Intern. J . Neuroscience, 1980, Vol. 10, pp. 211-216 0020-7454/80/1004-0211$04.50/0

0 1980 Gordon and Breach Science Publishers, Inc. Printed in Great Britain

TEMPORAL SEQUENCE DISCRIMINATION OF DICHOTIC TONES: THE EFFECT OF FREQUENCY

HARVEY BABKOFF and DANIEL ALGOM Department of Psychology, Bar-Ilan University, Ramat-Can, Israel

(Received August 31, 1979)

The present study was designed to investigate the effect of stimulus frequency on the ability to discriminate the temporal sequence of two 250 ms-duration dichotic stimuli. Dichotic stimulus pairs of three frequencies were tested, 493 Hz, 988 Hz and 3982 Hz, using a three-alternative forced-choice design. The results indicate that frequency does not affect the form, the slope or the location along the abscissa of the discrimination functions. All functions reached 67 % discrimination level with a dichotic temporal asymmetry of approxi- mately 4 0 4 5 ms.

Temporal order or sequence judgements of intra- and intermodal stimuli have been investi- gated by many authors over the past 20 years (see Babkoff, 1975; Green, 1973; Patterson & Green, 1970 for reviews). The original suggestion of Hirsh (1959) and Hirsh and Sherrick (1961) of a sensory modality-free general mechanism for the judgement of the temporal order of stimuli has not stood up well to the empirical findings which indicate the importance of the modality tested as well as the possible complex interactions of two stimuli separated by very short intervals (e.g., Babkoff, 1975; Babkoff & Sutton, 1963, 1971; Green, 1973; Oatley, Robertson & Scanlan, 1969; Patterson &Green, 1970; Robinson, 1967).

With regard to the auditory system, several stimulus parameters including the mode of stimu- lation (monaural vs. binaural) are relevant in the determination of the temporal separation required to discriminate sequence as well as in the form of the discrimination function (Babkoff, 1975; Babkoff & Sutton, 1963).

Among other stimuli, Hirsh (1959) also used monaurally presented low- medium- and high- pitched tone pairs to test sequence discrimination. The two tones of each pair differed from each other by a musical minor third and were asynchronous with respect to onset, however, both members of

The authors would like to thank Mr. J. Gutgold for technical help, and Mrs. H. Cohen, Ms. I. Barak and Ms. 0. Pesach for help in the experimentation. Send offprint request to H. Babkoff.

21 1

the tone pair ended simultaneously. Hirsh reported that a temporal separation (At) of 20ms is suf- ficient for a 75% correct judgement of order regardless of the frequency of the tones.

With monaural stimulation of transient and short sinusoidal signals, discrimination of the sequence of segments differing in frequency or intensity is possible with separations ( A t ) of approximately 2.0 ms (Babkoff & Sutton, 1971 ; Green, 1973; Patterson & Green, 1970; Wier & Green, 1975). With an increase in the interval separating the transient stimuli or in the duration of the sinusoids, discrimination decreases (4< At< 32 ms) and increases with separations or durations longer than 20 to 32ms (Green, 1973; Hirsh, 1959; Wier & Green, 1975). In summary, it appears that the frequency of the sinusoidal segments of short-duration tonal stimuli is irrel- evant to monaural stimulation as long as the difference in frequency ( A F needed to identify the two segments) is greater than the frequency discrimination limits of the auditory system (Wier & Green, 1975).

The data discussed so far are monaural and thus subject to peripheral auditory frequency processing mechanisms. Dichotic stimulation, on the other hand, provides the opportunity for testing temporal sequence discrimination with stimulus pairs which are equal along all dimensions. One can, therefore, test temporal sequence by presenting stimulus pairs whose members are equal with respect to frequency and intensity, but differ only with respect to the arrival time at the two ears.

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212 H. BABKOFF A N D D. ALGOM

The question raised in this paper is, therefore, does stimulus frequency affect the temporal sequence discrimination of dichotically presented pairs of long-duration tonal stimuli. With respect to other binaural phenomena arising from inter- aural time asymmetry, such as localization of long-duration tonal stimuli, frequency has been shown to be an important variable, interacting with interaural time (or phase) asymmetry. Localization of tones dichotic with respect to time is best at low frequencies and decreases as a function of increasing frequency to about 1500 Hz (Klumpp & Eady, 1956; Mills, 1972; Nordmark, 1976). It is of interest, therefore, to study the effect of tonal frequency on other binaural phenomena arising as a result of interaural time asymmetry. The purpose of this paper is to report the results of a study of tonal frequency on another binaural phenomenon, temporal sequence or temporal order discrimination.

