compared to single-channel presentation.then two signals were presented at once, but little or no

difference between single- and two-channel conditionson trials when no signal was present on the contralateralchannel.

Changes in Task Demands Associatedwith Appearance of the Effect

There are also differences in the task demands inMoray's experiments compared to those with alternatingpresentation of stimuli. Moray's studies comparedefficiency in responding to both of two simultaneousinputs with efficiency in responding to one of the two orto one presented alone. A decrement in the formercondition could be associated with changes in threeseparate task demands. (1) doubling the amount ofmaterial to be processed, (2) simultaneous rather than

52 HARVEY AND TREISMAN

successive presentation of items, and (3) thepresentation of relevant material on two separate inputchannels rather than one. Taking as the controlcondition attention to a single message presented on onechannel, effects of the first task demand could beisolated by doubling the input rate relative to thecontrol message, but still using successive presentationon one channel; effects of the second by presenting twomessages simultaneously on a single channel, each at halfthe rate of the control message; and effects of the thirdby presenting altenlate items to alternate channels, usingsuccessive presentation at the same overall rate as thecontrol message. This last condition tests how farattention can be divided or alternated between thechannels themselves rather than between competingstimuli on those channels. It is possible, of course, thattwo or more of these task demands either contributeadditively or interact in causing the difficulty of thestandard two-channel simultaneous input task. Thepresent experiment (like the earlier one with speechstimuli, Treisman, 1971) tests only the third.

Nature of Limitationwith Alternating Stimuli

Evidence of an attentional limitation with tones insuch stimulus conditions comes from an experiment byAxelrod, Guzy, and Diamond (1968). In order to matchthe perceived rates of alternating dichotic and successivemonaural clicks, Ss reqUired a faster actual rate for thedichotic clicks. Axelrod et al (1968) suggested that, atfast rates, Ss are unable to switch attention between theears quickly enough and so lose a certain proportion ofinput clicks. We attempted to test this possibility in aslightly different task. If a certain proportion of stimulifail to be analyzed, Ss should miss a certain proportionof target stimuli when monitoring for them in rapidalternating sequences. Alternatively, if they do attemptto follow the stimuli by switching attention, and if thisrequires a certain additional time, their responselatencies should increase progressively through the list asthey lag further and further behind. This predictionassumes that all stimuli are analyzed in the order inwhich they arrive, i.e., that they queue for attention in abuffer store. If stimuli can be dropped from the queueor taken out of order, response latencies should increaseat first and then decrease as stimuli and attention getback into phase. This would produce oscillation of RTswith serial position.

EXPERIMENT I

Method

Apparatus and Stimuli

Pure sinusoids were generated by Advance Instruments 12-£oscillators and delivered to 1052 Sony DR-3C stereo earphonesvia a multichannel gate unit. Signals were gated at random. The

gate unit and recording and timing of responses were controlledby a LINe-8 computer. The frequencies of nontarget and targetpulses were 1.000 and 1.050 Hz. respectively. Both had aduration of 80 msec and an intensity of 70 dB S1. Ss respondedby pressing a button on a keyboard which was connected to thecomputer. RTs were measured from the start of the target tone.

Subjects

Five male and three female students, aged between 18 and 24years. were paid for serving in one t/z-h practice session and twoexperimental sessions of approximately 65 min duration each.All Ss were right-handed.

Experimental Design

Each trial consisted of a sequence of six pure sinusoids, eitherall of the same frequency (the nontarget 1,000 Hz) or else five ofthis frequency and one of another frequency (the target1,050 Hz). Following each tone sequence, Ss were allowed a2-sec intertrial interval (offset to onset) to make a response. Ineach block of 120 trials, target and nontarget trials werepresented randomly with the constraint that each type of trialappeared equally often. Targets occurred equally often in eachof the six positions in the tone chain. There were two rates,10 tones/sec and 6 tones/sec, and two conditions, monaural anddichotic alternating. Speed and mode of presentation wereconstant within anyone block. Prior to each monaural block, Sswere told whether the left or right ear would be stimulated. Priorto each alternating block, Ss were told whether the left or rightear would be stimulated fust. There were eight blocks perexperimental session. One experimental session was devoted tothe fast rate and one to the slow rate of presentation. Order ofpresentation of dichotic and monaural blocks was balancedwithin each session for each S, whereas order of presentation offast and slow blocks was balanced over sessions and Ss.

