What's special about human speech? A student exercise for comparing speech production between humans and chimpanzees.

Modern humans and chimpanzees share a common ancestor on the phylogenetic tree, yet chimpanzees do not spontaneously produce speech or speech sounds. The lab exercise presented in this paper was developed for undergraduate students in a course entitled "What's Special About Human Speech?" The exercise is based on acoustic analyses of the words "cup" and "papa" as spoken by Viki, a home-raised, speech-trained chimpanzee, as well as the words spoken by a human. The analyses allow students to relate differences in articulation and vocal abilities between Viki and humans to the known anatomical differences in their vocal systems.

Cochlear tuning and the peripheral representation of harmonic sounds in mammals.

Albert Feng was a prominent comparative neurophysiologist whose research provided numerous contributions towards understanding how the spectral and temporal characteristics of vocalizations underlie sound communication in frogs and bats. The present study is dedicated to Al's memory and compares the spectral and temporal representations of stochastic, complex sounds which underlie the perception of pitch strength in humans and chinchillas. Specifically, the pitch strengths of these stochastic sounds differ between humans and chinchillas, suggesting that humans and chinchillas may be using different cues.

Perception of degraded speech by chinchillas (Chinchilla laniger): Word-level stimulus generalization.

One characteristic of human speech perception is a remarkable ability to recognize speech when the speech signal is highly degraded. It has been argued that this ability to perceive highly degraded speech reflects speech-specific mechanisms. The present study tested this hypothesis by measuring the ability of chinchillas to recognize noise-vocoded (NV) versions of naturally spoken monosyllabic words using operant conditioning in a stimulus generalization paradigm.

Perception of noise-vocoded tone complexes: A time domain analysis based on an auditory filterbank model.

When a wideband harmonic tone complex (wHTC) is passed through a noise vocoder, the resulting sounds can have spectra with large peak-to-valley ratios, but little or no periodicity strength in the autocorrelation functions. We measured judgments of pitch strength for normal-hearing listeners for noise-vocoded wideband harmonic tone complexes (NV-wHTCs) relative to standard and anchor stimuli. The standard was a 1-channel NV-wHTC and the anchor was either the unprocessed wHTC or an infinitely-iterated rippled noise (IIRN).

Acoustic analysis of the frequency-dependent coupling between the frog's ears.

The ears of anurans are coupled through the Eustachian tubes and mouth cavity. The degree of coupling varies with frequency showing a bandpass characteristic, but the characteristics differ between empirically measured data based on auditory nerve responses and tympanic membrane vibration. In the present study, the coupling was modeled acoustically as a tube connected with a side branch.

Perception of degraded speech sounds differs in chinchilla and human listeners.

The behavioral responses of chinchillas to noise-vocoded versions of naturally spoken speech sounds were measured using stimulus generalization and operant conditioning. Behavioral performance for speech generalization by chinchillas is compared to recognition by a group of human listeners for the identical speech sounds. The ability of chinchillas to generalize the vocoded versions as tokens of the natural speech sounds is far less than recognition by human listeners.

Pitch strength of noise-vocoded harmonic tone complexes in normal-hearing listeners.

To study the role of harmonic structure in pitch perception, normal-hearing listeners were tested using noise-vocoded harmonic tone complexes. When tested in a magnitude judgment procedure using vocoded versions generated with 2-128 channels, judgments of pitch strength increased systematically as the number of channels increased and reflected acoustic cues based on harmonic peak-to-valley ratio, but not cues based on periodicity strength. When tested in a fundamental frequency discrimination task, listeners correctly recognized the direction of pitch change with as few as eight noise-vocoded channels.

Processing pitch in a nonhuman mammal (Chinchilla laniger).

Whether the mechanisms giving rise to pitch reflect spectral or temporal processing has long been debated. Generally, sounds having strong harmonic structures in their spectra have strong periodicities in their temporal structures. We found that when a wideband harmonic tone complex is passed through a noise vocoder, the resulting sound can have a harmonic structure with a large peak-to-valley ratio, but with little or no periodicity in the temporal structure.

