The ABCs of categorical perception

Studies of speech perception first revealed a surprising discontinuity in the way in which stimulus values on a physical continuum are perceived. Data which demonstrate the effect in nonspeech modes have challenged the contention that categorical perception is a hallmark of the speech mode, but the psychophysical models that have been proposed have not resolved the issues raised by empirical findings. This study provides data from judgments of four sensory continua, two visual and two tactual-kinesthetic, which show that the adaptation level for a set of stimuli serves as a category boundary whether stimuli on the continuum differ by linear or logarithmic increments. For all sensory continua studied, discrimination of stimuli belonging to different perceptual categories was more accurate than discrimination of stimuli belonging to the same perceptual category. Moreover, shifts in the adaptation level produced shifts in the location of the category boundary. The concept of Adaptation-level Based Categorization (ABC) provides a unified account of judgmental processes in categorical perception without recourse to post hoc constructs such as implicit anchors or external referents.     

Categorical perception of speech sounds in illiterate adults

Children affected by dyslexia exhibit a deficit in the categorical perception of speech sounds, characterized by both poorer discrimination of between-category differences and by better discrimination of within-category differences, compared to normal readers. These categorical perception anomalies might be at the origin of dyslexia, by hampering the set up of grapheme-phoneme correspondences, but they might also be the consequence of poor reading skills, as literacy probably contributes to stabilizing phonological categories. The aim of the present study was to investigate this issue by comparing categorical perception performances of illiterate and literate people. Identification and discrimination responses were collected for a /ba-da/ synthetic place-of-articulation continuum and between-group differences in both categorical perception and in the precision of the categorical boundary were examined. The results showed that illiterate vs. literate people did not differ in categorical perception, thereby suggesting that the categorical perception anomalies displayed by dyslexics are indeed a cause rather than a consequence of their reading problems. However, illiterate people displayed a less precise categorical boundary and a stronger lexical bias, both also associated with dyslexia, which might, therefore, be a specific consequence of written language deprivation or impairment.     

Thresholds for color discrimination in English and Korean speakers

Categorical perception (CP) is said to occur when a continuum of equally spaced physical changes is perceived as unequally spaced as a function of category membership (Harnad, S. (Ed.) (1987). Psychophysical and cognitive aspects of categorical perception: A critical overview. Cambridge: Cambridge University Press). A common suggestion is that CP for color arises because perception is qualitatively distorted when we learn to categorize a dimension. Contrary to this view, we here report that English speakers show no evidence of lowered discrimination thresholds at the boundaries between blue and green categories even though CP is found at these boundaries in a supra-threshold task. Furthermore, there is no evidence of different discrimination thresholds between individuals from two language groups (English and Korean) who use different color terminology in the blue-green region and have different supra-threshold boundaries. Our participants’ just noticeable difference (JND) thresholds suggest that they retain a smooth continuum of perceptual space that is not warped by stretching at category boundaries or by within-category compression. At least for the domain of color, categorical perception appears to be a categorical, but not a perceptual phenomenon.     

Categorical results do not imply categorical perception

Categorical perception refers to the ability to discriminate between- but not within-category differences along a stimulus continuum. Although categorical perception was thought to be unique to speech, recent studies have yielded similar results with nonspeech continua. The results are usually interpreted in terms of categorical, as opposed to continuous, perception of both speech and nonspeech continua. In contrast, we argue that these continua are perceived continuously, although they are characterized by relatively large increases in discrim-inability near the category boundary. To support this argument, the amplitude rise time of a tone was varied to produce either an increase or a decrease in the intensity during the initial portion of the tone. A bipolar continuum of onset times increasing and decreasing in amplitude yielded traditional categorical results. However, when only half of this continuum was tested, subjects perceived the same sounds continuously. The finding of traditional categorical results along the bipolar continuum, when the sounds were shown to be perceived continuously in another context, argues against the use of traditional categorical results as evidence for categorical perception.     

Categorical perception of familiar objects

We report three experiments where the categorical perception of familiar, three-dimensional objects was investigated. A continuum of shape change between 15 pairs of objects was created and the images along the continuum were used as stimuli. In Experiment 1 participants were first required to discriminate pairs of images of objects that lay along the shape continuum. Then participants were asked to classify each morph-image into one of two pre-specified classes. We found evidence for categorical perception in some but not all of our object pairs. In Experiment 2 we varied the viewpoint of the objects in the discrimination task and found that effects of categorical perception generalized across changes in view. In Experiment 3 similarity ratings for each object pair were collected. These similarity scores correlated with the degree of perceptual categorization found for the object pairs. Our findings suggest that some familiar objects are perceived categorically and that categorical perception is closely tied to inter-object perceptual similarity.     