Procedure

5s were instructed to press the key with their right indexfingers as soon as they heard a target in the tone chain. If notarget was presented, they were told to withhold their responses.They were requested to respond as fast as possible, while tryingto keep their errors below five or six per block of 120 trials. Thepractice session consisted of 16 blocks of 20 trials each, given inthe same order as the experimental blocks and with one speed ofpresentation following the other. During practice, Ss were givenfeedback regarding their reaction times and error rates. The twoexperimental sessions followed the practice session on separatedays. Five additional practice trials were given before the start ofeach condition. Ss were given a I-min break between blockswhile data were stored on magnetic tape to be analyzed at theend of the session. In addition, they were allowed a 5-min breakhalfway through each session.

Results

Figure 1 gives the serial position curves for reactiontimes and percent omissions for each mode and rate ofpresentation. There were too few omissions and falsepositives to justify a statistical analysis over differentserial positions. An analysis of the total omissionsshowed a significant effect only of dichotic vs monauralpresentation [F(1,7) = 36.6, P < .001], as did ananalysis of false positive responses [F(1,7) = 6.54,p < .05]. The effect of rate was not significant.Although the increases in omissions and false positives

S\V1TCHIl\G ATTE~TIOl\ BETWEE~ THE EARS 53

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were seen consistently across 5s. they occurred inextremely few lists. averaging only 1.87c for omissionsand 0.8S:C for false positives.

A three-way analysis of variance on the reaction timesshowed that responses to dichotic lists were significantlyslower than to monaural lists [F(1.7) =6.11, p < .05] :the mean difference was 40 msec. The effect ofpresentation rate was not significant and neither was theinteraction between these variables. The effect of serialposition of the target in the tone chain was significant[F(5.35) = 20.37. p<.OOI], but there were nosignificant interactions of serial position with mode orwith rate of presentation. A Newman-Keuls test showedthat Position 1 gave significantly slower RTs than all theothers (p < .01) and that Position 6 gave slower RTsthan Positions 3,4, and 5 (p < .05).

When 5s were aSked for their comments on the task,they claimed that with the fast alternating lists theyheard only three or four of the tones: they had theimpression that these included the target whenever theyresponded to it.

L 3 4 5 6 FP"s. 2 3 4 5 6 FPs

Discussion Senal POSition

We look brietly at the serial position effects on RTsand then turn to the central question being investigated.TI1ere are two plausible reasons for the longer latency ofresponses to Position I. One is a difference in 5s' state ofexpectancy. Their preparation may not have beenmaximal at the onset of the first tone. while the firsttone itself could act as an additional warning signal forthe rest of the list. Another reason. which would alsoexplain the slower responses to Position 6. is that 5smade relative rather than absolute judgments offrequency, matching each tone to its predecessor and/orsuccessor rather than to a memorized target. RTs to thefirst and last tones would then be delayed because theylacked a predecessor and a successor, respectively. sothat less information \vas available on which to base therelative judgTI1ent.

The main aim of the experiment was to see whetherrapid alternation between the ears caused perceptualdifficulties with monitoring simple auditory stimuli, andwhether such difficul ties might be attributed to a limitedrate of switching attention. The slight decrease inaccuracy and increase in RTs for dichotic compared tomonaural lists at both presentation rates suggests thatalternating presentation does present some perceptualdifficul ty.

