Dissociation of orientation discrimination from form detection for motion-defined bars and luminance-defined bars: effects of dot lifetime and presentation duration.

A stationary bar-shaped area that was perfectly camouflaged within a dot pattern was rendered visible by moving the dots inside and outside the bar at equal and opposite speeds. Orientation discrimination for this motion-defined (MD) bar was compared with orientation discrimination for a luminance-defined (LD) bar created by switching off all dots outside the bar. The best values of orientation discrimination threshold were similar for MD and LD bars at long presentation durations and long dot lifetimes. But, as presentation duration or dot lifetime was reduced below 1.0 sec, orientation discrimination threshold for MD bars increased at an accelerating rate, while discrimination for LD bars was comparatively unaffected. However, these effects of presentation duration and dot lifetime were largely due to changes in bar visibility. When bar visibility was normalised relative to the relevant bar detection threshold, the effect of presentation duration upon orientation discrimination was abolished for both MD and LD bars, and the effect of dot lifetime was abolished or even reversed. These observations dissociate detection and discrimination for MD and LD form. We suggest that orientation discrimination for MD and LD bars is determined by opponent-orientation mechanisms whose performance is not directly affected by presentation duration, nor degraded by reducing dot lifetime.