APPARATUS AND PROCEDURE

The design of the psychoacoustic apparatus allowed for the physical independence and separate attenuation of the stimuli in each channel, control over the duration, rise- and decay-time of the stimuli, as well as the temporal interval separating them.

The stimuli were generated in each channel by a Heathkit Model IG-72 Audio Generator. Tone frequencies for each of the two audio gener- ators as well as tone durations and onset asym- metries (At) were calibrated by a Monsanto Type 120 A Counter-Timer. Three different frequencies were used in this experiment, generated separately in each channel by each of the two audio generators. The low frequency (L ) in both channels was 493 Hz. The medium frequency ( M ) in the right channel was 988 Hz, and 989 Hz in the left channel. The high frequency ( H ) was 3982 Hz in the right channel and 3988 Hz in the left channe1.t Onset and offset gating of the tones, controlled by locally designed circuitry,

?Because of use of two independent audio generators, we were unable to obtain exact frequency duplication at the two ears for the medium and high frequencies. With the settings of the audio generators producing 988 Hz in one channel and 989 Hz in the other, as well as 3982Hz in one channel and 3988 Hz in the other channel, i t was possible to maintain accurate calibration. These small frequency differences (1 Hz at 988 Hz, and 6 Hz at 3982 Hz) were subjectively indiscriminable.

was 1 ms. The duration of the tones was 250 ms including onset and offset times.

The stimuli were transduced by a pair of Scintrex MKIV earphones. Three males, ages 14 to 16, served as subjects.

Prior to the main experiment, the monaural threshold of each ear of each subject to each frequency was determined by the method of constant stimuli. For the main experiment, stimulus level was set at 35 dB above the monaural threshold for each frequency for each subject (i.e., 35 dB SL).

Seven values of the onset asymmetry (At) were used to generate a discrimination function for each subject at each of the three frequencies: 10, 20,40,80, 160,320, and 640 ms. The two members of each tone pair did not end simultaneously; thus, for each onset asymmetry there was an equivalent offset asymmetry. Each At for each frequency was presented 80 times. The order of the presentation of At was random.

One frequency was used in each session for each subject. Four sessions were devoted to the testing of each of the three frequencies. The order of frequencies tested was randomly and independently determined for each subject. Each subject was tested over a 12-session period in the main experiment. One training session pre- ceded the experiment.

All testing took place in a Medtechnic Silent Cabin. Subjects were seated facing a panel with a warning light. The experiment was conducted using a three-alternative temporal forced-choice technique with feedback for correct responses. On a given trial, the subject was presented with three pairs of dichotic tones. Each tone pair was presented within a one-second interval. Two of these pairs (same) consisted of tones of a given frequency (either L, M , or H ) with the tone presented to the right ear preceding the tone presented to the left ear by a given At. The third pair (different) consisted of the same frequency tone presented to the left ear preceding the tone presented to the right ear by the same At. Subjects were required to respond as to which of the three intervals had the different dichotic tone pair.

A warning light indicated the onset of a trial. Each of the three intervals was separated from the others by 1 sec. A trial, consisting of the three intervals, lasted 5 sec. The intertrial interval was approximately 8 sec, during which the subjects responded. Correct responses were signaled to the subject by another panel light.

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DICHOTIC TONE DISCRIMINATION 213

I NIB

1- M- H- MK R N

40 160 640 10 40 160 640 10 40 160 640

FIGURE 1. Data for each of the three subjects are shown separately in each of the three panels. Percentage discrimination is plotted on the ordinate as a function of dichotic temporal asymmetry (At) in ms on a logar- ithmic abscissa. Data are shown for thre: stimulus frequencies: L (493 Hz to both ears); M (988 Hz to the right ear, 989 Hz to the left ear); and H (3982 Hz to the right ear, 3988 Hz to the left ear). See text for explanation.

RESULTS

The data are shown for each subject separately in the three panels of Figure 1. Data for the low- medium- and high-frequency tones are shown separately. Percentage discrimination is plotted on the ordinate as a function of dichotic temporal asymmetry ( A t ) in ms on a logarithmic abscissa. The data indicate that for all the subjects, an increase in At results in a monotonic increase in discrimination level. For subjects NIB and R N (leftmost and center panel), the data for the three frequencies appear to overlap completely, and can perhaps be fitted by a single curve. For subject MK, the data for the three frequencies were more variable, overlapping at the shortest (10, 20 ms) and the longest At sec (80 to 640 ms) and separated at the mid At value (40ms). The order of the discrimination levels for the three frequencies at Ar = 40 ms is not systematic across the three subjects.

