TIMING OF VISUAL EVENTS FOR MOTION DISCRIMINATION

The observer's task was to identify the temporal order of the two adja cent luminance excursions one of which was a step-function and the oth er a Linear increase in luminance starting from zero and reaching vari ous final amplitude A after some period of time D. The interstimulus d elay, Delta t, between these two transitions was determined at which t hey appeared isochronous. The point of the subjective equality (PSE) d epended on both ramp parameters, the rise-time duration D and its ampl itude A. All of the data can be accounted for by supposing that judgem ents about the temporal order are based on the comparison of a simple attribute of these luminance excursions, the time moments when the lum inous energy concentrated on low temporal frequencies exceeds some lev el. The perceived temporal order, which was experienced as a leftward or rightward displacement of the whole pattern, was determined by the sequence in which low-frequency portion of these two luminance excursi ons reached the threshold value. The implications of this simple contr ast detection explanation for theories of motion analysis are discusse d.