Abstract
Based on their responses to small and large stimuli, MT neurons can be classified as surround-suppressed (SS) or non-surround suppressed (NS). Surround suppression in MT has often been associated with a loss of perceptual sensitivity for large, moving stimuli (Tadin et al. 03). Weakened spatial suppression in certain cohort of subjects is therefore thought to be due to reduced efficacy of GABAergic inhibition, but these links have not been probed experimentally. Here we examined the causal role of GABAergic inhibition in MT responses. Monkeys performed a 2AFC motion direction discrimination task with Gabor patches of various sizes at a fixed duration (typically 50 ms). We found that the performance of SS neurons generally correlated more strongly with perceptual reports, as monkeys found the motion direction of larger stimuli more difficult to discriminate. Surprisingly, NS neurons consistently outperformed the monkey for large stimuli, suggesting sub-optimal pooling of the neurons to perform the task. To interpret these results and establish a causal relationship, we first directly confirmed that MT neurons are causally involved in motion direction discrimination of Gabor patches by injecting muscimol to reversibly inactivate MT. We found that inactivation of MT reduces performance for Gabor and random dots stimuli to the same degree. While monitoring the neuronal response, we injected GABAergic agents to bidirectionally manipulate inhibitory efficacy. We found GABA suppresses untuned responses by increasing the spiking threshold, and that GABA antagonists have the opposite effects. This raises the possibility that the additional inhibition associated with surround suppression renders SS neurons more direction selective and thus more informative about motion direction. However, we found that local blockade of GABA receptors did not diminish surround suppression, as previously observed in V1. It thus appears that the contribution of inhibition to surround suppression is more complex and dynamic than previously thought.
Meeting abstract presented at VSS 2014