Academic literature on the topic 'Artificial Scotoma'

Conditioning human observers with an “artificial scotoma”—a small retinal area deprived of patterned stimulation within a larger area of dynamically textured noise—results in contractions and expansions of perceived space that are thought to reflect receptive-field changes among cells in the primary visual cortex (Kapadia et al., 1994). Here we show that one-dimensional counter-phase flickering grating patterns are also potent stimuli for producing artificial scotomata capable of altering three-element bisection ability analogous to those results reported earlier. Moreover, we found that the magnitude of the induced spatial distortions depends critically on the relative orientations of peri-scotomatous and test-stimulus spatial contrast. In addition, the perceptual distortions are found to be relatively short lived, decaying within 660 ms. The results support the hypothesis that artificial scotoma-induced perceptual distortions are generated by dynamic alteration of connection efficacy within a network linking cortical areas of similar orientation specificity, consistent with established anatomical and physiological results.

The position, size, and shape of the receptive field (RF) of some cortical neurons change dynamically, in response to artificial scotoma conditioning (Pettet & Gilbert, 1992) and to retinal lesions (Chino et al., 1992; Darian-Smith & Gilbert, 1995) in adult animals. The RF dynamics are of interest because they show how visual systems may adaptively overcome damage (from lesions, scotomas, or other failures), may enhance processing efficiency by altering RF coverage in response to visual demand, and may perform perceptual learning. This paper presents an afferent excitatory synaptic plasticity rule and a lateral inhibitory synaptic plasticity rule—the EXIN rules (Marshall, 1995)—to model persistent RF changes after artificial scotoma conditioning and retinal lesions. The EXIN model is compared to the LISSOM model (Sirosh et al., 1996) and to a neuronal adaptation model (Xing & Gerstein, 1994). The rules within each model are isolated and are analyzed independently, to elucidate their roles in adult cortical RF dynamics. Based on computer simulations, the EXIN lateral inhibitory synaptic plasticity rule and the LISSOM lateral excitatory synaptic plasticity rule produced the best fit with current neurophysiological data on visual cortical plasticity in adult animals (Chino et al., 1992; Pettet & Gilbert, 1992; Darian-Smith & Gilbert, 1995) including (1) the retinal position and shape of the expanding RFs; (2) the corticotopic direction in which responsiveness returns to the silenced cortex; (3) the direction of RF shifts; (4) the amount of change in response to blank stimuli; and (5) the lack of dynamic RF changes during conditioning with a retinal lesion in one eye and the unlesioned eye kept open, in adult animals. The effects of the LISSOM lateral inhibitory synaptic plasticity rule during artificial scotoma conditioning are in conflict with those of the other two LISSOM synaptic plasticity rules. A novel “complementary scotoma” conditioning experiment, in which stimulation of two complementary regions of visual space alternates repeatedly, is proposed to differentiate the predictions of the EXIN and LISSOM rules.

We investigated artificial scotomas created when a moving object instantaneously crossed a gap, jumping ahead and continuing its otherwise smooth motion. Gaps of up to 5.1 degrees of visual angle, presented at 18° eccentricity, either closed completely or appeared much shorter than when the same gap was crossed by two-point apparent motion, or crossed more slowly, mimicking occlusion. Prolonged exposure to motion trajectories with a gap in most cases led to further shrinking of the gap. The same gap-shrinking effect has previously been observed in touch. In both sensory modalities, it implicates facilitation among codirectional local motion detectors and motion neurons with receptive fields larger than the gap. Unlike stimuli that simply deprive a receptor surface of input, suggesting it is insentient, our motion pattern skips a section in a manner that suggests a portion of the receptor surface has been excised, and the remaining portions stitched back together. This makes it a potentially useful tool in the experimental study of plasticity in sensory maps.

