2000

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Neural representations of skilled movement

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Authors:

  • Kathleen Y. Haaland

  • Deborah L. Harrington

  • Robert T. Knight

Date: 2000

PubMed: 11050030

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Abstract:

The frontal and parietal cortex are intimately involved in the representation of goal-directed movements, but the crucial neuroanatomical sites are not well established in humans. In order to identify these sites more precisely, we studied stroke patients who had the classic syndrome of ideomotor limb apraxia, which disrupts goal-directed movements, such as writing or brushing teeth. Patients with and without limb apraxia were identified by assessing errors imitating gestures and specifying a cut-off for apraxia relative to a normal control group. We then used MRI or CT for lesion localization and compared areas of overlap in those patients with and without limb apraxia. Patients with ideomotor limb apraxia had damage lateralized to a left hemispheric network involving the middle frontal gyrus and intraparietal sulcus region. Thus, the results revealed that discrete areas in the left hemisphere of humans are critical for control of complex goal-directed movements.

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Transient interference of right hemisphere function due to automatic emotional processing

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Authors:

  • Kaisa M. Hartikainen

  • Keith H. Ogawa

  • Robert T. Knight

Date: 2000

PubMed: 11074080

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Abstract:

We examined the effects of emotional stimuli on right and left hemisphere detection performance in a hemifield visual discrimination task. A group of 18 healthy subjects were asked to discriminate between upright and inverted triangles (target). Targets were randomly presented in the left or right visual hemifield (150 ms target duration). A brief emotional picture (pleasant or unpleasant; 150 ms stimulus duration) or neutral picture selected from the International Affective Picture System was randomly presented either in the same (47%) or the opposite (47%) spatial location to the subsequent target. Emotional or neutral stimuli offset 150 ms prior to the subsequent target. Subjects were instructed to ignore the pictures and respond to the targets as quickly and accurately as possible. Independent of field of presentation, emotional stimuli prolonged reaction times (PB 0.01) to LVF targets, with unpleasant stimuli showing a greater effect than pleasant stimuli. The current study shows that brief emotional stimuli selectively impair right hemispheric visual discrimination capacity. The findings suggest automatic processing of emotional stimuli captures right hemispheric processing resources and transiently interferes with other right hemispheric functions.

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Prefrontal-cingulate interactions in action monitoring

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Authors:

  • William J. Gehring

  • Robert T. Knight

Date: 2000

PubMed: 10769394

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Abstract:

We found that medial frontal cortex activity associated with action monitoring (detecting errors and behavioral conflict) depended on activity in the lateral prefrontal cortex. We recorded the error- related negativity (ERN), an event-related brain potential proposed to reflect anterior cingulate action monitoring, from individuals with lateral prefrontal damage or age-matched or young control participants. In controls, error trials generated greater ERN activity than correct trials. In individuals with lateral prefrontal damage, however, correct-trial ERN activity was equal to error-trial ERN activ- ity. Lateral prefrontal damage also affected corrective behavior. Thus the lateral prefrontal cortex seemed to interact with the anterior cingulate cortex in monitoring behavior and in guiding compensatory systems.

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Prefrontal modulation of visual processing in humans

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Authors:

  • Francisco Barceló

  • Shugo Suwazono

  • Robert T. Knight

Date: 2000

PubMed: 10725931

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Abstract:

Single neuron, evoked potential and metabolic techniques show that attention influences visual pro- cessing in extrastriate cortex. We provide anatomical, electrophysiological and behavioral evidence that prefrontal cortex regulates neuronal activity in extrastriate cortex during visual discrimination. Event-related potentials (ERPs) were recorded during a visual detection task in patients with damage in dorsolateral prefrontal cortex. Prefrontal damage reduced neuronal activity in extrastriate cortex of the lesioned hemisphere. These electrophysiological abnormalities, beginning 125 ms after stimulation and lasting for another 500 ms, were accompanied by behavioral deficits in detection ability in the contralesional hemifield. The results provide evidence for intrahemispheric prefrontal modulation of visual processing.