Categorical perception of emotional facial expressions in preschoolers

Adults perceive emotional facial expressions categorically. In this study, we explored categorical perception in 3.5-year-olds by creating a morphed continuum of emotional faces and tested preschoolers’ discrimination and identification of them. In the discrimination task, participants indicated whether two examples from the continuum “felt the same” or “felt different.” In the identification task, images were presented individually and participants were asked to label the emotion displayed on the face (e.g., “Does she look happy or sad?”). Results suggest that 3.5-year-olds have the same category boundary as adults. They were more likely to report that the image pairs felt “different” at the image pair that crossed the category boundary. These results suggest that 3.5-year-olds perceive happy and sad emotional facial expressions categorically as adults do. Categorizing emotional expressions is advantageous for children if it allows them to use social information faster and more efficiently.     

Some properties of linguistic feature detectors

A series of experiments, using a selective adaptation procedure, investigated some of the properties of the linguistic feature detectors that mediate the perception of the voiced and voiceless stop consonants. The first experiment showed that these detectors are centrally rather than peripherally located, in that monotic presentation of the adapting stimulus and test stimuli to different ears resulted in large and reliable shifts in the locus of the phonetic boundary. The second experiment revealed that the detectors are part of the specialized speech processor, inasmuch as adaptation of a voicing detector (as measured by a shift in the phonetic boundary) occurred only when the voicing information was presented in a speech context. In the third experiment, the detector mediating perception of the voiced stops was shown to be more resistant to adaptation than the detector mediating perception of the voiceless stops.     

Categorical perception depends on the discrimination task

Speech sounds are said to be perceived categorically. 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.     

Categorical perception effects reflect differences in typicality on within-category trials

Many studies have shown better discrimination of two stimuli that cross a category boundary than of two stimuli belonging to the same category. This finding, known as categorical perception, is generally assumed to reflect consistently good performance on cross-category trials, relative to within-category trials. However, Roberson, D., Damjanovic, L., and Pilling, M. (Memory & Cognition, 35, 1814-1829, 2007) revealed that performance on within-category pairs of morphed facial expressions matched performance on cross-category trials when the target was a good exemplar of its category. Here, we investigate the generality of that finding by conducting new analyses of data from a series of studies of categorical perception in facial identity and color domains with speakers of different languages. Consistent with Roberson et al. (2007), the new analyses demonstrate that performance for central targets on within-category trials is as good as performance on cross-category trials. Participants perform badly on within-category items only when the target is closer to the category boundary than is the distractor. These results provide no support for the view that categorical perception is associated with increased perceptual sensitivity at category boundaries.     

Categorical perception of facial expressions by 7-month-old infants.

Recent research indicates that adults show categorical perception of facial expressions of emotion. It is not known whether this is a basic characteristic of perception that is present from the earliest weeks of life, or whether it is one that emerges more gradually with experience in perceiving and interpreting expressions. We report two experiments designed to investigate whether young infants, like adults, show categorical perception of facial expressions. 7-month-old infants were shown photographic quality continua of interpolated (morphed) facial expressions derived from two prototypes of fear and happiness. In the first experiment, we used a visual-preference technique to identify the infants' category boundary between happiness and fear. In the second experiment, we used a combined familiarisation-visual-preference technique to compare infants' discrimination of pairs of expressions that were equally physically different but that did or did not cross the emotion-category boundary. The results suggest that 7-month-old infants (i) show evidence of categorical perception of facial expressions of emotion, and (ii) show persistent interest in looking at fearful expressions.     

Adaptation of the category boundary between speech and nonspeech: A case against feature detectors

Two experiments were performed employing acoustic continua which change from speech to nonspeech. The members of one continuum, synthesized on the Pattern Playback, varied in the bandwidths of the first three formants in equal steps of change, from the vowel /α/ to a nonspeech buzz. The other continuum, achieved through digital synthesis, varied in the bandwidths of the first five formants, from the vowel /æ/ to a buzz. Identification and discrimination tests were carried out to establish that these continua were perceived categorically. Perceptual adaptation of these continua revealed shifts in the category boundaries comparable to those previously reported for speech sounds. The results were interpreted as suggesting that neither phonetic nor auditory feature detectors are responsible for perceptual adaptation of speech sounds, and that feature detector accounts of speech perception should therefore be reconsidered.     

