Retinal Scotomas Research Articles

Homeostatic plasticity in human extrastriate cortex following a simulated peripheral scotoma

Neuroimaging and patient work over the past decade have indicated that, following retinal deafferentation, the human visual cortex undergoes a large-scale and enduring reorganization of its topography such that the classical retinotopic organization of deafferented visual cortex remaps to represent non-classical regions of visual space. Such long-term visual reorganization is proposed to occur through changes in the functional balance of deafferented visual circuits that engage more lasting changes through activity-dependent neuroplasticity. Here, we investigated the short-term changes in functional balance (short-term plasticity; homeostatic plasticity) that occur within deafferented human visual cortices. We recorded electroencephalogram (EEG) while observers were conditioned for 6s with a simulated retinal scotoma (artificial scotoma) positioned 8.0° in the periphery. Visual evoked potentials (VEPs) evoked by the onset of sinusoidal visual probes that varied in their tilt were used to examine changes in cortical excitability within and around cortical representations of the simulated scotoma. Psychophysical orientation functions obtained from discrimination of visual probe tilt were used to examine alterations in the stimulus selectivity within the scotoma representations. Consistent with a mechanism of homeostatic disinhibition, an early extrastriate component of the VEP (the early phase P1) exhibited increased amplitude following the condition with a simulated scotoma relative to a stimulus-matched control condition. This increased visual cortical response was associated with a reduction in the slope of the psychophysical orientation function, suggesting a broader tuning of neural populations within scotoma representations. Together, these findings support a mechanism of disinhibition in promoting visual plasticity and topographical reorganization.

Visual rehabilitation in patients with myopic maculopathy: our experience

ObjectiveTo evaluate the efficacy of the MP-1 microperimeter (Nidek Technologies Srl, Padova, Italy) and Visual Pathfinder (LACE Inc) in improving visual function of patients with myopic maculopathy. DesignProspective, nonrandomized, interventional case series. ParticipantsSeventeen patients (34 eyes) between 36 and 58 years of age with myopic maculopathy and central retinal scotomas. MethodsAfter a complete eye examination, all patients underwent 10 training sessions with MP-1 biofeedback (7 minutes) and Visual Pathfinder (3 minutes) for each eye once a week. Statistical analysis was performed with Student t test. The p values less than 0.05 were considered statistically significant. ResultsThe mean best corrected visual acuity increased from 0.64 ± 0.22 to 0.38 ± 0.20 logMAR at the end of follow-up (p = 0.03); visual-evoked potential P100 amplitude increased from 3.54 ± 1.90 to 6.64 ± 2.91 μV at the end of follow-up (p = 0.04); average retinal sensitivity, calculated in the 12 degrees of the central retina, increased from 6.6 ± 2.6 to 14.6 ± 3.6 dB (p = 0.03). Fixation behaviour in the 2 degrees of the central retina increased from 45% ± 17% to 75% ± 23% (p = 0.04). The bivariate contour ellipse area (95%) increased from 10.34 ± 3.11 to 7.64 ± 2.71 square degrees (p = 0.04). ConclusionsThe combination of acoustic biofeedback training with MP-1 and Visual Pathfinder offers a reasonable improvement of visual function in patients with myopic maculopathy. This method might be considered as a rehabilitative strategy as a “therapeutic option” in these patients for whom most treatments usually do not work.

Neural Mechanisms of Short-term Plasticity in the Human Visual System

Following circumscribed retinal damage, extensive reorganization of topographically organized visual cortical areas has been demonstrated in several species of mammals (including humans). Although reorganization is often studied over extended time scales, neural response properties change within seconds of retinal deafferentation. Understanding the mechanisms underlying these short-term effects is essential for developing a complete picture of representational plasticity. One approach to the study of short-term plasticity has been to use an artificial scotoma, a stimulus-induced analog of a retinal scotoma, as a model. Here, we use event-related potentials in an artificial scotoma paradigm to examine 2 aspects of short-term plasticity in the human visual system. First, we investigated the changes within visual representations temporarily deprived of patterned visual input by probing the inner boundaries of an artificial scotoma. We found an enhanced early sensory P1, consistent with a reduction in inhibition (disinhibition), a proposed mechanism of short-term visual plasticity. Second, we investigated mechanisms through which representations of surrounding space invade a visually deprived area by probing the outer boundaries of an artificial scotoma. In this case, a later visual component, the N1, was enhanced, suggesting that feedback may provide a source of unmasked, or invading, activity to visually deprived representations.

