Vision-related quality of life according to location of visual field loss in patients with glaucoma.

PURPOSE: To assess influence of the location of visual field (VF) loss on vision-related quality of life (VRQOL) in patients with glaucoma. METHODS: We included 826 patients with primary open-angle glaucoma (POAG) enrolled in the prospectively designed Life Quality of Glaucoma Patients Who Underwent Treatment (LIGHT) study organized by the Korean Glaucoma Society. We divided the integrated binocular visual field (IVF) into four regions and evaluated the associations between Rasch-analysed 25-item National Eye Institute Visual Function Questionnaire (NEI VFQ-25) and location of the IVF using univariate and hierarchical multivariable linear regression. RESULTS: Mean deviation (MD) of the IVF at superior centre showed the lowest value among the four regions. Multivariable linear regression analysis revealed significant correlation of the composite score and all subscales of the NEI VFQ-25 except ocular pain and near activities at each of four IVF regions. Mean deviation at superior centre showed significant correlation with composite score (R2  = 0.181), near activities (R2  = 0.175), and social functioning (R2  = 0.166); MD at superior periphery showed highest correlation with role difficulties (R2  = 0.137); MD at inferior centre showed highest correlation with driving (R2  = 0.145); and MD at inferior periphery showed significant correlation with distance activities (R2  = 0.214) and dependency (R2  = 0.119). CONCLUSION: Four different regions of the IVF had a similarly important impact on subscales of the NEI VFQ-25 in glaucoma patients. Preservation of the superior field should be considered as much as that of the inferior field for maintaining a good VRQOL in patients with glaucoma.

[The scotomas as a metaphor in the work of Oliver Sacks]. / Los escotomas como metafora en la obra de Oliver Sacks.

INTRODUCTION: The use of medical metaphors is common in the social discourse and in the media. However, the use by physicians themselves to define different concepts to the original meaning of the medical word is rare. AIM: To analyze the term «scotoma¼ in its metaphorical sense in the works of the neurologist and writer Oliver Sacks. DEVELOPMENT: Sacks used scotoma metaphorically in two works, in the autobiographical book A leg to stand on and in an essay Scotoma: forgetting and neglect in science. In the first case, he used it to define the sensorial loss of his leg after an accident, which could be interpreted as a «mental scotoma¼. In the second case, Sacks analyzed the process and the reasons of forgetting the works of some early scientific discoverers and discussed why this happened. In this case, Sacks made an analogy with the process suffered by scientific discoveries and the reasons why some of them are largely ignored, in a situation that he called «historical¼ or «social scotoma¼. CONCLUSIONS: Sacks does not use the term «scotoma¼ uniformly. When used to describe the sensorial loss of his leg, it might be considered that scotoma is there a second, although minor but accepted, meaning of the word. However, its use in the definition of historical neglect of early discoveries can be clearly defined as a medical metaphor in full sense.

TITLE: Los escotomas como metafora en la obra de Oliver Sacks.Introduccion. La utilizacion de metaforas medicas es frecuente en el discurso social y en los medios de comunicacion. Sin embargo, su empleo por los propios medicos para definir conceptos distintos al significado original del termino resulta poco habitual. Objetivo. Analizar el empleo del termino «escotoma¼, en su sentido metaforico, en las obras del neurologo y escritor Oliver Sacks. Desarrollo. Sacks uso escotoma metaforicamente en dos obras, en el libro autobiografico A leg to stand on y en un ensayo, Scotoma: forgetting and neglect in science. En el primer caso, lo utilizo para describir la situacion de perdida sensorial de su extremidad inferior, que experimento despues de un accidente, lo que podria interpretarse como un «escotoma mental¼. En el segundo caso, Sacks analizo el proceso y las razones del olvido de las obras de algunos descubridores cientificos precoces y comento por que habia sucedido, una situacion que denomino «escotoma historico¼ o «social¼. Conclusiones. Sacks no utiliza el termino «escotoma¼ de forma uniforme y, si bien en el caso de su accidente podria considerarse como una acepcion menor, pero aceptada, es indudable su uso como metafora medica para describir el olvido y la ignorancia de algunos descubrimientos cientificos en determinadas situaciones historicas.

Association Between Undetected 10-2 Visual Field Damage and Vision-Related Quality of Life in Patients With Glaucoma.

