Perception of complex sounds by the green treefrog, Hyla cinerea: envelope and fine-structure cues

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1 J Comp Physiol A (1993) 173: Journal of Comparative and Physiology A Behavioral Physiology 9 Springer-Verlag 1993 Perception of complex sounds by the green treefrog, Hyla cinerea: envelope and fine-structure cues A.M. Simmons, R.C. Buxbaum, M.P. Mirin Department of Psychology, Brown University, Box 1853, 89 Waterman St., Providence, RI 02912, USA Accepted: 30 April, 1993 Abstract. 1. The envelope periodicity of communication signals is an important feature distinguishing advertisement and aggressive calls for the green treefrog (Hyla cinerea). Envelope periodicity, a cue for periodicity pitch perception in humans, is affected by the fine-structure of the signal, a cue for timbre perception in humans. The present study examined perception of two acoustic features affecting waveform fine-structure - harmonic structure and phase structure - in male green treefrogs. 2. We analyzed evoked vocal responses of male green treefrogs living in laboratory arenas to playbacks of digitally-generated signals resembling either conspecific advertisement or aggressive calls in their first harmonic periodicity. Systematic changes in the harmonic structure of these signals were achieved by varying the harmonic relations between frequency components in the signals, and changes in phase structure were achieved by varying the starting phases of harmonically-related components. 3. Calling was significantly influenced by the first harmonic periodicity of the signals. Males vocalized more to signals with the periodicity of the advertisement than the aggressive call. There were no differences in response to harmonic and inharmonic signals with similar spectral content. Phase structure did not significantly influence vocal responses. 4. These results suggest that the fine-structure ("timbre") of complex acoustic signals is not a significant feature guiding behavior tested using a communication response in this species. Key words: Treefrog - Acoustic communication plex sounds - Harmonic periodicity Introduction Com- Many species of anuran amphibians (frogs and toads) communicate using complex, often harmonically-structured acoustic signals. Both spectral and temporal properties of these signals play important roles in species recognition and in mediating female choice of appro- Correspondence to." Dr. A.M. Simmons priate mates. Of these different cues, some of the more salient include the frequency distribution of harmonics in the signal, waveform periodicity, call duration, and calling rate (Allan and Simmons 1993; Forester and Czarnowsky 1985; Gerhardt 1978, 1981, 1987, 1988; Klump and Gerhardt 1987; Schwartz 1986; Straughan 1975; Sullivan 1983; Wells 1988). Waveform periodicity is a particularly important cue for distinguishing between different functional categories of vocalizations in several anuran species. Acoustic features affecting waveform periodicity (the amplitude-time envelope of the signal) include the rate and depth of amplitude modulation as well as the fundamental frequency itself. The importance of these envelope cues is particularly evident in examination of the vocal repertoire of the green treefrog (Hyla cinerea). Males of this species use two distinct calls for communication, the advertisement call and the aggressive call. Both signals are harmonically-structured, with dominant spectral peaks in a low-frequency range around 900 Hz, and in a high-frequency range around 2700 and 3000 Hz (the exact values of these peaks vary both among individual frogs, and between groups of frogs in different parts of their range; Gerhardt, personal communication). These signals differ primarily in their waveform periodicity: The advertisement call has a pulsatile beginning (one to two cycles of Hz modulation) followed by a smooth (unmodulated) envelope with a first harmonic periodicity of about 300 Hz, and the aggressive call is modulated at a rate of 50 Hz, leading to a 50 Hz periodicity as well as to extra sidebands produced by the modulation process (Gerhardt 1978). Both female (Gerhardt 1978) and male (Allan and Simmons 1993) green treefrogs respond differently to, and presumably distinguish between, natural and synthetic versions of these signals on the basis of these differences in waveform periodicity. For human listeners, waveform periodicity is an acoustic cue for the psychological percept of periodicity pitch (Moore 1989). This temporal feature can be influenced by spectral information, represented as the detailed structure of the waveform under the envelope - the fine-structure. Important cues affecting waveform fine-

