Figure 6.

ON alpha cells receive feedforward inhibition at both positive and negative contrast. A, CRH-1 cell voltage response to two stimuli followed by a switch to darkness. Stimulus was a contrast-modulated spot (100% contrast, 1000 μm diameter) at 2 temporal frequencies (0.5 or 5 Hz) with a background set to the mean luminance. Inset in the middle column shows an expanded view of the 5 Hz response. Relative to the resting potential (solid horizontal line), the cell hyperpolarized near the dark potential (dashed horizontal line; i.e., membrane potential after the switch from mean luminance to dark background measured over 50 ms) during periods of negative contrast at both temporal frequencies. B, Same format as A for a CRH-3 cell recording. C, Same format as A for excitatory current recorded in an ON alpha cell. The current is suppressed (i.e., moves outward) near the dark current (dashed line) during periods of negative contrast at both temporal frequencies. D, Same format as C for inhibitory current recorded in the same ON alpha cell. During periods of negative contrast, the inhibition persisted and was not fully suppressed to the level near the dark current (dashed line). E, Summary data of NMI across cells showing that CRH-1 and CRH-3 voltage and ON alpha cell excitatory current are suppressed near the dark voltage/current in normalized coordinates (where 1 = dark current/potential; and 0 = resting current/potential). The ON alpha cell inhibitory current NMI was significantly less than the values measured in CRH-1 voltage, CRH-3 voltage, or ON alpha excitatory current at both 0.5 and 5 Hz. For 0.5 Hz stimulation, responses were measured within a 200 ms time window positioned within 500 ms after the transition to negative contrast. For 5 Hz stimulation, responses were measured within 15–20 ms time windows positioned within 100 ms after the transition to negative contrast. Population data show mean ± SEM across cells. F, Voltage-clamp recording of a CRH-1 cell. During negative contrast, the tonic excitatory current was suppressed, whereas the inhibitory current did not increase above baseline. Inhibition instead increased during positive contrast. G, Same format as F for a CRH-3 cell, which showed a similar pattern of synaptic input as the CRH-1 cell.