Importance: Recent evidence supports the presence of macular damage (within 8° of the central field) to retinal ganglion cells and associated central visual field (VF) defects in glaucoma, even in early stages. Despite this, to our knowledge, the association of 10-2 VF damage with vision-related quality of life (QOL) has not been well studied. Objective: To determine the association between QOL and visual function as measured by 24-2 and 10-2 VFs in patients with primary open-angle glaucoma and to test the hypothesis that patients with vision-related QOL disproportionate to their 24-2 VF status may exhibit 10-2 damage overlooked by the 24-2 test. Design, Setting, and Participants: In this cross-sectional analysis of observational cohort study data taken from a tertiary care specialty practice, 113 patients with glaucoma with the entire range of 24-2 VF damage completed the National Eye Institute Visual Function Questionnaire (NEI VFQ-25). Data were collected from May 2014 to January 2015 and were analyzed from March 2016 to May 2016. Interventions: Standardized binocular 24-2 and 10-2 VF sensitivities were calculated for each patient. Main Outcomes and Measures: Association of binocular 24-2 and 10-2 VF sensitivity with Rasch-calibrated NEI VFQ-25 scores. Detection of outliers was based on Cook distance of the regression of binocular 24-2 and NEI VFQ-25 score. Outlier association with QOL was then assessed using a linear regression model, with binocular 10-2 VF sensitivity as the independent variable. Results: Of the 113 patients, the mean (SD) age was 70.1 (10.9) years, and 51 (45.1%) were male and 71 (62.8%) were white. The composite NEI VFQ-25 score was associated with both binocular 24-2 (ß = 1.95; 95% CI, 0.47-3.43; P = .01) and 10-2 (ß = 2.57; 95% CI, 1.12-4.01; P = .001) sensitivities, but the 10-2 VF univariable model showed an almost 2-fold better fit to the data (R2 = 9.2% vs 4.9%). However, the binocular 10-2 sensitivities of 24-2 outliers had the strongest association with the composite NEI VFQ-25 scores (ß = 2.78; 95% CI, 0.84-4.72; P = .006.) and the best fit to the data (R2 = 18.2%.). Conclusions and Relevance: The 10-2 VF model showed a stronger association with NEI VFQ-25 score than the 24-2 VF model. Patients with disproportionately low quality of vision relative to patients with 24-2 VF damage may have damage on the central field missed by the 24-2 grid. Future prospective testing, including additional dimensions of quality of life, is indicated.

Visual exploration of objects and scenes in patients with age-related macular degeneration.

OBJECTIVE: Studies on people with age-related macular degeneration (AMD) have shown that they are able to detect briefly displayed objects and scenes with high accuracy (above 80%). However, in everyday life we explore our environment to search and to recognize objects. We assessed visual exploration in people with AMD during the identification of objects and scenes. METHOD: Twenty patients with AMD, fifteen age-matched and twelve young controls participated. We used colored photographs of isolated objects, natural scenes and objects in scenes, displayed centrally on a monitor. Participants were asked to name the objects and scenes. Ocular movements were recorded during the identification task. Scan paths, saccades, fixations, and accuracy were also recorded. RESULTS: People with AMD exhibited lower accuracy (by about 30%). Eye movement parameters were impaired with a larger number of saccades, shorter fixation durations and a larger scan path than controls. CONCLUSIONS: Our results are consistent with studies on artificial scotoma in normally sighted people showing that a central scotoma impairs oculomotricity. In contrast to detection tasks, people with central vision loss exhibit impaired performance in identification of objects and scenes (62 to 66%). Eye movement studies suggest that the lower accuracy in patients is likely due to the use of peripheral vision and instability of fixation.

The association between visual field defect severity and fear of falling in primary open-angle glaucoma.

PURPOSE: To determine if glaucomatous visual field defect severity is associated with fear of falling. METHODS: This is a cross-sectional study. A total of 387 consecutive subjects with POAG were enrolled in this study along with 293-ocular disease-free control subjects, who were screened at the same institutions. We defined mild POAG as MD of -6 dB or better, moderate POAG as MD of -6 to -12 dB, and severe POAG as MD of -12 dB or worse in the better eye. All participants were requested to answer a questionnaire on fear of falling. Associations between POAG severity and the prevalence of fear of falling were evaluated with the Cochran-Armitage trend test. Multivariable factors including age-adjusted odds ratios (ORs) for the prevalence of fear of falling and 95% confidence intervals (CIs) were evaluated with logistic regression models. RESULTS: The prevalence of fear of falling was 35/293 (11.9%) in the control group, 38/313 (12.1%) in the mild POAG group, 12/48 (25.0%) in the moderate POAG group, and 6/26 (23.1%) in the severe POAG group, and the trend was statistically significant (P = 0.028 Cochran-Armitage trend test). The adjusted ORs for prevalence in the mild, moderate, and severe POAG groups compared with that in the control group were 1.44 (95% CI: 0.83-2.51), 2.33 (95% CI: 1.00-5.44), and 4.06 (95% CI: 1.39-11.90), respectively. CONCLUSIONS: Among patients with POAG, the severity of visual field defects is associated with fear of falling. (http://www.umin.ac.jp/ctr/index.htm number, UMIN000005574.).

