RICCOS DIAMETER FOR LINE DETECTION INCREASES WITH STIMULUS VELOCITY

The purpose of this study was to determine whether Ricco's diameter, t he spatial extent within which sensitivity demonstrates a perfect reci procity between contrast and area, enlarges as the stimulus velocity i ncreases. Detection thresholds were measured for a single line of leng th 10 arcmin as a function of linewidth that varied between 0.31 and 2 1.7 arcmin and for velocity ranging from 0 to 6 deg/s. We fitted the d etection threshold versus linewidth data with two power functions of s lope 0 and 1 and defined the intersection of these two functions as Ri cco's diameter. For an increase in velocity from 0 to 6 deg/s, Ricco's diameter increases in dimension by approximately a factor of 4. Simil ar results were obtained when Ricco's diameter was estimated by compar ing detection threshold of a thin line to that of an edge. The increas e in Ricco's diameter with stimulus velocity suggests that the spatial -frequency mechanism that mediates line detection shifts progressively toward lower spatial frequencies for faster moving stimuli. (C) 1996 Optical Society of America.