Fig. 5.

Evidence that the hyperpolarizing response of an ON cell depends on high basal glutamate release. Thetrace shows 500 msec of mean luminance followed by 1.5 cycles of a step response (full contrast). Initially (39 s), l-AP-4 reduced the membrane variance at mean luminance and eliminated the hyperpolarizing response to dark (h). Next (48 s),l-AP-4 eliminated the depolarizing response to light. The recovery during wash was in the opposite order; first (46 s) the depolarizing response recovered, and then (133 s) the variance increased, and the hyperpolarizing response returned. Apparently the hyperpolarizing response depends on basal glutamate release (which causes increased membrane variance at mean luminance) so that an excitatory signal can be withdrawn. The recording electrode contained QX-314 so that membrane variance could be assessed independent of spiking.