Visual discrimination after the multisensory rehabilitation of hemianopia

Unilateral cortical blindness is a common consequence of damage to visual cortex on one side of the brain. This condition is often permanent and intractable to rehabilitative techniques. Recently, we have developed an effective, non-invasive multisensory ‘training’ paradigm that can ameliorate this condition within weeks. The paradigm involves repeated exposure to spatiotemporally congruent cross-modal (visual and auditory) stimuli in the blind (i.e., contralesional) hemifield. It is believed to restore visual responsiveness to neurons in the midbrain superior colliculus (SC) that were rendered unresponsive as a secondary consequence of the cortical lesion. Here we asked whether the functional restoration was restricted to the visuomotor detection/approach behaviors known to be subserved by the SC or also involved a capacity to distinguish spatial patterns. Cats were trained in a visual detection/approach task as well as a battery of visual pattern discriminations. They were required to indicate (via button press) whether a pair of simultaneous visual stimuli were the same or different along a given dimension such as size, the direction of movement, orientation, or shape. The two stimuli were either presented within or across the two hemifields while the animal fixated centrally. During prelesion testing animals discriminated patterns everywhere in space. After all contiguous areas of unilateral visual cortex had been removed, animals were blind across the entire contralesional hemifield. However, after weeks of cross-modal training, visual detection/approach was restored. The training paradigm also restored their ability to discriminate visual patterns presented together in the previously blinded hemifield or across the two hemifields. Post-rehabilitation performance was well above chance but still below pre-lesion performance levels. These data reveal that the visual processing capabilities restored by this multisensory training paradigm extended far beyond the visually-guided behaviors commonly associated with the SC to include the visual discrimination capabilities that are commonly associated with the neocortex.