Target edge sharpness and temporal brightness enhancement

Abstract

Brightness increases with duration for presentation times less than %I00 msec. and is independent of duration for times greater than about 200 msec.’ With sufficient luminance contrast. intermediate duration stimuli appear brighter than longer or shorter stimuli. a phenomenon termed “temporal brightness enhancement” or the “Broca-Sulzer effect.” Blurring the contours of a target causes a decrease in apparent brightness (Enoch. 1958: Thomas and Kovar. 1965) and an increase in the luminance required for threshold detection (Enoch. 1958: Ogle. 1960: Hood. 1973). Does blurring affect temporal brightness enhancement’! Arend (1973) reported that the Broca-Sulzer erect did not occur with targets whose contours were blurred. Other studies. investigating variations in the apparent contrast of grating patterns with the exposure duration of the grating (Kitterle and Rysberg. 1976: Kitterle and Corwin. L978) have shown that for sine-wave gratings of low spatial frequency (less than 0.95 c deg). apparent contrast was enhanced for exposures of 80-100 msec relative IO shorter and longer durations. Sine-wave gratings are inherently “blurred” targets. and this result suggests that sharp edges may not be a critical factor for obtaining temporal brightness enhancement. Further. temporal contrast enhancement is not evident for gratings of high spatial frequency (56cideg or more) (Kitterle and Corwin. 1978). Since by Fourier analysis the high spatial frequency components of a pattern represent sharp-edged contours. visual mechanisms sensitive to higher spatial frequencies may have no role in generating the Broca-Sulzer effect. A disc target contains both high and low spatial frequencies. Blurring selectively attentuates high spatial frequencies. Thus. the Arend and the Kitterle results are contradictory. Consequently we investigated the question of uhether the presence of sharp