Visual form integration and discontinuity detection.

Two experiments were conducted to determine if discontinuity detection limits the visual integration of two stimuli presented successively. If this is the case, then presenting two dim stimuli should permit better integration than presenting two bright stimuli. In Experiment 1, five observers named the position of the missing dot in a 5 x 5 dot matrix. Twelve randomly selected dots were presented in each of two stimulus presentations. The stimuli varied in intensity in a ratio of 15:1 and stimulus onset asynchrony (SOA) varied between 50 msec and 80 msec. At the longer SOAs, presenting two bright stimuli led to a higher percentage of correct responses than presenting two dim stimuli. There was no difference between those conditions at the shorter SOAs. In Experiment 2, 12 naive observers rated the amount of integration of the same stimuli on a 7-point scale, under the same presentation conditions. Rated integration was higher with two dim stimuli than with two bright stimuli. These apparently contradictory results are explained in terms of Hawkins and Shulman's distinction between two kinds of visual persistence. It is concluded that subjects can respond to integration and discontinuity detection separately. No evidence was found that discontinuity detection limits the integration.