Perceived Auditory Continuity with Alternately Rising and ......Warren & Obusek, 1971; Warren &...

Perceived Auditory Continuity withAlternately Rising and FallingFrequency Transitions*

GARY L. DANNENBRING

McGill University

ABSTRACT

Six experiments were conducted investigatingperceived auditory continuity with alternatelyrising and falling frequency glides, in which theglides were perceived as continuous when de-leted portions were replaced by white noisebursts. The first three experiments showed thatperceptual continuity could be obtained whenthe deleted portion came either in the middle ofthe glide, or at the top and bottom of the glides;continuity was actually better for the latter con-dition. Also, it was found that as glide durationincreased, the threshold between perceived con-tinuity and discontinuity increased; there was asimilar increase as the difference between high-est and lowest frequencies increased. It was alsofound, in Experiments IV-VI, that when the peakwas deleted and replaced with noise, there wasno perceptual extrapolation of the incompleteglides; rather, there seemed to be considerablerounding off of the trajectory of the glide.

An interesting auditory phenomenonwhich has recently been investigated is per-ceived auditory continuity, or auditory in-duction (Warren, Obusek, & Ackroff,1972). Basically, the phenomenon is one inwhich a soft, intermittent sound is per-ceived as continuous when the breaks arefilled in with a louder sound. This

phenomenon has been observed using avariety of stimulus materials, such as twotones of different frequencies and ampli-tudes (Thurlow, 1957; Thurlow & Elfner,1959; Elfner & Homick, 1967b); soft noisebursts with loud tones as the interpolateditems (Elfner & Caskey, 1965; Elfner &Homick, ig66; Elfner & Marsella, 1966;Elfner & Homick, 1967a; Elfner, 1969;Elfner, 1971); a soft tone with loud noisebursts as the interpolated signal (Warren,Obusek, & Ackroff, 1972); and soft speechstimuli with louder sounds filling in gaps inthe speech (Miller & Licklider, 1950;Cherry & Wiley, 1967; Holloway, 1970;Warren & Obusek, 1971; Warren & Sher-man, 1974). What all of these studies sug-gest is that there seems to be some sort ofgeneral mechanism which allows the audi-tory system to generate missing auditorystimuli under certain conditions.

One implication of studies such as that byCherry and Wiley (1967) is that missingformant transitions must be regeneratedfor the speech to sound smooth and con-tinuous. The importance of frequencytransitions in speech has recently been em-phasized by Cole and Scott (1973). Theyshowed that when the transition in a cvsyllable is removed, subjects hear the con-sonant perceptually segregated from thevowel. Rather than hearing a repeating cvsyllable, subjects heard a repeating vowelwith a consonant-like noise in the back-ground; the cv syllable was no longer per-ceived as a unit. Cole and Scott suggestedthat the important role which frequencytransitions play in speech perception is tohold the speech stream together, to preventthe sounds from becoming grouped on thebasis of frequency. Bregman and Dannen-

*Experiments i-m of this research were presented at the 1974 meeting of the Eastern Psychological Association,Philadelphia, Pa., and Experiments iv—vi were presented at the 1974 meeting of the Canadian PsychologicalAssociation, Windsor, Ontario. This paper is based on part of a PH D thesis by the author. The work was supportedby the following giants from the National Research Council of Canada awarded to Dr Albert Bregman: A-0127,E-3221, E-3338, and E-3413. Additional support was provided by Defence Research Board grant no. 9401-40 toDr Albeit Bregman, giants from the FCAC program of the Quebec Ministry of Education, the McGill UniversityGraduate Faculty, and a National Research Council of Canada Postgraduate Scholarship awarded to the author.The author wishes to thank Dr Albert Bregman for his advice throughout this research.

Canad. J. Psychol./Rev. Canad. Psychol., 1976, 30 (2) 99

bring (1973) have shown that frequencytransitions do seem to play such a role withnon-speech sounds. They found that arapidly presented sequence of high and lowtones tends to split into two streams basedon frequency, a high stream and a lowstream. However, adding frequency tran-sitions between high and low tones, oradding partial transitions which merely'pointed' at the neighbouring tones, re-duced the tendency for segregation tooccur, and enabled subjects to more accu-rately report the correct order of the tones.

Thus, it appears that frequency transi-tions are important in allowing the listenerto correctly perceive an ongoing stream ofauditory material. In addition, it seemsprobable that the auditory system must beable to generate frequency transitionswhich are missing or masked. The presentstudies, therefore, have been designed as apreliminary investigation of the ability ofthe auditory system to perceptually gener-ate missing portions of frequency transi-tions. A total of six experiments were con-ducted. The first three experiments lookedat perceived auditory continuity when thetonal stimuli were linear frequency transi-tions, varying the extent of the frequencychange (A/), the rate of change, and thelocation of the noise burst relative to thesine tone transitions. Experiments iv, v,and vi investigated the nature of the per-ceptually continuous signal which occurswhen noise bursts are located at the top andbottom of the frequency glides. The num-bering of the experiments indicates the his-torical order in which they were conducted.

E X P E R I M E N T S I—III

Experiment 1 was designed to investigateperceived auditory continuity with risingand falling frequency glides which hadnoise bursts located in the middle of eachglide. In Experiment 11, steady state toneswere used to look at the basic effect of in-creasing tone duration. Finally, in Experi-ment in, the stimuli were again like those in

Experiment 1, except that the noise burstswere located at the top and bottom of thetransitions rather than in the middle.

