Abstract
We employed an analogue of the Hess effect to study the temporal dynamics of rod- and cone-driven responses across the retina. Radial stimuli allowed us to study well-localised eccentricities. A high contrast reference bar rotated like a clock hand at 180–270 deg.s-1 about a central fixation point. A spot of equal velocity orbited the same centre. S adjusted its phase until it appeared aligned with the reference.
Isoluminant red (MBDKL 0deg) and blue (90deg) spots, eccentricity 3deg, on a 12.5cd.m-2 illuminant ‘C’ background, showed large phase shifts corresponding to a lag of about 60ms. Away from isoluminance, this decreased to about 10ms. Between 1 and 4deg eccentricity, the red lag increased by 35ms. Rod function from 2 to 40deg eccentricity was studied in dark-adapted Ss using a dim (0.006 to 0.12 cd.m-2 ) blue spot on a black background with a deep red reference bar. The dimmest target lagged by about 30ms at 2deg but started to lead as eccentricity increased, with an overall change of approximately 40ms.
These results show a central bias in the temporal dynamics of cone-driven mechanisms but a peripheral bias in rod-driven mechanisms, reflecting in part the underlying densities of these photoreceptor classes.