The role of the magnocellular pathway in serial deployment of visual attention

Normal human visual function often demands detection of a target among a number of other objects cluttering the scene, such as when searching for a known face in a crowd. In these and similar tasks, the search performed is a serial one, with an attentional spotlight scanning the objects of the scene. We have investigated whether one of the afferent channels in vision, the colour-blind magnocellular pathway, is essential in such serial searches. We did this by using items that were isoluminant with the background but of a different colour, for which the magnocellular cells would be blind. The search in these conditions required much longer reaction times than when even a very small luminance contrast (2%) was added to the items. Because such luminance contrasts can be detected only by magnocellular cells and not by neurons of the other channels (parvocellular and koniocellular), the magnocellular pathway appears vitally important for serial search. In contrast, in a feature search task, which does not require allocation of attentional resources, the search was as efficient with isoluminance as when luminance contrast was added to the items.