Abstract
Purpose: :
The characteristics and extent of spatial integration during detection in the dark-adapted fovea are not well defined, and are often modeled assuming either independent cone contributions to detection or as if subjects can match the pooling area to the exact extent of the stimulus. We test these assumptions using both conventional and adaptive optics with multiple spot stimulus sizes.
Methods: :
Dark-adapted foveal detection thresholds and color appearance were simultaneously measured for 550 nm stimuli of multiple sizes (from 1.7' to 25') displayed with conventional optics (2mm artificial pupil) with a rating scale method in 4 subjects. Two subjects repeated the experiment using adaptive optics (6mm artificial pupil) to measure foveal cone spacing and include stimuli smaller than possible with conventional methods (0.3' to 9.9'). Thresholds and color reports were compared as a function of stimulus size.
Results: :
Detection thresholds failed to improve linearly from the largest to the smallest stimulus, reaching a plateau between 4' and 10'. For the two subjects who also performed the adaptive optics experiment, the detection thresholds for smallest 0.3' stimulus were ~0.20 log units below this plateau, and were accompanied by shallower psychometric function slopes. Color reports incorporating hue and saturation varied significantly with stimulus size in both conditions.
Conclusions: :
Dark-adapted thresholds with and without adaptive optics are consistent with a fixed pooling area, and exclude both independent cone contributions to detection and a flexible (i.e., optimal), stimulus-matching pooling strategy. Reduced thresholds and shallow slopes for the smallest adaptive optics stimulus suggest increased variability due to the granularity of the cone mosaic at this scale may positively impact detection. Despite the fixed pooling area, color reports in both conditions suggest the visual system maintains a fine grained visual representation.
Keywords: photoreceptors: visual performance • detection • color vision