How does glaucoma look?: patient perception of visual field loss.

OBJECTIVE: To explore patient perception of vision loss in glaucoma and, specifically, to test the hypothesis that patients do not recognize their impairment as a black tunnel effect or as black patches in their field of view. DESIGN: Clinic-based cross-sectional study. PARTICIPANTS: Fifty patients (age range, 52-82 years) with visual acuity better than 20/30 and with a range of glaucomatous visual field (VF) defects in both eyes, excluding those with very advanced disease (perimetrically blind). METHODS: Participants underwent monocular VF testing in both eyes using a Humphrey Field Analyzer (HFA; Carl Zeiss Meditec, Dublin, CA; 24-2 Swedish interactive threshold algorithm standard tests) and other tests of visual function. Participants took part in a recorded interview during which they were asked if they were aware of their VF loss; if so, there were encouraged to describe it in their own words. Participants were shown 6 images modified in a variety of ways on a computer monitor and were asked to select the image that most closely represented their perception of their VF loss. MAIN OUTCOME MEASURES: Forced choice of an image best representing glaucomatous vision impairment. RESULTS: Participants had a range of VF defect severity: average HFA mean deviation was -8.7 dB (standard deviation [SD], 5.8 dB) and -10.5 dB (SD, 7.1 dB) in the right and left eyes, respectively. Thirteen patients (26%; 95% confidence interval [CI], 15%-40%) reported being completely unaware of their vision loss. None of the patients chose the images with a distinct black tunnel effect or black patches. Only 2 patients (4%; 95% CI, 0%-14%) chose the image with a tunnel effect with blurred edges. An image depicting blurred patches and another with missing patches was chosen by 54% (95% CI, 39%-68%) and 16% (95% CI, 7%-29%) of the patients, respectively. Content analysis of the transcripts from the recorded interviews indicated a frequent use of descriptors of visual symptoms associated with reported blur and missing features. CONCLUSIONS: Patients with glaucoma do not perceive their vision loss as a black tunnel effect or as black patches masking their field of view. These findings are important in the context of depicting the effects of glaucomatous vision loss and raising awareness for glaucoma detection. FINANCIAL DISCLOSURE(S): The author(s) have no proprietary or commercial interest in any materials discussed in this article.

Patient awareness of binocular central scotoma in age-related macular degeneration.

PURPOSE: To assess whether age-related macular degeneration (AMD) patients are aware of binocular central visual field defects. METHODS: One hundred fifty-three consecutive AMD patients in their initial low-vision rehabilitation evaluation were immediately asked at the beginning of their visit (1) whether they were able to see any blind spots or defects in their field of vision and (2) whether they had any evidence or experiences that led them to believe that they had defects in their field of vision. They then had their vision assessed by binocular central visual field testing using the California Central Visual Field Test, binocular reading performance evaluated using the Smith-Kettlewell Reading Test (SK Read) and MN Read charts, and visual acuity measured using the ETDRS chart at 1 meter. Mean diameters of the scotomas with borders near fixation were noted. RESULTS: Visual acuity median was 20/253 (range 20/40 to hand movements). Binocular scotomas were present in 88% of patients (66% had dense scotoma). Of patients with binocular scotomas, 56% were totally unaware of their presence, even with dense scotomas measuring up to 30° in diameter; 1.5% could fleetingly see a defect in their visual field on waking; and 44% related experiences of things "disappearing" on them. The median and range of scotoma diameters for those unaware vs. those with some awareness of their scotomas were comparable. There was no significant relationship of awareness of the scotoma with age, acuity, scotoma size, density, or duration of onset. Awareness of scotoma was associated with fewer errors on the SK Read (p < 0.01). CONCLUSIONS: Low vision clinicians cannot depend on patients to report the presence of significant scotomas; thus, appropriate testing must be performed. Presence of scotomas decreased reading accuracy, but some awareness of the scotomas had a tendency to improve accuracy. The value of rehabilitation programs aimed at increasing patient awareness of their scotomas may be supported by this evidence.

Relationship between vision-related quality of life and different types of existing visual fields in Japanese patients.

