The effects of contrast and size on orientation discrimination

Motivated by the recent physiological finding that a neuron's receptive field can increase in size by a factor of 2-4 fold at low contrast (Sceniak et al 1999; Kapadia et al. 1999), we sought to examine whether a psychophysical task might reflect the contrast dependent changes in the size/structure of a receptive field. We postulate that since spatial summation is not contrast invariant, a task which relies on the spatial structure of a receptive field, such as orientation discrimination, should also be affected by changes in contrast. Previously, orientation discrimination thresholds have been reported to be roughly independent of the contrast of a stimulus for most of the visible range of contrasts. In this experiment, subjects were presented with two patches of grating and were required to indicate whether the second was shifted clockwise or counter-clockwise relative to the first. Contrast and size of the gratings were varied independently and measurements were made in the fovea and near periphery. We found large improvements in orientation discrimination with contrast that were parametric in stimulus area. Furthermore, the apparent constancy of orientation discrimination for large area stimuli is likely a result of a floor effect on the threshold. Therefore we conclude that there is not strong evidence for contrast invariant orientation discrimination.