METHOD

Subjects

A total of 50 graduate and undergraduatepsychology students at McGill University par-ticipated in thses experiments; all were volun-teers and naive as to the purposes of the experi-ments. Twenty subjects participated in Experi-ment 1, and 15 each in Experiments 11 and in.

Apparatus

The sine tones used in these experiments weregenerated by a PDP- 11 computer (DigitalEquipment Corp.) operating a Wavetek model136 VCA-VCG tone generator by means of a D/Aconverter. To reduce switching transients in thetonal signal, all amplitude changes were made atzero voltage crossing of the signal. The whitenoise was generated by a Briiel and Kjaer ran-dom noise generator, type 1402, and recordedon a Sony TC-200 tape recorder. The tape re-corded noise was switched on and off by thecomputer at the proper locations in the tonalsignal. The resulting tone and noise signals werecombined through a microphone mixer,amplified, and presented binaurally to indi-vidual subjects through headphones. Because ofvarious problems, different headphones wereused in different experiments. The headphonesused in Experiment 1 were Sharpe HA-8 stereoheadphones; in Experiment 11, Sound DH-09-sstereo headphones; in Experiment iir, KossPro-4A headphones.

The subject adjusted a knob connected to anA/D converter, which caused the computer tomodify the duration of the break in the tone(and thus the white noise duration). When thesubject was satisfied with the setting, he presseda button which caused the computer to store thevalue, in msec, of the duration of the white noise,and start the next trial.Stimuli

Experiment I The stimuli for Experiment 1 con-sisted of alternately rising and falling linear fre-quency transitions centred at 1000 Hz and pre-sented to subjects at 75 dB SPL, with 90 dB SPLwhite noise bursts presented in the middle ofeach glide. All dB levels were measured using aGeneral Radio type 155 l-c sound level meterwith a flat plate coupler. At the offset of the tonethe amplitude of the frequency transition wasattenuated immediately to 12 dB for the dura-tion of the noise burst, which was perceptually

100 G.L. Dannenbring

uZ

o

SOO 1SOO 2BOOTIME (msec )

FIGURE i Example of a portion of a stimulus se-quence used in Experiment i. In this example, A/" =1000 Hz, and transition duration = 1000 msec.

silent. Background noise was approximately 54dB, and consisted primarily of low frequencies,produced by the building air conditioning sys-tem. The frequency glides were produced bychanging the frequency every msec, producingperceptually smooth glides; there was no per-ceptible 1000 Hz buzz, which one might expectdue to the step-like changes in frequency.Stimuli were generated on-line, with the noiseduration being controlled by the subject's ad-justment of a knob. The white noise completelyfilled in the break in the tonal signal. Within eachtrial the slope of the frequency transition waskept constant, so that an increase in white noiseduration caused a decrease in the duration ofthe audible portion of the frequency transition.A diagram of the stimuli is given in Figure 1.

Transition durations are given as the timefrom the middle of one white noise burst to themiddle of the next noise burst; i.e., the acutalduration of the transition when the noise burst isat a minimum. Thus, with a transition durationof 500 msec, and the knob set to produce a noiseburst of 100 msec, the acutal duration of theaudible frequency transition is 400 msec. How-ever, on all figures used in this paper, transitiondurations will be defined as stated above, or, inthis case, 500 msec. There were four durationsof frequency transitions: 250, 500, 1000, and2000 msec, and four levels of the extent of fre-quency change (A/): 100, 250,500, and 1000 Hz,for a total of 16 different trials. The subject's

knob was calibrated to produce white noise du-rations of 2-1000 msec. Since not all of the fre-quency glides were that long, the maximumwhite noise durations that the subject couldmake were 200, 450, and 950 msec for fre-quency glides of 250, 500, and 1000 msec re-spectively. The 16 different trials were pre-sented in blocks three times, in a different ran-dom order for each block, for a total of 48 trials.There was also a .5 sec 2000 Hz warning tone atthe beginning of each trial.

Experiment II The tonal stimuli used in this ex-periment consisted of a 70 dB steady-state sinetone of 1000 Hz, with 85 dB white noise burstspresented during the breaks in the tone. Thesebreaks came, as in Experiment 1, at regular in-tervals of either 250, 500, 1000, or 2000 msec,with the duration of the break, and thus thewhite noise, being controlled by the subject'sknob. There were, therefore, a total of four dif-ferent trials, which were presented at random 12times each, for a total of 48 trials.

Experiment III The stimuli used in this experi-ment were basically the same as those used inExperiment 1, except that the noise bursts oc-curred at the high and low points of the fre-quency transitions. A diagram of a portion ofone of the sequences used in this experiment isgiven in Figure 2.

u

O

000 1500 2500TIME ( msec )

FIGURE 2 Example of a portion of a stimulus se-quence used in Experiment in. In this example, Af =1000 Hz, and transition duration = 1000 msec.

Alternately rising and falling frequency transitions 101

Z 600

zO 500

AF IN HZ> I O O

> 2 5 0

• 5 0 0

. 1OOO

250 500 lOOO 3O0OTRANSITION DURATION

IN MSECFIGURE 3 Mean continuity thresholds for Experi-ment I.

