Spatially parallel processing of within-dimension conjunctions

Within-dimension conjunction search for red-green targets amongst red-blue, and blue-green, nontargets is extremely inefficient (Wolfe et al, 1990 Jou rnal of Experimental Psychology. Human Perception and Performance 16 879-89 2). We tested whether pairs of red-green conjunction targets can neverthele ss be processed spatially in parallel. Participants made speeded detection responses whenever a red-green target was present. Across trials where a se cond identical target was present, the distribution of detection times was compatible with the assumption that targets were processed in parallel (Mil ler, 1982 Cognitive Psychology 14 247-279). We show that this was not an ar tifact of response-competition or feature-based processing. We suggest that within-dimension conjunctions can be processed spatially in parallel. Visu al search for such items may be inefficient owing to within-dimension group ing between items.