This project can be divided in three parts: a first part in which we tried to develop a new type of rehabilitation for patients with hemianopia following post-chiasmatic lesions, a second part in which our goal was to investigate (experiment 1, 2 and 3) which hemisphere is responsible for vision in recovered areas (as a result of treatment) or in intact islands of the blind hemifield and a third part in which we analysed with a new approach the reliability of the unconscious responses to stimuli presented to the hemianopic hemifield (experiment 4 and 5). We developed a rehabilitation protocol that by means of a computerized visual training with moving stimuli can stimulate the residual visual abilities in the transitional area located between the blind and the spared area of the visual field while at the same time stimulating also the periphery of visual field. The aim is to activate the superior colliculus (SC) and the middle temporal area. These structures that are thought to be involved in implicit vision when primary visual cortex is lesioned, are both sensitive to moving stimuli. Moreover the SCs seem to process preferentially peripheral areas of the visual field. This approach is supposed to help reactivate the pre-existing brain connectivity responsible for conscious vision, or to induce the formation of new circuits that might yield unconscious vision (blindsight). The main clinical goal is to induce an enlargement of the visual field, with a consequent substantial improvement of patients’ quality of life. Functional magnetic resonance imaging (fMRI), visual field perimetry, and visual function questionnaire (VFQ25) have been used to evaluate the effect of treatment. Six hemianopic patients were included in the rehabilitation protocol. The treatment had positive effects on most of the patients. Three patients improved, at least slightly, in all measures. The literature on blindsight and on recovery of vision in the hemianopic field, focalizes on identifying the neural sites involved in these processes, but the role that the intact hemisphere could play, has not been often discussed. The focus of the second part is to investigate whether blindsight (experiment 1), or recovered (experiment 2), or residual vision (experiment 3) could be subserved by the damaged or the intact hemisphere. To establish which hemisphere mediates vision in the blind field, we used a manual visuo-motor reaction time (RT) paradigm which enables to assess interhemispheric transfer (IT), the so called Poffenberger Paradigm (PP). Healthy participants, generally, show a speed advantage for uncrossed with respect to crossed hemifield-hand combinations, because in the former no IT is required and a positive crossed-uncrossed difference (CUD) is found. In the affected hemifield, our prediction was to find a positive CUD if the damaged hemisphere was mediating the response and a paradoxical negative CUD (with the crossed condition faster than the uncrossed), if the intact hemisphere was mediating the response. To make sure that the unconscious responses obtained in the blind field (experiment 1) could not have been given by chance we compared the RTs distributions obtained in three conditions: stimuli presented to a) the blind field, b) the intact field and c) to the blind field with the eyes occluded (experiment 4). The rationale of this procedure is that unseen stimuli presented to the blind hemifield might trigger unconsciously a motor response in an automatic fashion and this would be a bias-free evidence of blindsight. Furthermore, we compared the performance of a patient showing blindsight in the blind and the intact field while manipulating eccentricity (experiment 5). The presence of an eccentricity effect in the blind field, would be a sign of implicit vision processed by a structure with a retinotopic organization. In sum, we found clear evidence of blindsight in one patient with a unilateral optic tract lesion. The RT distribution obtained in the blind hemifield showed a non-random peak similar to that found for the intact hemifield. Moreover, this patient showed an eccentricity effect not only in the intact but also in the hemianopic hemifield. Finally, we found in this patient a paradoxically negative CUD strongly suggesting that blindsight was mediated by the intact hemisphere through a mechanism that requires further investigation.
Study and rehabilitation of homonymous visual field defects
BARABAS, Marissa
2011
Abstract
This project can be divided in three parts: a first part in which we tried to develop a new type of rehabilitation for patients with hemianopia following post-chiasmatic lesions, a second part in which our goal was to investigate (experiment 1, 2 and 3) which hemisphere is responsible for vision in recovered areas (as a result of treatment) or in intact islands of the blind hemifield and a third part in which we analysed with a new approach the reliability of the unconscious responses to stimuli presented to the hemianopic hemifield (experiment 4 and 5). We developed a rehabilitation protocol that by means of a computerized visual training with moving stimuli can stimulate the residual visual abilities in the transitional area located between the blind and the spared area of the visual field while at the same time stimulating also the periphery of visual field. The aim is to activate the superior colliculus (SC) and the middle temporal area. These structures that are thought to be involved in implicit vision when primary visual cortex is lesioned, are both sensitive to moving stimuli. Moreover the SCs seem to process preferentially peripheral areas of the visual field. This approach is supposed to help reactivate the pre-existing brain connectivity responsible for conscious vision, or to induce the formation of new circuits that might yield unconscious vision (blindsight). The main clinical goal is to induce an enlargement of the visual field, with a consequent substantial improvement of patients’ quality of life. Functional magnetic resonance imaging (fMRI), visual field perimetry, and visual function questionnaire (VFQ25) have been used to evaluate the effect of treatment. Six hemianopic patients were included in the rehabilitation protocol. The treatment had positive effects on most of the patients. Three patients improved, at least slightly, in all measures. The literature on blindsight and on recovery of vision in the hemianopic field, focalizes on identifying the neural sites involved in these processes, but the role that the intact hemisphere could play, has not been often discussed. The focus of the second part is to investigate whether blindsight (experiment 1), or recovered (experiment 2), or residual vision (experiment 3) could be subserved by the damaged or the intact hemisphere. To establish which hemisphere mediates vision in the blind field, we used a manual visuo-motor reaction time (RT) paradigm which enables to assess interhemispheric transfer (IT), the so called Poffenberger Paradigm (PP). Healthy participants, generally, show a speed advantage for uncrossed with respect to crossed hemifield-hand combinations, because in the former no IT is required and a positive crossed-uncrossed difference (CUD) is found. In the affected hemifield, our prediction was to find a positive CUD if the damaged hemisphere was mediating the response and a paradoxical negative CUD (with the crossed condition faster than the uncrossed), if the intact hemisphere was mediating the response. To make sure that the unconscious responses obtained in the blind field (experiment 1) could not have been given by chance we compared the RTs distributions obtained in three conditions: stimuli presented to a) the blind field, b) the intact field and c) to the blind field with the eyes occluded (experiment 4). The rationale of this procedure is that unseen stimuli presented to the blind hemifield might trigger unconsciously a motor response in an automatic fashion and this would be a bias-free evidence of blindsight. Furthermore, we compared the performance of a patient showing blindsight in the blind and the intact field while manipulating eccentricity (experiment 5). The presence of an eccentricity effect in the blind field, would be a sign of implicit vision processed by a structure with a retinotopic organization. In sum, we found clear evidence of blindsight in one patient with a unilateral optic tract lesion. The RT distribution obtained in the blind hemifield showed a non-random peak similar to that found for the intact hemifield. Moreover, this patient showed an eccentricity effect not only in the intact but also in the hemianopic hemifield. Finally, we found in this patient a paradoxically negative CUD strongly suggesting that blindsight was mediated by the intact hemisphere through a mechanism that requires further investigation.File | Dimensione | Formato | |
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https://hdl.handle.net/20.500.14242/182502
URN:NBN:IT:UNIVR-182502