Categorical and coordinate spatial processing: more on contributions of the transient/magnocellular visual system

Observers were presented with stimuli consisting of a line and two horizontally separated dots. A categorical spatial task required observers to indicate whether the dots were above or below the line and a coordinate spatial task required observers to indicate whether the line could fit into the space between the two dots. Coordinate (but not categorical) spatial processing was less accurate and took longer with stimuli presented on a red background than with stimuli presented on a green background, even though the background color varied randomly from trial to trial and the viewing screen remained gray between trials. Because the color red attenuates processing in the transient/magnocellular visual system, these results suggest that coordinate spatial processing is dependent on that pathway. Furthermore, such effects do not involve mechanisms of perceptual adaptation that depend on the same color background being present throughout an experiment or for a prolonged period of time. As in earlier experiments, the effects of color condition were the same regardless of which visual field (and hemisphere) received the stimulus information. However, in contrast to the results of earlier experiments, there was no significant interaction of task and visual field.