The temporal properties of rod vision.

1. Profiles which represent rod thresholds for flickering fields seen against backgrounds of various intensity have shapes which depend on flicker frequency. Low frequency profiles rise smoothly as background intensity is increased. High frequency profiles are only affected by bright backgrounds, which cause them to rise steeply. Intermediate frequency profiles contain two distinct branches which resemble separate increment threshold functions. 2. The high intensity branches of two-branched threshold profiles cannot be attributed to cone intrusion. Instead, both branches of such profiles are mediated by visual mechanisms which have the spectral properties, the dark adaptation properties and the directional insensitivity of rod vision. Thus, the stimuli are detected by rods. 3. Plots of critical flicker frequency (c.f.f.) as a function of intensity contain two rising branches which are separated by a plateau (when modulation depth is large), or they form two enclosed lobes so that only intermediate frequencies, but neither high nor low ones, are seen (when modulation depth is small). C.f.f. is profoundly depressed by very bright light (above 100 scotopic trolands) which saturates rod vision. 4. In dim light rod modulation sensitivity functions resemble those of low-pass filters, but in bright light they resemble those of band-pass filters. 5. Several forms of rod mediated interference occur at moderate intensities, where rod vision's temporal properties ordinarily improve abruptly. With certain stimuli, rod signals conveying temporal information disrupt one another so completely that suprathreshold flicker cannot be seen within a ten-fold intensity range. 6. Many of these observations can be explained by the hypothesis that rod vision comprises two temporal channels which have different properties.