Figure 2.

The circuit that controls visual integration time can be shifted from one state to another by a change in the gap junction coupling of one of its cell classes. (A) Visual integration time is shaped, in large part, by a negative feedback loop in the outer retina: photoreceptors send signals forward to both bipolar cells and horizontal cells; the horizontal cells then, in turn, provide negative feedback to the photoreceptors (Baylor et al., 1971; Kleinschmidt and Dowling, 1975; Dowling, 1987). Note that the figure shows only one type of horizontal cell and a generic photoreceptor; this is consistent with our model system, the mouse retina, which has only one type of horizontal cell, and it acts on both rods and cones (Peichl and González-Soriano, 1994; Trumpler et al., 2008; Babai and Thoreson, 2009). (B, left) In daylight conditions, horizontal cell feedback is strong. This cuts photoreceptor integration time short, and the system shifts to high temporal frequency responses. At night (B, right), when the system needs longer integration times, a reduction in horizontal cell feedback is needed. The opening of the gap junctions provides a mechanism for achieving this. It produces a shunting of horizontal cell current that weakens or inactivates the horizontal cells. The photoreceptor integration time then becomes longer, and the system shifts to low temporal frequency responses. The change in the gap junction coupling acts, effectively, as a knob to regulate the strength of the negative feedback. (See Appendix 2 for a formalized version of the model.)