L'étude de la lecture s'intéresse, entre autres, aux facteurs déterminant la visibilité de lettres et les mouvements oculaires. Néanmoins, l'approche de ces mécanismes reste différente selon qu'elle concerne la vision centrale ou périphérique. Cette thèse proposait, au contraire, une approche conjointe dans le but (1) de mettre en évidence les effets des facteurs visuels limitant la reconnaissance de mots, (2) de mieux comprendre le comportement oculomoteur lors de la lecture en vision centrale et (3) d'étudier le rôle des facteurs oculomoteurs spécifiques à la lecture en vision périphérique. Trois expériences ont été menées. La première axée sur l'étude des facteurs visuels, a mis en évidence que l'effet OVP était moins important pour des mots présentés dans le champ visuel inférieur. Ce résultat pouvant s'expliquer par la géométrie de l'acuité visuelle et de l'encombrement, suggère que la lecture en périphérie est limitée par les mêmes facteurs qu'en vision centrale. La deuxième étude, concernant la stabilité de fixation durant la lecture en présence d'un scotome artificiel, a révélé que la vitesse de lecture pouvait être améliorée en présence d'une stimulation fovéale stable. Enfin, nous avons montré que les mouvements oculaires variaient selon la taille des lettres, et ce, différemment selon la position du regard par rapport au centre des mots. Ce résultat, nouveau, est vraisemblablement attribuable à l'effet de processus visuomoteurs précoces. En conclusion, l'étude conjointe de la lecture en vision centrale et périphérique est prometteuse, non seulement d'un point de vue fondamental, mais aussi pour l'aide aux patients atteints de DMLA<br>Reading is a large research field, which investigates for a great part the variables influencing letter visibility and eye movements. However, its study in central and peripheral vision has been conducted in parallel and with different approaches. Our work relied, on the contrary, on a joint approach in order (1) to uncover the visual factors limiting word recognition, (2) to better determine how the eyes move during reading in central vision, and (3) to study the role of oculomotor factors that might affect reading in peripheral vision. Three experiments were conducted. The first study focused on the role of visual factors in word identification. It showed that the OVP effect is reduced for words displayed in the lower visual field. This result, which can be explained by the geometry of visual acuity andl crowding, suggests that reading in peripheral vision is limited by the same visual factors as in central vision. Then we investigated the influence of fixation stability on sentence reading using an artificial scotoma. It revealed that reading speed could be improved with a stable foveal stimulation. The third experiment re-examined the possible influence of character size on eye movements in central vision. It revealed that eye movements varied depending on letter size, and differently as a function of the location of the eye relative to the center of words. This novel finding was very likely the effect of early visuomotor processes. In conclusion, the joint study of reading in central and peripheral vision is promising, not only at a fundamental level, but also for a better understanding of retinal visual deficits, such as AMD

Thesis (M. S.)--Psychology, Georgia Institute of Technology, 2009.<br>Committee Chair: Dr. Paul Corballis, Ph.D.; Committee Member: Dr. Daniel Spieler, Ph.D.; Committee Member: Dr. Eric Schumacher, Ph.D.; Committee Member: Dr. Krish Sathian, M.D., Ph.D.; Committee Member: Dr. Randall Engle, Ph.D.

Trabalho de Projeto do Mestrado Integrado em Engenharia Biomédica apresentado à Faculdade de Ciências e Tecnologia<br>Plasticidade neuronal é um tópico muito contemporâneo mas ao mesmo tempo bastante controverso. O conceito de plasticidade reflete mudanças na estrutura e função por adaptação ao ambiente onde se está inserido ao longo da vida. O sistema visual é um dos modelos mais robustos para estudar fenómenos de plasticidade devido ao vasto conhecimento sobre a sua estrutura e propriedades. De facto, estudos em humanos investigando as alterações causadas pela falta de informação vinda da visão periférica escasseiam na literatura. Neste trabalho questionamos se populações de neurónios visuais se reorganizam em resposta a alterações percetuais numa doença em que a perda visual é geneticamente pré-determinada resultando em escotomas (regiões cegas) periféricos em adultos. A Doença Pigmentar (RP) é uma doença genética rara que leva à degeneração dos fotorrecetores e consequentemente à perda de campo visual periférico, afetando progressivamente a visão central também. Foram adquiridos dados anatómicos e funcionais de doze doentes em diferentes estágios da doença e vinte-e-cinco indivíduos controlo saudáveis de modo a investigar as propriedades de uma população de campos recetivos (pRF) de neurónios visuais, cujas alterações podem sugerir reorganização em resposta à ausência de informação sensorial. Aplicou-se uma técnica recente denominada mapeamento de population receptive fields (pRFs), uma alternativa mais sofisticada à Retinotopia tradicional, de modo a analisar o tamanho dos campos recetores para cada hemisfério de cada participante. Em geral, e como esperado, os tamanhos médios dos pRFs aumentaram dentro das áreas visuais e ao longo de 11 graus de excentricidade. No entanto, o padrão de variação foi diferente do dos controlos. Além disso, a análise dos declives das retas de ajuste para cada uma das áreas visuais e entre os grupos RP e Controlo revelou uma interação significativa. Na realidade, ao contrário dos controlos, o grau de mudança dos tamanhos médios dos pRFs ao longo da excentricidade é muito similar nas diferentes áreas visuais de RP, o que sugere uma perda de convergência de informação de V1 para V2, e mais tarde para V3. Em adição, o tamanho médio dos pRFs em V1 na zona de representações periféricas foi significativamente maior nos pacientes com RP, o que indica uma reorganização funcional para compensar a falta de informação recebida por parte da periferia. De forma a clarificar se este remapeamento tem como origem uma adaptação a curto-prazo ou se é devido a fenómenos de plasticidade a longo-prazo, foram adquiridos dados de controlos estimulados com escotomas artificiais (AS) que simulam o campo visual de cada um dos pacientes com RP. Foi verificada uma baixa variância explicada e uma diminuição do tamanho dos pRFs no grupo AS, ao contrário dos doentes, confirmando a existência de plasticidade a longo-prazo em vez de mecanismos de adaptação rápida. Resumindo, descobrimos evidências de mecanismos de reorganização a longo-prazo na população de campos recetores de neurónios em resposta à degeneração do campo visual periférico em pacientes adultos com RP. Deste modo, estes acontecimentos devem ser levados em consideração devido às suas implicações no desenvolvimento de estratégicas de terapêutica e reabilitação nestes pacientes.<br>The topic of neural plasticity is very contemporary but is still very controversial. Plasticity reflects changes on the structure and function following environmental demands throughout life. The visual system is one of the finest models to study plasticity phenomena due to the extensive knowledge of its structure and properties. Indeed, there is a lack of human studies investigating the alterations caused by a loss of input from peripheral vision. In this work we questioned if populations of visual neurons do reorganize in response to a genetically-determined perceptual alteration due to peripheral loss of vision (scotomas) in adults. Retinitis Pigmentosa (RP) is a rare disease that leads to the degeneration of photoreceptors causing a loss of the peripheral visual field that progresses towards the center.We acquired anatomical and functional data from twelve patients in different stages of the disease and twenty-five healthy controls in order to study population receptive field (pRF) properties of visual neurons, whose alterations can be indicative of reorganization in response to the loss of sensory input. We used a recent technique called population receptive field (pRF) mapping, an optimized alternative to traditional Retinotopy, to estimate the sizes of population receptive fields for each hemisphere of each participant. In general, mean pRF sizes showed the expected increase within the visual areas along 11 degrees of eccentricity. However, the pattern of the variation was different from controls. Moreover, the analysis of slopes of the fitting lines for each visual area and between RP and Control groups revealed a significant interaction. In fact, in contrast to controls, the degree of change of mean pRF sizes along eccentricity was similar across visual areas in RP, suggesting a loss of input convergence from V1 to V2, and later to V3. Furthermore, the mean pRF size of V1 in the peripheral representations was significantly higher in RP patients, which indicates a functional reorganization to compensate the lack of peripheral visual input. In order to understand if this remapping is a short-term adaptation or along-term plasticity phenomena, we acquired data from controls stimulated with artificial scotomas (AS) simulating the visual field of each RP patient. We found a lower explained variance and lower pRF sizes in the AS group, as compared to RP, confirming that there is long-term plasticity rather than rapid adaptation mechanisms in the latter.To sum up, we found evidence for long-term reorganization mechanisms of neural pRFs in response to peripheral visual field degeneration in adult RP patients. Therefore, these plasticity phenomena should be taken in consideration due to their strong implications for therapeutic and rehabilitation strategies in these patients.<br>FCT<br>FCT<br>FCT

