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The contrast thresholds for sine-wave profile gratings were measured as a function of both the length of the bars and the number of bars of the grating. These factors were found to have a significant influence on threshold only when the resultant height and width was less than a size equivalent to ten periods of the grating at all spatial frequencies. This result suggests that there is a functional summation of responses of detecting elements at threshold over an area the size of which is reciprocally related to the spatial frequency.

These data were used to measure the threshold contrast sensitivity functions for sine-wave and square-wave gratings under conditions where there are comparable field-sizes at each spatial frequency.