Illusory Contours over Pathological Retinal Scotomas

Our visual percepts are not fully determined by physical stimulus inputs. Thus, in visual illusions such as the Kanizsa figure, inducers presented at the corners allow one to perceive the bounding contours of the figure in the absence of luminance-defined borders. We examined the discrimination of the curvature of these illusory contours that pass across retinal scotomas caused by macular degeneration. In contrast with previous studies with normal-sighted subjects that showed no perception of these illusory contours in the region of physiological scotomas at the optic nerve head, we demonstrated perfect discrimination of the curvature of the illusory contours over the pathological retinal scotoma. The illusion occurred despite the large scar around the macular lesion, strongly reducing discrimination of whether the inducer openings were acute or obtuse and suggesting that the coarse information in the inducers (low spatial frequency) sufficed. The result that subjective contours can pass through the pathological retinal scotoma suggests that the visual cortex, despite the loss of bottom-up input, can use low-spatial frequency information from the inducers to form a neural representation of new complex geometrical shapes inside the scotoma.

Measuring and modeling filling-in effects in retinal AMD scotomas

Measuring and modeling filling-in effects in retinal AMD scotomas

Étude en IRM fonctionnelle 3T de la projection corticale de scotomes rétiniens : résultats préliminaires

We used functional magnetic resonance imaging (fMRI) to precisely measure both the localization and size of the cortical projections of artificial scotomas in healthy subjects as well as the size of a reversible retinal scotoma in a patient with central serous chorioretinopathy (CSCR). Using a 3T MRI scanner, anatomical and functional data were acquired on two healthy subjects and a patient with CSCR. Retinotopic maps were first reconstructed using phase mapping techniques. Next, a block paradigm consisting of a grey background alternating with a full-field, flickering checkerboard was used to stimulate the complete central (19.5 degrees) visual field. A condition with artificial peri-foveal scotomas of different sizes and eccentricities was interleaved in healthy subjects. Differential maps were computed to obtain the cortical representation (size and location) of artificial scotomas. Full-field functional data were also acquired in the CSRC patient, at the acute stage and after recovery. Cortical projections of each scotoma were identified using differential maps and carefully characterized with quantitative analysis: the measured cortical positions of the inactivated cortical zones were compared with the known radius and eccentricity values in the scotomas in the visual field. We also compared the size of the inactivated cortical zones to the known size of scotomas. However, we found a consistent relationship between the size of the scotomas and their cortical projections, albeit with the absolute size smaller than expected from known cortical magnification factors. The cortical deactivation zone was also observed in the CRSC patient, which disappeared at recovery stage. Cortical retinotopic mapping can be performed successfully for both artificial and retinal scotomas. This study can serve as a basis for the future investigation of cortical plasticity in the visual cortex.

Spatial Alignment over Retinal Scotomas

Perceptual completion can mask the presence of physiological and pathologic retinal scotomas. This psychophysical study used a spatial alignment task to examine the processes underlying this perceptual completion. Similarities between the completion of pathologic and physiological scotomas would be consistent with large-scale reorganization of the visual system in eye disease In five control subjects with no eye disease, Vernier alignment thresholds were measured over the physiological blind spot at the optic nerve head and over equally eccentric temporal retina. For nine subjects with retinal scotomas, alignment thresholds were measured over the maximum vertical extent of the larger scotoma in one eye and at an equal separation and eccentricity in the eye with a smaller or no scotoma In control subjects, alignment thresholds were better over the physiological blind spot than over equally eccentric temporal retina (P < 0.05). Alignment thresholds were no better over pathologic retinal scotomas than more intact, equally eccentric retina (P = 0.9) These quantitative differences implicate different mechanisms for perceptual completion over pathologic and physiological retinal scotomas. Filling in across pathologic scotomas appears to involve higher level image processing-based mechanisms that operate even when their input is interrupted. Filling-in at the optic nerve head involves additional low-level processes that may be hardwired, in which receptive fields span the blind spot and support fine orientation discriminations. These results argue against low-level reorganization of the visual system in people with retinal disease.