La dégénérescence maculaire liée à l'âge (DMLA) touche actuellement un million de Canadiens, ce qui en fait la principale cause de perte de vision au Canada. La DMLA cause l’apparition d’une tache aveugle au niveau de la macula – la zone centrale de la vision. Cette tache aveugle affecte la vision centrale rendant les perceptions visuelles floues ou déformées. L’impact de cette condition est considérable puisqu’elle rend impossible la conduite, difficile la lecture et entraîne ultimement une cécité totale. La DMLA a également été identifiée comme facteur de risque de la dépression et de l’isolement sociale, ce qui porte d’autant plus atteinte à la qualité de vie des patients. Une des avenues de réadaptation pour les patients est d’améliorer leur utilisation de leur fonction visuelle résiduelle, notamment la vision périphérique. Le but de la présente recherche est d’investiguer le potentiel réadaptatif d’un entraînement imposant l’utilisation de la vision périphérique chez des participants en bonne santé en présence d’une perte de vision centrale simulée. Une compréhension des changements dans les comportements des mouvements oculaires en présence d'un scotome artificiel central aidera à développer des programmes de réadaptation pour les personnes atteint de DMLA et plus largement les personnes présentant une perte visuelle centrale. Les résultats démontrent une adaptation des stratégies visuelles des participants, entraînant une augmentation de la performance, une réduction des temps de réponse et une meilleure discrimination, suggérant ainsi que l’entraînement est susceptible de contribuer à la réadaptation des personnes atteintes de pertes visuelles centrales.<br>Age Macular Degeneration (AMD) currently affects one million Canadians, making it the leading cause of vision loss in Canada. AMD causes the appearance of a blind spot on the macula – the central area of vision. This blind spot affects the central vision making visual perceptions blurry or distorted. The impact of this condition is considerable since it impedes driving as well as reading and ultimately leads to total blindness. AMD has also been shown to be a risk factor for depression and social isolation, further compromising quality of life for patients. One of the avenues of rehabilitation for patients is to improve their use of their residual visual function, in particular their peripheral vision. The purpose of the present research is to investigate the rehabilitative potential of training requiring the use of peripheral vision in healthy participants in the presence of a simulated central vision loss. An understanding of changes in eye movement patterns in the presence of a central artificial scotoma will help develop rehabilitation protocols for people with AMD and more broadly people with central visual loss. Results demonstrate an adaptation of visual strategies among participants, resulting in increased performance, reduced response times and better discrimination, suggesting that training is likely to contribute to the rehabilitation of people with central vision loss.