Abstract
PURPOSE. Sally and Gurnsey (2001, ARVO) measured orientation discrimination thresholds for high contrast stimuli as a function of stimulus size and eccentricity. We ensured that orientation thresholds for stimuli of different sizes were not affected by changes in perceptual contrast. At all eccentricities, orientation thresholds decreased as stimulus size increased. However, thresholds obtained at all sizes and eccentricities could be collapsed onto a single function of stimulus size by dividing the sizes of peripherally presented stimuli by F = 1 + E/E2 where E2 = 1.29 . Sally and Gurnsey (2002, VSS) presented data suggesting that E2 associated with the orientation discrimination task might be larger for low contrast stimuli. The purpose of the present research was to replicate the procedure of Sally and Gurnsey (2001) but with perceived contrast for all stimuli matched to a low contrast standard.
METHODS. Stimuli were broad-band line patterns presented at a range of sizes (0.19 to 12 ) and eccentricities (0 to 15 ) in the temporal periphery. A 3 reference line presented at fixation (0 ) was set to two JNDs above detection threshold. Subjects matched the perceived contrast of all other stimuli to this reference line. Orientation discrimination thresholds were then obtained at these matching contrasts for all stimulus sizes and eccentricities.
RESULTS. At all eccentricities, orientation thresholds decreased as stimulus size increased. All data could be fit to a single function of stimulus size by dividing the sizes of peripherally presented stimuli by F = 1 + E/E2 where E2 = 3.36 .
CONCLUSIONS. Orientation discrimination thresholds obtained at low perceptual contrasts elicit an E2 that is 2.6 times greater than that elicited at at high perceptual contrast. This may reflect a change in the structure of orientation selective mechanisms as a function of stimulus contrast (Mareschal, Henrie, & Shapley, 2002, Vision Research).