2 322 A.M. Simmons et al.: Sound perception by treefrogs structure include the particular frequencies present in the sound, and their relative amplitudes and starting phases. These fine-structure cues, in humans, contribute to the psychological percept of timbre, the quality or tone color of a sound (Moore 1989; Plomp 1976). One way in which to affect the percept of timbre is by changing the starting phases of the individual harmonics in the sound; such manipulations in phase structure, by changing the shape of the waveform envelope, affect the salience of the periodicity pitch associated with the sound (Bilsen 1973 ; Lundeen and Small 1984; Moore 1977; Ritsma and Engel 1964). The psychological percept of timbre is also affected by changes in the harmonic relations between individual frequency components in the sound. Depending on the magnitude of the changes in harmonic structure and resulting changes in the shape of the waveform envelope, the sound can be considered to be quasiperiodic or inharmonic, and the salience and identity of the perceived periodicity pitch can be altered (de Boer 1956; Plomp 1976; Schouten et al. 1962). The perceptual salience of acoustic features related to timbre has been identified as important for understanding periodicity pitch perception in humans (Moore 1989; Patterson 1987; Schouten 1970). The focus of the experiments reported here is to examine the role of the fine-structure of complex, biologicallyrelevant vocal signals in mediating the perception of these signals by green treefrogs. Few studies (Gerhardt 1978) have examined the relative influence of envelope and fine-structure cues on call recognition, and have focussed primarily on the perceptibility of the waveform envelope itself (Allan and Simmons 1993; Gerhardt 1978). Whether anurans can perceive, and use for communication, acoustic features contributing to waveform fine-structure is important for two reasons. First, physiological studies suggest that the anuran auditory system represents a good model for studying the neural basis of periodicity phenomena. Eighth nerve fibers code the amplitude envelope of complex sounds (Rose and Capranica 1985; Schwartz and Simmons 1990; Simmons et al. 1992), and are sensitive to both harmonic structure (Simmons and Ferragamo 1993) and phase structure (Simmons et al. 1993). How and whether these physiological cues translate into behavior would provide crucial information in constraining models of complex sound perception in anurans. Second, several species of frogs are behaviorally sensitive to acoustic cues mediating the perception of pitch in humans, such as, as described above, waveform periodicity (Allan and Simmons 1993; Gerhardt 1978, 1988). Perception of phase structure by frogs has not been previously reported. Whether frogs can perceive differences in the harmonic structure of complex sounds is unresolved. Green treefrogs tested in a psychophysical paradigm show lower thresholds for detecting a harmonic than an inharmonic signal in noise (Simmons 1988). But, female green treefrogs do not apparently use this acoustic cue to guide their phonotaxis responses (Gerhardt et al. 1990). In these experiments, we test the ability of male green treefrogs to perceive acoustic cues related to timbre perception in humans. We assess the animals' recognition of synthetic versions of their advertisement and aggressive calls. The synthetic signals have the same waveform periodicity as natural calls of these animals but differ in either their phase structure or their harmonic structure. Perception of these cues is tested using the evoked calling technique, a behavioral assay based on a communication response of the animal. The use of this technique, rather than a psychophysical technique, allows us both to test the assertions by Gerhardt et al. (1990) on the usefulness of harmonic structure as a factor guiding communication and to measure the generalizability of the psychophysical results themselves. Experiments were conducted in the laboratory to allow precise control of the acoustic background. Materials and methods Six male green treefrogs were tested in these experiments. They were established in a circular arena (1.3 m diameter) housed in a soundattenuating room (3 x 3 m) lined with acoustic foam to minimize reverberations and extraneous noise. The room was maintained at temperatures of 23 to 25 ~ with a light/dark schedule of 14 h light/10 h darkness, simulating the natural breeding period of the green treefrog. Males were housed in the arena in groups of 2 to 5 males. In this colony, only one caller was present. If other males began vocalizing, either spontaneously or during the course of playbacks, they were removed in order to avoid