Multidimensional Auditory Stimulus Control in a Chimpanzee (Pan troglodytes)

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1 PRIMATES, 31(4): , October Multidimensional Auditory Stimulus Control in a Chimpanzee (Pan troglodytes) MASAKI TOMONAGA* Osaka University ABSTRACT. An adolescent female chimpanzee (Pan troglodytes) was trained to discriminate auditory compound stimuli differing in tonal frequency and/or tone on-off rate. Following acquisition training and overtraining, she was shifted to multidimensional stimulus control testing using redundant relevant auditory stimulus sets with discriminability of elements in each dimension varied systematically. Although the control by both dimensions changed significantly as a function of discriminability, the degree of dimensional control was stronger in the tone on-off rate than in the tonal frequency. These results clearly demonstrated "attentional" control of the chimpanzee's auditory discrimination behavior and the interaction between two dimensions of auditory stimuli. Key Words: Auditory compound discrimination; Attention; Multidimensional stimulus control; Discriminability; Key press; Chimpanzee. INTRODUCTION SKINNER (1953) defined the term"attention" as the controlling relations between a stimulus and a response. When behavioral changes accompany these along a given stimulus dimension, an animal is said to "attend to" that dimension. In this sense, attention is synonymous with stimulus control (TERRACE, 1966; RAY, 1969). Studies on such selective or multidimensional stimulus control include the pioneer experiments carried out by REYNOLDS (1961). He trained two pigeons to discriminate between a white triangle on a red background (positive stimulus) and a white circle on a green background (negative stimulus) with a free-operant discrimination procedure. In a resistance-to-extinction test where the elements of compound stimuli were presented separately (red, green, triangle, and circle), one bird responded only to the white triangle; the other bird responded only to the red background. On the basis of these results, REYNOLDS concluded that only one aspect or dimension of discriminative stimuli exerted control of behavior. However, many reexaminations of this experiment, in contrast to REYNOLDS' conclusion, indicated that the behavior could not be controlled by only one dimension of the compound stimuli, but by several dimensions to a different degree (e.g. FARTHING & HEARST, 1970; JOHNSON & CUMMING, 1968; KENDAL & MILLS, 1979; WILKIE & MASSON, 1976). Further investigations also revealed that interactions between the stimulus dimensions (e.g. discriminability or relative intensity of elements) exert some effects on the multidimensional stimulus control in operant discrimination (e.g. BLOUGH, 1972; CHASE & HEINEMANN, 1972; MILES & JENKINS, 1973) or in conditioned suppression (e.g. MACKINTOSH, 1971, 1976; MACKINTOSH & REESE, 1979). *The author is now at the Department of Psychology, Primate Research Institute, Kyoto University as a transfer student of the Doctor course.

2 546 M. TOMONAGA In most studies on multidimensional stimulus control, the compound stimuli used in the experiments consisted of several dimensions in a visual modality (e.g. hue and line orientation) or consisted of one dimension in a visual modality and another dimension in another sensory modality (e.g. luminance and sound intensity). Furthermore, the species employed as experimental subjects were mostly limited to pigeons or rats. Thus, it is worthwhile examining whether the effects described above have generality over various sensory modalities and the various species. In recent decades, many researchers have developed a strong interest in the auditory perception of primates (e.g. PETERSEN, 1981). All of the auditory stimuli possess multidimensional properties. They have different pitches, intensities, frequency and amplitude modulation rates, and spectral contents from each other. In comparison with other primates such as macaques, it is true to say that in chimpanzees, despite the psychophysical research using simple sounds such as pure tones which has been conducted for many years (e.g. ELDER, 1934; STEBBINS, 1971), there have been relatively few studies on the perception of multidimensional