Investigation of the neural mechanisms of residual vision (blindsight) in the blind field of hemispherectomized subjects

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Dr. Alain Ptito is interested in the visual abilities of neurological patients. As is the case for loss of an eye or optic nerve damage, destructions of the occipital cortex has traditionally been thought to lead to permanent blindness in the contralateral visual field presumably because this region is indispensable for vision. But, residual visual functions in a cortically blind field have been described and this ability to respond to visual stimuli in the blind visual field without consciously experiencing them has been called 'blindsight'. Dr. Ptito and his team are investigating residual visual abilities of patients in whom a whole cerebral hemisphere has been removed or disconnected from the rest of the brain for the treatment of intractable epilepsy. These patients are unique in that they report being unaware of vision in the half visual field opposite the side of their surgery. Dr. Ptito's research, supported by NSERC, has shown that these hemispherectomized patients in fact possess some residual visual abilities such as detecting moving targets or responding more quickly to two targets, one of which is in the blind field, than to a single target presented in the intact field. Dr. Ptito and his team are now investigating the neuronal correlates underlying the 'blindsight' phenomenon using brain imaging techniques such as functional magnetic resonance imaging which allows invivo visualization of cerebral activity following visual stimulation and diffusion tensor imaging to help identify the cerebral pathways involved in 'blindsight'. Depicting residual visual abilities in the blind field of individuals has important implications for rehabilitation and for understanding visual consciousness.