(A) Suppression indices of membrane potential for the iso-oriented surround (SIiso) in 47 simple cells (30 surround-suppressed; 17 non-suppressed) plotted against cortical input index (CII), which correlates well with percent of excitatory input received from cortex (vs. LGN), see text. Only cells with CII>0.2 (vertical dashed line), suggesting >20% cortical input, show strong suppression. Arrow shows mean SIiso for 18 LGN cells, which is comparable to mean values for cortical cells with little cortical input and for non-suppressed cortical cells. (B) Same as (A) for orientation selectivity of suppression (SIiso–SIcross), the difference between suppression induced by iso- and cross-oriented surrounds. Arrow shows mean value of SIiso–SIcross for 18 LGN cells. (C and D) SIiso and SIiso–SIcross for membrane potential plotted against latency of response to electrical stimulation of the LGN (C, 13 simple, 10 complex; D, 13 simple, 5 complex). Cells with short latencies (<2.3 ms) receive some excitation directly from LGN; cells with long latencies (>2.8 ms) receive no monosynaptic input from LGN. Regression lines are derived from surround-suppressed cells only. Arrows as in (A) and (B). In (A)-(D): circle and square symbols, simple and complex cells; black and cyan symbols, surround-suppressed and non-suppressed cells.