Effect of ramp-like contours upon perceived size and detection threshold

Perceived widths and detection thresholds were measured for foveally viewed stimuli which were edged on two sides by luminance ramps. Perceived width approximated the distance between the midpoints of the ramps. It varied with the width of the ramps, but was unaffected by a five-fold change in the luminance of the stimulus. The detection threshold was higher for a ramp stimulus than for a normal stimulus of the same apparent width. The relationship between perceived width and detectability is consistent with a theoretical treatment based upon receptive-field mechanisms.