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Rightward shifts in visual attention with reduced alertness: Is the persistence of unilateral neglect an exaggeration of a normal pattern
MANLY, T., DODDS, C., George, M. & Dobler, V.
Brain Impairment, 6(2). 123, 2005
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Objectives: Most patients with unilateral neglect recover from this severe spatial bias within days or weeks. Persistent forms of this debilitating condition are almost exclusively seen following right hemisphere damage and often co-occur with deficits in alertness/sustained attention. Observations, such as a reduction in neglect consequent upon pharmacological or auditory stimulation, have been argued to reflect a modulatory relationship between alertness and spatial attention. The specificity of this relationship is, however, difficult to test using only patient data in which attention to the left is originally deficient. The aim of the studies reported here was to examine changes in spatial bias in healthy participants as a consequence of naturally declining alertness. Methods: Adults from the healthy population performed a spatial task at two sessions. In one session they were sleep deprived, in the other they were well rested. In addition, changes in performance across each session were examined. In the second study, a new group of healthy participants performed the rather tedious and repetitive spatial task over an hour. Results: Relative sleep deprivation was associated with a significant rightward shift in normal spatial attention. Subsequent analysis revealed that, even in the 'alert' session, a significant rightward shift occurred as a function of time-on-task. The second study replicated the rime-on-task effect. Conclusions: It has been argued that the two hemispheres of the brain are engaged in a form of dynamic competition, each attempting to push attention into contra-lateral space. Conscious awareness of stimuli has been argued to involve the engagement of a 'general purpose' frontoparietal circuit in addition to activity in basic perceptual areas. Intrinsic alertness has been associated with predominantly right hemisphere activation within this circuit. This provides a mechanism whereby reductions in alertness may reduce conscious awareness of objects in left space and may reflect an analogous pattern to that seen in persistent unilateral neglect. Understanding recovery and persistence within conditions can lead to more appropriate targeting of rehabilitative and pharmacological interventions.