Fig. 8. Schematic representation of the effect of NMDA and AMPA receptors on direction selectivity and their regulation by GABA.A, In control conditions, direction-selective cells in layer 2/3 have both AMPA and NMDA components that contribute to preferred and nonpreferred responses. Shown here is the hypothetical contribution of the two components to a cell with direction index = 0.6, which is approximately the 50th percentile of cells in our sample (Fig. 2A,B). Hypothetical responses are shown normalized to the peak response, and the response scale applies to B–E as well. B, CNQX ejection removes the AMPA component and leaves the NMDA component, which contributes prominently to responses in the preferred direction. NMDA-mediated responses in the nonpreferred direction are reduced disproportionately by GABAergic inhibition, leading to a high direction index.C, Ejection of APV leaves the AMPA component, which contributes similarly to both preferred and nonpreferred directions, causing no change in the direction index. D, The NMDA-mediated response, particularly that in the nonpreferred direction, is modulated importantly by GABAergic inhibition. Simultaneous application of CNQX and bicuculline leads to a disproportionate increase in the nonpreferred response and to a decrease in the direction index. E, The AMPA-mediated response is increased proportionately in both directions by removal of inhibition. Simultaneous ejection of bicuculline and APV increases both the preferred and nonpreferred response and causes no change in the direction index. F, GABAergic inhibition reduces both AMPA- and NMDA-mediated responses and particularly the NMDA responses in the nonpreferred direction. Bicuculline thus increase the nonpreferred response more than the preferred response, leading to a decrease in the direction index. The hypothetical responses inF are normalized differently from those inA–E.