If this difficulty were due to a limited perceptualswitching rate. what other predictions would follow?Various strategies could be available to 5s: (1) Theymight simply fail to process a proportion of the stimuli.as suggested by A'\elrod et al (1968). The very smallnumber of omissions (47c in the fast alternating lists)shows that this very rarely occurred. (2) They mighthold the queue of stimuli in a fixed order to be dealtwiti1 seri:J1ly 3t ~1 sll)wer attention-switching rate. This

Fig. 1. RTs and percent omissions in detecting target tones.

theory would predict increasing lag with increasing listposition. so that the difference in RTs to alternating andmonaural stimuli should increase with list position. asshoul~ the difference in RTs to fast and slow alternatingstimuli. Neither of these predictions is borne out. Theslight increase in missed signals and false positives withfast compared to slow alternating lists cannot havecancelled an alternative increase in RTs (through achange in speed·error tradeoff). since the main increasein missed targets was due to three 5s only. while the fiveSs whose omissions remained constant (a mean increaseof only 0.257c) also showed no increase in RTs on thefast lists. (3) Reversals of processing order might occursuch that later stimuli received in phase with attentionwere analyzed before earlier ones waiting in the queue.However, unless the switching rate is very variable, thistheory would predict much greater oscillations in RTwith list position than we found. since the intef\'albetween stimuli in fast lists was 100 msec. (4) 5s mightprocess first all the tones on one ear and then all thoseon the other ear, This theory is also ruled out by thedata. since 5s would have to wait till the whole list hadbeen presented to complete their monitoring of the firstear before switching to the second. If they had followedthis strategy. RTs to target tones in Position 2 wouldhave been delayed by at least 500 msec with fast listsand 830 msec with slow lists. In fact. RTs to the secondtone in dichotic lists were only 2 and 34 msec slO\wfthan RTs to the second tone in monaural lists. Thus.none of the possible strategies associated \\"ith a limitedswitching fate appears to fit the results.

The only remaining exp13narion is that 5s ~He able'.

54 HARVEY AND TREISMAN

0- -0 Monaural

••--.. Alternating

III 5"c0...-0ti

4.c

5z3

J " 5 6Number of tones presented

Fig. 2. Number of tones reported in Experiment II.

with these simple stimuli at least, to "divide theirattention" between the channels to monitor thefrequency of all the tones, but that this in some wayreduces their efficiency, so increasing RTs by 40 msecand false positives by about 2%. But this account raises afurther question, namely how to reconcile it, togetherwith the very high detection rates, with Ss' reports thatthey clearly heard only three or four tones in the fastalternating lists. Taken together, these findings suggesteither that Ss in the fast alternating condition respondedto tones which they did not subjectively "hear" or thattargets were "heard" with a much higher probabilitythan were nontargets. The former possibility isreminiscent of Fehrer and Raab's finding (1962) thatsimple RTs to visual stimuli show no delay in conditionsin which the stimuli are masked by metacontrast fromsubsequent stimuli, and of Cumming's recentexperiments (1971) showing the same for choiceresponses. However, Ss' introspections favor the latterpossibility; they usually claimed that they heard thetarget when it received a manual response. Thedifference in the perception of target and nontargetitems may therefore be more closely related to Neisser'sfinding (1964) that 5s cannot report the identity ofbackground letters although they do detect the majorityof target letters in a visual search task.

This paradoxical combination of 5s' assertion thatthey heard only three or four of the fast alternatingtones with the fact that they detected almost all targettones seemed worthy of further investigation. Wetherefore decided to look more fully into 5s' perceptionof these fast alternating lists. Two further experimentswere carried out in which Ss were asked to report howmany tones they had heard.

EXPERIMENT II

Method

In order to explore further the nature of 5s' subjective

experience of the fast alternating lists used in Experiment I, weattempted to f'md monaural or dichotic lists which would soundperceptually equivalent to them. The Ss in Experiment I claimedthat at the fast rate they often heard only three out of the sixalternating tones, and that these occurred at regular intervals. InExperiment II, we therefore always presented three tones inIntervals 1, 3, and 5 within the framework of the six 10o-msecintervals previously used for fast lists, and we added 1, 2 or 3tones in the remaining three intervals. The aim was to seewhether these tones disappeared subjectively and whether thisdepended on how many were presented. When one"ortwo toneswere added, their temporal positions varied from list to list. Inall cases when presentation was dichotic, the tones within a listalternated between the ears. Ss were not told of theserestrictions. The tones were all of the same frequency andduration as the nontarget tones in the previous experiment.