Perception of the missing fundamental by chinchillas in the presence of low-pass masking noise.

The pitch of the missing fundamental (F0) is one of the principal psychological attributes of human pitch perception. Behavioral responses to harmonic tone complexes having missing F0s were measured in chinchillas using operant conditioning and stimulus generalization. Animals were trained to discriminate between tone complexes having a 500-Hz F0 and a 125-Hz F0.

Critical bands and critical ratios in animal psychoacoustics: an example using chinchilla data.

This paper suggests that critical ratios obtained in noise-masked tone studies are not good indicators of critical bandwidths obtained in both human and nonhuman animal subjects. A probe-tone detection study using chinchilla subjects suggests that they may be broadband processors in detection tasks as opposed to human subjects who use narrow-band, critical-band processing. If chinchilla and other nonhuman animal subjects are wideband processors, this can partially explain why their critical ratios are significantly greater than those measured in human subjects.

Representation of the spectral dominance region of pitch in the steady-state temporal discharge patterns of cochlear nucleus units.

Single-unit responses to infinitely iterated rippled noise and wideband noise were recorded from the cochlear nucleus of anesthetized chinchillas. Rippled noises had a fixed delay of 4 ms, and spectral depth was varied by attenuating the delayed version of the noise. Temporal discharge patterns were analyzed using neural autocorrelograms, and responses to rippled noises were compared to wideband noise responses.

Pitch perception in chinchillas (Chinchilla laniger): stimulus generalization using rippled noise.

Rippled noises evoke the perception of pitch in human listeners. Infinitely iterated rippled noise (IIRN) is generated when wideband noise (WBN) is delayed, attenuated, and added to the original WBN through either a positive (+) or a negative (-) feedback loop. The pitch of IIRN[+] is matched to the reciprocal of the delay, whereas the pitch of IIRN[-] for the same delay is an octave lower.

Pitch cue learning in chinchillas: the role of spectral region in the training stimulus.

Chinchillas were trained to discriminate a cosine-phase harmonic tone complex (COS) from wideband noise (WBN) and tested in a stimulus generalization paradigm with tone complexes in which phase differed between frequency regions. In this split-phase condition, responses to complexes made of random-phase low frequencies, cosine-phase high frequencies were similar to responses to the COS-training stimulus. However, responses to complexes made of cosine-phase low frequencies, random-phase high frequencies were generally lower than their responses to the COS-training stimulus.

Listening experience with iterated rippled noise alters the perception of 'pitch' strength of complex sounds in the chinchilla.

Behavioral responses obtained from chinchillas trained to discriminate a cosine-phase harmonic tone complex from wideband noise indicate that the perception of 'pitch' strength in chinchillas is largely influenced by periodicity information in the stimulus envelope. The perception of 'pitch' strength was examined in chinchillas in a stimulus generalization paradigm after animals had been retrained to discriminate infinitely iterated rippled noise from wideband noise. Retrained chinchillas gave larger behavioral responses to test stimuli having strong fine structure periodicity, but weak envelope periodicity.

Perception of the periodicity strength of complex sounds by the chinchilla.

The perception of periodicity strength was studied in chinchillas using a stimulus generalization paradigm in an operant-conditioning, positive reinforcement behavioral task. Stimuli consisted of cosine-phase and random-phase harmonic complex tones, infinitely iterated rippled noises, and wideband noise. These stimuli vary in periodicity strength as measured by autocorrelation functions and are known to generate a continuum in the perception of pitch strength in human listeners.

Pitch strength and Stevens's power law.

Recent studies have suggested that the saliency or the strength of pitch of complex sounds can be accounted for on the basis of the temporal properties in the stimulus waveform as measured by the height of the first peak in the waveform autocorrelation function. We used a scaling procedure to measure the pitch strength from 15 listeners for four different pitches of complex sounds in which the height of the first peak in the autocorrelation function systematically varied. Pitch strength judgments were evaluated in terms of a modification of Stevens's power law in which temporal information was used from both the waveform fine structure and the envelope.