A parametric fMRI study of overt and covert shifts of visuospatial attention

It has recently been demonstrated that a cortical network of visuospatial a nd oculomotor control areas is active for covert shifts of spatial attentio n (shifts of attention without eye movements) as well as for overt shifts o f spatial attention (shifts of attention with saccadic eye movements). Stud ies examining activity in this visuospatial network during attentional shif ts at a single rate have given conflicting reports about how the activity d iffers for overt and covert shifts. To better understand how the network su bserves attentional shifts, we performed a parametric study in which subjec ts made either overt attentional shifts or covert attentional shifts at thr ee different rates (0.2, 1.0, and 2.0 Hz). At every shift rate, both overt and covert shifts of visuospatial attention induced activations in the prec entral sulcus, intraparietal sulcus, and lateral occipital cortex that were of greater amplitude for overt than during covert shifting. As the rate of attentional shifts increased, responses in the visuospatial network increa sed in both overt and covert conditions but this parametric increase was gr eater during overt shifts. These results confirm that overt and covert atte ntional shifts are subserved by the same network of areas. Overt shifts of attention elicit more neural activity than do covert shifts, reflecting add itional activity associated with saccade execution. An additional finding c oncerns the anatomical organization of the visuospatial network. Two distin ct activation foci were observed within the precentral sulcus for both over t and covert attentional shifts, corresponding to specific anatomical landm arks. We therefore reappraise the correspondence of these two precentral ar eas with the frontal eye fields. (C) 2001 Academic Press.