To test for the effects of frequency, the data of the three subjects were analyzed by a two-way analysis of variance for repeated measurements. These results are shown in Table I. The results

TABLE I Results of two-way analysis of variance

(repeated measurements)

Source SS df MS F

A. Frequency 54.22 2 27.11 0.932 B. Dichotic temporal 29962.54 6 4993.76 87.64*

C. Subjects 380.03 2 190.02 A x B 113.56 12 9.46 0.259 A x C 116.36 4 29.09 B x C 683.75 12 56.98 A x B x C 876.15 24 36.51

separation (A?)

Q 0.001

indicate that only one main effect, dichotic temporal separation (A t ) , is highly significant ( p <O.OOl). Neither the frequency of the stimulus nor the frequency x At interaction term is significant.

The data of the three subjects were averaged and are shown in Figure 2. Percentage discrimination is plotted on the ordinate as a function of At in ms on a logarithmic abscissa for the low, medium, and high frequencies separately.

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214 H. BABKOFF A N D D. ALGOM

so I 60 4

c J E 0.

- 1 - M - H

4 0 t

10 2 0 4 0 80 160 320 640

AT [ M S f C l

FIGURE 2. Percentage discrimination is plotted on the ordinate as a function of dichotic temporal asymmetry ( A t ) in ms on a logarithmic abscissa. Data are shown for three stimulus frequencies: L (493 Hz to both ears); M (988 Hz to the right ear, 989 Hz to the left ear); H (3982 Hz to the right ear, 3988 Hz to the left ear). These data are based on the average of the data shown in Figure 1.

With a three-alternative forced-choice experi- mental design, the 67% level of discrimination is equivalent to a 50% level of discrimination corrected for chance. The three functions overlap and cross the ordinate at a 67% discrimination level at a At of approximately 40 to 45 ms. From the curves drawn in Figure 2, it appears that the average data can be fitted by one function. This corresponds to the results of the analysis of variance, indicating no effect of frequency and no interaction between frequency and At .

DISCUSSION

The results of the present experiment indicate a temporal sequence discrimination threshold of approximately 40-45 ms (temporal interval asym- metry) for a pair of 250 ms dichotic tones. These data are comparable to data for stimuli of similar durations as reported by Green (1973) on the discrimination of the order of two monaurally presented sinusoidal segments differing by 10 dB i n intensity. Green (1973) reported finding non- monotonic functions with high discrimination levels at approximately 4 ms, and a drop in

discrimination level with duration increases of from 4-32ms, followed by an improvement in discrimination level occurring only for durations between 32 and 64 ms. Since the shortest A t used in this study was 10 ms, we are unable to compare the absence of a nonmonotonic function (i.e., high discrimination levels at At = 4 ms) found for the dichotic tonal data to the presence of a nonmonotonic function reported by Green ( 1 973) for monaural tonal data.

Of further interest is the finding reported by Green (1973) on the effect of three frequencies on monaural temporal order discrimination, 1000, 2000 and 4000Hz. Regarding the monaural presentation of short-duration (2 ms) sinusoidal signals, Green concluded “. . .the value of temporal acuity appears to be largely independent of frequency over a wide range.. . .”

The data reported in this paper indicate a frequency-independent mechanism for the auditory system for discriminating temporal sequence when stimuli are presented dichotically. These data may pose somewhat of a problem for models of temporal order discrimination which posit an overall frame- work within which all of the binaural phenomena occurring as a result of an increase in interaural temporal asymmetry result for sequential oper- ations on the temporal continuum separating the dichotic stimuli (Babkoff, 1975; Sternberg & Knoll, 1973).

Sternberg and Knoll (1973) presented a general model to explain temporal order judgements in which a “decision function” converts a difference in central “arrival time” of two sensory signals into an order judgement.

Several phenomena are associated with a monotonic increase in interaural temporal asym- metry ( A t ) of equally intense auditory stimuli: lateralization of a fused stimulus i n the direction of the lead ear when At is in the microsecond range, the subsequent breakup of the fused stimulus with At in the millisecond range, followed by temporal order judgements at longer Ats.