auditory stimuli such as speech sotmds (cf. KOJXMA, 1988). In examining the evolution of human speech perception, it is important to investigate the auditory perception of the chimpanzee. Furthermore, to elucidate the detailed properties of the perception of multidimensional complex sounds in chimpanzees, we need to analyze the characteristics of "attention" to multidimensional auditory stimuli in chimpanzees. The purpose of the present experiments was to investigate the effects of discriminability of elements on multidimensional auditory stimulus control in the chimpanzee. METHOD In the present experiments, an adolescent female chimpanzee was first trained to discriminate auditory compound stimuli consisting of two dimensions, tonal frequency and tone on-off rate, with a conditional position discrimination procedure. She was then tested for the stimulus control exerted by each dimension using various stimulus sets differing irt discriminability between the elements of each dimension. SUBJECT An adolescent female chimpanzee (Pan troglodytes), approximately 8 years old, served as the subject. She had been raised at Osaka University since July 1980, from about 2 years of age. She had been employed in kinesiological experiments on bipedal locomotion three years previously, and in visual go/no-go discrimination one year before the present experiments (TOMONAGA & OHTA, 1990). Although she was maintained at her free-feeding weight, she was deprived of food for approximately 19 hr, since she fed after the two daily experimental sessions. APPARATUS The experimental box panel measured cm in size. The front panel, 60 cm in width and 50 cm in height, was made of aluminum, and the other walls were made of wood. The front panel contained three illuminated keys, each cm in size. One of the keys was mounted 25 cm below the top and 30 cm from both sides of the panel. This key, illuminated in red, was the "start" key for presentation of the discriminative stimulus. The two

3 Auditory Stimulus Control in a Chimpanzee 547 other keys were situated 27.5 cm below the top and 13 cm from each side, separated by 29 cm. These side keys, illuminated in white, were employed for the choice response. A yellowilluminated lamp, 5 5 cm in size, was mounted 7 cm below the top and 30 cm from both sides of the panel. This lamp, called the "feedback lamp," was used to tell the chimpanzee whether her response to the side key was correct or incorrect. A loudspeaker was mounted 2 cm below the feedback lamp. A food tray ( cm), connected with a general feeder, was equipped at the bottom of the panel. Maeda Confectionery baby cookies for human infants, 1 cm in diameter, were delivered to the food tray, to serve as reinforcers. This box panel was installed at the subject's home cage during a 6-month experimental period. A 5-ram thick iron plate covered the front panel, except during the daily experimental sessions. A microcomputer (NEC, PC-9801 VM2) controlled all of the experimental events, and also collected and analyzed the data. STIMULI The pseudo-pure tones which were used as the discriminative stimuli were generated by a programmable digital sound generator (NEC, PC ) containing the FM sound source LSI (YAMAHA, YM-2203), and were presented through the loudspeaker on the panel. Compound stimuli, which consisted of elements in the tonal frequency and in the tone onoff rate dimension, were employed. The three elements in the tonal frequency were of 1, 2, and 4 khz, and were designated as L(ow), M(iddle), and H(igh), respectively. Three of the four elements in the rate dimension were pulse tones, and one was a continuous tone. The continuous tone, designated as c(ontinuous), was presented without interruption from the last response to the start key to the response to one of the side keys. The pulse tones were repeatedly turned on and off. The slow-rate pulse, designated as s(low), was presented with a 100-msec on-time and 160-msec off-time, so that thus 3.8 pulses/sec (3.8 Hz) were presented. The medium-rate pulse, designated as m(edium), was repeatedly turned on for 50 msec and off for 80 msec (7.7 Hz). The fast-rate pulse, designated as f(ast), had a 25-msec on-time and 40-msec off-time (15.4 Hz). The rate element c was employed only in preliminary (acquisition) training, and the rate elements f and m were used only in stimulus control testing. In the preliminary training, 6 of 12 (3 frequency elements 4 rate elements) compound stimuli designated as Hc, Hs, Mc, Ms, Lc, and Ls, respectively, were employed. In the stimulus control testing, 9 of 12 compound stimuli, designated as Hf, Hm, Hs, Mf, Mm, Ms, Lf, Lm, and Ls, respectively, were used. The mean intensity of these stimuli, measured on the B scale of a Matsushita Electric sound-level meter (Model VS-3701A), was 78 db in SPL. The environmental noise level in the laboratory was about 60 db in SPL. By pairing two different stimuli, 15 stimulus sets for preliminary training and 9 for stimulus control testing were prepared. Each of the sets consisted of one stimulus corresponding to the left key (SL) and another to the right key (S ). PROCEDURE Initially, the subject was trained to press the start key of the front panel. She was then trained in the chain of responses to the start key and to the side keys. She received 51 daily pretraining sessions. Preliminary (Acquisition) Training: Following the pretraining sessions, the subject was shifted to the acquisition training of the discrimination task, employing Hc as SL and Ls as SR

4 548 M. TOMONAGA (stimulus set designated as Hc-Ls). A typical trial proceeded as follows. The trial began with illumination of the start key light. Two consecutive responses to the start key resulted in the presentation of one of the discriminative stimuli(sl or SR)through the loudspeaker. Following a 1-sec "listening period," during which the stimulus and the start key light remained on and responses to any of the keys had no effect, both side key lights were turned on and the start key light was turned off. Within a 5-sec limited hold, the subject had to respond to one of the side keys. Response to the side key resulted in immediate termination of the discriminative stimulus and both side key lights. A correct response, defined as a response to the left key in the presence of SL, and to the right key in the presence of Srt, was followed by a 2-sec illumination of the feedback lamp (correct signal) and reinforcement in every correct trial. If the subject responded incorrectly or did not respond within the limited hold, the feedback lamp was turned on and off for 2 sec (incorrect signal), followed by a 2-sec timeout. After a reinforcement or timeout, the next trial began with reillumination of the start key light. In principle, each session consisted of 80 trials. These trials were randomized with the restriction that identical stimuli were never presented in more than three consecutive trials, and that each of the stimuli appeared with equal frequencies. Two sessions were conducted per day, and 12 sessions per week. During the acquisition training, a correction method was used: if the subject responded incorrectly or did not respond within the limited hold in the presence of a stimulus, the same stimulus again appeared in the next trial (the correction trial). Acquisition training was continued until the chimpanzee's performance reached the criterion of more than 85 ~o correct trials for two successive sessions. After the subject had acquired the task, for overtraining, she was given 14 stimulus sets successively employing the same procedure and the same criterion. In some of these sets, either the frequency or rate dimension was relevant to discrimination, and in others, both dimensions were redundantly relevant. As in the acquisition training, the correction method was also used in all problems. Stimulus Control Testing: After the subject had completed overtraining, multidimensional stimulus control testing was then introduced. The procedure for this testing was derived from that employed by DOBRZECKA et al. (1966). They used a modified forced two-choice task to examine the effects of stimulus quality and location on the response quality and location in dogs. They tested the stimulus control by presenting stimuli combining the quality and location in reverse to the original training stimuli. The testing procedure utilized in the present study was almost the same as that of DOBRZECKA et al. Nine stimulus sets were employed for testing, and given in the order shown in Table 1. The