vocal interactions between frogs. We noted that males housed singly seldom vocalized. The frogs were tested for about 1 h/day, 5 days/week, during the months of September-November 1990 and September-December Testing began near the beginning of the dark period in the light/dark cycle. Stimuli. Acoustic stimuli were digitally synthesized on an IBM PC/AT computer with a Data Translation DT2801A board at a sampling rate of 20 khz. Two different sets of stimuli were synthesized. For the experiments testing perception of harmonic structure, the stimuli were 4 two-tone complexes with frequencies of Hz (harmonic sound with periodicity of 300 Hz); Hz (harmonic sound with periodicity of 276 Hz); Hz (inharmonic sound without a stable first-harmonic periodicity); and Hz (inharmonic sound without a stable firstharmonic periodicity). For the first 3 stimuli, the changes in finestructure did not obviously change the shape of the waveform envelope, but the envelope shape was altered in the second inharmonic stimulus. Three of these stimuli were the same as those used by Simmons (1988) and two of them were used by Gerhardt et al. (1990). The frequencies in these signals were chosen either to match dominant spectral peaks in the advertisement call of the green treefrog, or to be within the same critical ratio-bands as these frequencies (Moss and Simmons 1986). The individual frequency components in the complexes were of equal amplitude, and were summed together in sine phase. Stimuli were 160 ms in duration with rise/fall times of 10 ms. For the experiments testing perception of phase structure, stimuli were synthetic 12-component harmonic complexes with frequencies based on green treefrog vocalizations recorded in the field in eastern Georgia. These natural vocalizations were digitized with an RC Electronics isc-67 interface board and accompanying software on an IBM PC/AT computer. Spectral analyses of the vocalizations were computed using ILS software from Signal Technology Inc. Twelve harmonics were chosen from this analysis (see Table 1). Their amplitudes and frequencies were set to be integer multiples of a (missing) 300 Hz fundamental frequency, and matched those of the digitized vocalizations. Six different variations of this basic stimulus were produced. Three stimuli were unmodulated, but maintained a waveform periodicity of 300 Hz to match that of the

3 A.M. Simmons et al. : Sound perception by treefrogs 323 Table 1. Composition of the stimuli in phase-shift experiments Frequency (Hz) Relative Phase shifts (degrees) Amplitude (db) SNE ALT RND SNE phase-coherent stimulus; ALT alternating phase stimulus; RND random phase stimulus. These 3 basic signals were left unmodulated to produce 300 Hz-type stimuli, or were modulated at 50 Hz to produce 50 Hz-type stimuli conspecific advertisement call. Three other stimuli were modulated at a rate of 50 Hz (depth 100%) to match the waveform periodicity of the conspecific aggressive call. The stimuli further differed in the starting phases of the individual harmonics. In one variant, all starting phases were set at sine phase (phase-coherent waveform, labeled SNE). In a second variant, starting phases varied between consecutive harmonics between sine and cosine phase (alternatingphase waveform, labeled ALT). In a third variant, starting phases were chosen using a 4th moment algorithm for flattening a waveform, based on that of Pumplin (1985). The resulting waveform is termed random-phase, labeled RND. Starting phases of each harmonic in the 6 stimuli are given in Table 1. Stimulus durations were 160 ms with 10 ms rise/fall times. Procedure. Stimulus presentation was controlled by an IBM PC/AT computer using custom-written software. The digitally-generated sounds were filtered (20 to 6000 Hz) with a Krohn-Hite model 3550 filter, attenuated (Hewlett-Packard audio attenuator and Coulbourn electronic attenuator), amplified (first channel of a Coulbourn $82-24 mixer/amplifier and Onkyo A-15 stereo amplifier), and presented to the frogs via a loudspeaker (Realistic, minimus-7) suspended 1 m above the center of the arena. The frog's responses were picked up by an Azden microphone placed at the approximate position of the calling male and recorded on the right channel of a Marantz (PMD 430) stereo tape deck. Stimuli were simultaneously recorded onto the left channel of the same tape deck. Calibration of RMS intensity levels of the acoustic stimuli was performed before each individual testing session. This was done using a Briiel & Kjaer 2230 digital precision sound level meter ("fast" setting, linear scale) placed at the approximate position of the calling male. The intensity levels of the stimuli did not vary more than 3 db for any