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The contribution of recollection and familiarity to yes-no and forced-choice recognition tests in healthy subjects and amnesics

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Authors:

  • Wayne Khoe

  • Neal E. A. Kroll

  • Andrew P. Yonelinas

  • Ian G. Dobbins

  • Robert T. Knight

Date: 2000

PubMed: 10869576

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Abstract:

Recent reports suggest that some amnesic patients perform relatively normally on forced-choice recognition memory tests. Their preserved performance may re ̄ect the fact that the test relies more heavily on assessments of familiarity, a process that is relatively preserved in these patients, than do other recognition tests such as yes±no tests, which may rely more on recollection. The current study examined recognition memory using yes±no and forced-choice procedures in control and amnesic patients in order to determine whether the two tasks di€erentially relied on recollection and familiarity, and whether the extent of the recognition memory de®cit observed in amnesia was dependent upon the type of recognition test used to measure performance. Results using the remember±know procedure with healthy subjects showed that there were no substantial di€erences in recognition accuracy or in the contribution of recollection to these two tasks. Moreover, amnesic patients were not found to perform better on a forced-choice test than on a yes±no test, suggesting that familiarity contributed equally to these two types of recognition test.

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An analysis of atrophy in the medial mammillary nucleus following hippocampal and fornix lesions in humans and nonhuman primates

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Authors:

  • Mirela Loftus

  • Robert T. Knight

  • David G. Amaral

Date: 2000

PubMed: 10785457

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Abstract:

Lesions of the hippocampal formation or transec- tions of the fornix are followed by shrinkage of the medial mammillary nucleus (MMN). We determined whether the shrinkage of this nucleus was due to loss and/or shrinkage of neurons in addition to the loss of neuropil. We examined the MMN in a patient (KB) with an infarct that led to marked atrophy of the left hippocampus and subiculum, leaving the right MMN intact. Unbiased, stereological measure- ment techniques were used to compare the total cell number and individual neuronal cross-sectional ar- eas in both left and right MMN in this patient and in two control human brains. We also analyzed the MMN in four macaque monkeys that underwent ex- perimental unilateral transections of the fornix. The volume of the MMN on the lesioned side in KB was 55% of the unlesioned side (2.8 mm 3 vs 5.1 mm 3 ); the MMN in the monkey cases were reduced to 47–58% of the volume of the nonlesioned side. Neurons in the deafferented MMN of KB and of the monkey subjects were decreased in cross-sectional area (16 –20%, P < 0.0001). There was a trend toward decreased cell numbers (11–15%) on the lesioned side in all cases. We have estimated that the loss in cell number and shrinkage of remaining cells contribute negligibly to the 45% reduction in MMN volume. Therefore, the loss of neuropil (dendrites and afferent and efferent axons) appears to be the major contributor to the change in MMN volume.

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Prefrontal cortex, time and consciousness

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Authors:

  • Robert T. Knight

  • Marcia Grabowecky

Date: 2000

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Abstract:

A central feature of consciousness is the ability to control the fourth dimension, time. Humans can effortlessly move their internal mental set from the present moment to a past remembrance and just as easily project themselves into a future event. It is proposed that this capacity to extract oneself from the present and fluidly move forward or backward in time is dependent on the evolution of the human prefrontal cortex. Prefrontal cortex modulates activity in multi-modal association and limbic cortices through widely distributed inhibitory and excitatory pathways. Prefrontal cortex also has a selective bias to novelty, crucial for detecting change and hence for the correct temporal coding of events. These extensive modulatory pathways coupled with an intrinsic link to temporal coding provide a mechanism for rapid engagement of distributed neural networks critical for seamless transitions through the time continuum. Support for this hypothesis is found in the fact that prefrontal damage results in a failure in the ability to extract oneself from the present. Indeed, the hallmark of the severe prefrontal syndrome is persevatory and stimulus-bound behavior--a classic example of a failure in temporal control.

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Neural Origins of the P300

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Authors:

  • Maryam Soltani

  • Robert T. Knight

Date: 2000

PubMed: 12645958

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Abstract:

A review of the literature investigating the neural origins of detection behavior in humans reveals two event-related potential components, P3a and P3b, each with a distinct neural organization and cognitive func- tion. The P3a is involved in automatic novelty detection and characterized by a more anterior cortical distribution, whereas the P3b is concerned with volitional target detection and has a more posterior cortical distribution. Intra- cranial investigation, studies with patients with focal brain lesions, and functional neuroimaging (fMRI) studies converge with the scalp-recorded ERP data in suggesting that a widespread cortical network gives rise to both automatic and controlled detection behavior. The main regions consistently attributed to generating detection related brain activation include the temporal-parietal junction, medial temporal complex, and the lateral prefrontal cortex. The extant human and animal literature addressing the neural networks, neuropharmacological underpinnings, and behavioral signicance of “the P300” potential will be reviewed.