stimuli in all sets consisted of one of three frequency elements and one of three rate elements (without continuous tone). In all sets, both dimensions were redundantly relevant, and the discriminability of elements in each dimension was systematically varied. That is, one of three possible combinations of the frequency elements (H-L, H-M, and M-L) and one of three possible combinations of the rate elements (f-s, f-m, and m-s), each of which was operationally defined as discriminability in each dimension, were combined in a stimulus set. In the pretest training, a higher frequency element and a faster rate element always corresponded to the left key. For the pretest training, the subject was first trained in each set to reach the criterion of

5 Auditory Stimulus Control in a Chimpanzee 549 Table 1. Trials to reach the criterion in pretest training sessions, the percent correct trials in baseline trials of the test sessions, the percent responses to the left key in the presence of each stimuli in the test sessions, and the percent responses to the rate dimension in the test trials for each testing set. Stimulus control testing Pretest training ~ responses to left key ~ responses to rate Stimulus set Trials to criterion Baseline (~ correct) Test dimension Hf-Ls 160 Hf Lf 85.0 Ls 1.7 Hs 55.0 (99.2) 65.0 Hm-Ms 160 Hm 95.0 Mm 30.0 Ms 10.0 Hs 85.0 (92.5) 22.5 Mf-Ls 160 Mf 98.3 Lf 85.0 Ls 6.7 Ms 10.0 (95.8) 87.5 Hf-Lm 160 Hf 86.7 Lf 55.0 Lm 3.3 Hm 10.0 (91.7) 72.5 Hf-Ms 160 Hf 96.7 Mf 85.0 Ms 1.7 Hs 30.0 (97.5) 77.5 Mm-Ls 215 Mm 91.7 Lm 40.0 Ls 15.0 Ms 60.0 (88.4) 40.0 Hf-Mm 180 Hf 96.7 Mf 85.0 Mm 13.3 Hm 65.0 (91.7) 60.0 Hm-Ls 160 Hm Lm 45.0 Ls 3.3 Hs (98.4) 22.5 Mf-Lm 160 Mf Lf 85.0 Lm 0.0 Mm 30.0 (100.0) 77.5 more than 90 ~ correct trials in 160 consecutive trials. Each session consisted of 80 trials in general. After reaching the criterion in each set, two test sessions were introduced immediately. Each test session consisted of 60 baseline trials and 20 test trials. In the baseline trials, the same stimuli as those in the pretest training were presented. In the test trials, stimuli with reversed element combinations of the training stimuli were presented. Thus, for example, in the set Hf-Ls, the test stimuli were Hs and Lf. In the baseline trials, the same contingency of reinforcement as in the pretest training was in effect, while in the test trials, a response to any of the side keys was nondifferentially reinforced. The baseline trials were randomized with the same restrictions as in the preliminary training, and test trials randomly appeared twice out of eight trials. Such training-testing sequences as described above were given successively to the subject. RESULTS PRELIMINARY TRAINING The subject acquired the conditional position discrimination task with auditory cues in

6 550 M. TOMONAGA 24 sessions (1,920 trials), and completed 14 overtraining sets in 86 sessions (491 trials averaged for one set). STIMULUS CONTROL TESTING The left part of Table 1 shows the trials to reach the criterion in the pretest training sessions for nine stimulus control testing sets. Seven of the nine sets reached the criterion in 160 trials, and the mean percent correct baseline trials in two test sessions exceeded 90 ~o except for one set (Mm-Ls, 88.4~). Insertions of test stimuli in the test sessions did not lower the accuracy of discrimination of training stimuli in the test sessions with one exception, Mf-Lm, which revealed significant increase in accuracy of discrimination in the test sessions (ff = 5.3, p<.05). The middle part of Table 1 shows the percentage responses to the left key in each of the bas~dine and test trials in the test sessions for each testing set. Chi-square tests revealed that the subject responded significantly to the rate dimension in Mf-Ls (Z 2 = 22.5, p<.001), Hf-Lm (Zz = 8.1, p<.01), Hf-Ms (ff = 12.1, p<.001), and Mf-Lm (~2 = 12.1,p<.001), and to the frequency dimension in Hm-Ms (ff = 12.1, p<.001) and Hm-Ls (ff = 12.1, p<.001). The right part of Table 1 shows the percentage responses to the rate dimension for each testing set, calculated from the following formula: p _ (NL+NR) 100, N7 where P is the percent responses to the rate dimension; NL is the number of test trims in which the subject responded to the