position the frog took in the arena. Because animals were not restrained, the exact position of the caller throughout each individual testing session was unknown. We observed, however, that the caller appeared to remain within a small circumscribed area (< 0.3 m) during the experiment. Three frogs were tested for perception of harmonic structure. The 4 different stimuli were presented in 8 blocks (2 blocks for each stimulus) of 270 repetitions each. Interstimulus interval was 760 ms. Within each block, only one stimulus was presented. The order of stimulus presentation (blocks) was organized such that the same stimulus was not presented in consecutive blocks and all 4 stimuli were presented in each half of each playback session. Within these constraints, the order of presentation was randomized. There was a 3 min silent interval between blocks, during which spontaneous calling was measured. Stimulus intensity was set at 70 db SPL, a level at which males reliably vocalize in the laboratory (Allan and Simmons 1993). Background noise was present continually throughout each experimental block. This was generated with a Coulbourn $81-02 noise generator, lts level was set, in different experimental sessions, at spectrum levels of 15, 25 or 35 db RMS/ Hz, the latter two being the same used in the experiment of Simmons (1988). Three frogs were tested for perception of phase structure. The 6 different stimuli were presented in 6 blocks (1 block per stimulus) of 200 repetitions each, with an interstimulus interval of 760 ms. Each block lasted about 2-3 min. The order of stimulus presentation (blocks) was randomized. Silent intervals of 3 min duration separated individual blocks; spontaneous calling was measured during these intervals. Stimulus intensities were set at either 70 or 80 db SPL on separate testing sessions, and stimuli were presented under ambient noise conditions only. Analysis. The tape-recorded vocal responses to the playbacks were passed through a Schmitt trigger and digitized (IBM PC/AT computer with RC Electronics series 200 acquisition board, sampling rate 25 khz). They were analyzed by a custom-designed software program which converted each response into a digital display and computed the total number of responses evoked by each stimulus presentation; the latency of the first response to a given stimulus, measured from the onset of stimulus presentation; and the time-ofoccurrence of each vocal response with respect to each stimulus. The timing data were used to compute vector strength (VS), a measure of synchronization of response to stimulus (Batschelet 1981). VS varies between 0 (indicating no synchronization) and 1 (indicating perfect synchronization). The statistical significance of VS was calculated using the Rayleigh test of circular data (z distribution; Batschelet 1981). Differences in responding across stimulus type were analyzed using Friedman two-way analysis of variance by ranks (chi-square distribution) and the Kruskal-Wallis test. Results Perception of harmonic structure. We analyzed data only from those playback sessions (N= 21) during which the frogs responded to at least 4 blocks of stimuli. Across these 21 playback sessions, there were no significant differences in the proportion of blocks during which responding occurred between harmonic (87 % of blocks) or inharmonic (85% of blocks) stimuli, or between the two harmonic or the two inharmonic stimuli taken separately. There were 35 playback sessions in which the animals responded in at least one stimulus block (we removed from these analyses all sessions in which the frogs vocalized only during the first stimulus block). Within these 35 sessions, there were again no significant differences in proportion of blocks eliciting responding between harmonic (63%) or inharmonic (59%) stimuli. Figure 1 shows how the males' vocal responses were influenced by the harmonic structure of the stimuli and by the background noise level present during playbacks. Data were included only from those 21 sessions meeting our criteria for responding, and are shown as means (+ standard deviations) calculated from responses to the two harmonic stimuli and the two inharmonic stimuli summed together. There were no consistent individual differences between frogs in the pattern of response. The intensity of background noise significantly influenced the number of evoked vocal responses [H (31,63,58)=8.1, P<0.05; Fig. 1A]. Males called significantly more in

