The neural basis of lip-reading capabilities is altered by early visual deprivation

Abstract

The present study investigated the neural basis of lip-reading in patients treated for congenital bilateral cataracts using functional magnetic resonance imaging (fMRI). These patients represent a model to study the role of visual experience in early infancy for the development of visual functions. Short video clips with an adult speaker's lips mouthing different words were presented. The participants were asked to indicate whether the current word was the same as the previous one (one-back matching task). A control condition consisted of the same stimuli but with the task to judge whether the position of a small black dot superimposed on the lips changed location between trials. During both tasks, neural activity as indexed by fMRI, and behavioral data were recorded. The cataract patients' lip-reading performance was worse than that of a group of normally sighted controls, matched for age, gender, and education. By contrast, these groups did not differ in the visual control task. Only the control group showed reliable lip-reading specific activations in superior and middle temporal areas and in right parietal cortex, resulting in a significant group effect for these brain areas. Additional control participants with a late onset of visual impairments matching those of the cataract group showed comparable behavioral performance and similar fMRI activations in superior temporal areas as the normally sighted controls. These results suggest that a sensitive phase in early infancy might exist during which visual acuity must be sufficiently high to discriminate lip movements in order to allow for the emergence of a regular neural lip-reading system.