left key in the presence of the same rate element as baseline SL ; NR is the number of test trials in which the subject responded to the right key in the presence of the same rate element as baseline SR; and Nr is the total number of test trials. In the case of this index, 100 ~ indicates perfect control by the rate dimension, 0 ~ indicates perfect control by the frequency dimension, and 50 ~ shows random performance or position preference. Figure 1 gives a three-dimensional representation of the percent responses to the rate dimension. The percent responses to the rate dimension increased consistently from the stimulus sets containing m and s, through those containing f and m, to those containing f and s when the frequency elements were H-M and M-L. Nonparametric analysis of variance by ranks for ordered hypotheses (PAGE, 1963) revealed significant changes along the rate dimension (p<.05). On the other hand, there were some changes in percent responses to the rate dimension accompanied by changes in the discriminability of frequency elements (PAGE'S test,.05<p< 1.0). DISCUSSION The subject was able to acquire the conditional position discrimination task with frequency-rate multidimensional auditory cues. Stimulus control testing further revealed that she acquired control by both dimensions, and that the extent of control exerted by each dimension could be determined mainly by the discriminability in each dimension. For example, in Mf-Ls, the subject's behavior was controlled by the rate dimension, elements

7 Auditory Stimulus Control in a Chimpanzee M-OL f-s FREQUENCY,-t,-~ RATE ELEMENTS ELEMENTS Fig. 1. Three-dimensional representation of the percent responses to the rate dimension for each stimulus set in stimulus control testing: Left margin, three combinations of the frequency elements, right margin, three combinations of the rate elements. Each of the points shows the percent responses to the rate dimension for each testing set. For example, the point at the intersection of m-s and H-L indicates the percent responses to the rate dimension for Hm-Ls. of which were easier to discriminate than those in the frequency dimension, whereas, in Hm- Ls, where frequency elements were more discriminable than rate elements, her behavior was conversely controlled by the frequency dimension. Such effects of the discriminability, however, were stronger in the rate dimension than in the frequency dimension. The data in Figure 1 show that as the discriminability of rate elements increased, so the extent of control by the rate dimension also increased. On the other hand, the extent of control by the frequency dimension did not increase consistently as the discriminability of the frequency elements increased. One possible reason for this is that the rate elements were more discriminable than the frequency elements. Another plausible reason is that the discriminability in the frequency dimension was masked by those in the rate dimension. In other words, the salience or perceptual dominance of the rate dimension could influence the discriminability in the frequency dimension. If the rate dimension is more salient than the frequency dimension for the chimpanzee, this could be due to the temporal property of the rate dimension. CHURCH and MECK (1984) defined stimulus rate as the number of stimuli per unit time. Following this definition, the discrimination of rate depends on temporal perception. The temporal properties of stimuli, as well as the spatial properties, are often referred to as amodal, since "perceiving them does not depend on a specific modality" (MENDELSON, 1979, p. 334). In auditory discrimination in monkeys, their behavior is more rapidly controlled by the location of the sound than by the quality of the sound (HARRISON, 1984; HARRISON et al., 1971 ; SEGAL t~ HARRISON, 1978). The present results, taken together with the findings reported by HARRISON and his colleagues, suggest that it is easier for primates to acquire control by the amodal properties of dimensions of auditory stimuli such as location or presentation rate than that by the modal dimensions such as spectral contents, intensity, or frequency. Although further studies will be needed to elucidate the characteristics of the multidimensional control, the present study clearly demonstrated that in auditory frequency-rate discrimination, the chimpanzee shifted her "attention" as a function of discriminability in