Procedure

Subjects were told that the frequency glideswould sound continuous for very short whitenoise durations, and discontinuous for very longwhite noise durations. They were asked to adjustthe duration of the white noise to find the pointat which their perception of the frequency glidechanged from continuous to discontinuous, orvice versa. They were free to turn the knob inboth directions, and were urged to use the fullrange of the knob, turning it back and forth untilthey centred on the point between continuityand discontinuity. This point will be referred toas the threshold for continuity. When satisfiedwith the adjustment, the subject pushed a buttonwhich stopped the trial and recorded the whitenoise duration. He then turned the knob back tothe shortest white noise duration, which causedthe computer to start the next trial. Thus, eachtrial began with the knob in this position, and thecomputer waited for the knob to be reset beforebeginning the next trial. It took most subjectsapproximately one half hour to complete the 48trials. In addition, each subject was given a prac-tice trial consisting of 1000 msec frequencyglides between 1000 and aooo Hz (in Experi-ment 11, the tone was a steady 1000 Hz), in whichhe adjusted the white noise duration until thethreshold between continuity and discontinuitywas reached. All subjects demonstrated an abil-ity to hear continuity of the tonal signal in thepractice trial.

RESULTS

Experiment I

Mean white noise durations for thethreshold between continuity and discon-tinuity are presented in Figure 3. Thethreshold at which continuity was no longerheard increased as the duration of the fre-quency glide increased, and the effect washighly significant, F(3,57) = 37.35, p <.001. In addition, the threshold also in-creased as kf increased, ̂ "(3,57) = 5-77,p <

.005. There was no significant interactionbetween these two effects. There was alsono significant difference between blocks,suggesting that increased practice with thestimuli probably did not affect the results.

Experiment II

The mean thresholds between perceivedcontinuity and discontinuity for this ex-periment are shown in Figure 4. As can beseen, the threshold increased as the dura-tion between noise burst onsets increased,and the effect was significant, 7 (̂3,42) =13.06,/? < .001.

uUl 700to

Z eoo

9 500

5a 400

IU

5300

z

2so 500 1000 aoooTONE DURATION IN

MSEC

FIGURE 4 Mean continuity thresholds for Experi-ment n.


2 6OO

zO 500

250 500 1000 20O0

TRANSITION DURATIONIN MSEC

FIGURE 5 Mean continuity thresholds for Experi-ment in.

Experiment III

The mean white noise durations for thethreshold for continuity for Experiment inare presented in Figure 5. As in Experi-ment i, the threshold increased as the dura-tion of the frequency transitions increased,and the effect was highly significant,^(3,42) = go.46.jb < .001. In addition, as inExperiment 1, the threshold increased as Afincreased,^(3,42) = 17.88,/? < .001. Therewas also a significant interaction betweenthese two effects, ^(9,126) = 3.79,p < .005.An analysis of variance comparing theoverall continuity thresholds found in Ex-periment 11 with Experiment in revealed asignificant difference,F( 1,28) = 13.93,/? <

.001. The mean continuity threshold forthis experiment was also significantly dif-ferent from Experiment 1,^(1,33) = 4.45, p< .05. It should be noted that these tests areprobably not statistically valid owing tonon-random assignment of subjects to thedifferent experiments. However, the bestcontinuity seems to have occurred with ris-ing and falling frequency transitions hav-ing noise bursts located at the top and bot-tom of the glides.

DISCUSSION

These results demonstrate, first of all, thatperceived continuity occurs with frequencytransitions as well as with steady-state tones,when the breaks in the transitions are filledin with white noise. Subjects perceived thetransitions as being continuous over a sur-prisingly long break; for example, a meanwhite noise duration of 288 msec over allconditions in Experiment 1. Similarly, War-ren et al. (1972) found that subjects per-ceived steady-state tones as being continu-ous over 300 msec breaks. Elfner (1969),however, found thresholds for continuitywhich were less than 35 msec, even withsignals as long as 950 msec. Probably themain reason for this discrepancy is that Elf-ner was investigating perceived continuityof soft white nosie signals with louder in-terpolated tones, while Warren et al., andthe present study, are investigating con-tinuity of soft tonal signals with loudernoise bursts filling the spaces. Hellman(1972) found that noise masks a tone muchbetter than a tone masks noise. Since War-ren et al. (1972) have shown that maskingand auditory induction seem to be related,the difference is probably due to the inabil-ity of tonal signals to efficiently mask noisein Elfner's case, and thus the inability toproduce continuity of the noise signal.

A second major result was that in all ex-periments, the threshold for continuity in-creased as the duration of the tonal signalincreased. This increase might be expected,to an extent, from earlier studies (Elfner &Homick, 1966; Elfner & Marsella, 1966;Elfner, 1969) which showed that continuitythresholds for noise signals slightly in-creased as the duration of the noise in-creased. However, the increase in thosestudies was not as great as that found in thepresent experiments. For example, thegreatest increase was from 17 to 34 msec inone condition of the study by Elfner (1969),which was considerably less than that foundin the present study. This difference couldbe due to the fact that the present experi-ments investigated continuity of a sine tone


signal rather than a white noise signal, asdiscussed earlier.

The effect of Af on the continuitythreshold cannot be predicted from earlierwork, since no previous studies have usedstimuli which varied along this dimension.A possible reason for this effect of Af is thatit might reflect certain organizational prin-ciples by which the auditory system oper-ates. Recent work by Bregman and Dan-nenbring (1973) suggests that the auditorysystem codes incoming stimuli into streamsaccording to various 'rules' of the sensorysystem. They found that a rapid sequenceof tones was organized into a single streammore readily when frequency transitionson the ends of the steady-state tones'pointed' toward the neighbouring tones. Itmight be hypothesized that with a greaterAf there is a greater pointing effect, result-ing in an increased tendency for the tonalstimuli to be included together in a singlestream and thus be perceived as a continu-ous series of rising and falling frequencyglides.