To investigate the vision-related quality of life (VRQOL) in relation to the type of existing visual field (EVF) in the presence of visual field defect. We evaluated the VRQOL of 95 patients by using Visual Function Questionnaire-25 (NEI VFQ-25). Patients fulfilled the following criteria: (1) they underwent a Goldmann perimetry (GP) test within 3 months of the initial visit, (2) the results of the GP test could be classified into three categories, and (3) the best-corrected decimal visual acuity (VA) was ≤0.3. The EVF was assessed by composite measurement of both eyes for the V-4e or I-4e isopters, as obtained using the GP test. We compared the VFQ-25 scores between three groups: (1) the "center and peripheral" group in which the EVF of the V-4e area was >30º and the I-4e area was <30º, (2) the "central VF" group in which the EVF of the V-4e area was <20º, and (3) the "central VF loss" group in which the central scotoma was >5º and the EVF of the V-4e area was >30º in peripheral vision. There were significant differences between the three groups (P = 0.02, ANOVA). The total score of the "central VF loss" group was significantly lower than those of the "center and peripheral" and "central VF" groups. The central vision influenced the VRQOL to a greater extent than peripheral vision.

Gray matter alterations in visual cortex of patients with loss of central vision due to hereditary retinal dystrophies.

In patients with central visual field scotomata a large part of visual cortex is not adequately stimulated. Over time this lack of input could lead to a reduction of gray matter in the affected cortical areas. We used Voxel Based Morphometry to investigate structural brain changes in patients with central scotomata due to hereditary retinal dystrophies and compared their results to those of normal sighted subjects. Additionally we correlated clinical and demographic characteristics like duration of disease, scotoma size, visual acuity, fixation stability and reading speed to the amount of gray matter in whole brain analyses within the patient group. We found a decrease in gray matter around the lesion projection zone in visual cortex of patients in comparison to controls. Gray matter loss along the posterior and middle portions of the calcarine sulcus is also correlated with scotoma size, indicating that indeed the lack of functional input provokes the gray matter alterations. In whole brain regression analyses within the patient group we found an additional cluster in the right superior and middle frontal gyri, slightly anterior to the frontal eye fields, where gray matter correlated positively with fixation stability. This could be regarded as a consequence of oculomotor learning.

Line bisection in homonymous visual field defects - Recent findings and future directions.

Homonymous visual field defects are a frequent consequence of brain damage occurring in some 20-40% of stroke patients. They are often accompanied by a peculiar spatial bias termed "Hemianopic Line Bisection Error" (HLBE). Although known for more than 100 years the explanations for the HLBE put forward remain controversial. One explanation holds that the HLBE is a direct consequence of the field defect itself and reflects a compensatory shift of attention towards the scotoma. Another, contradicting position states that although the HLBE is frequently found in any type of homonymous visual field defect - not only hemianopia - it is not simply a direct consequence of the field defect itself, although it does contribute to it. According to this position, the HLBE arises from additional damage to extrastriate cortex, thus causing the spatial bias towards the blind field. In the present article we summarize the main arguments of both theoretical positions and argue that although both accounts are valid, they are incomplete and several important issues remain unresolved. These include the potential contribution of eccentric fixation to the HLBE, the question of multimodal impairments, the role of (visuo)-motor processes, the relation between the HLBE and visual field recovery, and the exact clinical significance of the HLBE. Thus, far from concluding the research on the line-bisection error in hemianopia, the recent series of publications on this topic serve as a welcome reminder of how much more research is needed.

Line bisection in unilateral homonymous visual field defects.

The contralesional line bisection error in unilateral homonymous hemianopia is a frequent but neglected clinical phenomenon. Our knowledge about this bisection error is based on small samples of hemianopic patients. Moreover, horizontal line bisection has never been investigated in other unilateral visual field defects. The present study is the first to examine line bisection in a large, representative sample of patients with unilateral homonymous visual field defects. We investigated horizontal line bisection in 129 patients with left- or right-sided homonymous hemianopia (60.5%), upper and lower quadranopia (24.8%), and paracentral scotoma (14.7%), and determined the magnitude and direction of line bisection error. The contralesional horizontal line bisection error was present not only in patients with hemianopia but also in those with upper or lower quadranopia or paracentral scotoma. Neither the type nor the severity of the visual field defect was found to determine the bisection error. Only the side of the field defect seemed to determine the horizontal direction of the bisection error (left-/rightward). The contralesional bisection error is not a specifically "hemianopic" phenomenon. It is frequently associated with any unilateral homonymous visual field defect, i.e., hemianopia, upper/lower quadranopia, paracentral scotoma. Moreover, our results further support the recent finding that the contralesional bisection error is not a direct consequence of the visual field defect. Yet, they also suggest that, although the visual field defect does not seem to be the primary cause of the contralesional bisection error, it may nevertheless contribute to it.

