Figure 7. Surprisingly persistent cone-driven response in ipRGCs.

Data are from a Cx36 knockout mouse retina, in which the primary and secondary rods paths are eliminated and ON channel synaptic input to ipRGCs comes mainly or exclusively from cones. A, B: Raster plots (above) and peri-stimulus time histograms (below) of responses of a single ipRGC (A) and of multiunit responses of conventional RGCs (B) to 10 sec light steps of various intensities (500 nm narrowband; n = 7 cells in A; n = 14 multiunit recording sites in B). Firing rates of each cell (or recording site) were first normalized to the maximal firing rate at any intensity, and then averaged over cells (or recording sites) to plot the histograms. Light intensities are listed at the left (in Rh*/rod/sec ). Light responses are always much more sustained in the ipRGCs than in the conventional ganglion cells, even at the two lower intensities, which are below the threshold for intrinsic, melanopsin-mediated phototransduction. Timing of light stimulus is indicated by the marker above the traces. C–F: Plots comparing of steady-state firing during the stimulus (right point) to the prestimulus spontaneous rate (left point) for individual ipRGCs (C, E) and for multiunit recordings of conventional RGCs (D, F). Spontaneous rate was assessed during the one-second interval preceding the stimulus, and steady state rate during the last second of the 10 sec stimulus. All data in C-F obtained simultaneously from a single Cx36 KO retina. Light intensities (in Rh*/rod/sec) are 22.7 in C and D; and 227 in E and F. Both of the intensities are below melanopsin threshold. Note that all ipRGCs showed elevated steady-state firing during prolonged stimulation, whereas nearly all conventional RGCs returned to baseline firing rates during the stimulus.