In comparing the results of Experiment 1with those of Experiment m, there is a sug-gestion that the perception of continuitymay be enhanced when the noise bursts arelocated at the top and bottom of the transi-tions rather than in the middle of them.Although it is possible that slight differ-ences in the subject populations could becreating the different results, it is also pos-sible that the difference is a perceptualphenomenon. It should be noted that asimilar effect has been found by Glynn(1954) in a study of apparent transparency.This is a phenomenon in which, under theproper conditions, a narrow screen which islocated in front of a moving rectangle ap-pears to be transparent; i.e., subjects statethat they seem to be able to see the form andcolour of the object under the physicallyopaque screen. Thus, there seems to be aperceived continuity of the moving visualobject, much like the perceived continuityof the tonal signal in the present experi-ments. Glynn found that continuity was

tuzw

oUJOf

C. Possibleperceivedstimulus

TIME - >

FIGURE 6 A portion of a standard sequence fromExperiment iv, showing the theoretical extrapolatedpoint and a possible perceived stimulus.

enhanced when the moving stimulus ap-peared to change directions somewhatwhile hidden by the screen, similar to theenhanced auditory continuity which seemsto occur when the direction of the sine tonetransition changes at the point of the noiseburst.

The significant interaction between Afand transition duration in Experiment msuggests that the effect of Af was greater forlonger transition durations than for shorterones. However, it is not clear at this pointwhy this occurred in this experiment andnot in Experiment 1. It might be noted thata scale transformation on theK-axis, such asa square root transformation, could elimi-nate this interaction (Winer, 1962). Sincethere is no reason to assume that linearincreases in the white noise duration areperceived as linear, such a transformationmight be appropriate, and thus the ob-tained interaction may be psychologicallymeaningless.

Since one might expect, from the Gestaltprinciple of good continuation (Koffka,1935) that linear frequency transitionsshould continue in a linear fashion, onemight expect that subjects would hear atonal signal in Experiment m which con-


tinues to a point that is an extrapolation ofthe adjacent transitions. This can be seen aspoint A in Figure 6. Such an expectationconforms well with the subjects informaldescriptions of what they heard in Experi-ment m. However, some additional, infor-mally gathered evidence suggested that thisnight not be the case. Rather than hearingpoint A in Figure 6 as the highest point,several persons, including the author, feltthat point B was the highest frequency per-ceived. Thus, Experiments IV-VI were con-ducted in an attempt to determine the na-ture of the perceived continuous signalwhich occurs when the peaks of alternatelyrising and falling frequency transitions areremoved and replaced with noise bursts.

EXPERIMENT IV

METHOD

Subjects

Subjects were five students from McGill Univer-sity. Three were graduate students in psychol-ogy, one an undergraduate in psychology, andone an undergraduate music student. All Ss hadan extensive musical background, which was theprimary criterion for selecting the subjects be-cause of the difficulty of the experiment. Two ofthe subjects were familiar with some of the pre-vious work in perceived auditory continuity;however, all subjects were naive as to the pur-pose of the present experiment. All subjectswere paid for their services.

Apparatus

The basic apparatus used in this experiment wasthe same as that used in the first three experi-ments. The headphones used in this experimentwere Koss P10-4A stereo headphones.

Stimuli and Procedure

In each trial, 5s were presented with a standardsequence and an adjustable sequence. The stan-dard sequence consisted of alternately risingand falling frequency glides centred at 1000 Hz,in which the upper peaks of the glides wereremoved and replaced with white noise bursts ofeither 100 or 200 msec duration. The tonal sig-nals were presented at 75 dB SPL; the noisebursts were go dB SPL. The frequency glidesextended the same distance below 1000 Hz asabove it, with the lower portion of the glides

uzaUJa:

TIME

tuz

EU

UI

B

/

/ , • '

/s

" \

'"N \\ \

\

/

TIME *•

FIGURE 7 (a) Example of a portion of a standardsequence, (b) A portion of an adjustable sequence. Thedashed line represents the sequence at a lower knobsetting.

being connected by a steady-state tone of thesame duration as the white noise burst (see Fig-ure 7). The adjustable sequence consisted of ris-ing and falling frequency glides, as in the stan-dard sequence, except that there was no noiseburst at the peaks of the glides. Instead, the topsof adjacent glides were connected by steady-state tones. In addition, the peaks of the adjusta-ble sequences were controlled by the subjectusing a knob. When the knob was in the extremeclockwise position, the peak frequency was at aminimum (the same as the lowest portion of thefrequency glide), and the frequency was thussimply a steady-state tone having the lowest fre-quency of the standard sequence glides. As theknob was turned counter-clockwise, the peakfrequency moved up. Diagrams of typical se-quences are shown in Figure 7. The adjustablesequence always matched the rhythmic struc-ture (i.e., transition duration) of the standardsequence.

In each trial, the subject was asked to firstlisten to the standard sequence and rate howcontinuous the tonal portion of the signalsounded by placing a mark on a 150 mm scalewith the extremes labelled 'not continuous' and'continuous.' Such a rating was necessary to de-termine whether or not subjects perceived con-tinuity of the tonal signal in these trials. He wasthen asked to adjust the upper peak of the fre-


The results of Experiment iv, showing mean adjusted tops of the frequency glides, along with the actual tops of thestandard transitions and the theoretical, extrapolated peaks.

