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Although visual spatial attention has been shown to increase activity as measured with both fMRI and electrophysiological techniques, significant differences in the results have been shown. fMRI studies have routinely demonstrated large signal increases to an attended versus unattended stimulus in early visual areas (V1-V3) whereas some previous electrophysiology research has either shown very small or no differences in spike rate. One possible explanation for this discrepancy is that most previous fMRI studies have not differentiated between stimulus-evoked and baseline-shift changes in the response to an attended stimulus. Here, fMRI was used to separately measure stimulus-evoked and baseline-shift responses. In the first experiment, contrast-response functions to grating stimuli that were either attended or unattended were measured. The results show that the increases in fMRI signal associated with spatial attention are accounted for by a baseline shift. In a second experiment, spatial attention was fixed in a single location that isolated possible stimulus-evoked changes with attention. Consistent with the first experiment, no stimulus-evoked changes were found. These results potentially reconcile previous discrepant findings between fMRI studies and some neurophysiology studies of attention by demonstrating that the effects of spatial attention in early visual areas can be dominated by stimulus-independent shifts in baseline responses.

The effects of spatial attention in early human visual cortex are stimulus independent