Linear and nonlinear responses to speech rate in left and right temporal cortex

Abstract

Aspects of speech processing in the temporal lobes were investigated in a functional neuroimaging study using a parametric design, where the rate of hearing single words was varied. A previous study distinguished regions in left and right temporal cortex where activity varied linearly with rate (up to 90 wpm) from a single region, in the left posterior temporal cortex, where activity was maximal at 10 wpm [Price et al., Neurosci. Lett. 146, 179–82 (1992)]. A fMRI study of left and right primary auditory cortex (PAC) and posterior dorsolateral temporal cortex [Dhankhar et al., J. Neurophysiol. 77(1), 476–483 (1997)] showed a decline in activity at rates above 90 wpm. In the current PET study, speech rate was varied up to 180 wpm, and the consequent curves in activity were investigated for the shape of the consequent nonlinearity in activity. These curves are described and related to hypothesized hemisphere specialization in speech processing. This result raises important issues about the responsiveness of the auditory system in relation to the rate at which it can process speech input.