Condition

A/ = 100 HzGlide duration

250500

10002000


250500

10002000


250500

10002000


250500

10002000

Noise duration

100 msec

Adjusted

1026.71041.21049.61044.4

1059.01086.91117.11114.6

1126.21186.91219.01246.8

1246.61380.91451.31465.1

Actual

1030104010451048

1075110011131119

1150120012251238

1300140014501475

Theoretical

1050105010501050

1125112511251125

1250125012501250

1500150015001500

200 msec

Adjusted

1028.61032.91046.21043.9

1024.81062.81094.31114.6

1065.51138.71193.41222.5

1112.21275.21374.01424.1

Actual

1010103010401045

1025107511001113

1050115012001225

1100130014001450

Theoretical

1050105010501050

1125112511251125

1250125012501250

1500150015001500

quency transitions of the adjustable sequenceuntil it matched the highest point heard in thestandard sequence. Each sequence was activatedby holding down a switch, and subjects wereencouraged to switch back and forth betweenstandard and adjustable sequences until the ad-justment was made as accurately as possible. Thesubject then pushed a button to record the ad-justed frequency as set by the knob, and thenturned the knob back to the lowest frequency,starting the next trial.

There were four different durations of fre-quency transitions: 250, 500, 1000, and 2000msec, and four levels of Af: 100, 250, 500, and1000 Hz. All measurements are given from thelowest point of a transition to a theoretical 'peak'(point A in Figure 6). As mentioned before,there were also two white noise durations, 100and 200 msec, for a total of 32 different trials.Subjects received these 32 trials in random ordera total of four times, with 64 trials in one ex-

perimental session and 64 trials the next day. Allsubjects demonstrated an ability to hear con-tinuity during 10 practice trials.

RESULTS

The adjusted peak frequencies of the ad-justable sequences are shown in Table 1,along with the theoretical, extrapolatedpeak frequencies and the actual tops of thefrequency glides. This table shows that forall conditions, the frequency which the sub-ject seemed to hear as the highest point ofthe standard was the actual high point ofthe frequency glide rather than the theoret-ical peak frequency. In addition, it showsthat the subjects generally seem to have setthe peak of the adjustable sequencesslightly lower than the top of the frequency


TABLE II

Rated judgments of continuity for Experiment iv.

A/in Hz

100250500

1000

Transition duration (msec)

250

104111111116

500

102112114119

1000

106110116123

2000

110109115123

glide. However, an analysis of variance per-formed on the difference scores (the dif-ference between the adjusted frequencyand the actual high point of the glide) re-vealed that there was no significant differ-ence from o.

The overall mean score made by the sub-jects for continuity judgments was 113. Ascore of 75 was the midpoint between dis-continuous and continuous on the 150 mmscale. The rated continuity judgments forall conditions are shown in Table 11.Analysis of variance revealed no significantmain effects nor any interactions. How-ever, all effects generally showed slighttrends in the expected directions. For ex-ample, as Af increased, the rated continuityalso increased slightly, as might be expectedfrom Experiments 1 and in.

DISCUSSION

The most interesting finding of this ex-periment was that subjects matched thehigh point of the adjustable sequence to theactual high point of the frequency transi-tions of the standard rather than to thetheoretical, extrapolated peak, as had beenexpected on the basis of the Gestalt princi-ple of good continuation. However, sub-jects generally described the tonal signalwhich they perceived under the white noiseburst as being a peak in the frequency. Yet,it is evident from the data that the per-ceived peak must have been somethingsimilar to C in Figure 6 rather than the peakat A. Experiment v was designed to investi-gate this possibility.

Finding no significant differences acrossconditions for the rated continuity judg-ments was not totally surprising since, fromthe results of the first three experiments,one would generally expect subjects to hearcontinuity in all conditions (except possiblyfor transition duration = 250 msec, noise =200 msec). In addition, there is no reason toassume that all subjects were using the samecriteria in making their judgments. Theimportant result of these ratings is simplythat subjects did perceive the tonal signal tobe more or less continuous through thenoise burst.

EXPERIMENTS V AND VI

After finding that subjects do not perceivethe gliding frequency transitions in Exper-iment iv as continuing beyond their actualterminal frequencies, one question whicharises concerns the nature of the tonal sig-nals which are perceived under the whitenoise bursts. At one extreme, subjects mayhave perceived something similar to C inFigure 6. At the other extreme, subjectsmay have correctly perceived the frequencytransition until its termination, and thenperceived a steady-state tone connectingthe high point of one transition with theadjacent one through the white noise burst.Or, the perceived stimulus could be some-thing between these extremes. Experimentv attempted to answer this question by hav-ing subjects modify various adjustable se-quences until they matched a standard aswell as possible, and then rate how well theoverall structure of the perceived tonalsignal of the standard matched that particu-lar adjustable sequence.

Experiment vi was designed to investi-gate the perceived terminal pitch of thefrequency glides in a situation in which con-tinuity did not occur (no noise bursts werepresent in the standard sequences), to de-termine whether or not slight perceptualovershoot occurs in an auditory continuitysituation.


METHOD

Subjects

Subjects were 10 psychology students (five ineach experiment) who were paid for their ser-vices. All subjects had a musical background, asin Experiment iv. One subject had also partici-pated in Experiment m; however, he and allother subjects were naive as to the purpose of thepresent experiments.

Apparatus

The apparatus used was the same as in Experi-ment iv, except that the headphones wereSennheiser HD-414 stereo headphones.