Eight right-handed undergraduate Ss each heard 30 monauraland 30 dichotic lists of Lengths 4, 5, and 6, and 10 lists of eachtype of Length 3. These were presented in two sessions, onemonaural and one dichotic, with order counterbalanced acrossSs. They were as~ed to listen to each list and immediatelyafterwards to ma~k m a response matrix a tick for each tone theyhad heard, showmg the temporal and spatial position in whichthey had heard it. They were given matrices of 6 by 2 cells forthe ~ichotic lists with left- and right-ear cells labeled, andmatnces of 6 by 1 for the monaural lists. The order of writingtheir responses was not controlled.

Results

Figure 2 shows the averaged judged number of tonesas a function of the presented number, together with themean of the variances for individual 5s. Analysis ofvariance showed no significant effect of dichotic vsmonaural presentation on the mean estimates ofnumber, but a significant effect on the variances ofindividual 5s' means [F(1,7) = 17.6, P < .01]. Thus,although the reduction in perceived number withdichotic presentation failed to reach significance, 5swere considerably less accurate in judging the number oftones in dichotic lists.

Figure 3 shows, separately for dichotic lists whichstarted on the right and the left ears, the totalpercentage of tones reported in each serial position, andhow this percentage was divided between correct andincorrect ears. The peaks and troughs on alternatepositions are due to the fact that two of the eight 5sfilled some cells representing temporally simultaneoustones, although none were every presented. While theserial position curves for ear errors are relatively flat,those for omissions drop steeply on the last position:only about half the tones in Position 6 were reported. 5smade slightly less ear errors in lists which started on theleft ear; otherwise, there were no systematic differencesbetween the tones on the right and left ears, either in theprobability of detection or of ear errors.

5s were also much less accurate in judging thetemporal positions of the tones and the gaps in thedichotic than in the monaural lists. The proportion oflists in which at least one error of temporal position wasmade was 71% for the dichotic lists compared to 29%for the monaural lists. (This difference was not due toerrors in which tones were attributed to the wrong ear indichotic lists, since ear errors were ignored in this

S\\"ITCHI~G ATTE~TION BETWEH\ THE EARS 55

Method

Fig. 3. Percent tones reported in lists of sLx as a function ofear and serial position.

oL.-~'-~2---:3~-':4-~5~6~-------.JSerial position

-- Leftear first

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possible source of bias introduced by the fixed responsematrices used in Experiment II, and to separate theeffects of rate and of number of tones presented. Wealso asked whether the results would vary with the formof response required, verbal or motor. since the differentresponses required might have induced a different choiceof perceptual strategy in the two previous experiments.Guzy and A.xelrod (1971) have shown that differentresponse requirements can affect performance in thistype of task, although the differences in theirexperiment appeared to be due to response rate ceilingsrather than to changes in perceptual capacity. Half the5s in Experiment III were asked to write down thenumber of tones they had heard and half were asked totap out the number they had heard immediately afterthe list was presented.

The experiment was divided into three sections: (i) In thetirst. 24 lists of six tones were presented at the fast rate used inExperiment I. Half began on the right and half on the left ear. Sswere not told that these lists would all be the same length. (ii) Inthe second section. 48 lists varying in length from three to eighttones were presented, all at the same fast rate as in Experiment I.and without gaps. so that the total duration of the lists variedfrom 300 to 800 msec for lists of three to eight tones.respectiwly. (iii) In the third section. the same lists as inSection ii were used again. but randomly interspersed w'ith 60lists in which the rate was varied irregularly as follows: thetemporal positions of tones in the regular lists of dght \\er"taken as a framcwork and two. thrt't'. or four wert' delt'tt'd. Tht'rcmaining tones werc madt' tc' altcrnate betwt'en tht' Celr'. The'

analysis.) Even for lists where the /lumber of tones wasjudged correctly, Ss made many temporal positionerrors. For example, 60'iC of the correctly judged lists offour and five tones contained temporal position errorswith dichotic presentation, while only 9'iC did withmonaural presentation. Judgments of number wereincorrect in 44<;1 of the dichotic lists and 20<;1 of themonaural ones. Thus, temporal judgments suffer morethan judgments of lIumber when presentation is dichoticrather than monaural.