Discrimination of speech sounds by children with dyslexia: comparisons with chronological age and reading level controls

Previous studies have shown that children suffering from developmental dyslexia have a deficit in categorical perception of speech sounds. The aim of the current study was to better understand the nature of this categorical perception deficit. In this study, categorical perception skills of children with dyslexia were compared with those of chronological age and reading level controls. Children identified and discriminated /do-to/ syllables along a voice onset time (VOT) continuum. Results showed that children with dyslexia discriminated among phonemically contrastive pairs less accurately than did chronological age and reading level controls and also showed higher sensitivity in the discrimination of allophonic contrasts. These results suggest that children with dyslexia perceive speech with allophonic units rather than phonemic units. The origin of allophonic perception in the course of perceptual development and its implication for reading acquisition are discussed.     

Categorical Perception of Morphed Facial Expressions

Using computer-generated line-drawings, Etcoff and Magee (1992) found evidence of categorical perception of facial expressions. We report four experiments that replicated and extended Etcoff and Magee's findings with photographic-quality stimuli. Experiments 1 and 2 measured identification of the individual stimuli falling along particular expression continua (e.g. from happiness to sadness) and discrimination of these stimuli with an ABX task in which stimuli A, B, and X were presented sequentially; subjects had to decide whether X was the same as A or B. Our identification data showed that each expression continuum was perceived as two distinct sections separated by a category boundary. From these identification data we were able to predict subjects' performance in the ABX discrimination task and to demonstrate better discrimination of cross-boundary than within-category pairs; that is, two faces identified as different expressions (e.g. happy and sad) were easier to discriminate than two faces of equal physical difference identified as the same expression (e.g. both happy). Experiments 3 and 4 addressed two new issues arising from Etcoff and Magee's (1992) data and the results of our own Experiments 1 and 2: (1) that they might reflect artefacts inherent in the use of single continua ranging between two prototypes- for example, a range effect or an anchor effect, (2) given that the ABX procedure incorporates a short-term memory load, discrimination data obtained with this task might reflect a short-term memory rather than a perceptual phenomenon. We found no support for either of these reinterpretations and further evidence of categorical perception.     

Visual perception of orientation is categorical near vertical and continuous near horizontal

Four experiments were conducted to examine whether visual-orientation information is perceived categorically. In experiments 1 and 3, adult participants sorted oriented line stimuli into broad oblique and narrow vertical or horizontal categories. Experiments 2 and 4 showed that categorical discrimination of orientation occurred only near the vertical-oblique boundary. The data indicate that there is categorical perception near vertical and more continuous perception near horizontal. The results are relevant to the debate over whether categorical perception is derived from perceptual structure, verbal coding, or within-task learning. In addition, the asymmetrical perception of orientation around vertical and horizontal is consistent with the possibility that there may be differences in the functional significance of orientation near the two main axes.     

Categorical and dimensional perceptions in decoding emotional facial expressions

We investigated whether categorical perception and dimensional perception can co-occur while decoding emotional facial expressions. In Experiment 1, facial continua with endpoints consisting of four basic emotions (i.e., happiness-fear and anger-disgust) were created by a morphing technique. Participants rated each facial stimulus using a categorical strategy and a dimensional strategy. The results show that the happiness-fear continuum was divided into two clusters based on valence, even when using the dimensional strategy. Moreover, the faces were arrayed in order of the physical changes within each cluster. In Experiment 2, we found a category boundary within other continua (i.e., surprise-sadness and excitement-disgust) with regard to the arousal and valence dimensions. These findings indicate that categorical perception and dimensional perception co-occurred when emotional facial expressions were rated using a dimensional strategy, suggesting a hybrid theory of categorical and dimensional accounts.     

Categorical and dimensional perceptions in decoding emotional facial expressions

We investigated whether categorical perception and dimensional perception can co-occur while decoding emotional facial expressions. In Experiment 1, facial continua with endpoints consisting of four basic emotions (i.e., happiness–fear and anger–disgust) were created by a morphing technique. Participants rated each facial stimulus using a categorical strategy and a dimensional strategy. The results show that the happiness–fear continuum was divided into two clusters based on valence, even when using the dimensional strategy. Moreover, the faces were arrayed in order of the physical changes within each cluster. In Experiment 2, we found a category boundary within other continua (i.e., surprise–sadness and excitement–disgust) with regard to the arousal and valence dimensions. These findings indicate that categorical perception and dimensional perception co-occurred when emotional facial expressions were rated using a dimensional strategy, suggesting a hybrid theory of categorical and dimensional accounts.     

