Society for Neuroscience, 2003, New Orleans, LA

AN ANALYSIS OF SPIKE-TRIGGERED COVARIANCE REVEALS SUPPRESSIVE MECHANISMS OF DIRECTION SELECTIVITY IN MACAQUE V1 NEURONS

Nicole C Rust1, Odelia Schwartz2, Eero P Simoncelli1 and J Anthony Movshon1 1. HHMI and Ctr Neural Sci, NYU, New York, NY, USA; 2. Salk Inst, La Jolla, CA USA

In the macaque visual system, direction selectivity (DS) is first found in neurons in primary visual cortex, area V1. It remains unclear whether this selectivity arises purely from the integration of excitatory inputs with appropriately arranged spatiotemporal offsets, or if additional suppressive influences are also involved. To address this issue, we measured the responses of V1 neurons in opiate anaesthetized, paralyzed macaques to spatiotemporal binary noise stimuli, and applied a spike-triggered covariance analysis (STC) to the data. The analysis extracts a set of excitatory and suppressive linear components as well as the nonlinear rules by which they are combined. In some DS cells the STC was dominated by excitatory components, and a model based on these components accurately predicted the cells directionality as measured with gratings. For other DS neurons, STC revealed strong excitatory and suppressive components with opposite direction preferences. For these neurons, a model that included only excitatory components overestimated the response to gratings moving in the nonpreferred direction; incorporating suppressive components in the model yielded accurate predictions of selectivity. To further explore these suppressive influences, we constructed stimuli which systematically varied the contribution of the excitatory and suppressive components. Stimuli tailored to activate the excitatory components alone elicited vigorous responses. As predicted, these responses were substantially reduced by the presence of stimuli tailored to activate the suppressive components. These results suggest that V1 may implement a multistage computation in which directional selectivity is initially but imperfectly established by suitable spatiotemporal filters, and then refined by suppressive signals that eliminate unwanted responses to non-preferred stimuli.