Deriving numerosity and shape from identical visual displays.

We presented identical displays of three to five dots in a functional magnetic resonance imaging (fMRI) experiment with normal volunteers. Two distinct directed attention tasks were performed on these displays: In one condition, subjects assessed the numerosity of the display; in the other condition, they assessed the shape of the display. Decisions based on numerosity activated differentially striate and extrastriate visual processing areas as well as left inferior frontal cortex. Decisions based on shape derived from arrangement activated differentially temporoparietal cortex bilaterally, medial posterior cingulate cortex, and left dorsolateral prefrontal cortex. These divergent neural activations in response to identical stimuli suggest that attentional mechanisms are deployed in very different ways in rapid enumeration of visual objects and in linking spatially discrete elements to one form. Copyright 2001 Academic Press.