Unilateral papilledema caused by a fronto-temporo-parietal arachnoid cyst

PURPOSE: To report a case of unilateral papilledema caused by an arachnoid cyst. METHODS: Interventional case report. RESULTS: A 36-year-old woman presented with unilateral papilledema in the left eye. Magnetic resonance imaging (MRI) of the brain demonstrated a large fronto-temporo-parietal arachnoid cyst in the left hemisphere. The papilledema resolved after placement of a cysto-peritoneal shunt that drained the cerebrospinal fluid directly from the arachnoid cyst into the peritoneum. CONCLUSION: Papilledema caused by increased intracranial pressure is usually bilateral; unilateral papilledema is possible under special anatomical circumstances.     

Two-dot alignment across the physiological blind spot

Three competing hypotheses have been proposed for the cortical representation of the blind spot. These are: (i) the regions surrounding the blind spot maintain their spatial values; (ii) the opposite sides of the blind spot are represented adjacently at the cortex, so that the blind spot is “sewn-up”; and (iii) the blind spot is sewn-up with compensation occurring in the immediate surround of the blind spot, so that spatial values are distorted only in the immediate surround of the blind spot. To distinguish between these hypotheses we used a two-dot alignment task, with the two dots straddling the blind spot at varying dot separations. Thresholds in the two-dot alignment task are limited by the cortical separation of the two dots. When thresholds for alignment across the blind spot are compared with thresholds over intact retina at the same eccentricity, the three hypotheses predict: (i) no change in thresholds; (ii) a lowering of thresholds; and (iii) a lowering of thresholds but only at separations slightly greater than the diameter of the blind spot. Thresoolds across the blind spot were closely similar to thresholds across intact retina. The results do not support a sewing-up (with or without compensation) of the blind spot. Rather, our results are consistent with a preservation of spatial values around the blind spot.     

Perceptual filling-in from the edge of the blind spot

Looking at the world with one eye, we do not notice a scotoma in the receptor-free area of the visual field where the optic nerve leaves the eye. Rather we perceive the brightness, color, and texture of the adjacent area as if they were actually there. The mechanisms underlying this kind of perceptual filling-in remain controversial. To better understand these processes, we determined the minimum region around the blind spot that needs to be stimulated for filling-in by carefully mapping the blind spot and presenting individually fitted stimulus frames of different width around it. Uniform filling-in was observed with frame widths as narrow as 0.05 degrees visual angle for color and 0.2 degrees for texture. Filling-in was incomplete, when the frame was no longer contiguous with the blind spot border due to an eye movement. These results are consistent with the idea that perceptual filling-in of the blind spot depends on local processes generated at the physiological edge of the cortical representation.     

Characteristics of the filled-in surface at the blind spot

Our visual system can restore information missing within the portion of the retinal image corresponding to the blind spot where the optic nerve exits the eye. Previous studies of the properties of filled-in surfaces at the blind spot have found similarities and dissimilarities between filled-in and real surfaces and have therefore not provided a consistent view of the characteristics of the filled-in surface. First, we investigated whether filling-in utilizes a contour integration mechanism. Gratings with collinear lines filled in the blind spot more effectively than those both with orthogonal lines and without any line, suggesting that collinear facilitation underlies the filling-in of the blind spot. Second, the dynamics of binocular rivalry was examined by comparing the dominance duration distributions of filled-in and real surfaces. The results indicated that the strength of the filled-in surface was attenuated compared to that of the real surface during rivalry. Lastly, we tested whether travelling waves of dominance in rivalry could occur at the blind spot. The travelling waves could propagate through a hole only at the blind spot, suggesting that the filled-in surface helps perceptual waves to travel across the blind spot. These results suggest that the filled-in surface shares a common mechanism via a horizontal connection but that it has weak strength to suppress the opposite eye during binocular viewing.     

Bilateral slowly progressive big blind spot syndrome

A 33-year-old woman was found with bilateral enlargement of the blind spot without optic disc edema. Her bilateral peripapillary retinas appeared slightly decolored and elevated with obscure margins and fluorescein angiography indicating only slight hyperfluorescence in the late venous phase. Four years after her initial visit, both of her blind spots became enlarged further and dark adaptation was remarkably difficult. Peripapillary retinal lesions appearing as retinal pigmentary degeneration were evident with very definitive margins. The lesions showed strong hyperfluorescence in the arteriovenous phase owing to window defects. From these findings, it appears that bilateral big blind spots without optic disc edema may possibly result from peripapillary retinal degeneration, which may become evident at a much later date.     

Global interactions in perceptual completion at the blind spot

Nonlinearity and asymmetrical property in the filling-in process at the blind spot are investigated. Perceptual completion of a pattern with a gap is deteriorated, i.e. gap information inhibits completion across the gap. Filled-in patterns, observed from complementary patterns, are not the same in complementary sense. These suggest that perceptual completion cannot simply be predicted from a superimposition of completions, obtained from local stimulus conditions, and seems to be mediated by global processes.     

Anisotropy of tolerance of perceptual completion at the blind spot

Three experiments examined anisotropies of tolerance of perceptual completion at the blind spot when a pair of line segments was presented on opposite sides of the blind spot. The tolerance of perceptual completion is defined as the maximum difference in a stimulus attribute between the line segments on opposite sides of the blind spot when perceptual completion of a line has occurred. The misalignment, orientation difference, and luminance difference between the line segments were used as the stimulus attributes in Experiments 1, 2, and 3, respectively. The results showed anisotropies of the tolerance of perceptual completion between horizontal and vertical configurations of the line segments. Vertical superiorities, which imply a greater extent of tolerance in the vertical configuration than that in the horizontal configuration, were observed for misalignment and orientation difference, while horizontal superiority, which implies a greater extent of tolerance in the horizontal configuration than that in the vertical configuration, was observed for luminance difference. We discussed possible origins of the anisotropy of the tolerance of perceptual completion at the blind spot.     

Filling-in at the natural blind spot contributes to binocular rivalry

The human natural blind spot is usually filled in based on the contextual information. When two sufficiently different images are presented to the two eyes, observers typically perceive an alternation between the two images (binocular rivalry). Both the filling-in process and binocular rivalry have been the subject of considerable research. This study investigates whether filled information in one eye's natural blind spot can contribute to binocular rivalry. A radial grating (D=12 degrees ) was presented to one eye, centered on the natural blind spot. Observers perceived a complete figure in monocular view; the blind spot area was filled-in based on the surrounding information. Simultaneously, a circular grating smaller than the blind spot (D=4 degrees ), was presented to the fellow eye in the region corresponding to the other eye's blind spot. The amount of rivalry as indexed by how often the smaller circular grating remained visible was measured. The results suggest that the filled information in the area of the blind spot does contribute to the rivalry process.     

Acute, idiopathic blind spot enlargement syndrome.

An otherwise healthy 12-year-old boy was evaluated for an enlarged blind spot in his left eye. Neither optic nerve edema/neuritis nor a retrobulbar mass explained this finding. Consultation with a neuro-ophthalmologist over a period of 14 months resulted in a diagnosis of acute, idiopathic blind spot enlargement syndrome, a rare and poorly understood ocular condition. To the author's knowledge, this is the youngest case ever reported.