Abstract

Membrane current was recorded from single rod outer segments of human eye donors. Light responses were compared to psychophysical properties of human dark-adapted vision. 1) The spectral sensitivity of human rods could account for the scotopic luminosity function. 2) The “integration time” (or average duration) of the response to single photons in dark-adapted human rods was 300 msec. The analogous psychophysical parameter, “critical duration,” has a value of about 100 msec for human dark-adapted vision, suggesting that rod signals are high-pass filtered by later neural processing. 3) Background illumination reduced the amplitude of photon responses in human rods according to Weber’s law. The half-desensitizing intensity of 120 photoisomerizations/sec (∼14 sc td) was 100 to 10,000 times greater than the intensity needed to reduce human visual sensitivity by a factor of two. This indicates that background desensitization of rod-mediated vision cannot be attributed to a reduction in the size of the photon signals in the rods themselves. Background light also produced a modest reduction in the duration of rod photon signals.

© 1993 Optical Society of America