Brain : a journal of neurology
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Review Comparative Study
Potentials evoked in human and monkey cerebral cortex by stimulation of the median nerve. A review of scalp and intracranial recordings.
Somatosensory evoked potentials (SEPs) are generated in afferent pathways, subcortical structures and various regions of cerebellar and cerebral cortex by stimulation of somatic receptors or electrical stimulation of peripheral nerves. This review summarizes current knowledge of SEPs generated in cerebral cortex by stimulation of the median nerve, the most common form of stimulation for human research and clinical investigations. Major sources of data for the review are intracranial recordings obtained from patients during diagnostic or neurosurgical procedures, and similar recordings in monkeys. ⋯ Old World monkeys such as Macaca mulatta and M. fascicularis provide an excellent model for human short-latency SEPs. All the potentials described above in humans have corresponding monkey analogues, with similar distributions over the cortical surface. The squirrel monkey, a New World species, exhibits the same potentials, but due to the different morphology of sensorimotor cortex, the surface distribution of SEPs is quite different.
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Current knowledge of cognitive dysfunction in Parkinson's disease (PD) has largely been obtained from studies of chronically treated patients in whom effects of disease chronicity, treatment, depression and dementia are confounding factors. Studies of untreated patients have examined few cognitive domains and relationships between cognition, depression and motor disability have been incompletely explored. Accordingly, we studied 60 consecutive patients with newly diagnosed, untreated, idiopathic PD and 37 matched, healthy control subjects; no subject had clinical dementia or depression. ⋯ Some, but not all, of the frontal lobe deficits of chronic disease are detectable in early, untreated PD. The pathogenesis of the cognitive deficits shown here appears to involve extrastriatal dopamine systems or non-dopaminergic pathology. Longitudinal study is necessary to determine whether increasing disease duration exacerbates the early cognitive deficits and affects new cognitive domains, in addition to producing increasing motor disability.
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In recent years, there has been a growing consensus among investigators that the presence or absence of external cues guiding behaviour and attention is an important factor in determining whether or not deficits are found in patients with Parkinson's disease (PD). In an earlier study, the authors suggested that the pattern of impaired and intact performance could be explained in terms of differential resource demands of the tasks, combined with depleted levels of central processing resources in PD patients. Two experiments are reported, both employing dual-task paradigms. ⋯ The patients also showed an interfering effect with concurrent foot tapping but not with an articulatory suppression task. The results were taken to support the hypothesis that PD patients have depleted central processing resources. In considering the present data, alternative explanations for the results are considered, in particular the possibility that they represent a deficit in switching processing resources between two tasks as the combined demands outweigh available resources.
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In normal subjects, heterotopic painful stimuli induce simultaneous and parallel decreases in the sensation of pain and in the spinal nociceptive flexion (RIII) reflex evoked by electrical stimulation of the sural nerve. This inhibition of the RIII reflex is not seen in tetraplegic patients with clinically complete spinal cord transection, suggesting that supraspinal structures are involved in this type of inhibition, mediated through 'Diffuse Noxious Inhibitory Controls' (DNIC). In the present study, the effects of heterotopic nociceptive stimuli on the RIII reflex were examined in 3 patients with unilateral thalamic vascular lesions and in 3 with Wallenberg's syndrome (WS). ⋯ The same conditioning procedure applied to the nonanalgesic hand of the WS patients resulted in inhibition and after-effects similar to those observed in normal subjects. The fact that noxious but nonpainful stimuli triggered DNIC in the patients with thalamic lesions excludes the possibility that masking of pain by a second painful focus is mainly due to attentional processes. It is also concluded that lemniscal and spinothalamic pathways are not involved in the triggering of DNIC in man and it is suggested that the brainstem and probably the spinoreticular tract are key neuronal links in the loop subserving DNIC in man.
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Patients with unilateral spatial neglect fail to report or respond to stimuli contralateral to the lesion which usually involves the right parietal lobe. When asked to mark the centre of a horizontal line, these patients place the mark to the right of the true midpoint. It has been considered that they neglect the left part of the line and bisect the perceived line segment. ⋯ These findings suggest that the left hemisphere has the ability to estimate the midpoint of the line through the right visual field and that visuospatial disorder in the line bisection test is attributable to the pathological change in the right hemisphere. The results are interpreted to mean that left hemianopic patients with unilateral spatial neglect see a totalized image of a line extending equally to either side of the point where they are going to mark the subjective midpoint. We considered that the right hemisphere completes the line, using the visual input relating to the right part of the line perceived by the left hemisphere.