Filling in at partially deafferented visual cortex

Methods:Perceptual visual filling in at the corresponding area of toxoplasmic retinochoroiditis scotomas in the fellow eye was studied in 14 patients. Targets were programmed to appear inside the scotoma corresponding...

Illusory Stimuli Can Be Used to Identify Retinal Blind Spots

BackgroundIdentification of visual field loss in people with retinal disease is not straightforward as people with eye disease are frequently unaware of substantial deficits in their visual field, as a consequence of perceptual completion (“filling-in”) of affected areas.MethodologyWe attempted to induce a compelling visual illusion known as the induced twinkle after-effect (TwAE) in eight patients with retinal scotomas. Half of these patients experience filling-in of their scotomas such that they are unaware of the presence of their scotoma, and conventional campimetric techniques can not be used to identify their vision loss. The region of the TwAE was compared to microperimetry maps of the retinal lesion.Principal FindingsSix of our eight participants experienced the TwAE. This effect occurred in three of the four people who filled-in their scotoma. The boundary of the TwAE showed good agreement with the boundary of lesion, as determined by microperimetry.ConclusionFor the first time, we have determined vision loss by asking patients to report the presence of an illusory percept in blind areas, rather than the absence of a real stimulus. This illusory technique is quick, accurate and not subject to the effects of filling-in.

Étude en IRM fonctionnelle 3T de l’organisation rétinotopique du cortex visuel

INTRODUCTION. We used high-field (3T) functional magnetic resonance imaging (fMRI) to map the retinotopic organization of human cortical areas. Retinotopic maps were reconstructed using existing mapping techniques. Stimuli were made of a rotating wedge stimulus, which provided angular coordinate maps, and an expanding or contracting ring, which provided eccentricity coordinate maps. Stimuli consisted of a grey background alternating with a flickering checkerboard. A Brucker 3T scanner equipped with a head coil and a custom optical system was used to acquire sets of echoplanar images of 20 occipital coronal slices within a RT of 2.111 ms and an 8 mm3 voxel resolution. Surface models of each subject's occipital lobes were constructed using the Brainvisa software from a sagittal T1-weighted image with a 1 mm3 voxel resolution. The cortical models were then inflated to obtain unfolded surfaces. Statistical analyses of the functional data were made under SPM99, and the response amplitudes were finally assigned to the cortical reconstructed surfaces. We identified boundaries between different early visual areas (V1, V2, V3) using eccentricity and polar angle retinotopic maps and detection of reversals in the representation of the polar angle. Both complete maps and reversals corresponding to the V1/V2 borders were clearly visible with a single recording session. Also, we were able to compare data from subjects across various fMRI acquisitions and found that there was a strong correlation between maps acquired at different sessions for the same subject. We developed a quick (<40 min) retinotopic cortical area mapping method at 3T, which makes it possible to study the cortical remapping in patients with retinal scotomas.

