The incremental sensitivity curve of turtle cone photoreceptors

where AZ is the incremental threshold intensity measured at each background intensity, Zb. The constant I0 is photopic absolute threshold and factor C is a proportionahty constant relating threshold elevations to increases in background intensity {the Weber fraction). While this relation provides a quantitative description of the psychophysical process of background adaptation, the neural mechanisms which underty this ~enom~on are as yet ~n~ompleteiy understood. Intmcel~u~ar recordings made in the vertebrate retina have demonstrated that cone photoreceptors are desensitized by background illumination (Baylor and Hodgkin, 1973; Baylor and Hodgkin, 1974; Normann and Werblin, 1974; Normann and Perhnan, 1979a). Baylor and Hodgkin (1974) studied how cone sensitivity is affected by both I.lOsec brief exposures of tight, and by steady state exposures of light. Most of their work was done with brief ~~kg~unds where two forms of cone de~nsjti~tion are present; a desensitization due to thecone’s ins~n~n~us ~n~~n~rity (the nonlinear relation between light intensity and cone voltage response), and a desensitization due to the background light itself. The instantaneous nonlinearity is time dependent and can be minimized by using steady state backgrounds, a condition also used by