Sternberg and Knoll include binaural lateraliz- ation phenomena in this general order-judgement model by adding the provision that several decision functions may be interspersed along the way in a sensory system which receive and weigh arrival- time differences of inputs from two channels (in this case from the two ears). Each channel per- forms processing operations on the signal prior to its arrival at a given decision function. The arrival-time difference at the nth decision function

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DICHOTIC TONE DISCRIMINATION 21 5

is determined by the sum of the latencies of the n - 1 processing operations prior to it.

When applying this model to binaural stimu- lation, one notes that the stimulus parameters which affect the lateralization of dichotic transient stimuli separated by very short tempxal intervals (of the order of microseconds), such as frequency and interaural intensity asymmetry (Babkoff er a/., 1973; David et al., 1959; Deatherage & Hirsh, 1959; Green, 1973; Harris, 1960), also affect the breakup of the fused image into two dichotic stimuli (Babkoff & Sutton, 1966). This lends support to a general model in which the various lateralization phenomena, including the breakup of the fused image which occurs as a function of increased Ar, can be considered within the framework of various processing operations on the tempxal continuum separating di,chotic stimuli, beginning with the phenomenon of “centering” (A? = 0).

In a previous paper (Babkoff, 1975), we reported on the use of an experimental paradigm designed to obtain quantitative data on two of these binaural phenomena generated by transient stimuli: the breakup of the fused sound into two separately perceived stimuli and the judgement of temporal order. Discrimination level is related to interaural temporal asymmetry by a V-shaped function. The left segment of this function, decreasing as At increases from 2 to 8 ms, reflects the breakup of the fused lateralized stimulus and the right, rising segment of this function, increasing fram 12 to 128 ms, reflects temporal order discrimin- ation. Those data were discussed in terms of the model presented by Sternberg and Knoll (1973) and considered supportive of that model (Babkoff, 1975).

However, the data on pure tone stimuli repxted in this paper pose a problem for the applicability of the general framework. There exists an upper frequency limit on the ability to lateralize pure tones on the basis of interaural time (or phase) asymmetries. This limit appears to be around 1500-2000Hz (Klumpp & Eady, 1956; Mills, 1972; Nordmark, 1976; Perrott & Nelson, 1969; Tobias, 1972). Interaural time discrimination decreases as a function of increasing frequency from 200 Hz to about 1500 Hz.

Were a similar discrimination function to be generated with the stimuli used in this study on a temporal continuum beginning with At = 0, “centering” and discrimination from center, one would expect stimulus frequency to affect the

results significantly. The low-frequency stimuli would generate the lowest threshold for discrimi- nation from a “central” position, while the high- frequency stimuli would generate the highest thresholds for such discrimination. The absence of an effect of frequency in the discrimination of the temporal sequence of these tones, therefore, raises doubts about the validity of placing the various perceptual lateralization phenomena on the same temporal continuum. This, in turn, raises doubts about the general applicability of a model positing a sequence decision function, n, which weighs the arrival times in tke two channels in terms of the summation of the n- 1 latencies prior to it. If frequency affects the early n-1 decision processes, why is the effect not measurable at the nth decision function (sequence discrimi- nation) ?

Several comments should be made, however, tefore rejecting the general order-judgement model because of the data on the temporal sequence discrimination of pure tones.

First, perhaps the general order-judgement model, hypothesizing summation of all n - 1 decision functions (latencies) prior to the nth decision, .is applicable to transient stimuli but not to pure tone stimuli. Transients, in addition to teing acoustic stimuli with broad frequency spectra, are also excellent time markers, since such stimuli cause a highly synchronized response of the auditory nerve ( N I ) ; therefore, neural time- coding, in terms of a synchronized VIII nerve discharge, is available even a t the periphery. The transient is, therefore, the preferred stimulus to test the hypothesis of additivity of the n- 1 decision functions along the dichotic temporal separation continuum. With the transient stimuli, dichotic temporal separations ( A t ) greater than 1 ms are always interstimulus separations, with no stimulus temporal overlap. With pure tone stimuli of 250 ms duration as used in this study, most of the Ats, except for the longest sepsrations, 320 and 640 ms, involve overlapping dichotic stimulation. The time-marker aspect of these stimuli may be less relevant than the time or phase differences appear- !ng in the ongoing structure of the tones (Searle e? al., 1976) or in their offset (Efron, 1973). In summary, perhaps the general order-judgement model is applicable t o transient stimuli and should be tested with such stimuli only.