EXPERIMENT III

In this experiment. we attempted to correct the

Discussion

The variance of judgments made by our Ss was muchgreater with dichotic than with monaural presentation,although the mean judged numbers did not differsignificantly. Our results with the lists of six tones differfrom those of Axelrod et al (1968) in that their Ssshowed considerably greater underestimation of the rateof tones in dichotic lists: the proportion D/M (dichoticheard number over monaural heard number) for our Sson the lists of six was 0.93, while their Ss matched a rateof l6.8/sec dichotic to a monaural rate of 10/sec. givinga ratio MID of 0.6. Guzy and Axelrod (1972) testedjudged numerosity rather than rate, but only up to a rateof 8/sec, for which they found D/M = 0.81 for lists ofsix. However. as the variances indicate. it is unlikely thatthe difference reflects more accurate perception in ourexperiment. An alternative account is that range effectsdistorted the mean estimates in our experiment. Ss knewthat the maximum number presented would be six andmay have been biased towards filling in a certainproportion of sixes in the dichotic as well as themonaural lists.

Many of our lists contained temporal gaps whichproduced an objectively irregular rhythmic structure. Wefound that 5s were very bad at judging the presence andthe temporal positions of these gaps. This finding is ofsome interest in relation to a theoretical issue raised byAxelrod et aI's (1968) theory and by A.xelrod (1972personal communication): this is the question of whathappens to the "missed" tones in fast regular sequences,Why do these unlleard tones not leave temporal gaps inthe perceived sequence. and so produce apparentarrhythmia') Our suggestion is that the temporalarrangemen t perceived depends on the au tpu t times ofthe perceptual mechanism rather than on the physicalarrival times of the subset of stimuli that it is able toaccept. On this hypothesis, if some of the stimuliqueueing for the limited-capacity perceptual analyzersdrop out through decay or displacement. their absencewill not be detected by the S. The poor judgment oftemporal patterns that we found with objectivelyarrhythmic sequences is consistent with this account.

56 HARVEY AND TREISMAN

sectIOns Ill) and tllIl WIthout gaps Section (lirl

Fig. 4. Number of tones reported inExperiment III.I~

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actual patterns used are shown in Fig. 4 together with theresults, A~ain. each S heard the tones both dichotically andmonaurall;'. with the order counterbalanced across Ss. Sections i.ii. and iii ,,'ere. however. ltiven in this fixed order.

Four S5. chosen rand~mly. verbally reported the number oftones heard and four "'ere asked immediately after the list to tapa number of times ,,'hich matched the number of tones they hadheard, T,,'o of the first four Ss gave unusual patterns ofresponses in that they underestimated the monaural lists muchmore than the dichotic ones, This pattern of responses had neveroccurred in the previous experiment. or in the informal reportsin Experiment 1. One of the two admitted that, with the dichoticlists. she had listened to one ear only and doubled the numbershe heard there, These TWO SS were therefore replaced with twomore. and their results are shown separately in Fig. 4.

Results

There was no systematic difference between the fourSs who responded verbally (after the two unusual Ss hadbeen replaced) and the four who responded by tapping.This may be because the latter Ss verbally mediated theirresponses. Results from the two groups weresubsequently pooled for further analyses. The meanestimates of number for the lists of six in Section i were4.1 for the dichotic lists and 4.9 for the monaural. Bothare appreciably lower than in Experiment II, supportingour belief that range effects introduced artifactually highjudgments in that experiment. There were no systematicchanges with practice over these 24 trials or across thesame lists of six consecutive tones in any of the threesections.

Figure 4 shows the mean estimates of number foreach type of list in Sections ii and iii and the means ofindividual variances. The dichotic lists gave lowerestimates of number than the monaural lists (DIM forSection ii averaged 0.85). An analysis of variance on thelists without gaps from Sections ii and iii showed asignificant effect of dichotic vs monaural presentation[F(I,7) = 6.59, p < .05] and also a significantinteraction of this factor with the number of tones per

list [F(5,35) = 4.58, P < .01). Again, Ss were much lessaccurate on the individual lists in the dichotic condition,as we see from the larger variances for these lists. Theeffect of monaural vs dichotic presentation on individualvariances was significant [F(I,7) = 8.01, P < .025] , andagain this interacted with the number of tones [F(5,35)= 3.16, p < .025], showing that accuracy decreasedmore on dichotic than on monaural lists as list lengthincreased.

