Effect of aging on visual shape distortion

Neuroplasticity pathophysiological mechanisms underlying neuro-optometric rehabilitation in ischemic stroke – a brief review

Neuroplasticity is an essential phenomenon underlying on neurorehabilitation process, by which the brain can remodel the dysfunction consequent to a lesion. Ischemic brain lesions are the most frequent brain lesions often associated with visual function disability. Experimental and clinical studies established that visual function disability can impede the neurorehabilitation therapy efficiency. Neuro-optometric therapy has been proved to significantly improve the patient outcome after brain lesions. The pathophysiological mechanisms underlying this process are yet to be deciphered. Current knowledge regarding the pathophysiological mechanisms involved in ischemic lesions and neuroplasticity as a reparation process offers real support to a more efficient neurorehabilitation therapy that can contribute to the improvement of life quality in stroke patients. Keywords: neuroplasticity, neuro-optometric rehabilitation, ischemic stroke,

Plasticity in the human visual cortex: an ophthalmology-based perspective

Neuroplasticity refers to the ability of the brain to reorganize the function and structure of its connections in response to changes in the environment. Adult human visual cortex shows several manifestations of plasticity, such as perceptual learning and adaptation, working under the top-down influence of attention. Plasticity results from the interplay of several mechanisms, including the GABAergic system, epigenetic factors, mitochondrial activity, and structural remodeling of synaptic connectivity. There is also a downside of plasticity, that is, maladaptive plasticity, in which there are behavioral losses resulting from plasticity changes in the human brain. Understanding plasticity mechanisms could have major implications in the diagnosis and treatment of ocular diseases, such as retinal disorders, cataract and refractive surgery, amblyopia, and in the evaluation of surgical materials and techniques. Furthermore, eliciting plasticity could open new perspectives in the development of strategies that trigger plasticity for better medical and surgical outcomes.

Adaptation and visual coding

Visual coding is a highly dynamic process and continuously adapting to the current viewing context. The perceptual changes that result from adaptation to recently viewed stimuli remain a powerful and popular tool for analyzing sensory mechanisms and plasticity. Over the last decade, the footprints of this adaptation have been tracked to both higher and lower levels of the visual pathway and over a wider range of timescales, revealing that visual processing is much more adaptable than previously thought. This work has also revealed that the pattern of aftereffects is similar across many stimulus dimensions, pointing to common coding principles in which adaptation plays a central role. However, why visual coding adapts has yet to be fully answered.

The effect of aging on contour integration

Perception of global patterns requires the integration of local orientation information across space. The present study examined whether this integration ability declines in older age. We measured contrast thresholds for discriminating the global orientation of a "C" shaped contour against a blank background in younger and older subjects. Performance of younger subjects improved when the elements composing the "C" were aligned along the contour path, as opposed to being orthogonal to it or of mixed orientations. However, older subjects' performance was not affected by the orientation of the local elements. This indicates an age-related decline in contour integration.

Aging and blur adaptation

Color appearance remains remarkably stable in the aging visual system despite large changes in the spectral distribution of the retinal stimulus and losses in chromatic sensitivity (P. B. Delahunt, J. L. Hardy, K. Okajima, & J. S. Werner, 2005; J. S. Werner, 1996). This stability could reflect adaptive adjustments in peripheral or central chromatic mechanisms that compensate for sensitivity losses in senescence. We asked whether similar compensatory adjustments play a role in maintaining spatial vision--and whether the adaptation itself shows changes with aging-by examining the effects of adaptation on judgments of image focus. Perceptual aftereffects following adaptation to a uniform field and blurred or sharpened images were compared between younger adults and older observers. Subjects adapted to a sequence of blurred or sharpened images for 120 s, and a two-alternative forced-choice staircase task was used to vary the filter exponent of the test to define the subjective point of best focus. There was a small but significant difference between younger and older observers in the level perceived as best focused in all three adaptation conditions, possibly reflecting differences in the ambient blur level the groups are routinely exposed to. However, the magnitude of the blur aftereffect did not differ between the two age groups. These results suggest that although there may be small differences in the long-term adaptation to blur, younger and older observers do not differ in the strength of adaptation to transient changes in blur. The neural processes mediating adaptation to blur thus appear to remain largely intact with aging.