Resting and stimulated states in functional imaging studies: evidence of differences in attentional and intentional set.

ABSTRACT

In a [99mTc]-hexamethylpropyleneamine oxime single photon emission computed tomography study of cerebral blood flow (CBF) in a visual activation paradigm (awake, eyes closed versus eyes open viewing a reversing checkerboard pattern), the authors systematically measured previously observed qualitative alterations in frontal blood flow associated with visual stimulation (experiment 1). They confirmed a trend toward reductions in CBF throughout precentral cortex that approached significance in areas 9 and 46, in conjunction with significant increases in CBF in postcentral cortices, including visual association area PO, and areas 3-1-2, 22, and 23. The authors posited that these changes may be related to differences in attentional and intentional state in the eyes-closed and eyes-open conditions. Such differences should be associated with alterations in motor preparedness, leading to changes in response times and to alterations in thalamocortical gating of somatosensory information, which in turn lead to changes in somatosensory-evoked potential amplitudes. In experiment 2, the authors measured simple motor response times to a 1500-Hz tone stimulus and early components of somatosensory-evoked potentials under the same experimental conditions. In the visual stimulation condition, there was a significant increase in the evoked potential amplitude (t = 2.686, p = 0.021), and a significant decrease in response time (t = -2.464, p = 0.031). These observations provided tentative support for their hypothesis. The authors also demonstrated the major effect of normalization assumptions on regional blood flow measurements.