4 324 A.M. Simmons et al.: Sound perception by treefrogs response to stimuli presented against a noise level of 25 db RMS/Hz compared to either lower or higher levels. The drop in calling at high levels of background noise suggests that male responses were not stimulated by the noise alone. Spontaneous calling was essentially nonexistent. There were no significant differences in number of responses attributable to differences between harmonic and inharmonic stimuli either across noise levels [Z 2 (3)= 1.1, P< 0.9] or within any particular noise level. Latency to first response also varied with noise intensity (Fig. 1B). For these analyses, a latency was included only for those blocks during which the frogs responded. This allowed us to test for consistent differences in latency apart from differences in number of responses, and it means that these data are based on unequal sample sizes. Latencies to responding were significantly shorter at a noise level of 25 db RMS/Hz [H (29,61,49)=22.2, P<0.01] than at either higher or lower noise levels. Latency did not differ significantly across noise levels with differences in harmonic structure [Z2(3)=1.5, P<0.75), nor within any noise level. Males tended to synchronize their responses to the playbacks, indicating that responding was not controlled by the presence of the continuous background noise alone (Fig. 1C). At all noise levels and for all stimuli, vector strength was statistically different from 0. (These data only include a value for vector strength when the frogs actually responded to a particular stimulus). Responding was significantly more synchronized at a noise level of 15 db RMS/Hz than at a noise level of 35 db RMS/Hz [H (29,61,49)=42.4, P<0.01]. Across noise levels, vector strength did not vary significantly with differences in harmonic structure [Z 2 (3) = 7.1, P < 0.1]. Perception of phase structure. Data could be analyzed from 35 separate experimental sessions during which the animals vocalized during at least one stimulus block, excluding the first. The frogs responded more often to playbacks at 80 db SPL than at 70 db SPL; however, the pattern of response (relative responding across stimuli) was similar across playback intensity, so data presented here are summed across intensity. The frogs tended to respond most consistently in response to playbacks of the 300 SNE stimulus, vocalizing to this stimulus on about 90% of the test sessions, as compared to the 50 SNE stimulus, to which responding occurred in only about 50% of the test sessions. Differences in the proportion of test sessions during which the frogs vocalized as a func- A ~, I loo 150 l - o 50 0 B, (J ]Harmonic A ~ 200 o ~ 150 ~ 1 O0 ~S 5o B ~" 60 o l"- C ~ 20 0 C 1.0 0,8,.- 0,6 ~ ~ 0,2 0,0 Noise!j!, )ectrum Level (db RMS/Hz) Fig. 1A-C. Influence of harmonic structure on evoked vocal responses. Data are means and standard deviations of responses to the sum of the 2 harmonic and the sum of the 2 inharmonic stimuli across different background noise levels. A Number of vocal responses. B Latency to first vocal response. C Vector strength of response o ~ -~ ~ 40 ~ c- o o t- ~.. m ~ e 20 _j rl I % ~ 0.8 o ~ 0.6 ~ 0.4 ~ ~ g g g Fig. 2A-C. Influence of phase structure on evoked vocal responses. Data are means (:k standard deviation) averaged over individual frogs and displayed over stimulus type. Stimuli differ in both waveform periodicity (50 and 300 groups) and in phase structure (SNE, ALT and RND groups). A Number of vocal responses. B Latency to first response. C Vector strength of response

5 A.M. Simmons et al.: Sound perception by treefrogs 325 tion of type of stimulus were, significantly different as a function of waveform periodicity [50 Hz vs 300 Hz groups of stimuli; Z~(5)= 11, P<0.05]. There were no significant differences within the 50Hz and 300 Hz groups according to the phase characteristics of the different stimuli. There were no significant differences among individual frogs in the pattern of responding. The evoked calling responses of the males were influenced by the waveform periodicity (50 or 300 Hz) rather than phase structure (SNE, ALT, RND variations) of the stimuli (Fig. 2). As shown in Fig. 2A, the frogs vocalized more in response to playbacks of stimuli modulated at 300 Hz than at 50 Hz. Statistical analyses revealed significant effects by stimuli [Z 2 (5)= 32.9, P < 0.001] attributable to differences between the 50 Hz and 300 Hz groups of stimuli. There were no differences in number of responses among the 3 types of 50 Hz modulated stimuli [Z 2 (2)= 1.95, P= 0.38] or among the 3 different types of 300 Hz stimuli [Z 2 (2)= 0.47, P = 0.06]. Latency data were included only for those testing sessions in which an animal responded to a particular stimulus. There were no significant differences in latency to respond across the different stimuli [Z2(5)=9.1, P=0.10; Fig. 2B]. The data in Fig. 2C show that when the frogs responded to a stimulus, they tended to respond consistently at