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Alan C. Evans
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Journal Articles
Publisher: Journals Gateway
Journal of Cognitive Neuroscience (1997) 9 (3): 392–408.
Published: 01 May 1997
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Vigilance behavior, or watch keeping, involves the focusing of attention on the detection of subtle changes in the environment that occur over a long period of time. We investigated the time course of changes in brain activity during the continuous performance of a 60-min auditory vigilance task. The task required the detection of an intensity drop that occurred in 5% of the auditory stimuli. Six 1-min samples of cerebral blood flow (CBF) and electroencephalographic (EEG) activity were obtained at l0-min intervals during the vigilance performance. Changes in CBF were measured by means of positron emission tomography (PET). Performance data (hits, false alarms, reaction time) were analyzed across six 10-min blocks. Eight healthy male volunteers participated in the study. During the 60-min test, the number of correct detections (hits) did not change, but both the reaction time and EEG activity in the theta (4 to 7 Hz) range progressively increased across testing. CBF in several subcortical structures (thalamus, substantia innominata, and putamen) and cortical areas (ventrolateral, dorsolateral, and orbital frontal cortex; parietal cortex; and temporal cortex) decreased as a function of time-on-task; changes in the cortical regions were limited to the right hemisphere. Blood flow also decreased in the temporalis muscles. At the same time, CBF increased in several visual cortical areas including the left and right fusiform gyri. Furthermore, the thalamic blood-flow response co-varied with that in the substantia innominata, the ponto-mesencephalic tegmentum, and the anterior cingulate cortex. The right ventrolateral-frontal blood-flow response covaried with that in the right parietal, orbitofrontal, and dorsolateral frontal cortex. 'Iko main conclusions are drawn from the obtained data. First, we suggest that the observed time-related changes in reaction time, EEG activity, and blood flow in the temporalis muscles are related to changes in the level of arousal (alertness) and that CBF changes in the thalamus-related neural circuitry represent a brain correlate of such changes. Second, we speculate that time-related CBF decreases in cortical regions of the right hemisphere underlie a shift from controlled to automatic attentional processing of the auditory stimuli.
Journal Articles
Publisher: Journals Gateway
Journal of Cognitive Neuroscience (1996) 8 (6): 588–602.
Published: 01 November 1996
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A plethora of studies, across many species, have now demonstrated that the hippocampal region plays a critical role in memory for spatial location. In spite of this compelling evidence, a number of important neuropsychological and neuroanatomical issues remain unresolved. In the present study, the functional anatomy of object-location memory was investigated using positron emission tomography (PET) with magnetic resonance imaging (MRI). Regional cerebral blood flow (rCBF) was measured while normal volunteers encoded, and then retrieved, the locations of eight familiar objects presented on a computer screen. In two analogous conditions, designed to fractionate object-location memory into its component processes, the subjects were simply required to encode, and then to retrieve, eight distinct locations represented by identical white boxes on the screen. An increase in rCBF was observed in the region of the right parahippocampal gyrus corresponding to entorhinal cortex when the Retrieving Location condition was subtracted from the Retrieving Object-Location condition. In contrast, when the Encoding Location condition was subtracted from the Encoding Object-Location condition, no significant rCBF changes were observed in the hippocampal region although significant activation was observed, bilaterally, in the anterior fusiform gyrus. In addition, the two encoding conditions activated left-hemisphere regions preferentially, whereas the two retrieval conditions activated right-hemisphere regions. Together, these findings suggest that the human right hippocampal region is critically involved in retrieving information that links object to place. The secondary finding that encoding and retrieval appear to be lateralized to the left and right hemispheres respectively, is discussed with reference to current models of episodic memory, and alternative hypotheses are considered.
Journal Articles
Publisher: Journals Gateway
Journal of Cognitive Neuroscience (1996) 8 (1): 29–46.
Published: 01 January 1996
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Neuropsychological studies have suggested that imagery processes may be mediated by neuronal mechanisms similar to those used in perception. To test this hypothesis, and to explore the neural basis for song imagery, 12 normal subjects were scanned using the water bolus method to measure cerebral blood flow (CBF) during the performance of three tasks. In the control condition subjects saw pairs of words on each trial and judged which word was longer. In the perceptual condition subjects also viewed pairs of words, this time drawn from a familiar song; simultaneously they heard the corresponding song, and their task was to judge the change in pitch of the two cued words within the song. In the imagery condition, subjects performed precisely the same judgment as in the perceptual condition, but with no auditory input. Thus, to perform the imagery task correctly an internal auditory representation must be accessed. Paired-image subtraction of the resulting pattern of CBF, together with matched MRI for anatomical localization, revealed that both perceptual and imagery. tasks produced similar patterns of CBF changes, as compared to the control condition, in keeping with the hypothesis. More specifically, both perceiving and imagining songs are associated with bilateral neuronal activity in the secondary auditory cortices, suggesting that processes within these regions underlie the phenomenological impression of imagined sounds. Other CBF foci elicited in both tasks include areas in the left and right frontal lobes and in the left parietal lobe, as well as the supplementary motor area. This latter region implicates covert vocalization as one component of musical imagery. Direct comparison of imagery and perceptual tasks revealed CBF increases in the inferior frontal polar cortex and right thalamus. We speculate that this network of regions may be specifically associated with retrieval and/or generation of auditory information from memory.