Binocular versus monocular viewing in patients with age-related macular degeneration

This paper compares monocular versus binocular viewing in AMD patients during fixation and the potential of binocular function. Twenty-seven patients with bilateral AMD were recruited. A scanning laser ophthalmoscope was used to identify PRLs and map retinal scotomas monocularly. An infrared eye tracker was used to evaluate gaze position changes (and indirectly retinal locus changes) between monocular and binocular fixation. Through their combined use the retinal loci used for fixation under binocular viewing conditions were identified. Binocular function was tested with Bagolini striated glasses and fusion at the PRLs was tested with a computer-driven display. It was found that only five patients used the same PRL to fixate under monocular and binocular conditions for both eyes. 44.4% of the patients with symmetrical scotomas but only one patient with asymmetrical scotomas showed no shift in gaze position. However, there was no difference between them (chi-square, p=0.03). Binocular PRLs fell on retinal areas that could be fused in 85.1% of the patients. Still, in 25.9% of the patients binocular PRLs fell within the scotoma in the worse eye. All patients exhibited fusion when tested with the Bagolini glasses, but only 33% showed evidence of fusion near the PRL. These results demonstrate that AMD patients could use a different PRL when viewing binocularly versus monocularly. Although retinal correspondence of binocularly used fixation locus is retained, fusion near the PRL seems impaired in the majority of the cases. These results provide an insight into how people with bilateral scotomas operate in the real world.

Fixation stability using central and pericentral fixation targets in patients with age-related macular degeneration

To determine fixation stability for central and pericentral fixation targets in patients with age-related macular degeneration (AMD). Comparative study. Twelve patients having late-stage AMD involving the fovea and 10 age-matched controls having no other eye diseases and visual acuity better than 20/25. Six different fixation targets (1 degrees cross; 1 degrees filled circle; 1 degrees letter x; small 4-point diamond; large 4-point diamond using dimensions as in a field analyzer; large-crossover whole-image diagonal with open 1 degrees center) were presented on a high-resolution monitor. Before examination, subjects were given verbal instructions to move their eye to see the center of the target best. Fixation stability was measured for the preferred eye, with the fellow eye occluded, using a gaze tracker. Fixation stability was quantified by calculating the bivariate contour ellipse area (BCEA) over 30 seconds for each target. For statistical analysis, BCEA values (minutes of arc2) were converted into their logarithms. The absolute retinal scotoma for the study eye was determined using a scanning laser ophthalmoscope. Bivariate contour ellipse area. Visual acuity in patients (age range, 57-87 years) ranged from 20/32 to 20/600. The lowest BCEA values were found for the 1 degrees letter x in patients (mean, 12052.2%+/-254.0%) and for the 1 degrees cross in normal subjects (mean, 1286.9%+/-47.8%); the highest BCEA values were found for the small 4-point diamond in patients (mean, 23109.5%+/-298.3%) and for the large 4-point diamond in normals (age range, 62-79 years) (mean, 3229.2%+/-105.4%). The difference between the targets was significant for normal subjects (analysis of variance [ANOVA], P<0.01) but not for patients (ANOVA, P>0.05). In normals, BCEA values were significantly lower for central fixation targets than for pericentral fixation targets (P<0.01). Fixation is significantly less stable for pericentral fixation targets in normal subjects, indicating an advantage for central fixation targets. These results are particularly significant for any clinical and experimental testing method that requires the patient to maintain stable fixation.

Filling-in of retinal scotomas

In this study we examined the perception of one- and two-dimensional patterns across central retinal scotomas, caused by age-related macular degeneration. In contrast with previous studies of disrupted visual input that used the blind spot and artificial scotomas, the current study used large central scotomas caused by physical retinal damage. Such damage is associated with atrophy and long-term cortical reorganization, and it was therefore unclear whether perceptual completion in the damaged system will be similar to that reported for artificial scotomas and the blind spot. In addition, the scotomas under study were much larger and more central than artificial scotomas for which perceptual completion has been reported. For 1-D line and grating patterns, we found perceptual completion across large central scotomas (up to radius of 7°), which is significantly beyond the range of perceptual completion in artificial scotomas. Gratings completion was better than that of a single line, and increased with bars density. The use of central scotomas allowed us to test the completion of 2-D patterns that are difficult to study in peripheral vision. We found completion of two-dimensional dot arrays over large regions that improved with pattern density and regularity. The results show that in the physically damaged system the range of perceptual completion is increased compared with artificial scotomas, they strongly support the view of an active filling-in process rather than simply ignoring the damaged location, and they show that perceptual completion of physical scotomas is likely to involve cortical processing at multiple levels. We finally discuss implications of the results to the possible use of image enhancement techniques to facilitate the perception of low-vision individuals.

