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      Visual activation of extra-striate cortex in the absence of V1 activation

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          Research highlights

          ▶ Gray matter of V1 shows abnormal T1 characteristics and its perfusion is reduced. ▶ Damage is confined to gray matter with no adjacent white matter involvement. ▶ BOLD activation levels in the calcarine sulcus are drastically reduced. ▶ Activation of extrastriate regions to visual stimulation is preserved. ▶ Pathway between LGN and V1 shows degeneration; between LGN and V5/MT is intact.

          Abstract

          When the primary visual cortex (V1) is damaged, there are a number of alternative pathways that can carry visual information from the eyes to extrastriate visual areas. Damage to the visual cortex from trauma or infarct is often unilateral, extensive and includes gray matter and white matter tracts, which can disrupt other routes to residual visual function. We report an unusual young patient, SBR, who has bilateral damage to the gray matter of V1, sparing the adjacent white matter and surrounding visual areas. Using functional magnetic resonance imaging (fMRI), we show that area MT+/V5 is activated bilaterally to visual stimulation, while no significant activity could be measured in V1. Additionally, the white matter tracts between the lateral geniculate nucleus (LGN) and V1 appear to show some degeneration, while the tracts between LGN and MT+/V5 do not differ from controls. Furthermore, the bilateral nature of the damage suggests that residual visual capacity does not result from strengthened interhemispheric connections. The very specific lesion in SBR suggests that the ipsilateral connection between LGN and MT+/V5 may be important for residual visual function in the presence of damage to V1.

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          Most cited references15

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          Blindsight depends on the lateral geniculate nucleus

          Injury to the primary visual cortex (V1) leads to the loss of visual experience. Nonetheless, careful testing shows that certain visually guided behaviors can persist even in the absence of visual awareness1–5. The neural circuits supporting this phenomenon, often termed blindsight, remain uncertain5. Here we demonstrate a causal role of the thalamic lateral geniculate nucleus (LGN) in V1-independent processing of visual information. By comparing fMRI and behavioral measures with and without temporary LGN inactivation, we assessed the contribution of the LGN to visual functions of macaque monkeys with chronic V1 lesions. Prior to LGN inactivation, high contrast stimuli presented to the lesion-affected visual field (scotoma) produced significant V1 independent fMRI activation in extrastriate cortical areas V2, V3, V4, V5/MT, FST, and LIP, and were correctly located by the animals in a detection task. However, following reversible inactivation of the LGN in the V1-lesioned hemisphere both fMRI responses and behavioral detection were abolished. Taken together, these results demonstrate a critical functional contribution of the direct LGN projections to extrastriate cortex in blindsight, and suggest a viable pathway mediating fast detection during normal vision.
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            Non-invasive assessment of axonal fiber connectivity in the human brain via diffusion tensor MRI.

            A technique for assessing in vivo fiber connectivity in the human brain is presented. The method utilizes a novel connectivity algorithm that operates in three spatial dimensions and uses estimates of fiber tract orientation and tissue anisotropy, obtained from diffusion tensor magnetic resonance imaging, to establish the pathways of fiber tracts. Sample in vivo connectivity images from healthy human brain are presented that demonstrate connections in the white matter tracts. White matter connectivity information is potentially of interest in the study of a range of neurological, psychiatric, and developmental disorders and shows promise for following the natural history of disease.
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              Retinotopy and functional subdivision of human areas MT and MST.

              We performed a series of functional magnetic resonance imaging experiments to divide the human MT+ complex into subregions that may be identified as homologs to a pair of macaque motion-responsive visual areas: the middle temporal area (MT) and the medial superior temporal area (MST). Using stimuli designed to tease apart differences in retinotopic organization and receptive field size, we established a double dissociation between two distinct MT+ subregions in 8 of the 10 hemispheres studied. The first subregion exhibited retinotopic organization but did not respond to peripheral ipsilateral stimulation, indicative of smaller receptive fields. Conversely, the second subregion within MT+ did not demonstrate retinotopic organization but did respond to peripheral stimuli in both the ipsilateral and contralateral visual hemifields, indicative of larger receptive fields. We tentatively identify these subregions as the human homologues of macaque MT and MST, respectively. Putative human MT and MST were typically located on the posterior/ventral and anterior/dorsal banks of a dorsal/posterior limb of the inferior temporal sulcus, similar to their relative positions in the macaque superior temporal sulcus.
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                Author and article information

                Journal
                Neuropsychologia
                Neuropsychologia
                Neuropsychologia
                Pergamon Press
                0028-3932
                1873-3514
                December 2010
                December 2010
                : 48
                : 14
                : 4148-4154
                Affiliations
                [a ]FMRIB Centre, University of Oxford, John Radcliffe Hospital, Oxford OX3 9DU, United Kingdom
                [b ]Department of Clinical Neurology, University of Oxford, John Radcliffe Hospital, Oxford OX3 9DU, United Kingdom
                [c ]Department of Experimental Psychology, University of Oxford, Parks Road, Oxford OX1 3UD, United Kingdom
                Author notes
                [* ]Corresponding author at: FMRIB Centre, University of Oxford, John Radcliffe Hospital, Oxford OX3 9DU, United Kingdom. holly.bridge@ 123456clneuro.ox.ac.uk
                Article
                NSY3857
                10.1016/j.neuropsychologia.2010.10.022
                2998000
                20974160
                f8d03108-e11f-4245-8df9-ba5c93aa918b
                © 2010 Elsevier Ltd.

                This document may be redistributed and reused, subject to certain conditions.

                History
                : 19 July 2010
                : 15 October 2010
                : 15 October 2010
                Categories
                Article

                Neurology
                lateral geniculate nucleus,blind,primary visual cortex,fmri,structural imaging
                Neurology
                lateral geniculate nucleus, blind, primary visual cortex, fmri, structural imaging

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