a particular time within the interstimulus interval (high VS). Synchronization was significantly greater in response to the 300 Hz stimuli than to the 50 Hz stimuli [Z~z(5)=12.4, P< 0.05]. There were no differences in synchronization of responding that could be reliably attributed to differences in the phase structure of the stimuli (in the 50 Hz group of stimuli: Z2(2)=0.93, P=0.63; in the 300 Hz group of stimuli: Z2(2)=5.5, e = 0.06]. Discussion The two main signals in the male green treefrog's repertoire, the advertisement call and the aggressive call, differ in both their waveform periodicity (shape of the amplitude-time envelope) and in their fine-structure. These differences come about from differences in modulation rate superimposed on the complex waveform, which produce changes in waveform fine-structure from the extra sidebands produced by the modulation process. Both female (Gerhardt 1978) and male (Allan and Simmons 1993) green treefrogs perceive these differences in modulation rate and waveform periodicity. The present data (Fig. 2) confirm those of Allan and Simmons (1993) in showing that differences in waveform periodicity can significantly influence evoked calling behavior in the male green treefrog. In these experiments, we found no significant differences in male calling responses that could be attributed to differences in the fine-structure of the signal, whether expressed as shifts in harmonic structure (Fig. 1) or as shifts in phase structure (Fig. 2). Changing the harmonic relationship between components in a two-tone signal to an inharmonic relation (no stable first harmonic periodicity) did not significantly influence male responses to these signals when presented against background noise. When the starting phases of consecutive harmonics in a 12-component signal were shifted, the vocal responses of the males did not differ significantly with these changes in fine-structure. Previous research has dealt with the issue of perception of fine-structure in these animals, with mixed results. Gerhardt (1978) showed that female green treefrogs prefer a three-component synthetic call with a 300 Hz periodicity over a two-component synthetic call with the same periodicity. The differences in the fine-structure of these signals were reflected in the shape of the waveform envelope, and Gerhardt argued that it was the shape of the envelope, rather than the fine-structure difference itself, that motivated the females' responses. Consistent with this explanation, Gerhardt et al. (1990) found no preferences for harmonic over inharmonic signals when these changes in harmonic structure did not also produce obvious changes in the shape of the waveform envelope. The data presented here suggest that differences in harmonic structure, when individual frequency components remain within the same critical bands as in the harmonic situation, do not produce significant differences in evoked vocal responses. They suggest that the males' responses were guided by the amplitude-time envelope (waveform shape) and periodicity of the signals. The shifts in the envelope of one of the inharmonic stimuli were apparently not salient enough to affect vocal behavior when presented against background noise. The gross temporal features of the waveform thus appeared to have perceptually overshadowed any changes in waveform fine-structure. The differences in the salience of harmonic structure as a perceptual cue as now noted in different experiments may be due to several factors. First, these experiments all use different techniques with different underlying assumptions. Neither the study by Gerhardt et al. (1990), based on a selective phonotaxis response by females, nor the present study, based on an evoked calling response by males, can necessarily be used to answer questions about sound detection." if an animal tested using either of these two techniques does not respond to a particular stimulus, or does not seemingly differentiate between two stimuli, we cannot with certainty conclude that the animal does not detect differences between these stimuli. The animal's behavior suggests a lower or more inclusive criterion for responding in a communication situation than in a psychophysical situation. Second, both the study by Gerhardt et al. (1990) and the present study used playback levels above threshold in order to reliably motivate responding, while the study by Simmons (1988) was concerned with differences at threshold. We were unable to elicit evoked vocal responding at the lower playback levels where the psychophysical differences were found. Third, the negative results both of the present study and of the study by Gerhardt et al. (1990) may have been influenced by the presence of other acoustic features besides those being studied. Specifically, in these experiments, the harmonic and inharmonic stimuli were of the same duration as natural calls and were

