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Abstract

function of the interstimulus interval (ISI) between target and surrounding mask. We hypothesized that there might be an increase in latency of perception along with reduction
of perceived intensity.
We displayed two horizontal bars (in one line, height: 0.5 deg, width 3 deg) on a PC monitor, each for 25 ms with a variable ISI, the left one was presented first. To achieve maximal metacontrast there was no gap at the neighbored edges of the two bars.
Subjects perceive two moving lines, both start at the left and seem to ‚grow‘ to the right.
The motion seen in the right line is the well known line motion illusion, generated by cue induced visual focal attention (Steinemann et al. 1997). The motion illusion perceived in the left line is due to metacontrast. This conclusion is supported by the finding that the effect is strongest when the left line as a target is presented 40 - 80 ms before the right line acting as a mask. At equivalent time differences, metacontrast in classical paradigms
is strongest. The motion illusion results from the fact that the degree of dimming and the prolongation of latency decreases as the distance separating target from mask increases.
There is a close relationship between metacontrast and cue induced focal attention, both show, however, opposite actions: Focal attention intensifies perception of an object and reduces its latency of perception, whereas metacontrast diminishes intensity and prolongs latency.