Temporal summation in the achromat.

We investigated temporal summation of the rods in a complete achromat, who lacks cone vision. Critical duration (tc) was estimated both at the achromat's preferred area of fixation and at an area 12 deg laterally in the nasal visual field. Comparable tc determinations were made in a normal trichromat. At background luminances of 0.0 and 0.6 scot. td, where the rods mediate detection, the values of tc were similar for the achromat and the normal. At a luminance of 813 scot. tds, however, where the middle-wavelength sensitive cones mediate detection in the trichromat, the tc for the achromat was much longer than that for the trichromat.