Abstract

Performance in many visual tasks depends on visual field location and generally declines with increasing retinal eccentricity. The expression [F = 1 + E/E 2 ] represents a theory of how one may compensate for eccentricity dependent variations in sensitivity by appropriate scaling (F) of the stimuli at each eccentricity. E 2 indicates the eccentricity ( E ) at which the size of the stimulus must be doubled, relative to the foveal standard, to achieve equivalent performance. The parameter E 2 is considered to reflect changes in the spatial scale of mechanisms that subserve task performance. Unfortunately, E 2 s tend to vary widely, even for the same task. The present experiments have addressed this issue using orientation discrimination. These findings suggest that considerably heterogeneity of size-scaling E 2 s results from uncontrolled variations in perceptual contrast. Initial experiments indicated that the commonly used spatial-scaling technique of presenting stimuli at a fixed high level of contrast can result in inflated E 2 s. A method of controlling for perceptual contrast at high levels of stimulus contrast was therefore introduced. E 2 s recovered under these conditions were in the range of 1.29 ̕to 1.83 ̕(Sally & Gurnsey, 2003a). These estimates are in line with those of the classic spatial-scaling study on orientation discrimination of Mäkelä, Whitaker and Rovamo (1993) conducted at high contrasts. The E 2 for orientation discrimination was then determined using two methods (Melmoth, Kukkonen, Mäkelä & Rovamo, 2000a,b and Poirier & Gurnsey, 2002) that provide estimates of size- and contrast-scaling at near-threshold levels of stimulus contrast. Size-scaling E 2 s were reasonably consistent across methods and were substantially larger than those obtained at high stimulus contrasts, ranging from 3.71 ̕to 6.86.̕ The high-contrast experiment (Sally & Gurnsey, 2003a) was then replicated at low levels of stimulus contrast; perceptual contrasts were equated through a matching procedure. Size-scaling E 2 s were again large: 3.42 ̕and 3.50.̕ In conclusion, E 2 s for orientation discrimination depend critically on stimulus contrast and are small at high contrasts and large at low contrasts. This may be due to dynamic changes in the spatial structure of orientation selective mechanisms at low stimulus contrasts.