Scotoma and fixation patterns using scanning laser ophthalmoscope microperimetry in patients with macular dystrophy

PURPOSE: We used scanning laser ophthalmoscope microperimetry to evaluate the retinal scotoma and the fixation points in the patients with macular dystrophy. METHODS: We studied 10 eyes of five patients with macular dystrophy (three patients with cone dystrophy and two patients with Stargardt disease). The mean patient age was 37 years (range, 13 to 64 years). An estimation of scotoma and fixation points on the retina was performed using scanning laser ophthalmoscope microperimetry. RESULTS: All 10 eyes (100%) had one of two types of dense scotoma: type one was a dense ring scotoma (five eyes, 50%), and type two was a dense central scotoma (five eyes, 50%) that included the center of the fovea. In all eyes with a dense ring scotoma, the fixation points were stable and did not shift. In all eyes with a dense central scotoma, the fixation shifted. The logarithm of minimal angle of resolution of the visual acuity in the eyes with the dense central scotoma was significantly worse than that of eyes with the dense ring scotoma type ( P = .005). CONCLUSIONS: Scanning laser ophthalmoscope microperimetry findings demonstrate two types of dense scotoma (dense ring scotoma and dense central scotoma) in the patients with macular dystrophy. The two types of dense scotoma affect the shifting of the fixation points and the stability of fixation and may result in the difference in visual acuity in the patients with macular dystrophy.

Chapter 15 Cortical plasticity revealed by circumscribed retinal lesions or artificial scotomas

We review the work of others in which the effects of circumscribed, topographically corresponding binocular retinal lesions on the topographic organization of the visual cortex revealed that there is a substantial degree of topographical plasticity in the primary visual cortices of adult cats and macaque monkeys. Despite the evidence indicating that the reorganization of the topographic map in primary visual cortices of adult cats and macaques related to the input from one eye could be suppressed for a long time by inputs related to the other eye, we observed a substantial degree of topographical plasticity in the primary visual cortices of adult cats in which we have made circumscribed monocular retinal lesions. Overall, in both binocularly and monocularly lesioned adult animals, most cells recorded in the cortical projection zone of the retinal lesion (LPZ), several hours, several weeks or several months after placement of the lesions exhibited 'ectopic' excitatory visual receptive fields (RFs) which were displaced to the normal retina in the immediate vicinity of the lesion. The presence of ectopic RFs in cells recorded in the cortical LPZ, combined with the presence of normal cortical representation of the part of the retina in the vicinity of the lesion, indicate a clear expansion of the cortical representation of the part of the retina surrounding the lesion. When stimulated via the ectopic RFs, cortical cells exhibited normal orientation tuning and in the case of animals with monocular lesions, the orientation tuning of binocular cells when stimulated via ectopic RFs appeared to be very similar to that when the cells were stimulated via the RFs in the normal, unlesioned eye. In both binocularly and monocularly lesioned animals, the responses evoked by optimal visual stimuli from the ectopic RFs were substantially weaker than those evoked from their normal counterparts. Similarly, upper velocity limits were significantly lower when visual stimuli were presented via the ectopic RFs. In contrast to cats in which the retinal lesions were made in adulthood, in cats lesioned monocularly in adolescence (8-11 weeks postnatal), both the peak discharge rates and upper velocity limits of responses to photic stimuli presented via the ectopic RFs were very similar to those to stimuli presented via the normal eye. The intracortical mechanism(s) underlying the long-term cortical plasticity revealed by retinal lesions are likely to be closely linked to the mechanism(s) underlying the short-term reversible enlargement of cortical receptive fields observed with artificial scotomas. Furthermore, a similar putative intracortical mechanism(s) appears to underlie psychophysical phenomena observed in studies of retinal scotomas in humans. Overall, the research reviewed here strongly challenges the view that receptive fields of neurons in mammalian visual cortices are 'hard-wired'.