Neural processing of acoustic duration and phonological German vowel length: Time courses of evoked fields in response to speech and nonspeech signals

Recent experiments showed that the perception of vowel length by German listeners exhibits the characteristics of categorical perception. The present study sought to find the neural activity reflecting categorical vowel length and the short-long boundary by examining the processing of non-contrastive durations and categorical length using MEG. Using disyllabic words with varying /a/-durations and temporally-matched nonspeech stimuli, we found that each syllable elicited an M50/M100-complex. The M50-amplitude to the second syllable varied along the durational continuum, possibly reflecting the mapping of duration onto a rhythm representation. Categorical length was reflected by an additional response elicited when vowel duration exceeded the short-long boundary. This was interpreted to reflect the integration of an additional timing unit for long in contrast to short vowels. Unlike to speech, responses to short nonspeech durations lacked a M100 to the first and M50 to the second syllable, indicating different integration windows for speech and nonspeech signals.     

Eye-contact perception in schizophrenia: relationship with symptoms and socioemotional functioning

Accurately perceiving self-referential social signals, particularly eye contact, is critical to social adaptation. Schizophrenia is often accompanied by deficits in social cognition, but it is unclear whether this includes gaze discrimination deficits. This study investigated whether eye-contact perception is preserved or impaired and if it is related to symptoms and broader socioemotional functioning in schizophrenia. Twenty-six participants with schizophrenia (SCZ) and 23 healthy controls (HC) made eye-contact judgments for faces in varying gaze direction (from averted to direct in ten 10% increments), head orientation (forward, 30° averted), and emotion (neutral, fearful). Psychophysical analyses for forward faces showed that SCZ began endorsing eye contact with weaker eye-contact signal and their eye-contact perception was less of a dichotomous function, as compared with HC. SCZ were more likely than HC to endorse eye contact when gaze was ambiguous, and this overperception of eye contact was modulated by head orientation and emotion. Overperception of eye contact was associated with more severe negative symptoms. Decreased categorical gaze perception explained variance of socioemotional deficits in schizophrenia after taking basic neurocognition into consideration, suggesting the relationship was not solely due to a general deficit problem. These results were discussed in relation to the nature of categorical gaze perception and its significance to clinical and functional presentations of schizophrenia.     

Speech perception deficits by Chinese children with phonological dyslexia

Findings concerning the relation between dyslexia and speech perception deficits are inconsistent in the literature. This study examined the relation in Chinese children using a more homogeneous sample—children with phonological dyslexia. Two experimental tasks were administered to a group of Chinese children with phonological dyslexia, a group of age-matched control children, and a group of adults. In addition to a categorical perception task, a selective adaptation task was carried out. The results indicated that Chinese children with phonological dyslexia were less consistent than both the child and adult control groups in identifying stimuli within a given phonetic category. Furthermore, they did not show any significant adaptation effects in the selective adaptation task even when the adapting stimulus was identical to an endpoint stimulus in the test continuum. It seems that children with phonological dyslexia have a general deficiency in representing and processing speech stimuli.     

汉语元音对声调感知边界的影响

本项研究基于现代汉语普通话声调感知具有范畴性这一结论(Wang, 1976), 以普通话中6个单元音(/a//?//F//i//u//y/, 即拼音a、o、e、i、u、ü)阴平(T1)到阳平(T2)调的连续统(continuum)为实验语料, 用专业语音学软件Praat合成连续统语音样本, 对20名平均年龄为22.90岁的普通话母语者进行声调感知实验, 即让被试对所听刺激进行T1或T2的二选一鉴别测试(identification test), 从而探究单元音的内在音高(intrinsic pitch)与声调感知边界(boundary)的关系。本文借鉴Wang (1976)和Peng等(2010)对元音/i/声调连续统的合成方法, 使声调感知研究拓展到普通话中除/i/之外的其他5个单元音, 对以上两项研究起到一定的对照作用。研究发现, 元音会对T1到T2的声调范畴感知边界位置产生影响, 低元音/a/的内在音高较低, 其边界位置亦始终低于其他元音, 且具有显著性差异, 实验结果证明, 内在音高会对感知边界产生影响, 从而支持了Ohala和Eukel (1976)的“舌位拉伸”假说。