An analysis of variance on the mean numberjudgments for the lists with 0, 1, or 2 gaps in Section iiialso showed a significant effect of dichotic vs monauralpresentation [F(I,7) = 36.5, P < .001], a significanteffect of number of gaps [F(2,14) =8.32, p < .01] , anda significant effect of number of tones [F(2,14) = 75.8,p < .001] . In an analysis of the individual variances forthe same lists in Section iii, the effect of monaural vsdichotic presentation and the effect of the number ofgaps failed to reach significance, as did the interactionbetween these factors. Only the number of tonesproduced a significant effect [F(2,14) = 8.19, P < .01].Thus, in these lists, dichotic presentation led toconsistent underestimation, but not to increasedvariance relative to the monaural presentation.

Comparable lists were less accurately responded to inthis experiment than in Experiment II. For the lists ofsix without gaps, the mean of the variances in thealternating condition was 0.58 compared to the previous0.34. As pointed out earlier, range effects probablyreduced the variance for these lists in Experiment II.However, Ss in Experiment II were also more accurate(though not significantly so) on three types of lists offour or five tones which happened to be the same in thetwo experiments and so could be directly compared; themean of their variances was 0.35 in Experiment IIcompared to 0.49 in Experiment III. Accuracy on theselists was not affected by the range effect, since no S inExperiment III gave a judgment greater than 6 for any of

SWITCHIl\G ATTENTION BETWEEN THE EARS 57

these lists. This tlnding is of some interest. since theresponses required in Experiment II were much morecomplex (ear and temporal position had to be specifiedas well as number) and also took longer to make.

GENERAL DISCUSSION

Our tlndings clearly support the conclusion thatrapidly alternating dichotic lists give rise to lower andless consistent estimates of number than do monauralones, although they differ somewhat from those ofAxe lrod and his colleagues in the degree ofunderestimation of dichotic relative to monaural lists.Moreover, we have shown that Ss' accuracy is muchlower when they are asked to report how many tonesthey heard than when they press a key on detecting atarget tone. Ss can detect almost all target tones in listsin which they are unable to "hear" more than two-thirdsof the tones. One reason may be that the former taskinvolves memory storage: Ss can respond only after thewhole list has been presented rather than immediatelyafter a particular critical item has been detected.However, the more complex responses reqUired inExperiment II actually produced, if anything, moreaccurate numerical judgments than the simpler responsesof Experiment III. This suggests that the effects of delayand of response interference with memory are unlikelyto have played much part.

A more plausible explanation is that the tasks ofExperiments II and III are more demanding perceptuallythan that of Experiment I. The fact that theseexperiments also exaggerate the relative diftlculty ofdichotic compared to monaural lists suggests that thedifference between dichotic and monaural presentationalso affects primarily the perceptual level. How can oneaccount for this difference? We will consider threepossible explanations.

Cross-Channel Inhibition

It is possible that there are sequential inhibitoryeffects as well as the simultaneous inhibitory effectsbetween the ears suggested by Sorkin et al (1971).Schubert and Parker (1955) found a decrease in thediftlculty of understanding alternating speech when thecontralateral ear simultaneously received a maskingnoise, and attributed this decrease to a reduction in apostulated "contralateral inhibitory off-effect." Inphysiological studies, Rozenzweig and Rosenblith(1953) and Barrett (1972) have shown sequentialcontralateral inhibitory effects in some cases extendingup to 90-msec lSI. If the inhibitory effect werefrequency-specitlc, it would account neatly for therelative absence of effect on target detections inExperiment I. since the target tones differed infrequency. However. we can probably reject thisexplanation on the grounds (l) that monotic backwardmasking effects :He greater than dichotic ones (Elliott.

1962): (2) that backward masking effects typicallydecrease the duration and intensity of suprathresholdtones rather than making them disappear (Gold'dburt,1961): (3) that, if masking did indeed eliminate sometones completely, this theory would predict perceivedarrhythmia. An acceptable explanation should becompatible with the queueing plus drop-out hypothesissuggested earlier.

