M. E. H. Schouten

Categorical perception depends on the discrimination task

Speech sounds are said to be perceivedcategorically. This notion is usually operationalized as the extent to which discrimination of stimuli is predictable from phoneme classification of the same stimuli. In this article, vowel continua were presented to listeners in a four-interval discrimination task (2IFC with flankers, or 4I2AFC) and a classification task. The results showed that there was no indication of categorical perception at all, since observed discrimination was found not to be predictable from the classification data. Variation in design, such as different step sizes or longer interstimulus intervals, did not affect this outcome, but a 2IFC experiment (without flankers, or 2I2AFC) involving the same stimuli elicited the traditional categorical results. These results indicate that the four-interval task made it difficult for listeners to use phonetic information and, hence, that categorical perception may be a function of the type of task used for discrimination.

Modeling phoneme perception. I: Categorical perception

On the basis of a number of vowel and stop-consonant discrimination experiments (AX and 2IFC fixed and roving) with natural stimuli, it is concluded that stop-consonant perception is highly categorical: there were few significant differences between the discrimination results and the phoneme identification results. Moreover, the discrimination and identification response maxima differed significantly from the other data points. Vowel perception was much less categorical: the maxima in the functions were much less significant, and there were significant differences between the various paradigms. Consonant discrimination was much less (if at all) subject to range effects than vowel discrimination. All these results point to different memory types for stop consonants and vowels, and, consequently, to a combination of two different theories of speech sound discrimination: dual-process theory (DPT) for consonants, and trace-context theory (TCT) for vowels.

Identification and discrimination of sweep tones

In Experiment 1, subjects were asked to identify the direction (up or down) of sweep tones centered around frequencies of 400, 1300, and 2700 Hz. Durations were 20, 30, 40, or 50 msec, and the rates of the (unidirectional) sweeps were 0, 5,10, 20, 40, and 60 octaves/sec. The main result was that, on the whole, stimuli with zero- or low-sweep rates were judged to move \lddown,\rd irrespective of the actual direction. Experiment 2 was a discrimination experiment, in which subjects had to discriminate between falling, rising, and level sweep tones centered around 1300 Hz. It turned out that discrimination between rising and falling tones did not differ significantly from discrimination between rising and level tones, indicating again that level tones tend to be perceived as going down. The parameters were chosen in such a way that they resembled those of formant transitions in plosive consonants; some implications with regard to speech perception are discussed.

Modeling phoneme perception. II: A model of stop consonant discrimination

Combining elements from two existing theories of speech sound discrimination, dual process theory (DPT) and trace context theory (TCT), a new theory, called phoneme perception theory, is proposed, consisting of a long-term phoneme memory, a context-coding memory, and a trace memory, each with its own time constants. This theory is tested by means of stop-consonant discrimination data in which interstimulus interval (ISI; values of 100, 300, and 2000 ms) is an important variable. It is shown that discrimination in which labeling plays an important part (2IFC and AX between category) benefits from increased ISI, whereas discrimination in which only sensory traces are compared (AX within category), decreases with increasing ISI. The theory is also tested on speech discrimination data from the literature in which ISI is a variable [Pisoni, J. Acoust. Soc. Am. 36, 277-282 (1964); Cowan and Morse, J. Acoust. Soc. Am. 79, 500-507 (1986)]. It is concluded that the number of parameters in trace context theory is not sufficient to account for most speech-sound discrimination data and that a few additional assumptions are needed, such as a form of sublabeling, in which subjects encode the quality of a stimulus as a member of a category, and which requires processing time.

Perceptual development of phoneme contrasts : How sensitivity changes along acoustic dimensions that contrast phoneme categories

Listeners discriminate acoustic differences between phoneme categories at a higher level than similarly sized differences within phoneme categories. The question this paper aims to answer is how this pattern in perceptual sensitivity develops along an acoustic dimension that contrasts two non-native speech sounds: through acquired distinctiveness, through acquired similarity, or through a combination of the two. A pretest-training-post-test experiment was designed to study perceptual development directly, i.e., by including (i) a discrimination task to measure perceptual sensitivity, (ii) a transfer test to ensure language learning instead of stimulus learning, and (iii) a control group to exclude task repetition as an explanation of improvement. It is shown that the typical peak in perceptual sensitivity near a phoneme boundary that native listeners show is not found in relatively inexperienced language learners, despite their ability to classify a continuum in a nativelike way after short laboratory training. Experiment II indicates that a discrimination peak may be achieved by language learners, but only after much more language experience than short-term laboratory training can offer. Furthermore, reasons are given why classification improvement in the laboratory should not be taken as evidence for (i) increased discrimination of the newly learned phonemes and (ii) learning of phoneme representations.

