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 'subitizing' of small sets of objects is preattentive whereas 'counting' of over four items demands spatial shifts of attention. We evaluated this link at a neural level, using H2 15-O positron emission tomography to measure changes in regional cerebral blood flow while subjects enumerated the number of target vertical bars that 'popped out' of a 16-bar visual display consisting of both horizontal and vertical bars. Relative to a condition with a single target, subitizing (one to four targets) activated foci in the occipital extrastriate cortex, consistent with involvement of early, preattentive visual processes. Relative to subitizing, counting (five to eight targets) activated a widespread network of brain regions, including multiple foci implicated 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-counting dichotomy onto separable processes mediating preattentive vision and shifts of visual attention.