This study investigates perceptual effects of discharge rate adaptation in the auditory-nerve fibers. Discrimination tests showed that brief synthetic stimuli with stationary formants and periodic source were better discriminated when they had an abrupt as opposed to a gradual onset (non-adapted vs adapted condition). This effect was not observed for corresponding stimuli with noise source. Discrimination among synthetic /da/ stimuli (abrupt onsets) was worse than among /ad/ stimuli when the respective onset and offset frequencies of the second formant (F2) were varied. Similar results were obtained for /ba/ and /ab/. The low discrimination rate in consonant-vowel stimuli (CV) was explained in terms of sensory smearing of spectral information due to rapid formant transitions. Discrimination improved when the smearing effect was reduced by holding the onset formant pattern over a certain period of time of about 1 6ms. The relatively high discrimination score for the VC stimuli was explained by residual masking; extending the VC offset did not improve discrimination. Discrimination of place of articulation in CV syllables was examined in the light of sensory smearing. Two continua of /bu-du/ and /ba-da/ utterances were used in discrimination and identification experiments. It was observed that the discrimination peak for /Cu/ was displaced from the /b/-/d/ boundary, towards a flat F2 transition, suggesting that optimal place discrimination is related to the stability of the auditory representations generated at onset. This result is discussed in relation to current views of categorical perception. / För att köpa boken skicka en beställning till exp@ling.su.se/ To order the book send an e-mail to exp@ling.su.se

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Although there is a wealth of knowledge on categorization in infancy, there are still many unanswered questions about the nature of category representation in infancy. For example, it is yet unclear whether categories in infancy have well-defined boundaries or what knowledge about species categories young infants have before entering the lab. Using a morphing technique, we linearly altered the proportion of cat versus dog in images and observed how infants reacted to contrasts between pairs of images that either did or did not cross over the categorical boundary. This was done while equating between-category and within-category similarity. Results indicate that infants’ pre-existing categories of cats and dogs are discrete and mutually exclusive. Experiment 2 found that inversion caused a disruption in processing by 6.5- but not 3.5- month-old infants, indicating a developmental change in category representation. These findings demonstrate a propensity to dichotomize early in life that could have implications for social categorizations, such as race and gender. Furthermore, this work extends previous knowledge of infant categorical perception by demonstrating a priori knowledge of familiar species categories and the boundaries between them.

Individuals perceive emotional facial expressions in categories. Specifically, for basic emotional expressions, discrimination performance is better for pairs of stimuli that fall in either side of a perceptual category boundary than for those within a perceptual category. In this thesis I have examined categorical perception of emotional facial expressions from a number of different perspectives. In Chapter 2, I found in two experiments that categorical perception of emotional facial expressions of a robust phenomenon with a few consistent individual differences. There findings highlight some of the important caveats that categorical perception researchers face, not only in the area of emotional facial expressions, but across domains as well. In Chapter 3 I show that context has an effect on the visual perception of emotional facial expressions. A surprise-fear continuum was perceived categorically only in the case in which a context story was provided for the surprise face. In Chapter 4 I demonstrate categorical perception of a happy-sad continuum in 3.5-year-olds. This is an important study in development because the current literature is limited by studies that do not compare identification and discrimination performance in these age groups. The experiment in this chapter uses both identification and discrimination tasks and compares the results of 3.5-year-olds to adults who do the exact same task. The results suggest that 3.5-year-olds perceive happy and sad expressions as adults do, categorically. In Chapter 5 I develop a powerful new methodology for the study of the category boundary using non-verbal methods. This investigation with emotional facial expressions shows it can be reliably used to identify category boundary information in adults. Perception of emotional facial expressions is an essential part of successful social cognition, and the phenomenon of categorical perception specifically allows individuals to quickly and accurately respond to expressions. The research in this thesis is a further step in understanding the processes that allows individuals to be successful in a social environment. / Thesis / Doctor of Philosophy (PhD)

At the heart of statistical learning lies the concept of uncertainty. Similarly, embodied agents such as robots and animals must likewise address uncertainty, as sensation is always only a partial reflection of reality. This thesis addresses the role that uncertainty can play in a central building block of intelligence: categorization. Cognitive agents are able to perform tasks like categorical perception through physical interaction (active categorical perception; ACP), or passively at a distance (distal categorical perception; DCP). It is possible that the former scaffolds the learning of the latter. However, it is unclear whether DCP indeed scaffolds ACP in humans and animals, nor how a robot could be trained to likewise learn DCP from ACP. Here we demonstrate a method for doing so which involves uncertainty: robots perform ACP when uncertain and DCP when certain. Furthermore, we demonstrate that robots trained in such a manner are more competent at categorizing novel objects than robots trained to categorize in other ways. This suggests that such a mechanism would also be useful for humans and animals, suggesting that they may be employing some version of this mechanism.

