Threshold recognition of phantom-contour objects requires constant contrast velocity

Recognition of phantom objects--those with contours defined by rapid contrast reversal of adjacent fields of dark and light random dots--was investigated under conditions of abrupt or ramped onset and offset. Discrimination contrast thresholds were determined for a random-dot phantom letter in four possible orientations. For abrupt onset or offset, thresholds were almost independent of the duration of presentation time, over a range that varied tenfold, from 34-340 msec. However, when the onset and offset were shaped by a triangular envelope, thresholds were raised, so that form blindness occurred even when peak d ot contrasts exceeded 60%. Also under ramped onset and offset conditions, threshold contrast varied strictly linearly with stimulus duration in all subjects, suggesting a new construct--contrast velocity, the rate of change of contrast critical for phantom-object recognition.