Abstract
Visual spatial summation describes contrast sensitivity as a function of stimulus size. Despite its ubiquity in clinical perimetry, its physiological basis remains controversial. The purpose of this study was to relate the critical area of complete summation with a perceptive field center-surround model based on results from a psychophysical Westheimer paradigm. Three subjects were selected from a previous spatial summation experiment. Westheimer functions were generated from contrast detection thresholds as a function of surround pedestal size. Two target sizes (within the complete and partial summation areas of achromatic spatial summation) and eight pedestal sizes were tested. The target and pedestal were either a luminance or chromatic increment from the background. Thresholds as a function of pedestal diameter were fit with a 4-parameter ratio of gaussians (ROG) model to predict the strength and spatial extent of center and surround perceptive fields. The model could account for all conditions tested, revealing expected desensitization and subsequent sensitization with increasing pedestal size, typical of Westheimer functions. Achromatic functions for the larger target however, lacked an initial desensitization phase. Mean bootstrapped model parameter estimates for the small achromatic target revealed center diameters from 0.35° to 0.41° and surround diameters from 1.78° to 2.04°. Achromatic magnocellular center estimates were comparable to the critical area found in spatial summation experiments under similar conditions for each subject. For the chromatic parvocellular targets, center diameter estimates were above 2.19°, and surround diameter estimates were above 2.88°. Our findings suggest differences in size and response characteristics between magnocellular and parvocellular driven perceptive fields. Interestingly, parvocellular perceptive fields measured in this experiment were larger than magnocellular, suggesting a cortical, rather than retinal locus. Furthermore, we provide evidence that the critical area of spatial summation is related to perceptive field center size in the pathways tested.