Figure 9. Bipolar cells can directly evoke sustained extrinsic light responses in ipRGCs.

Voltage responses of a single well-isolated ipRGC, recorded extracellularly with an MEA electrode, to a series of light intensities all of which were subthreshold for intrinsic phototransduction. In the presence of the amacrine blocker cocktail with TTX omitted (right), the extrinsic light response of this ipRGC was sustained, just as it was in control Ames' medium (left, ‘−4’ and ‘−3’). Under amacrine blockade, the extrinsic response became more robust and the responses to the −4 and −3 log I light flashes were so strong that depolarization block became evident. Amacrine blockade also increased the frequency of spontaneous spikes. Under both conditions, −6 log I light flashes did not evoke any response (not shown).