Stimuli ami Procedure

Experiment V As in Experiment iv, the subjectwas presented with a standard and an adjustablesequence in each trial, each activated by holdingdown a switch. The standard sequences were thesame as those in Experiment iv, except thatthere were only three levels of A/: 250, 500, and1000 Hz, and three transition durations: 250,500, and 1000 msec. The white noise bursts were100 or 200 msec in duration. There were fivedifferent types of adjustable sequences, the dif-ference being in the steady-state duration at thetop of the frequency glides. The steady-stateduration was either o (a peak, or point), 25%,50%, 759?, or 100% of the duration of the noiseburst of the standard sequence. Thus, for the100 msec white noise burst, the steady-state du-ration of the adjustable sequence (the durationof the tone in that sequence which correspondsto the noise burst of the standard) was either o,25, 50, 75, or 100 msec. For standard sequenceswith 200 msec noise bursts they were o, 50, 100,150, or 200 msec. There were, therefore, a totalof 80 different trials, which subjects receivedtwice (in 2 sessions) for a total of 160 trials.

The subject was asked, for each trial, to mod-ify the adjustable sequence until the high pointof the frequency transition matched the highpoint of the standard as well as possible. Afterthe adjustment was made, the subject was askedto rate how the overall form of the adjustablesequence sounded compared to their perceptionof the tonal signal in the standard. Since thesubject had already adjusted the frequency, anydifference between the two should be due todifferences in the perceived duration of the highpoint of the standard sequence as comparedwith the adjustable sequence. This rating wasdone on a 7-point scale with 1 being 'exactlyalike' and 7 being 'quite dissimilar.' The subjectwas given an example of two sequences exactlyalike (the 'standard' in this example did not have

any noise bursts at the peaks; instead, the fre-quency came to a point, or o steady-state dura-tion, and then came down), and was told thattonal stimuli this similar should be rated ' 1.' Hewas also given the same standard and an adjust-able sequence witha 200 msec steady-state dura-tion at the peaks, and was told that this should bea '7.' All subjects felt that they could hear definitedifferences between the two examples, andseemed to understand the rating procedure.

After the subject had made his rating onpaper, he pushed a button which correspondedin number to his rating. This number was re-corded by the computer along with the knobsetting. As in the previous experiments, the sub-ject then turned the knob back to the extremeclockwise position (the lowest frequency), andthe next trial began.

Experiment VI The stimuli and procedure forExperiment vi were the same as for Experimentv, with the following exceptions: (1) there wereno noise bursts in the standard sequences, only abreak at the peaks of the frequency transitions,and (2) subjects did not rate the similarity be-tween the standard and adjustable sequences,since auditory continuity was not being investi-gated in this experiment.

RESULTS

Experiment V

The first and most obvious result of thisexperiment is that, as in Experiment iv,subjects matched the adjustable sequencesto approximately the actual top of theglides of the standard sequences ratherthan extrapolating beyond this point. Thiscan be seen in Table m. An analysis of var-iance performed on the difference scoresagain revealed no significant differencesfrom o. There was, however, a considerabledifference between the different compari-son steady-state durations. This differenceis shown in Figure 8. When the steady-statepeak frequency of the adjustable sequencewas o, subjects adjusted the frequencyabove the actual top of the frequency glideof the standard. The adjusted value de-creased in frequency as the steady-statevalue increased. An analysis of varianceshowed this to be highly significant, F(4,16)= 44.97, p < .001. The analysis also re-vealed a significant interaction between


The results of Experiment v, showing mean adjusted tops of the frequency glides, along with the actual tops of thestandard transitions and the theoretical, extrapolated peaks.

Condition


250500

1000


250500

1000


250500

1000

Noise duration

100 msec

Adjusted

1069.31102.51116.1

1145.31201.51227.7

1291.91400.91452.8

Actual

107511001113

115012001225

130014001450

Theoretical

112511251125

125012501250

150015001500

200 msec

Adjusted

1029.31077.01105.3

1057.81162.81207.7

1131.31304.21417.3

Actual

102510751100

105011501200

110013001450

Theoretical

112511251125

125012501250

150015001500

1300T

125O

11OO

1OO0

Mean top of glidesAdjusted frequencies(Exp V)Extrapolation of

frequency glides

Adjusted frequencies (Exp VI)

O 25 50 75 100ADJUSTABLE SEQUENCE STEADY-

STATE DURATION(%OF WHITE NOISE DURATION)

FIGURE 8 Mean adjusted frequencies of Experimentsv and vi for the 5 different types of adjustable se-quences, along with the actual peaks of the frequencyglides and the imaginary, extrapolated points.

steady-state duration and A/, ^(8,32) =3.86, p < .01 (the slope of the function inFigure 8 increased as A/ increased), andbetween steady-state duration and transi-tion duration,7^(8,32) = 338,/) < .01 (theslope of the function decreased as transi-tion duration increased).

The results of the rated similarity be-tween the adjusted sequences and the stan-dard sequences were highly ambiguous.There was no significant difference be-tween the five percentage steady-state du-rations as had been expected. There was,however, a significant interaction betweentransition duration and steady-state time,^(8,32) = 2.43,/? <.05, which can be seen inTable iv; there seemed to be a slightu-shaped function for a transition durationof 250 msec, while for transition durationsof 500 and 1000 msec the functions arebasically flat.

