No abstract

SUMMARY1. The ability of rhesus monkeys to detect the gap in Landolt ring testobjects that were presented against background luminances between 5 x 10-5 cd/M2 and 5 x 103 cd/M2 was compared with similar human data.2. At high luminance-levels the acuity of human observers is slightly better than that of rhesus, but rhesus have better acuity at scotopic luminance-levels. Both species have distinct photopic and scotopic acuity functions that cross at 6 x 103 cd/M2.3. The threshold for light detection is estimated to be the same for both species when specified in quanta incident on the retina.4. It is concluded that the receptor and neural mechanisms that mediate visual-acuity function similarly in rhesus and man, and that the differences in acuity that were measured in the two species may be attributed to optical rather than to physiological factors.

Small-field color-naming performance of two protanopes over a 4-log luminance range was impoverished in comparison with that of normal trichromats, and was more strongly affected by changes in luminance. At 200 cd/m2 responses to mid-spectral lights were dominated by 'yellow'; with lowering luminance, 'green' and 'red' were increasingly used. In the color spaces derived from these data the first two dimensions for trichromats are red-green and yellow-blue: those of the protanopes appear to be brightness and 'red-blue'. In the protanopes' color space the greater separation of stimuli at 0.2 cd/m2 suggests that with low luminance their color discrimination improves.

SUMMARY1. Contrast sensitivity functions of isolated colour mechanisms were measured at spatial frequencies from 0-2 to 32 c/deg.The contrast sensitivity vs. spatial-frequency functions of the red (nr5) and green (br4) mechanisms are similar, while the blue (iT3) mechanism has lower absolute sensitivity and lower resolving power. Isolation of a single mechanism never increases its maximum sensitivity. 2. The shape of the contrast sensitivity function of a colour mechanism is established within the mechanism. Little if any inhibitory interaction takes place among colour mechanisms.3. Differences that have been reported between the sensitivities of the red and green mechanisms, as well as the apparent 'supersensitivity' of the isolated green mechanism, may be artifacts that result from the extrapolation procedures that were used to estimate the absolute sensitivities of the colour mechanisms.