Exchange thresholds for long-wavelength incremental flashes.

Thresholds of 1-deg, 200-msec, 641-nm foveal test flashes rise after an exchange of II5-equated 536- and 626-nm fields, taking about 30 sec to recover. Silent substitution (no rise of threshold after the exchange) occurs, however, if these fields are alternated during adaptation. Thresholds for a 1-deg, 20-msec test rise similarly after an exchange but recover in only 0.5 sec and are not influenced by alternation of the 536- and 626-nm fields. These results can be accounted for if the 641-nm tests are detected not through pathways controlled by long-wavelength cones alone but through nonopponent (20-msc) and opponent (200-msec) pathways whose sensitivities may be reduced by transient inputs from other cones.