The temporal properties of rod vision.

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Abstract
Profiles which represent rod thresholds for flickering fields seen against backgrounds of various intensity have shapes which depend on flicker frequency [in humans]. 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 2 distinct branches which resemble separate increment threshold functions. The high intensity branches of 2-branched threshold profiles cannot be attributed to cone intrusion. 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. The stimuli are detected by rods. Plots of critical flicker frequency (cff) as a function of intensity contain 2 rising branches which are separated by a plateau (when modulation depth is large), or they form 2 enclosed lobes so that only intermediate frequencies, but neither high nor low ones, are seen (when modulation depth is small). Cff is profoundly depressed by very bright light (> 100 scotopic trolands) which saturates rod vision. In dim light rod modulation sensitivity functions resemble those of low-pass filters, but in bright light they resemble those of band-pass filters. 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 10-fold intensity range. Apparently, rod vision comprises 2 temporal channels which have different properties.