Influence of field size on pupil diameter under photopic and mesopic light levels.

PURPOSE: We investigated the interaction between adapting field size and luminance on pupil diameter when cones alone (photopic) or rods and cones (mesopic) were active. METHOD: Circular achromatic targets (1° to 24° diameter) were presented to eight young participants on a rectangular projector screen. The accommodative influence on pupil diameter was minimised using cycloplegia in the foveally fixating right eye and the consensual pupillary reflex was measured in the left eye. Target luminance was adjusted for each stimulus such that corneal flux density (product of field area and luminance) was constant at 3,600 cd.deg(2) m(-2) (photopic condition) and 1.49 cd.deg(2) m(-2) (mesopic condition). RESULTS: There were no statistically significant effects of adaptive field size on pupil diameter for either condition. CONCLUSION: If corneal flux density is kept constant, there will be no change in pupil diameter as the size of the stimulus field increases at either mesopic or photopic lighting levels up to at least 24° diameter.

Magnocellular and parvocellular pathway mediated luminance contrast discrimination in amblyopia.

To evaluate whether luminance contrast discrimination losses in amblyopia on putative magnocellular (MC) and parvocellular (PC) pathway tasks reflect deficits at retinogeniculate or cortical sites. Fifteen amblyopes including six anisometropes, seven strabismics, two mixed and 12 age-matched controls were investigated. Contrast discrimination was measured using established psychophysical procedures that differentiate MC and PC processing. Data were described with a model of the contrast response of primate retinal ganglion cells. All amblyopes and controls displayed the same contrast signatures on the MC and PC tasks, with three strabismics having reduced sensitivity. Amblyopic PC contrast gain was similar to electrophysiological estimates from visually normal, non-human primates. Sensitivity losses evident in a subset of the amblyopes reflect cortical summation deficits, with no change in retinogeniculate contrast responses. The data do not support the proposal that amblyopic contrast sensitivity losses on MC and PC tasks reflect retinogeniculate deficits, but rather are due to anomalous post-retinogeniculate cortical processing of retinal signals.