Categorical perception as a function of stimulus quality.

A number of experiments were carried out in order to test the hypothesis that categorical perception of speech stimuli is a function of synthetis quality – specifically, that the greater complexity of more natural speech stimuli makes it difficult for listeners to focus on particular stimulis parameters as psychoacoustic cues. The results show that there is an increase in categorical perception as synthesis quality improves from a simple synthesis by rule, via LPC synthesis, to a much more complex type called sinewave generation.

Identification and discrimination of sweep formants

Earlier identification experiments with sweep tones are repeated with rising and falling single formant (band) sweeps, with durations ranging from 15 to 40 msec and sweep rates from 0 to 40 oct/sec. Steady-state portions of 100-msec duration are then added to the sweeps. The general conclusions are that the tendency to perceive level and slightly rising tones as falling, which was such a prominent feature of the earlier results, disappears as the stimuli become more com-plex, and that sweep discrimination seems to be a function of the difference between the initial and the final frequency of a sweep.

Perceptual development of the Finnish /t-tː/ distinction in Dutch 12-year-old children: A training study

Abstract This paper addresses the issue of how perceptual sensitivity develops in child L2 learners along an acoustic dimension that contrasts two non-native speech sounds, and of how their language learning compares to that of adult learners, as investigated in Heeren and Schouten (2008) . A pretest–training–posttest design was used to tap into the learning process, and development was predicted to occur mainly through increased sensitivity in the newly trained category boundary region, i.e. acquired distinctiveness, rather than through a decrease in sensitivity within the new categories, i.e. acquired similarity. This prediction was borne out by both adult and 12-year-old learners of the Finnish quantity contrast, but changes remained small. Even though the manner and speed of learning were comparable between age groups, adults showed higher discrimination scores than children did.

Perception of Tone, Band, and Formant Sweeps

Natural speech can be described as an ongoing sequence of dynamic events with a certain linguistic meaning. Whatever speech perception model one prefers, undoubtedly these dynamic events have to be analyzed and interpreted by the listener. There is a substantial literature about how specific dynamic events, such as formant transitions, are labelled, identified, discriminated, memorized, masked, matched, or adapted to, in the context of speech-like stimuli. However, surprisingly little has been published about the basic properties of our hearing for analyzing and perceiving such dynamic events in the form of psychophysical stimuli. What is the just-noticeable-difference, what is the detection threshold, what is the internal dynamic spectrum? It almost seems that the use of dynamic (speech) stimuli is more common practice in electro-physiological studies (e.g. Proceedings edited by Carlson and Granstrbm, 1982) than in psychophysics. The locus theory of plosive perception, for instance, requires some form of extrapolation of formant transitions, whether or not via an innate language module which refers to articulatory gestures (Liberman and Mattingly, 1985). The model proposed by Lindblom and Studdert-Kennedy (1967) about perceptual compensation for formant frequency undershoot, implies similar perceptual capabilities.

Searching for an explanation for diphthong perception : Dynamic tones and dynamic spectral profiles

The aim was to find a psychophysical explanation for the perception, by naive listeners, of diphthongs as single vowels, even though they are essentially formant movements. Subjects were asked to match sinusoidal tone and resonance glides around 1,000 Hz with two connected steady-state tones or resonances whose frequencies could be controlled independently. The expectation was that short glides (below 120 ms) would give rise to single perceptual events without any movement in a particular direction, so that the two matching steady-state patterns would not show any frequency direction either; long resonance glides (above 120 ms), on the other hand, were expected to be perceived as rising or falling and matched accordingly. The results showed an effect of duration, although it interacted with glide width. At durations shorter than about 120 ms, subjects placed the two steady profiles with which they had to match the dynamic profile closer together than with durations over 120 ms; however, this only occurred if a glide covered more than 500 Hz, and is therefore irrelevant to diphthong perception.