Deriving numerosity and shape from identical visual displays

We presented identical displays of three to five dots in a functional magne tic resonance imaging (fMRI) experiment with normal volunteers. Two distinc t directed attention tasks were performed on these displays: In one conditi on, subjects assessed the numerosity of the display in the other condition, they assessed the shape of the display. Decisions based on numerosity acti vated differentially striate and extrastriate visual processing areas as we ll as left inferior frontal cortex. Decisions based on shape derived from a rrangement activated differentially temporoparietal cortex bilaterally, med ial posterior cingulate cortex, and left dorsolateral prefrontal cortex. Th ese divergent neural activations in response to identical stimuli suggest t hat attentional mechanisms are deployed in very different ways in rapid enu meration of visual objects and in linking spatially discrete elements to on e form. (C) 2001 Academic Press.