Processing Load

Our results are incompatible with the idea thatperception is a unitary event \vith different tasksdiffering only at the response selection stage: rather.they agree with the view that perception consists of anumber of distinct operations, not all of which areobligatory (cf. Laberge, 1971). The task demands in thepresent experiments differ for the detection and thenumber judgment experiments. Thus, we need todistinguish here at least three separate results ofperceptual processing: (1) the detection of target tones.(2) numerical reports of the number of tones heard, and(3) conscious experience of "hearing the tones." We hadhoped to make numerical reports depend on what was"heard" in Experiments II and III. but were probablynot always successful. (An extreme example was the Swho admitted listening to one ear in the dichoticcondition and doubling the number she heard.)

Tones in the dichotic lists could differ along threedimensions: spatial location, time of occurrence, andfrequency (target vs background), of which only the lastwas relevant to the detection task. One hypothesis isthat conscious perception of the tones must follow a fullanalysis and requires S to specify and to recombinecorrectly the outputs of analysis along each of the threeseparate dimensions for each particular stimulus tone.Now, if this full analysis is impossible at the fastpresentation rate, S would be unable to "hear" all thetones in the fast dichotic lists and unable to respondaccurately when asked to count them or to give theirtemporal and spatial positions. On the other hand. adetection response could be triggered by the outputs ofa single analyzer-in this case for frequency.

Does this account conflict with other results onmultidimensional stimulus processing? Treisman (1969)reviewed findings suggesting that different analyzers canbe used in parallel and that interference or difficulty individing attention arises chiefly when rapid orsimultaneous stimuli compete for a single analyzer. Thepresent suggestion that increased processing load in1pairsthe fuller analysis required for conscious hearing seemsinconsistent with this conclusion. However. it may bethe recombination of the different analyzer outputs wproduce a single integrated percept which is difficult l~r

time-consuming rather than their parallel use.The number of monauralh' presented tones was ::llsl~

consistently underestimated by 5s in Experiments II andIII. a finding \\hich replicates those re\ie\\ed by \\'hite

58 HARVEY AND TREISMAN

(1963). However. White's explanation in terms of alimiting frequency of cortical excitability cycles cannotaccount for the greater degree of underestimation withdichotic presentation. In the present account, theexplanation would be the same as for the dichotic tones,namely that at the fast rates, Ss are unable to analyzeand to recombine all the attributes (such as frequency,temporal position, and duration) that must be specifiedbefore conscious perception can occur. The fact thattheir spatial location is constant reduces the difficulty ofanalyzing that attribute and so increases the numberwhich can be consciously perceived.

How does this hypothesis explain the 40·msecincrease in latency for dichotic lists in the detectiontask? Ss' observation that a detected target was usuallyamong the three or four tones consciously heard suggeststhe following explanation: after detecting the target byits frequency, Ss in fact completed the analysis of itsother properties before making their response. This tooklonger when the spatial location was variable.

Perceptual Switching

A difficulty for any theory which denies the reality ofa switching limit is raised by a finding of Axelrod andPowazek (1972). They showed that the apparent rate ofalternating clicks varied monotonically with theirangular ~ separation. This result is difficult toaccommodate in a processing load account, which wouldpresumably predict that the more discriminable thespatial locations of the stimuli, the easier they should beto process. Axelrod and Naka0 2 have shown a similardegree of apparent slowing with alternating tactilestimuli presented bimanually compared to unimanually.They suggest (following Kahneman, 1970) that acommon limit to performance is set by the rate at whichSs can make an underlying motor change, such as alateral eye movement or some other motor componentof the orienting reflex. However, it is not entirely clearto us how such a motor process could help or hinderperception when clicks arrive over headphones, or why,if they do hinder the counting task, they should havemuch less effect on detections.

