DIRECTIONALITY AND INHIBITION IN CRAYFISH TANGENTIAL CELLS

The purpose of this study was to characterize the inhibitory mechanism (s) associated with directionally selective motion detection (DS) in n onspiking tangential dells of crayfish optic lobe. The experiments emp loyed intracellular recording of synaptic potentials elicited with sin ewave gratings and pharmacological techniques. Previous studies establ ished that tangential cells are subject to bicuculline-sensitive GABA- mediated inhibition. In this study DS was reduced by 90% by bicucullin e. The reduction in DS was accompanied by a substantial increase in th e response to null-direction motion. Bicuculline also altered the resp onse to pulses of illumination. The magnitude and time course of inhib ition were derived from the time varying difference between the contro l light response and that elicited during bicuculline perfusion. Both the inhibitory delay (relative to excitation) and the inhibitory ampli tude are close to the expectations of a linear model of DS. The inhibi tion is not prolonged with respect to excitation but its risetime is s imilar to 2.5 times longer. The result implies a longer time constant in the inhibitory pathway relative to that in the excitatory pathway a nd places limits on the frequency response of inhibition and DS. The v elocity-dependence of DS is related to the time course of inhibition. The stimulus drift velocity eliciting maximum directionality is invers ely proportional to the inhibitory delay. Bicuculline did not influenc e orientation selectivity. It is concluded that the quantitative featu res of bicuculline-sensitive, GABA-mediated inhibition are consistent with a linear model of DS.