Categorical speech perception in cerebellar disorders

Keele and Ivry (1991) considered the cerebellum an ''internal clock'' responsible for temporal computations both in the motor and in the perceptual domain. These authors, therefore, expected that the processing of durational parameters of the perceived acoustic speech signal such as voice onset time (VOT) depends upon the cerebellum as well. However, a preliminary investigation of Ivry and Gopal (1992) revealed unimpaired phoneme-boundary effects in cerebellar patients along a continuum of monosyllabic stimuli with systematically varied VOT (/ba/-/pa/). Since the energy of the aspiration noise provides additional cues for the discrimination of voiced and voiceless stops, the present study used a series of disyllabic stimuli differing in a purely durational parameter. Both controls and patients with unilateral cerebellar lesion identified the endpoints of this continuum in nearly all instances as the counterparts of a minimal pair (Boten, /bo:tn/, ''messengers'' versus Boden /bo:dn/, ''floor''). Subjects with bilateral pathology of the cerebellum, in contrast, did not show a comparable phoneme-boundary effect. Our data corroborate the hypothesis of the cerebellum as an internal clock and implicate a role of this structure in speech perception. (C) 1997 Academic Press.