How important is lateral masking in visual search?

Five experiments are presented, providing empirical support of the hypothesis that the sensory phenomenon of lateral masking may explain many well-known visual search phenomena that are commonly assumed to be governed by cognitive attentional mechanisms. Experiment I showed that when the same visual arrays are used in visual search and in lateral masking experiments, the factors (1) number of distractors, (2) distractor density, and (3) search type (conjunction vs disjunction) have the same effect on search times as they have on lateral masking scores. Experiment II showed that when the number of distractors and eccentricity is kept constant in a search task, the effect of reducing density (which reduces the lateral masking potential of distractors on the target) is to strongly reduce the disjunction-conjunction difference. In experiment III, the lateral masking potential of distractors on a target was measured with arrays that typically yield asymmetric search times in visual search studies (a Q among Os vs. an O among Qs). The lateral masking scores showed the same asymmetry. Experiment IV was a visual search study with such asymmetric search arrays in which the number of distractors and eccentricity was kept constant, while manipulating density. Reducing density (i.e., reducing lateral masking) produced a strong reduction of the asymmetry effect. Finally in experiment V, we showed that the data from experiment IV cannot be explained due to a difference between a fine and a coarse grain attentional mechanism. Taken together with eye movement data and error scores from experiment II and with similar findings from the literature, these results suggest that the sensory mechanism of lateral masking could well be a very important (if not the main) factor causing many of the well-known effects that are traditionally attributed to higher level cognitive or attentional mechanisms in visual search.