Abnormal Visual Input Research Articles

Spatial interactions in amblyopia: Effects of stimulus parameters and amblyopia type

Adults with amblyopia were recently shown to perform abnormally in tasks requiring integration of local features into global percepts. Moreover, spatial interactions in amblyopic patients, though often found to be abnormal, showed marked variability. Here we measured collinear lateral interactions using Gabor patches in a large number of amblyopic ( N = 75) and normal subjects ( N = 25), testing four spatial frequencies (1.5, 3, 6, 9 cpd). We used the lateral masking paradigm, in which the contrast-detection threshold is measured in the presence of high-contrast flankers at different distances from a central target. Whereas in normal subjects spatial interaction patterns were evident across all spatial frequencies, amblyopic subjects showed abnormal spatial interactions and increasing deficiencies with increasing spatial frequencies. These abnormalities depended on the axis of astigmatism (in meridional amblyopia) and were more pronounced in strabismic than in anisometropic amblyopia. Spatial interactions were independent on the contrast-detection thresholds. Thus, adults with amblyopia might perform as well as normal observers for some stimulus parameters and abnormally for others. Our results indicate a close relationship between abnormal visual input to the visual cortex during development and abnormal functionality of the collinear spatial interactions in adults with amblyopia.

The Development of Abnormal Axon Trajectories after Rotation of One Eye inXenopus

The targeting of isthmotectal axons in the Xenopus binocular pathway is guided by both activity-dependent cues and activity-independent cues. Abnormal visual activity induced by unilateral eye rotation overrides activity-independent cues and causes isthmotectal axons to arborize at new locations during a critical period of development that ends approximately 3 months postmetamorphosis (PM). Horseradish peroxidase staining of isthmotectal axons reveals that they normally run rostrocaudally in the tectum; in contrast, those axons in animals with early eye rotation have circuitous trajectories. In this paper, by studying the trajectories and branching patterns of isthmotectal axons at different times after eye rotation, we aimed to investigate when and how activity cues determine the projection pattern of isthmotectal axons. As suggested by electrophysiological recording, isthmotectal axons initially grow normally and make arbors according to activity-independent cues despite the presence of abnormal visual input. Our findings demonstrate that the development of abnormal trajectories starts by 2 weeks PM in response to eye rotation and is a protracted process. It begins in the tectal regions in which the initial connections of isthmotectal axons are first formed according to activity-independent cues. At transitional stages (5 and 10 weeks), axons with arbors at two different locations are observed, with locations corresponding to the old and new termination sites, respectively. Later, at 10 weeks of age, the fainter horseradish peroxidase staining in arbors at old termination sites suggests that the older arbors are undergoing withdrawal.

Abnormal visual input leads to development of abnormal axon trajectories in frogs.

Throughout the normal vertebrate brain, visual maps form the left and right eyes overlap and are in register with one another. Visual input has a major role in the development of the pathways which mediate these binocular projections. A dramatic example of the developmental role of sensory input occurs in the isthmo-tectal projection, which is part of the polysynaptic relay from the eye to the ipsilateral tectum of the frog, Xenopus laevis. If one eye is rotated when the animal is still a tadpole, the isthmic axons respond by changing the topography of their terminations in the tectum; for example, a given isthmo-tectal axon which normally would connect with medial tectum can be induced to terminate in lateral tectum. Such rearrangements bring the ipsilateral visual map into register with the contralateral retinotectal map, even though one eye has been rotated. Indirect evidence has suggested that after early eye rotation, isthmo-tectal axons do not grow directly to their new tectal targets but instead reach those targets by routes which pass through their normal termination zones. Here I have used anterograde horseradish peroxidase labelling of isthmo-tectal fibres to show the trajectories of such axons and to compare them with the routes which axons take when allowed to develop normally. Tracings of individual axons in flat-mounted, unsectioned tecta show that most axons in normal Xenopus follow fairly straight paths in the tectum. In contrast, early eye rotation causes many isthmo-tectal axons to follow crooked, circuitous pathways before they terminate.

A neural effect of partial visual deprivation in humans.

Certain human subjects have considerable sensitivity differences in the visual resolution of vertical gratings as compared to horizontal gratings. Although only subjects with pronounced ocular astigmatism exhibit this effect, the differences are of neural, rather than optical, origin. It is argued that the resolution anisotropies result from early abnormal visual input caused by astigmatism. This abnormal input permanently modifies the brain.