4 T-FMRI STUDY OF NON-SPATIAL SHIFTING OF SELECTIVE ATTENTION: CEREBELLAR
AND PARIETAL CONTRIBUTIONS.
Tuong Huu Le, Jos‚ V. Pardo, and Xiaoping Hu.
Center for Magnetic Resonance Research, Department of Radiology, University
of Minnesota, Minneapolis, Minnesota 55455, USA; Cognitive Neuroimaging Unit,
Psychiatry Service, Veterans Affairs Medical Center and Division of
Neuroscience Research, Department of Psychiatry, University of Minnesota,
Minneapolis, Minnesota 55417.
APStracts 4:336N, 1997.
ABSTRACT
Regional blood oxygenation in the cerebellum and posterior cerebral cortices
was monitored with functional magnetic resonance imaging (fMRI) at 4 Tesla
while sixteen normal subjects performed three tasks with identical visual
stimulation: fixation; attention focused upon either stimulus shape or color
and sustained during blocks of trials (sustained attention); and rapid, serial
shifts in attention between stimulus shape or color within blocks of trials
(shifting attention). The stimuli were displayed centrally for 100 ms followed
by a central fixation mark 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 following each target detection and
occurred on average once every three seconds. Subjects pressed a response key
upon detecting the target; reaction time and response accuracy were recorded.
Two protocols for T2*-weighted echo-planar imaging were optimized, one with a
surface coil for the cerebellum alone and the other with a volume coil for
imaging both cerebellum and posterior brain structures (parietal, occipital,
and part of temporal cortices). Since fMRI of the cerebellum is particularly
susceptible to cardiac and respiratory fluctuations, novel techniques were
applied to isolate brain activation signals from physiological noise.
Functional activation maps were generated for contrasts of 1) sustained
attention to color minus fixation; 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, subjects performed with greater than 80% accuracy during both
sustained attention and shifting attention. Analysis of variance did not show
significant differences in false alarms or true hits across either attentional
condition. A subgroup of subjects whose performance data were recorded during
ten minutes of continuous practice did not show significant changes over time.
Both contrasts between the conditions of sustained attention to color or to
shape as compared to the fixation condition showed significant bilateral
activation in occipital and inferior temporal regions (Brodmann areas 18, 19,
and 37). The anterior medial cerebellum was also significantly activated
ipsilateral to the finger used for responding. The principal comparison of
interest, the contrast between the condition of shifting attention and the
condition of sustained attention produced significant and reproducible
activation: lateral cerebellar hemisphere (ansiform lobule: Crus I Anterior
and Crus I Posterior; left Crus I Posterior); cerebellar folium; posterior
superior parietal lobule (R & L); and, cuneus and precuneus (R & L).
Received 12 May 1997; accepted in final form 18 November 1997.
APS Manuscript Number J393-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 12 December 1997