Objectives: The hypothesis that a natural auditory psychophysical discontinuity contributes to a perceptual category boundary between voiced and voiceless English stop consonants was examined. Design: The relationships between voice onset time (VOT) phonetic boundary and gap-detection thresholds for conditions in which the sounds delimiting the gap were acoustically identical or different were examined in 18 native young adult English speakers. It was specifically hypothesized that between-channel gap-detection thresholds, in contrast to within-channel gap thresholds, would be better predictors of categorical VOT values for listeners. The stimuli used in the between-channel gap-detection task were designed such that dissimilar leading and trailing markers of the gap approximated a stop burst and a following vowel, both in terms of temporal and spectral relationships, while being devoid of phonetic identity. The stimuli used to examine the VOT measure were synthesized bilabial stop syllables in a continuum ranging from /ba/ to /pa/. Results: Statistically significant larger gap thresholds were found for the between-channel conditions than for the within-channel condition (p < 0.05). The center frequency of the trailing marker affected the between-channel gap thresholds with the thresholds improving as the center frequency increased (p < 0.05). Statistically significant positive correlations and predictive linear relations were found between VOT phonetic boundaries and between-channel gap thresholds (p < 0.05) but not within-channel gap thresholds (p > 0.05). Conclusions: A relationship between the phonetic boundary of voiced–voiceless speech sounds and the auditory temporal resolution task of detecting gaps placed within dissimilar markers, regardless of the center frequency of the trailing marker noise burst, was demonstrated. Detection of gaps between different nonspeech acoustic markers and categorical perception of VOT seems to share the same underlying perceptual timing mechanisms in native English speakers.

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The current study lays the groundwork for a model of Mandarin tones based on both native speakers' and non-native speakers' perception and production. It demonstrates that there is variability in non-native speakers' tone productions and that there are differences in the perceptual boundaries in native speakers and non-native speakers. There are four experiments in this study. Experiment 1 utilizes native speakers' production data from a published speech database to explore the features of tone production by native speakers. Inter-speaker normalization is used to analyze the data. Experiment 2 synthesizes 81 tones that are carried by four sentences to measure perception by native and non-native speakers. The intra-speaker and inter-speaker normalization is used to investigate the perceptual space of T1, T2, T3, and T4. The researcher also explores the salient features distinguish native speakers' and non-native speakers' perception of the four principal tones. Experiment 3 uses both synthesized tones and natural tones that are carried by sentences to explore how pitch values of tones create overlapping areas in the perceptual map. Experiment 4 examines tone production by non-native speakers to identify the differences between native speakers' perception and non-native speakers' production; and the differences between non-native speakers' perception and their production of tones. The results of the perception and production experiments with native speakers show the perceptual boundaries and tonal categories in the perceptual space and the production space. The difference of native speakers' perception and production shows us the perceptual cue for perception. Meanwhile, the similarities of native speakers' perception and production reveal the acoustic cues, including register and contour, for tone perception and production. The results of the perception experiments with non-native speakers indicate that there are no clear boundaries, and that tone overlap in the perceptual space. Register plays an important role in the perception of tones by non-native speakers. The results of non-native speaker production also show overlapping tones in the acoustic space. The non-native speaker production appears to be determined by the contour of the tones in contrast both the contour and register determine the tonal categories of native speaker.

We argue that one of the early goals of color vision is to distinguish one kind of material from another. Accordingly, we show that when a pair of image regions is such that one region has greater intensity at one wavelength than at another wavelength, and the second region has the opposite property, then the two regions are likely to have arisen from distinct materials in the scene. We call this material change circumstance the 'opposite slope sign condition.' With this criterion as a foundation, we construct a representation of spectral information that facilitates the recognition of material changes. Our theory has implications for both psychology and neurophysiology. In particular, Hering's notion of opponent colors and psychologically unique primaries, and Land's results in two-color projection can be interpreted as different aspects of the visual system's goal of categorizing materials. Also, the theory provides two basic interpretations of the function of double-opponent color cells described by neurophysiologists.

We present a novel scheme ("Categorical Basis Functions", CBF) for object class representation in the brain and contrast it to the "Chorus of Prototypes" scheme recently proposed by Edelman. The power and flexibility of CBF is demonstrated in two examples. CBF is then applied to investigate the phenomenon of Categorical Perception, in particular the finding by Bulthoff et al. (1998) of categorization of faces by gender without corresponding Categorical Perception. Here, CBF makes predictions that can be tested in a psychophysical experiment. Finally, experiments are suggested to further test CBF.