Measurement of DC and AC spectral sensitivities of retinal horizontal cells by "voltage clamp by light".

The method of "voltage clamp by light" was applied to measure spectral sensitivities of second-order visual neurons, namely L- and R/G-type horizontal cells in the carp retina. The present equipment employs (i) a ceramic photomodulator to facilitate a fast servomechanical control of retinal illuminance, (ii) an electronic circuit to compensate for the synaptic transmission delay, and (iii) a manual selection switch for the system to operate on negative feedback for either depolarizing or hyperpolarizing responses to light. These features allowed us to determine quickly and simultaneously both DC and AC spectral sensitivities, although the AC case was examined only at 1 Hz in this report. In L-type cells, the AC spectral sensitivity was similar in shape to the DC result. These sensitivity curves differed from microphotospectrometric absorption of red-sensitive cones: in L-type cells at both ends of the visible spectrum and in R/G units with deep-red light.