Learning sound categories: A neural model and supporting experiments

Our ability to discriminate sounds such as vowels is not uniform throughout acoustic space. That is, our auditory perceptual spaces are warped representations of acoustic space. One example of auditory space warping, the perceptual magnet effect, arises from exposure to the phonemes of an infant’s native language. We have developed a neural model that accounts for this effect. The model is based on the idea that category learning during infancy changes the distribution of the firing preferences of neurons in auditory cortical maps and thus changes the discriminability of sounds from different parts of acoustic space. The model predicts that it should be possible to induce a perceptual magnet effect for non-speech stimuli. This prediction was verified by a psychophysical experiment in which subjects underwent categorization training involving non-speech auditory stimuli that were not ‘‘categorical’’ prior to training. The model further predicts that the magnet effect arises because prototypical vowels have smaller auditory cortical representations than non-prototypical vowels. This prediction was supported by a functional magnetic resonance imaging (fMRI) experiment involving prototypical and non-prototypical examples of the vowel /i/. The model thus provides an account of phoneme category learning that unifies observations from auditory psychophysics, cortical neurophysiology, and neural modeling.