Transient and permanent deficits in motion perception after lesions of cortical areas MT and MST in the macaque monkey

We examined the nature and the selectivity of the motion deficits produced by lesions of extrastriate areas MT and MST. Lesions were made by injecting ibotenic acid into the representation of the left visual field in two maca que monkeys. The monkeys discriminated two stimuli that differed either in stimulus direction or orientation. Direction and orientation discrimination were assessed by measuring thresholds with gratings and random-dots placed in the intact or lesioned visual fields. At the start of behavioral testin g, we found pronounced, motion-specific deficits in thresholds for all type s of moving stimuli, including pronounced elevations in contrast thresholds and in signal-to-noise thresholds measured with moving gratings, as well a s deficits in direction range thresholds and motion coherence measured with random-dot stimuli. In addition, the accuracy of direction discrimination was reduced at smaller spatial displacements (i.e. step sizes), suggesting an increase in spatial scale of the residual directional mechanism. Subsequ ent improvements in thresholds were seen with all motion stimuli, as behavi oral training progressed, and these improvements occurred only with extensi ve behavioral testing in the lesioned visual field. These improvements were particularly pronounced for stimuli not masked by noise. On the other hand , deficits in the ability to extract motion from noisy stimuli and in the a ccuracy of direction discrimination persisted despite extensive behavioral training. These results demonstrate the importance of areas MT and MST for the perception of motion direction, particularly in the presence of noise. In addition, they provide evidence for the importance of behavioral trainin g for functional recovery after cortical lesions. The data also strongly su pport the idea of functional specialization of areas MT and MST for motion processing.