Cross-modal plasticity of verbal memory in the blind

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Gross-modal playsticity refes to the concept that the cortex normally responsive to ohne sensory modality, when depreived of its usual input, becomes repsonsive to inputs from other sensory modalities. In blind subjects, the visual cortex processes somatosensory and auditory information. A recent functional imaging study foundt that verbal memory recall and verb generation significantly activated the primary visual cortex in early blind humas. The magnitude of occipital activation was positively correlated with subsequent performence on verbal memory and verb generation tasks. These relusts raised the untested hypothesis that the primary visual cortex may be the neural substrate mediating superior verbal memory performance in early blind individuals. We want to test this hypothesis with two experiments. The first experiment (using functional Magnetic Resonance Imaging, fMRI) will identify the magnitude of activation in corical areas associated with successful verbal memory encoding and retrieval, and with verb generation, in early and late blind individuals and sighted controls. The expected outcome is incrased involvement of the visual cortex in the early blind group. In the second experiment, we Transcranial Magnetic Stimulation; TMS) will habe deleterious effects on verbal memory encoding and verb generation in early blind subjecs, but not in late blind subject nor sighted controls. The TMS experiment is necessary to identify a cause-effect link between occipital activation and improved memory performance.