8 552 M. TOMONAGA each dimension, the effects of which were stronger in the rate dimension than in the frequency dimension. Acknowledgements. This paper is based partially on a thesis submitted to the Faculty of Human Sciences, Osaka University, for the MA degree. The author wishes to thank Drs. S. MATANO, H. ISHIDA, and H. OHTA of Osaka University, and Drs. K. MUROFUSHI and T. MATSUZAWA of Kyoto University for their useful advice and help with preparation of the manuscript. Ms. E. ONo-VINEBERG critically read the English draft, and Mr. S. YAMAMOTO provided technical advice. Thanks are also due to Dr. K. NAGASE of the Osaka Municipal Tennoji Zoological Garden for taking care of the subject. REFERENCES BLOUGH, D. S., Recognition by the pigeon of the stimuli varying in two dimensions. J. Exp. AnaL Behav., 18: CHASE, S. & E. G. HEINEMANN, Choices based on redundant information: An analysis of twodimensional stimulus control. J. Exp. Psychol., 92: CHURCH, R. M. & W. H. MECK, Numerical attribute of stimuli. In: Animal Cognition, H. L. ROITBLAT, T. G. BEVER, & H. S. TERRACE (eds.), Lawrence Erlbaum Associates, Hillsdale, New Jersey, pp DOBRZECKA, C., C. SZWEJKOWSKA, & J. KONORSKI, Qualitative versus directional cues in two forms differentiation. Science, 153 : ELDER, J. H., Auditory acuity of the chimpanzee. J. Comp. Psychol., 17: FARTHING, G. W. ~; E. HEARST, Attention in the pigeon: Testing with compounds or elements. Learn. Motiv., 1: HARRISON, J. M., The functional analysis of auditory discrimination. J. Acoust. Soc. Amer., 75 : , P. DOWNEY, M. SEGAL, M. HOWE, Control of responding by the location of auditory stimuli: Rapid acquisition in monkeys and rats. J. Exp. Anal. Behav., 15: JOHNSON, D. F. & W. W. CUMMING, Some determiners of attention. J. Exp. Anal. Behav., 11 : KENDAL, S. B. & W. A. MILLS, Attention in the pigeon: Testing for excitatory and inhibitory control by the weak elements. J. Exp. Anal. Behav., 31 : KOJIMA, S., Audition, speech perception and phonation of the chimpanzee: A search for the origin of human speech. Prim. Res., 4: (in Japanese with English abstract) MACKINTOSH, N. J., An analysis of overshadowing and blocking. Quart. J. Exp. Psychol., 23: , Overshadowing and stimulus intensity. Anita. Learn. Behav., 4: & B. REESE, One-trial overshadowing. Quart. J. Exp. Psychol., 31 : MENDELSON, M. J., Acoustic-optical correspondence and auditory-visual coordination in infancy. Canad. J. Psychol., 33 : MILES, C. G. & H. M. JENKINS, Overshadowing in operant conditioning as a function of discriminability. Learn. Motiv., 4: PAGE, E. B., Ordered hypotheses for multiple treatments: A significance test for linear ranks. J. Amer. Stat. Ass., 58: PETERSEN, M. R., Perception of acoustic communication signals by animals. In: Development of Perception: Psychobiological Perspectives. R. N. ASLIN, J. R. ALBERTS, M. R. PETERSEN (eds.), Academic Press, New York, pp RAY, B. A., Selective attention: The effects of combining stimuli which control incompatible behavior. J. Exp. Anal. Behav., 12: REYNOLDS, G. S., Attention in the pigeon. J. Exp. Anal. Behav., 4: SEGAL, M. t~ J. M. HARRISON, The control of responding by auditory stimuli: Interactions between different dimensions of the stimuli. J. Exp. Anal. Behav., 30: SKINNER, B. F., Science and Human Behavior. Macmillan, New York. STEBBINS, W. C., Hearing. In: Behavior of Nonhuman Primates: Modern Research Trends, Vol. 3, A. M. SCHRIER t~ F. STOLLNIZ (eds.), Academic Press, New York, pp

9 Auditory Stimulus Control in a Chimpanzee 553 TERRACE, H. S., Stimulus control. In: Operant Behavior: Areas of Research and Application, W. K. HONIG (ed.), Appleton-Century-Crofts, New York, pp TOMONAGA, M. & H. OHTA, Acquisition and transfer of visual Go/No-go discrimination by a chimpanzee. Primates, 31 : WILKIE, O. M. M. E. MASSON, Attention in the pigeon: A reevaluation. J. Exp. Anal. Behav., 26: Received October 26, 1989; Accepted January 18, 1990 Author's Name and Address: MASAKl TOMONAGA, Department of Biological Anthropology and Human Ecology, Faculty of Human Sciences, Osaka University, Yamada-oka, 1-2, Suita, Osaka, 565 Japan.