Fig. 2. Spontaneous and light-evoked activity recorded from the separated proximal optic stalk of a dark-adapted eye before and after injection of 10 pmol serotonin at daytime. Responsiveness increased with increments in light intensity. Injection of serotonin into the optic lobe reduced the spontaneous activity as well as the light-induced responses. Duration of light pulses to the contralateral eye was 1000 ms, as shown at the bottom of the waveform. The highest intensity tested was 0.4mW cm^-2 designated as logI=0.