Looking behind a pathological blind spot in human retina.

Recent work suggests that dichoptic lateral interactions occur in the region of the visual field of one eye that corresponds to the physiological blind spot in the other eye (Tripathy, S. P., & Levi, D. M. (1994). The two-dimensional shape of spatial interaction zones in the parafovea. Vision Research, 34, 1127-1138.) Here we ask whether dichoptic lateral interactions occur in the region of the visual field of one eye that corresponds to a pathological blind spot, a retinal coloboma in the other eye. To address this question we had the observer report the orientation of a letter 'T' presented within this region in the presence of flanking 'T's presented to the other eye around the coloboma. A large drop in performance was seen due to the flanks, showing the existence of dichoptic lateral interactions in this monocular region. The presence of these dichoptic interactions in a region lacking direct retinal afferents from one eye is consistent with the proposition that long-range horizontal connections of the primary visual cortex mediate these interactions.

Reading with central field loss: number of letters masked is more important than the size of the mask in degrees.

When the center of a readers, visual field is blocked from view, reading rates decline and eye movement patterns change. This is true whether the central visual field is blocked artificially (i.e. a mask) or through disease (e.g. a retinal scotoma due to macular degeneration). In past studies, when mask size was defined in terms of the number of letters masked from view, reading rates declined sharply as number of letters masked increased. Patients with larger central scotomas (in degrees of visual angle) also read slower. We sought to determine whether number of letters masked or size of the mask in degrees is the predominant factor affecting reading rates and eye movement behavior. By matching number of letters masked across several mask sizes (and compensating for reduced acuity in the periphery), we found that number of letters masked is the more important factor until mask size is quite large (> or = -7.5 degrees) and number of letters masked from view is more than seven.

Visual scotoma and visual afterimages: some evidence that the perceived visual afterimage may not be a purely retinal phenomenon: a single case study.

Observations of the differential appearance and behaviour of a deliberately induced visual scotoma and a patterned visual afterimage are reported. Although a retinal scotoma behaves like a visual afterimage in some ways, there are sufficient differences to suggest that the perceived visual afterimage may not be just the simple consequence of prolonged retinal stimulation.

Receptive field dynamics in adult primary visual cortex.

The adult brain has a remarkable ability to adjust to changes in sensory input. Removal of afferent input to the somatosensory, auditory, motor or visual cortex results in a marked change of cortical topography. Changes in sensory activity can, over a period of months, alter receptive field size and cortical topography. Here we remove visual input by focal binocular retinal lesions and record from the same cortical sites before and within minutes after making the lesion and find immediate striking increases in receptive field size for cortical cells with receptive fields near the edge of the retinal scotoma. After a few months even the cortical areas that were initially silenced by the lesion recover visual activity, representing retinotopic loci surrounding the lesion. At the level of the lateral geniculate nucleus, which provides the visual input to the striate cortex, a large silent region remains. Furthermore, anatomical studies show that the spread of geniculocortical afferents is insufficient to account for the cortical recovery. The results indicate that the topographic reorganization within the cortex was largely due to synaptic changes intrinsic to the cortex, perhaps through the plexus of long-range horizontal connections.

SIMULATION OF A RETINAL SCOTOMA BY A STABILIZED RETINAL IMAGE

Two methods are described for simulating a retinal scotoma by means of a stabilized retinal image, hence allowing some aspects of the impairment of visual function associated with scotomas to be studied in observers with normal visual fields. In the first method, an optical lever arrangement is used to generate a small artificial scotoma at a controllable position with 15 degrees field of view. The second, direct attachment, method uses a tight-fitting scleral contact lens, to which is attached a very light, aluminium-alloy tube. This carries a small piece of lightweight black card, the stabilized image of this card generating the scotoma. Although precise positioning of the scotoma is more difficult with the second method, the induced scotoma is absolute and the method is relatively cheap and easy to implement.