6 326 A.M. Simmons et al. : Sound perception by treefrogs presented in series at species-typical calling rates. In the psychophysical experiment, stimuli were presented singly and aperiodically, at interstimulus intervals ranging from 20 to 40 s, well outside the species-typical calling rate. Call rate is an important acoustic feature for green treefrogs and other anurans (Forester and Czanrowsky 1985; Gerhardt 1988; Klump and Gerhardt 1987; Schwartz 1986; Wells 1988). It may be a salient enough cue to override the subtler differences in the signals tested in these studies. Indeed, Allan and Simmons (1993) observed that male green treefrogs will vocalize in response to unmodulated noise bursts presented at a species-typical calling rate and duration. Fourth, the psychophysical results were strongly influenced by individual differences among animals. The inhibition level functions presented by Simmons (1988; Fig. 2) differed in their slope both across animals and across stimuli, and the actual magnitude of the inhibition effects, although statistically significant, was small and also differed across individual animals. Neither the study by Gerhardt et al. (1990) nor the present study observed any reliable individual differences among the animals being tested. Other evidence relevant to the issue of the importance ofwaveform fine-structure is available. The vocalizations of individual male green treefrogs, whether recorded in the natural environment or under laboratory conditions, are relatively stereotyped within individuals but can vary between individuals even within the same geographic area (Gerhardt, personal communication; Allan, personal communication). For example, the dominant lowfrequency spectral peak in the advertisement call of 29 individual males at the same calling site was found to vary from around 700 to 1160 Hz (Gerhardt et al. 1987), and males often produce signals that are aperiodic (Gerhardt et al. 1990). These differences in fine-structure cues are potentially available for recognition of specific individual frogs, while envelope cues might be more stereotyped and useful for species and call recognition. How any of these acoustic features influence mating success is uncertain (Gerhardt et al. 1987). Moreover, fine-structure cues may be seriously degraded by acoustic interference in natural choruses (Schwartz 1987). These observations argue against specific reliance on fine-structure cues in natural choruses, particularly on the part of male frogs where such fine discriminations may not confer any reproductive advantage. In a series of studies on the coding of the conspecific advertisement call by auditory nerve fibers in the bullfrog, Schwartz and Simmons (1990) and Simmons et al. (1992, 1993) showed that acoustic features mediating pitch and timbre perception in humans are reflected in temporal responses of the anuran eighth nerve. Specifically, at sound intensities of 70 to 90 db SPL, most fibers, regardless of their frequency tuning, preferentially synchronize to the waveform periodicity (amplitude envelope) of the synthetic call. At high intensities, or at low signal-to-noise ratios (high levels of background noise), many fibers shift their synchronization away from the waveform periodicity to a low-frequency spectral peak in the stimulus. This means that the coding strategy changes from one of envelope detection to one of fine-structure detection. Changing the phase relations of harmonics within the signal also affects fiber response most consistently at high sound levels. Simmons and Ferragamo (1993) further showed that bullfrog eighth nerve fibers are sensitive to the harmonic structure of complex sounds. Shifts in fine-structure related to shifts in the harmonic relationship between frequency components are evident in fiber response to signals with lowfrequency components. These data suggest that, at least in the bullfrog, phase and harmonic structure are coded by the eighth nerve, and are potentially available for central nervous system processing. How this neural code affects sound perception and communication behavior is still uncertain but worthy of further study. Acknowledgements. This research was supported by NIH grant NS28565 to AMS. We thank Susan Allan for providing the fieldrecorded treefrog vocalizations, and Yan Shen for programming assistance. References Allan SE, Simmons AM (1993) Temporal features mediating call recognition in the green treefrog, Hyla cinerea: Amplitude modulation. 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