Neural evidence linking visual object enumeration and attention

Visual object enumeration is rapid and accurate for four or fewer items but slow and error-prone for over four items. This dichotomy has recently been linked to visual attentional phenomena by findings suggesting that "subiti zing" of small sets of objects is preattentive whereas "counting" of over f our items demands spatial shifts of attention. We evaluated this link at a neural level, using (H2O)-O-15 positron emission tomography to measure chan ges in regional cerebral blood flow while subjects enumerated the number of target vertical bars that "popped out" of a 16-bar visual display consisti ng of both horizontal and vertical bars. Relative to a condition with a sin gle target, subitizing (one to four targets) activated foci in the occipita l extrastriate cortex, consistent with involvement of early, preattentive v isual processes. Relative to subitizing, counting (five to eight targets) a ctivated a widespread network of brain regions, including multiple foci imp licated in shifting visual attention-large regions of the superior parietal cortex bilaterally and a focus in the right inferior frontal cortex. These results offer the first direct neural support for mapping the subitizing-c ounting dichotomy onto separable processes mediating preattentive vision an d shifts of visual attention.