The physiological effects of monocular deprivation and their reversal in the monkey's visual cortex.
(9/1471)
1. 1127 single units were recorded during oblique penetrations in area 17 of one normal, three monocularly deprived and four reverse sutured monkeys. 2. In all animals most cells outside layer IV c were orientation-selective, and preferred orientation usually shifted from cell to cell in a regular progressive sequence. 3. The presence in layer IV c of non-oriented, monocularly driven units, organized in alternating right-eye and left-eye 'stripes' (LeVay, Hubel & Wiesel, 1975) was confirmed. 4. Early monocular deprivation (2--5 1/2 weeks) caused a strong shift of ocular dominance towards the non-deprived eye. However, even outside layer IV c, neural background and some isolated cells could still be driven from the deprived eye in regularly spaced, narrow columnar regions. In layer IV c the non-deprived eye's stripes were almost three times wider, on average, than the deprived. 5. Later monocular deprivation (11--16 months) had no detectable influence on layer IV c but seemed to cause a small shift in ocular dominance outside IV c. Deprivation for 6 1/4 months in an adult had no such effect. 6. After early reverse suturing (at 5 1/2 weeks) the originally deprived eye gained dominance over cells outside layer IV c just as complete as that originally exercised by the eye that was first non-deprived. 7. The later reverse suturing was delayed, the less effective was recapture by the originally deprived eye. Reversal at 8 weeks led to roughly equal numbers of cells being dominated by each eye; fewer cells became dominated by the newly open eye after reverse suturing at 9 weeks and most of them were non-oriented; reversal at 38 1/2 weeks had no effect. 8. Binocular cells, though rare in reverse sutured animals, always had very similar preferred orientations in the two eyes. The columnar sequences of preferred orientation were not interrupted at the borders of ocular dominance columns. 9. Even within layer IV c there was evidence for re-expansion of physiologically determined ocular dominance stripes. After early reverse suture, stripes for the two eyes became roughly equal in width. Possible mechanisms for these changes are discussed. (+info)
Reasons for variability in the reported rate of occurrence of unilateral spatial neglect after stroke.
(10/1471)
BACKGROUND AND PURPOSE: We sought to determine the frequency of occurrence of contralesional unilateral spatial neglect (USN) after stroke and to investigate the effect of side of lesion, nature of assessment tool used, and timing of assessment relative to stroke onset. METHODS: We performed a systematic review of published reports, identified by a search of electronic databases (MEDLINE 1966-1997, PSYCHLIT 1974-1996, and CINAHL 1982-1997) and by searching reference lists of the reports selected. Excluded were unpublished, non-English language, and nonhuman studies. RESULTS: Thirty published reports met the selection criteria, 17 of which directly compared right brain damage (RBD) and left brain damage (LBD). Contralesional USN appeared to occur more frequently after RBD than LBD in 16 of these. Both the assessment tool used and the time of assessment relative to stroke onset affected the reported rate of occurrence, although recovery rate data were inadequate (4 reports). CONCLUSIONS: The clinical belief that USN occurs more frequently after RBD than LBD was apparently supported by a systematic review of published data. However, an accurate estimate of the rates of occurrence and recovery after stroke could not be derived. Four reasons for the variability among studies were discussed, including subject selection, lesion localization, and nature and timing of assessment. Different USN disorders may exist, which may require type-specific rehabilitation approaches. This may have implications for epidemiological studies and for the development of new treatments. Theoretically driven epidemiological studies are required before adequately powered randomized controlled trials of rehabilitation can be conducted. (+info)
Semantic integration in reading: engagement of the right hemisphere during discourse processing.
(11/1471)
We examined the brain areas involved in discourse processing by using functional MRI in 10 individuals as they read paragraphs, with or without a title, word by word for comprehension. Functional data were collected from 20 adjacent 5 mm axial slices. Discourse processing was associated with activation in inferior frontal and temporal regions of both cerebral hemispheres in the titled and untitled conditions. Moreover, there was substantially more right hemisphere activation for untitled than for the titled paragraphs. More specifically we found: (i) greater activation in the inferior temporal sulcus of both hemispheres for untitled than titled paragraphs; (ii) greater average volume of activation in response to untitled than titled paragraphs in the middle temporal sulcus of the right hemisphere and the reverse pattern in the left middle temporal sulcus. Consistent with previous studies of individuals with right hemisphere damage, we suggest that the right middle temporal regions may be especially important for integrative processes needed to achieve global coherence during discourse processing. (+info)
Right prefrontal cortex and episodic memory retrieval: a functional MRI test of the monitoring hypothesis.
