Processing steps in perception of speech sounds]

The aim of this study is to investigate whether speech sounds--as is stated by the widely accepted theory of categorical perception of speech--can be perceived only as instances of phonetic categories, or whether physical differences between speech sounds lead to perceptual differences regardless of their phonetic categorization. Subjects listened to pairs of synthetically generated speech sounds that correspond to realizations of the syllables "ba" and "pa" in natural German, and they were instructed to decide as fast as possible whether they perceived them as belonging to the same or to different phonetic categories. For 'same'-responses reaction times become longer when the physical distance between the speech sounds is increased; for 'different'-responses reaction times become shorter with growing physical distance between the stimuli. The results show that subjects can judge speech sounds on the basis of perceptual continua, which is inconsistent with the theory of categorical perception. A mathematical model is presented that attempts to explain the results by postulating two interacting stages of processing, a psychoacoustical and a phonetic one. The model is not entirely confirmed by the data, but it seems to deserve further consideration.