Second, the methodology used in this study is a forced-choice technique in which reversal of the sequence of the dichotic stimuli serves as the basis

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216 H. BABKOFF AND D. ALGOM

for the difference judgement. The forced-choice method by its very nature allows for the use of any and all available cues to discriminate between the stimuli. In contrast, classical studies of sequence judgement have used techniques in which subjects judged pairs of temporally separated stimuli and reported the perceived temporal order of the members of the pair. A three-alternative forced-choice technique may have introduced a different basis for the judgement of temporal sequence than the perceived order, for example, that of dichotic tone pattern. Perhaps this basis of judgement is not determined by the mechanism operating on an additive decision-function prin- ciple. In summary, perhaps tests of the general order-judgement model should use methodologies based on temporal order judgements, rather than methodologies based on the discrimination of differences between pairs of stimuli whose order is reversed.

REFERENCES

Babkoff, H. Dichotic temporal interactions: Fusion and temporal order. Perception and Psychophysics,

Babkoff. H. & Sutton, S. Perception of temporal order and loudness judgments for dichotic clicks. Journal of the Acoustical Society of America, 1963,35, 574-577.

Babkoff, H. & Sutton, S. End point of lateralization for dichotic clicks. Journal of the Acoustical Society of America, 1966,39,87-102.

Babkoff. H. & Sutton, S. Monaural temporal interactions. Journal of the Acoustical Society of America, 1971, 50.459465.

Babkoff, H., Sutton. S. & Barris, M. Binaural interaction of transients: Interaural time and intensity asym- metry. Journal of the Acoustical Society of America,

David, E. E., Jr., Guttman, N. & Van Bergeijk, W. A. Binaural interaction of high-frequency complex stimuli. Journal of the Acoustical Society of America,

1975,18,267-272.

1973,53,1028-1036.

1959,31,774-782.

Deatherage, B. H. & Hirsh, I. J. Auditory localization of clicks. Journal of the Acoustical Society of America, 1959,31,486-492.

Efron, R. Conservation of temporal information by perceptual systems. Perception and Psychophysics,

Green,D. M. Temporal acuity as a function of frequency. Journal of the Acoustical Society of America, 1913,

Harris, G. G. Binaural interaction of impulsive stimuli and pure tones. Journal of the Acoustical Society of America, 1960.32, 685-692.

Hirsh, I. J. Auditory perception of temporal order. Journal of the Acoustical Society of America, 1959, 31,759-767.

Hirsh, I. J. & Sherrick. C. E.. Jr. Perceived order in different sense modalities. Journal of Experimental Psychology, 1961,62,423-452.

Klumpp, R. G. & Eady, H. B. Some measurements of interaural time difference thresholds. Journal of the Acoustical Society of America, 1956.28,859-860.

Mills, A. W. Auditory localization. In Tobias, J. V. (Ed.): Foundations of Modern Auditory Theory, New York: Academic Press, 1972.

Nordmark, J. 0. Binaural time discrimination. Journal. of the Acoustical Society of America, 1976,60,870-880.

Oatley, K., Robertson, A. & Scanlan, P. M. Judging the order of visual stimuli. Quarterly Journal of Experi- mental Psychology, 1969,21, 172-179.

Patterson, J. H. & Green, D. M. Discrimination of transient signals having identical energy spectra. Journal of the Acoustical Society of America, 1970, 48,894-905.

Perrott, D. B. & Nelson. M. A. Limits for the detection of binaural beats. Journal of the Acoustical Society of America, 1969,46, 1477-1481.

Robinson, D. Visual discrimination of temporal order. Science, 1967, 156, 1263-1264.

Searle, C. L., Braida, L. D., Davis, M. F. & Colburn. H. S. Model for auditory localization. Journal of the Acoustical Society of America, 1976,60, 178-1 8 1.

Sternberg, S. & Knoll, R. L. The perception of temporal order: Fundamental issues and a general model. In Kornblum, S. (Ed.): Attention and Performance IV, New York: Academic Press. 1973.

Tobias, J. V. Curious binaural phenomena. In Tobias, J. V. (Ed.): Foundations of Modern Auditory Theory, New York: Academic Press, 1972.

Wier, C. C. & Green, D. M. Temporal acuity as a function of frequency difference. Journal of the Acoustical Society of America, 1975,52, 15 12-1 5 15.

1973,14,518-530.

54,373-319.

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