4-T-FMRI STUDY OF NONSPATIAL SHIFTING OF SELECTIVE ATTENTION - CEREBELLAR AND PARIETAL CONTRIBUTIONS

Regional blood oxygenation in the cerebellum and posterior cerebral co rtices was monitored with functional magnetic resonance imaging (fMRI) at four Tesla while 16 normal subjects performed three tasks with ide ntical visual stimulation: fixation; attention focused upon either sti mulus shape or color and sustained during blocks of trials (sustained attention); and rapid, serial shifts in attention between stimulus sha pe or color within blocks of trials (shifting attention). The stimuli were displayed centrally for 100 ms followed by a central fixation mar k for 900 ms. Each stimulus was either a circle or a square displayed in either red or green. Attention shifting required switching between color and shape information after each target detection and occurred o n average once every three seconds. Subjects pressed a response key up on detecting the target; reaction time and response accuracy were reco rded. Two protocols for T2-weighted echo-planar imaging were optimize d, one with a surface coil for the cerebellum alone and the other with a volume coil for imaging both cerebellum and posterior brain structu res (parietal, occipital, and part of temporal cortices). Because fMRI of the cerebellum is particularly susceptible to cardiac and respirat ory fluctuations, novel techniques were applied to isolate brain activ ation signals from physiological noise. Functional activation maps wer e generated for contrasts of I) sustained attention to color minus fix ation; 2) sustained attention to shape minus fixation; and 3) shifting attention minus sustained attention (to color and shape; i.e., summed across blocks of trials). Consistent with the ease of these tasks, su bjects performed with >80% accuracy during both sustained attention an d shifting attention. Analysis of variance did not show significant di fferences in false alarms or true hits across either attentional condi tion. A subgroup of subjects whose performance data were recorded duri ng ten minutes of continuous practice did not show significant changes over time. Both contrasts between the conditions of sustained attenti on to color or to shape as compared with the fixation condition showed significant bilateral activation in occipital and inferior temporal r egions (Brodmann areas 18, 19, and 37). The anterior medial cerebellum was also significantly activated ipsilateral to the finger used for r esponding. The principal comparison of interest, the contrast between the condition of shifting attention and the condition of sustained att ention produced significant and reproducible activation: lateral cereb ellar hemisphere (ansiform lobule: Crus I Anterior and Crus I Posterio r; left Crus I Posterior); cerebellar folium; posterior superior parie tal lobule (R and L); and cuneus and precuneus (R and L).