Figure 7. Rbp4 R-cells show sustained irradiance-encoding light-evoked responses. A, Light-evoked firing, obtained in whole-cell current-clamp configuration, to a series of stimulus intensities (irradiance) under control conditions (retina bathed in Ames’ medium only). Irradiances in log photons cm−2 s − 1 are indicated above traces. Horizontal black bar below the leftmost trace indicates the duration of a 15 s wide-field light step from darkness. B, PSTHs averaged across cells (n = 4 cells; average ± SEM) for various irradiance levels. C, IR curve based on data presented in B. Data points and error bars indicate the steady state response (average ± SEM). The coefficient of determination (R
2) of the fit is indicated. D, Light-evoked currents, obtained in whole-cell voltage-clamp configuration, to a series of stimulus irradiance levels under control conditions. The membrane voltage was clamped at the chloride reversal potential. E, IR curve based on data presented in D. F–N, Pharmacological analysis of circuitry underlying R-cell light responses. F–H, RGC responses, obtained in whole-cell current-clamp configuration, while blocking the ON pathway using L-AP4. F, Example of a cell’s light-evoked voltage response (black trace) to the brightest stimulus tested (13 log photons cm−2 s − 1). The lower envelope of the voltage trace is shown in magenta. Inset, Same trace at higher gain showing a sustained hyperpolarization in the voltage response as well as the lower voltage envelope throughout the stimulus duration. G, Voltage envelope (n = 3 cells; average ± SEM) for various irradiance levels. H, IR curve based on data presented in G. I–K, RGC responses (n = 5 cells), obtained in whole-cell current-clamp configuration, while blocking both the ON and OFF pathways using L-AP4, D-AP5, and DNQX. Conventions in individual plots are the same as in F–H. L–N, R-cell responses (n = 4 cells), obtained in whole-cell current-clamp configuration, after more complete synaptic blockade by further addition of the OFF channel blocker ACET to the L-AP4, D-AP5, and DNQX already in the bath. Conventions in individual plots are the same as in F–H. O, Firing rate in response to a series of bright spots on a dark background at varying sizes (main plot, 50-400 μm; inset, 50-1200 μm; n = 3 cells). P, Example of direction and orientation selectivity tuning of two R-cells. Thick blue lines show the average cells’ response to sinusoidal gratings drifting in each of eight different directions, 45° apart. Thin blue lines show data from each of four single trials. N, nasal; D, dorsal; T, temporal; V, ventral. Red lines indicate the preferred direction based on four individual repetitions (thin lines), and their average (thick line). The DSI, OSI, and preferred direction (PD) of the cell are indicated. Q, Average ± SEM DSI and OSI across tested cells (n = 5 cells).