A part of the reason for the ambiguity ofthe rating data seems to be due to the factthat for certain conditions none of the ad-justable sequences were very good exam-ples of the perceived tonal signal of the


TABLE IV

Rated similarity between standard and adjustable sequences forExperiment v.(1 = exactly alike; 7 = quite dissimilar.)

Transitionduration

250500

1000

Steady-state duration (% of standard noise duration)

0

3.252.232.57

25

3.072.602.82

50

3.232.432.53

75

3.602.482.55

100

3.932.252.47

standard. For example, there was asignificant interaction between Af and noiseduration, F(2,8) = 569, p < .05, in whichthe overall rated similarity between thestandard and adjustable sequences de-creased for noise = 200 msec but remainedthe same for noise = 100 msec. In addition,rated similarity was better as glide durationincreased for noise = 200 msec, but re-mained the same for noise = 100 msec,F(2,8) = 48,29,/) <.001.

Experiment VI

As in Experiment v, subjects adjusted thetop of the adjustable sequence above theactual top of the frequency glide for the 0%condition, and this frequency dropped assteady-state duration increased (shown inFigure 8). An analysis of variance showedthis to be significant, ^(4,16) = 39.42, p <.001. There was also a significant interac-tion between the adjustable sequencesteady-state duration and transition dura-tion (as also occurred in Experiment v),^(8,32) = 2.78,/* < .025. Although the ad-justed frequencies were slightly lower thanin Experiment v, an analysis of varianceconducted to describe the relationship be-tween the two experiments revealed nosignificant difference between the two.

DISCUSSION

As has already been stated, Experiment vbasically replicates the results of Experi-ment iv, in that subjects again matched theadjustable sequence approximately to the

actual top of the standard sequence fre-quency glide rather than to a theoretical,extrapolated peak. In fact, glides in thisexperiment were adjusted closer to the ac-tual tops of the standard glides than in Ex-periment iv. This was apparently due tovarying the duration of the steady-stateportion of the adjustable sequence, with anovershoot in adjustment when the adjusta-ble sequence had no steady-state duration,to an undershoot for the ioo% steady-stateduration. This last type of adjustable se-quence, of course, is the same as that usedin Experiment iv, and the results are quitesimilar.

In addition, this experiment demon-strates that subjects did not match theslopes of the standard and adjustable se-quences rather than the perceived highpoints. The frequencies to which the ad-justable sequences would have been set,had subjects been matching for slope, areshown in Figure 8 as the extrapolations ofthe frequency glides. Thus, subjects wereapparently doing the task required ofthem.

It is known from several studies (Brady,House, & Stevens, 1961; Nabelek, Nabelek,& Hirsh, 1970, 1973) that subjects perceivethe pitch of a frequency transition to besome sort of average of the frequencies oc-curring over a brief period of time. Thisprocess would affect the perception of boththe standard and comparison sequences inExperiment vi, thus making accuratemeasurement of the perceived end pointsof the standard sequences difficult. How-


ever, the importance of the results of Ex-periment vi is their similarity to the resultsof Experiment v. It seems clear that sub-jects in Experiment v did not hear the sinetone, which they perceived as continuousbeneath the noise bursts, as extendingbeyond the actual end points of the fre-quency glides.

Very little useful information can be ob-tained from subjects ratings of the similar-ity between adjustable and standard se-quences. Apparently, the adjustable se-quences often never sounded similar to thestandard, in spite of changes in the dura-tion of the steady-state portion. Thisseemed to be especially true of the 200 msecnoise condition, and resulted in several in-teractions described in the results. The bestexplanation that can be offered is that sub-jects may have perceived a tonal stimulus inthe standard which was considerablyrounded off, not the abrupt changes as inthe adjustable sequences, and thus none ofthem may have sounded like the standard.

GENERAL DISCUSSION

The present experiments, together withthe previous research investigating thisphenomenon, provide a body of evidencein which a number of parameters have beenmanipulated, and the resulting effectsupon perceived auditory continuity ob-served. For example, the effects of varyingthe frequencies of the two signals (Thur-low, 1957; Thurlow & Elfner, 1959; War-ren et al., 1972), the direction in space ofthe two signals (Thurlow & Marten, 1962;Elfner, 1969; 1971), and the relative loud-ness of the two signals (Thurlow, 1957;Thurlow & Elfner, 1959; Warren et al.,1972) have all been previously investigated.In the present experiments, some of theresults of interest were the effects of signalduration and Af on perceived auditory con-tinuity.

As was stated earlier, the duration of thesofter signal has been demonstrated in pre-vious studies to affect the threshold for con-

tinuity (Elfner & Homick, 1966; Elfner &Marsella, 1966; Elfner, 1969). The presentexperiments demonstrated a very strongeffect of the duration of the softer signal onthe threshold between perceived continuityand discontinuity; as the duration of thesignal increased, the duration of the breakover which continuity of the tonal signalwas still perceived also increased.

The effect of Af on perceived auditorycontinuity is difficult to explain. Experi-ments 1 and in very clearly demonstratethat increasing the differences betweenhigh and low points of the frequency transi-tions resulted in an increase in the durationof the break which can occur in the signaland still result in a perceptually continuoussignal. As was suggested earlier, this may bedue to an increased 'pointing' effect, result-ing in an increased tendency for the streamto be perceived as an unbroken, continuousstream.