We concluded in discussing Experiment I that theconcept of switching was incompatible with the resultsin the detection task. However, it could still be involvedin other forms of perceptual processing. One majordifference between the detection and the counting taskswas that the latter [like Treisman's (1971) verbal recalltask with alternating digits] required a response to thelist as a whole, while the former reqUired a response onlyto one item within it. Therefore, the counting task mayhave forced Ss to send all the items through the samesubsystem, while the detection task allowed the three oneach ear to be processed separately and in parallel. Forexample, to tie this possibility more concretely tounderlying neural mechanisms, the detection responsemight be triggered independently from either cerebral

hemisphere, while number judgments required all thetones to be processed by the same hemisphere. It islikely that tones reaching the same hemisphere fromdifferent ears or spatial locations will differ in neuralpathway, quality, or latency, and thus requireadjustments to be made for subsequent stages ofprocessing. Switching time, on this hypothesis, becomesthe time necessary to reset the parameters appropriatelyfor the serial analysis of tones arriving from differentsources. If this time is appreciable, processing will bedelayed and items queueing for attention may be lost. Inthe detection task (for which this hypothesis assumes noswitching is necessary), the increased latency fordichotic lists could be due simply to uncertainty aboutwhich hemisphere will, in fact, trigger the response.There were no differences between left- and right-earlatencies, but, since simple manual responses can betriggered for either hand from either hemisphere, thesewould not be expected.

Guzy and Axelrod (1971) attempted to test a similaridea by asking Ss to tap in synchrony with right-earclicks using the right hand and with left-ear clicks usingthe left hand. They argued that this provision ofindependent response channels might eliminate the needto shift attention, but their results gave no evidence of achange in the form of preresponse processing. Thedifference between bimanual rates to monotic anddichotic clicks was as great as the difference between thecorresponding unimanual rates. Their results could betaken as evidence against this account of the discrepancybetween our detection and counting tasks, unless somefactor in Guzy and Axelrod's experiment prevented theirSs' adopting a parallel mode of processing. For example,the fact that each S received randomly mixed trials onmonotic and dichotic sequences, and the requirement tolisten to the clicks for some time to get a clear estimateof rate before starting to respond, may have biased theirSs towards listening to the sequence as a whole ratherthan as two independent subsequences.

A difficulty for any spatial switching theory,including this one, is that it requires a separateexplanation for the lesser, but consistent,underestimation of monaural clicks at fast presentationrates. The processing load hypothesis covers this findingwithin the same general account.

It might be possible to test these alternative accountsby using lists which alternated randomly rather thansystematically between the ears, and comparingperception of tones which followed a tone on the sameear with perception of tones which followed a tone onthe other ear. If the task required analysis of spatiallocation in both cases and if this were the main source ofthe difficulty produced by alternating tones, efficiencyof perception should not differ for tones which followeda switch and for tones which did not. If some forms ofswitching or of contralateral inhibition were involved,however, perception should be worse on tones whichfollowed a switch.

SWITCHING ATTENTION BETWEEN THE EARS 59

REFERENCES

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Axelrod, S., & Powazek, M. Dependence of apparent rate ofalternating clicks on azimuthal separation between sources.Psycho nomic Science, 1972, 26, 217-218.

Barrett, Y. W. Multiple use of the auditory cortex: Interaction ata single point. Experimental Neurology, 1972,34, I-IS.

Cumming, G. Visual perception and metacontrast at rapid inputrates. Unpublished doctoral thesis, Oxford University, 1971.

Elliott, L. L. Backward masking: Monotic and dichoticconditions. Journal of the Acoustical Society of America,1962,34,1108-1115.

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Kahneman, D. Remarks on attention control. Acta Psychologica,1970,33,118-131.

Laberge, D. On the processing of simple visual and auditorystimuli at distinct levels. Perception & Psychophysics. 1971, 9,331-334.

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Moray, N. Introductory experiments in auditory time sharing:Detection of intensity and frequency increments. Journal ofthe Acoustical Society of America, 1970,47,1071-1073.

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NOTES

1. Moray, N., & Fitter, M. 1. Attention and auditoryprocessing: The detection of intensity increments duringauditory time sharing. In preparation.

2. Axelrod, S., & Nakao, M. Apparent slowing of bimanuallyalternating pulse trains. In preparation.

(Received for publication October 25,1972;revision received March 15, 1973.)