Philip A. Morse

An investigation of categorical speech discrimination by rhesus monkeys

The categorical discrimination of synthetic human speech sounds by rhesus macaques was examined using the cardiac component of the orienting response. A within-category change which consisted of stimuli differing acoustically in the onset of F2 and F3 transitions, but which are identified by humans as belonging to thesame phonetic category, were responded to differently from a no-change control condition. Stimuli which differed by the same amount in the onset of F2 and F3 transitions, but which human observers identify as belonging toseparate phonetic categories, were differentiated to an even greater degree than the within-category stimuli. The results provide ambiguous data for an articulatory model of human speech perception and are interpreted instead in terms of a feature-detector model of auditory perception.

The use of auditory and phonetic memory in vowel discrimination

A commonly held assumption about memory for speech is that auditory memory is referred to only if phonetic memory does not contain the information needed for a particular trial. However, this assumption is in conflict with recent evidence [Crowder, J. Exp. Psychol.: Learning, Memory, Cognition 8, 153-162 (1982); Repp et al., J. Exp. Psychol.: Human Perception Performance 5, 129-145 (1979)]. The present study provides additional data to help determine how auditory and phonetic memory are used in a vowel discrimination task, and what happens during memory decay. Experiment 1 was conducted to determine whether performance levels decline at similar rates on between- and within-category AX vowel comparison trials when certain methodological problems are removed. This was confirmed. Experiment 2 demonstrated that in the AX task there is a vowel order effect, as Repp et al. found, but that this effect increased across interstimulus delay intervals, in contrast to their findings. The results can be accommodated with a model in which the memory for a vowel is represented as a small, bounded area within the vowel space, and in which memory decay is represented by the expansion of that bounded area over time.

The categorization of male and female voices in infancy

The present set of studies explored the nature of the 7-month-old infants perception of human voices. In Experiment I, infants learned to respond discriminatively to groups of male vs. female voices. That this was evidence of male/female categorization was supported in Experiment II, in which it was shown that infants did not learn to respond discriminatively to the same voices when they were randomly organized into “categories” containing both male and female voices. The extent to which fundamental frequency may have contributed to this male/female classification was investigated in Experiment III. The combined results of these three studies suggested that, although pitch is possibly one cue to which infants are attending when classifying these voices, it could not account fully for this ability. It remains for future research to identify other cues which may contribute to male/female categorization, as well as to investigate the developmental course of speaker recognition and classification in general.

Selective adaptation of vowels

Two experiments investigating the selective adaptation of vowels examined changes in listeners’ identification functions for the vowel continuum [i-I-∈] as a function of the adapting stimulus. In Experiment I, the adapting stimuli were [i], [I], and [∈]. Both the [i] and [∈] stimuli produced significant shifts in the neighboringand distant phonetic boundaries, whereas [I] did not result in any adaptation effects. In order to explore the phonetic nature of feature adaptation in vowels, a second experiment was conducted using the adapting stimuli [gig] and [g ∈ g], which differed acoustically from the [i] and [∈] vowels on the identification continuum. Only [gig] yielded reliable adaptation effects. The results of these experiments were interpreted as suggesting arelative rather than a stableauditory mode of feature analysis in vowels and a possibly more complex auditory feature analysis for the vowel [i].

Discrimination of voice onset time in infancy

In an effort to determine whether infants can discriminate speech sounds on the basis of a single acoustic cue, timing onset of periodic voicing, two experiments were conducted employing synthetic speech sounds. Naturally produced syllable pairs were also used for comparison. In the first experiment infants evidenced discrimination of a naturally produced /ba/ versus /pa/ pair and a naturally /du/ versus /tu/ pair. In addition, infants discriminated a synthetic /ba/ versus /pa/ contrast that was cued by several acoustic differences in addition to timing onset of periodic voicing but failed to evidence an ability to discriminate a synthetic /du/ versus /tu/ contrast that contained flat first formants and differed only in timing onset of periodic voicing. A second experiment was conducted in which infants once again evidenced discrimination of naturally produced /du/ versus /tu/ stimuli but not of synthetic /du/ versus /tu/ stimuli containing slight first-formant transitions. These results suggest that timing onset of periodic voicing alone may not be a sufficient cue for infant discrimination of English voicing contrasts.

Specific attentional effects reflected in the cardiac orienting response

Twenty-four undergraduates listened to 60-sec strings of synthetic speech syllables containing a syllable change in the middle of the string. No response was required but half of the subjects were instructed to “listen for a change.” Instructions did not affect cardiac orienting to onsets or offsets of the strings but did determine whether or not orienting occurred to a change that all subjects reported hearing. The findings imply that a failure of nonverbal subjects to orient to stimulus change cannot be accepted as evidence that the change is not discriminated.

Cardiac indices of infant speech perception: orienting and burst discrimination.

The present study investigated burst cue discrimination in 3- to 4-month-old infants with the natural speech stimuli [bu] and [gu]. The experimental stimuli consisted of either a [bu] or a [gu] burst attached to the formants of the [bu], such that the sole difference between the two stimuli was the initial burst cue. Infants were tested using a cardiac orienting response (OR) paradigm which consisted of 20 tokens of one stimulus (e.g. [bu]) followed by 20 tokens of the second syllable (20/20 paradigm). An OR to the stimulus change revealed that young infants can discriminate burst cue differences in speech stimuli. Discussion of the results focused on asymmetries observed in the data and the relationship of these findings to our previous failure to demonstrate burst discrimination using the habituation/dishabituation cardiac measure generally employed with older infants.

The Infancy of Infant Speech Perception: The First Decade of Research

Studies of infant speech perception have focused primarily on the ability of young infants (a) to make simple auditory discriminations, (b) to categorically discriminate speech sounds and (c) to exhib

Influence of task demands on the categorical versus continuous perception of vowels

Various task constraints were imposed on subjects making same/different, AX discriminations within and between phonetic categories. The experiments employed four 50‐ms, synthetic vowels from an/i/‐/I/continuum and five interstimulus delays (0‐2000 ms). In the first experiment, stimuli were presented with all trial types randomized together or with between‐ and within‐category shifts in separate trialblocks. Between‐category performance was superior with separate blocks, whereas within‐category performance did not differ in the two procedures. A second experiment involved AX presentations to subjects who were or were not initially informed of the intended category labels. Surprisingly, category knowledge improved within‐category discriminations. In a third experiment, subjects were required to identify both vowels in an AX pair. Between‐category discrimination was higher, and within‐category discrimination lower, with AX identification as compared to a same/different response. The results of these experime...

Selective adaptation effects in infant speech perception paradigms

The present studies were conducted to explore the possible role of selective adaptation in infant speech perception. In the first study, in which the growth of adaptation was examined by presenting adult listeners with successive blocks of 20 repeating stimuli, reliable adaptation effects were observed after only 80 stimulus presentations. In addition, recovery following adaptation was relatively rapid and complete by the end of a postadaptation identification sequence. Experiment II constituted a more direct investigation of the possible role of adaptation in infant speech perception in which actual protocols from heart-rate (HR) and non-nutritive high-amplitude sucking (HAS) infant testing sessions were presented to adult listeners. Reliable adaptation effects were obtained within the HAS protocol, but not within the HR format. This pattern of results was consistent with that observed in experiment I. The implications of these adult adaptation results for the processes underlying the infants responsiveness to these stimulis were discussed.