(12/1471)
Though the right prefrontal cortex is often activated in neuroimaging studies of episodic memory retrieval, the functional significance of this activation remains unresolved. In this functional MRI study of 12 healthy volunteers, we tested the hypothesis that one role of the right prefrontal cortex is to monitor the information retrieved from episodic memory in order to make an appropriate response. The critical comparison was between two word recognition tasks that differed only in whether correct responses did or did not require reference to the spatiotemporal context of words presented during a previous study episode. Activation in a dorsal midlateral region of the right prefrontal cortex was associated with increased contextual monitoring demands, whereas a more ventral region of the right prefrontal cortex showed retrieval-related activation that was independent of task instructions. This functional dissociation of dorsal and ventral right prefrontal regions is discussed in relation to a theoretical framework for the control of episodic memory retrieval. (+info)
Right hemispheric dominance of inhibitory control: an event-related functional MRI study.
(13/1471)
Normal human behavior and cognition are reliant on a person's ability to inhibit inappropriate thoughts, impulses, and actions. The temporal and spatial advantages of event-related functional MRI (fMRI) were exploited to identify cortical regions that showed a transient change in fMRI signal after the withholding of a prepotent motor response. The temporal specificity of the event-related fMRI design also minimized possible contamination from response inhibition errors (i. e., commission errors) and other extraneous processes. Regions identified were strongly lateralized to the right hemisphere and included the middle and inferior frontal gyri, frontal limbic area, anterior insula, and inferior parietal lobe. Contrary to the prominence traditionally given to ventral frontal regions for response inhibition, the results suggest that response inhibition is accomplished by a distributed cortical network. (+info)
When the left brain is not right the right brain may be left: report of personal experience of occipital hemianopia.
(14/1471)
OBJECTIVES: To make a personal report of a hemianopia due to an occipital infarct, sustained by a professor of neurology. METHODS: Verbatim observation of neurological phenomena recorded during the acute illness. RESULTS: Hemianopia, visual hallucinations, and non-occipital deficits without extraoccipital lesions on MRI, are described and discussed. CONCLUSIONS: Hemianopia, due to an occipital infarct, without alexia, is not a disability which precludes a normal professional career. Neurorehabilitation has not been necessary. (+info)
The anterior thalamic head-direction signal is abolished by bilateral but not unilateral lesions of the lateral mammillary nucleus.
(15/1471)
Head-direction (HD) cells are neurons that signal a rat's directional heading in the horizontal plane. Evidence suggests that the lateral mammillary nucleus (LMN) may play an important role in generating the HD signal. Here, we examined the role of LMN in the HD circuit by comparing the effects of unilateral and bilateral LMN lesions on the activity of HD cells in the anterodorsal thalamus (AD). HD cells were recorded from AD in freely behaving rats. In the middle of the recording session, the rat received either bilateral or unilateral lesions of LMN. Immediately after the lesion, we continued recording the same HD cell in AD that had been recorded before the lesion. Additional cells were recorded from lesioned animals for up to 3 weeks after the lesion. We found that bilateral lesions of LMN permanently abolish HD cells in AD. After bilateral lesions, AD exhibits unusual rhythmic oscillations and velocity-correlated spike activity. Unilateral lesions of LMN did not abolish HD cells in AD. After unilateral lesions, the firing properties of HD cells in AD become more similar to those of HD cells in the intact hemisphere of LMN. We discuss the implications of these findings for understanding the role of LMN in the HD circuit. (+info)
Cognitive asymmetry patterns in schizophrenia: active and withdrawn syndromes and sex differences as moderators.
(16/1471)
Recognition memory for words and faces was examined in male and female schizophrenia patients for evidence of associations between putative left-right hemisphere asymmetry patterns and active (positive) versus withdrawn (negative) syndromes. Ninety-five normal controls and 104 schizophrenia patients with active, withdrawn, and mixed syndromes or in symptom remission were examined, including an unmedicated subgroup. Memory was poorer in patients than controls, while the remitted group had superior memory to psychotic patients. Active and withdrawn patients showed the hypothesized syndrome-dependent cognitive asymmetries: active (word > faces); withdrawn (faces > words), except active females who showed a word deficit. The results support selective lateralized temporoparietal impairment of either hemisphere in schizophrenia, with laterality related to active (face memory/right-sided impairment) and withdrawn (word memory/left-sided impairment) syndromes, except active syndrome females. These syndrome-related asymmetries moderated the sexually dimorphic asymmetries found in normal subjects. Consideration of individual differences both in sex and syndromes based on activity and withdrawal, and of left and right hemisphere memory modality, may assist in unraveling heterogeneity in schizophrenic cognition. The superior memory of recovered patients indicates that some memory impairment in schizophrenia is functional. (+info)