Plasticity and Cross-modal Interactions in Profoundly Deaf Adults

A rightward asymmetry during face processing is well documented in hearing participants, in particular in the Fusiform Face Area, a cortical area underlying face processing. This rightward asymmetry seems to be very robust, present even with inverted faces or line drawings, and is already present at five years of age. If the brain asymmetry for face processing emerges during development, early deprivation or dramatic differences the infant's experience with the world should affect it. For example, infants treated for bilateral congenital cataracts after 7 weeks of age, who are deprived of patterned visual input for this duration, fail to develop some of the aspects associated with typical adult levels of face recognition. Non visual early deprivation can also affect the development of visual abilities. Several studies showed that deaf participants could detect targets at larger eccentricities, thus indicating larger visual field . Furthermore, better abilities have been found in deaf participants for the detection of motion, in particular in the visual periphery . Higher-level visual abilities have also been shown to be modified by early deafness, such as visual imagery (image generation, or rotation) or the processing of faces . Using the Benton Test of Facial Recognition, a previous study tested the recognition of individual faces in deaf participants. They obtained better scores than hearing non signers, but only in a difficult condition, in which faces were shadowed. This enhanced processing in deaf people could thus concern very particular aspects of face processing; it was found that deaf and normal hearing differed only by the detection of subtle facial features. Investigating configural processing during face perception, it was found an increased dependency on this mode of processing in deaf participants.

If visual processing is affected by early deafness, what about visual asymmetries? Several experimental studies examining hemispheric asymmetry in congenitally deaf individuals found that it differs from the one observed in hearing individuals . Concerning face processing, few studies investigated hemispheric lateralization during the perception of faces in deaf people. When presenting briefly unfamiliar faces in either the left or right visual field and no differences between deaf and hearing participants were found. In a previous experiment, using chimeric faces (faces vertically split into two different parts, ie half male, half female) it was found a reduction of leftward asymmetry for face processing in deaf participants during a gender categorization task.

The aim of the present study is to broaden the knowledge about asymmetry for face processing in deaf people, by evaluating if this reduction of asymmetry observed at the behavioral level results from a reduction of cerebral asymmetry at the level of Fusiform Face Area.

Deaf participants and normal hearing controls will be presented face stimuli, composed of either chimeric male/female and female/male faces, entire faces or entire face presented in divided visual field, to investigate Fusiform Face Area asymmetry in these two groups. Participants will lie in an fMRI scanner, and their cerebral activity will be recorded while they perform a gender categorization task on our face stimuli.