One of the most interesting results of thepresent study was that of Experiment in:that not only did subjects perceive con-tinuity of the tonal signal when the peakswere replaced with noise, but that theseconditions seemed to be better for per-ceived continuity than when the noise camein the middle of the glides, as in Experi-ment 1. The work of Glynn (1954) on theapparent transparency effect, cited ear-lier, demonstrated a similar phenomenonin vision. Experiments iv, v, and vi sug-gested that subjects listening to thesestimuli perceived a tonal signal which wasnot an extrapolation of the frequencyglides, but rather was a continuous risingand falling tone having the upper pitch ofthe end points of the frequency glides.

An important question which needs to beanswered concerns the reason that the au-ditory system organizes these tonal signalsinto one continuous tone rather than a tonealternating with a noise burst. The answermay be in the perceptual significance ofboundaries between auditory events. Ingeneral, the most important informationfor the perceptual system concerns the lo-


cation of boundaries between stimuli in anincoming stream. This is true even at verylow levels in the nervous system (Ratliff &Hartline, 1959). However, it also may be ageneral principle of the perceptual systemto ignore or reduce the discontinuities inthe incoming signal as much as possible,especially when the discontinuities are oc-curring at a rapid rate.

One important principle which seems tobe used in this process is that of segregatingitems which differ along some dimensioninto different streams. This is the phe-nomenon of auditory stream segregation.For example, rapid alternation betweentones of widely differing frequencies re-sults in extreme discontinuities in the streamof sounds, and the auditory system seemsto respond to this situation by splittingthe sounds into two streams based onfrequency (Bregman & Campbell, 1971).Although such a procedure by the auditorysystem serves to reduce discontinuities in astream of sounds, the ultimate reduction indiscontinuity is when one of a series of al-ternating sounds is heard as perceptuallycontinuous, the situation that occurs withauditory continuity. In fact, auditory con-tinuity might be considered a special case ofauditory stream segregation, in that theauditory system organizes the input intotwo streams of sound. Subjects subjectiveimpression of the stimuli in the present ex-periments was generally that one of thesounds was heard as continuous, and thatthere seemed to be no temporal relation-ship between the two sounds; i.e., theyseemed to form separate streams. For ex-ample, given the stimuli used in Experi-ments 1 and in, a number of persons ininformal sessions have described difficultyin identifying differences between the twokinds of stimuli, despite the fact that in onecase the noise burst is located in the middleof the frequency transition, and in theother case at the top and bottom of thetransition. The most reasonable explana-tion for this is that the tonal signal formsone continuous perceptual stream, with the

noise forming another stream, and thusjudgments of temporal relationships be-tween the two streams become difficult.This, of course, can possibly explain thedifficulty subjects have in locating in thespeech stream an extraneous sound that isreplacing a phoneme (Warren & Obusek,1971; Warren & Sherman, 1974). Similarexperiments might also be conducted,using non-speech stimuli, to see whether ornot subjects can make judgments of tem-poral relationships between the two soundsin a perceived auditory continuity situation.

Given a situation in which two objects (Aand B) have a common boundary, threeevents are possible in the real world: (1) theboundary represents the end of object Aand the beginning of object B; (2) theboundary is only a property of object A,and object B exists behind A; (3) the bound-ary is only a property of object B, with ob-ject A extending behind B. In a perceivedauditory continuity situation, there is acommon boundary between the two audi-tory events. However, the stimulus condi-tions suggest that the boundary only be-longs to one of the auditory objects; theother object is perceptually continuous.Those conditions, as shown by Warren et al.(1972), are conditions in which onestimulus would ordinarily mask the other.The boundary then seems to be assigned tothe masking stimulus, while the other, lack-ing a boundary, is perceived as continuous.Warren et al., however, do not seem to feelthat a compensation for masking is thecomplete explanation for auditory con-tinuity; they also state that contextual evi-dence must suggest that a sound could bepresent for the duration of the loudersignal.

Bregman and Dannenbring (1975) haverecently demonstrated such context effectsusing sine tones alternating with noisebursts, in which the amplitude of the tonalsignal was varied just before and after thenoise bursts (either increasing or decreas-ing linearly). It was found that when thetone suddenly gets softer before the noise, a


situation in which the context is suggestingthat the tone might turn off during thenoise, continuity thresholds were consider-ably reduced, below the level for controlstimuli having steady-state amplitudes. Inaddition, somewhat smaller decreases incontinuity thresholds were obtained forstimuli having increases in the total am-plitude before the noise. These findingshave since been replicated in a more exten-sive, unpublished study conducted in ourlaboratory. It appears, then that these am-plitude changes are giving the auditory sys-tem evidence that a discontinuity is prob-ably occurring during the noise bursts, thusreducing the perception of tonal contin-uity.

RESUME

Six experiences sur la perception auditive de lacontinuite avec pour stimuli des coules sonoresde frequence alternativement ascendante etdescendante, ou les coules sont percus commecontinus lorsqu'on en detruit certaines portionspour les remplacer par des bruits Wanes. Lestrois premieres experiences montrent qu'onpeut obtenir une perception continue quand laportion detruite intervient au milieu ou ausommet et la base du coule, la continuite etant enfait meilleure en cette derniere condition. Deplus, a mesure que s'accroit la duree du coule, leseuil entre continuite et discontinuity augmente;la meme augmentation s'observe a mesure ques'accroit la difference entre les plus hautes et lesplus basses frequences. Dans les trois autresexperiences, on observe egalement que, si Tonremplace par un bruit le sommet du coule, il nese produit pas d'extrapolation dans les coulesincomplets; mais il semble plutot qu'il se pro-duise un adoucissement considerable de la tra-jectoire du coule.

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