Effect of depression on actual and perceived effects of reading rehabilitation for people with central vision loss.

To investigate the relationship between depression and quantitative measures of visual function, we recruited 18 subjects with central scotomas from macular degeneration who were enrolled in a reading rehabilitation program. Psychological batteries and reading assessments were administered prior to rehabilitation; reading assessments and a measure of adaptation to vision loss were administered following rehabilitation. We investigated relationships between reported levels of depressive symptoms and reading and adaptation outcome measures by using Pearson product moment correlation analysis. Results revealed a significant relationship between depression levels and reading acuity difference scores (r(16) = 0.54, p = 0.02) and changes in adaptation to vision loss levels (r(16) = 0.62, p = 0.01), suggesting that those who reported greater depressive symptoms did not respond as well functionally to reading rehabilitation but reported greater improvement in levels of adaptation to vision loss following rehabilitation. Future research should focus on defining standard methods to assess and remediate depression as part of the rehabilitation process.

The mechanisms of vision loss associated with a cotton wool spot.

We characterized the perceptual, functional, and structural abnormalities associated with retinal ischemia during a cotton wool spot episode and its sequelae. The border of the visually salient field anomaly mirrored the quantitatively measured relative scotoma. Results of resolution perimetry and high resolution imaging indicated that there was a substantial loss of retinal ganglion cells within the affected region. A disruption in retinal nerve fiber arrangement was found at the cotton wool spot and within the arcuate relative scotoma. The presence of the arcuate relative scotoma is consistent with the hypothesis of failed signal transmission along the axons that pass through the cotton wool spot. The different levels of loss associated with the arcuate and focal scotomas indicate different underlying pathologies.

Neural correlates of hemianopic completion across the vertical meridian.

Hemianopic completion refers to the perceptual completion of figures located across the vertical meridian in the context of hemianopia, such that one half of the figure falls within the blind hemifield. It can occur whether the figure is itself complete (veridical completion) or incomplete (paracompletion). Psychophysical evidence suggests that this phenomenon may be a constructive one, and may share features with completion phenomena in normal vision. The neural structures mediating hemianopic completion are unknown. Here we studied the neural activity evoked by hemianopic completion using event-related fMRI in an individual (POV) with a large right visual field homonymous hemianopic scotoma due to left occipital damage. Either a large achromatic circular contour straddling the vertical meridian or a semicircular contour within the left hemifield just crossing the vertical meridian was presented to POV on each trial. POV indicated by button press whether he perceived a semicircular contour, a patchy circular contour or a complete circular contour. On trials where he reported perceiving a complete circular contour despite being presented with a semicircular contour (paracompletion), activity was increased in a region of ipsilateral extrastriate cortex (contralateral to the lesion, ipsilateral to the illusory edge of the circle). These results are discussed in the context of illusory contour completion in healthy subjects and more generally in the recovery of function after brain damage.

Traffic gap judgment in people with significant peripheral field loss.

PURPOSE: Subjects with significant peripheral field loss (PFL) self report difficulty in street crossing. In this study, we compared the traffic gap judgment ability of fully sighted and PFL subjects to determine whether accuracy in identifying crossable gaps was adversely affected because of field loss. Moreover, we explored the contribution of visual and nonvisual factors to traffic gap judgment ability. METHODS: Eight subjects with significant PFL as a result of advanced retinitis pigmentosa or glaucoma with binocular visual field <20 degrees and five age-matched normals (NV) were recruited. All subjects were required to judge when they perceived it was safe to cross at a 2-way 4-lane street while they stood on the curb. Eye movements were recorded by an eye tracker as the subjects performed the decision task. Movies of the eye-on-scene were made offline and fixation patterns were classified into either relevant or irrelevant. Subjects' street-crossing behavior, habitual approach to street crossing, and perceived difficulties were assessed. RESULTS: Compared with normal vision (NV) subjects, the PFL subjects identified 12% fewer crossable gaps while making 23% more errors by identifying a gap as crossable when it was too short (p < 0.05). The differences in traffic gap judgment ability of the PFL subjects might be explained by the significantly smaller fixation area (p = 0.006) and fewer fixations distributed to the relevant tasks (p = 0.001). The subjects' habitual approach to street crossing and perceived difficulties in street crossing (r > 0.60) were significantly correlated with traffic gap judgment performance. CONCLUSIONS: As a consequence of significant field loss, limited visual information about the traffic environment can be acquired, resulting in significantly reduced performance in judging safe crossable gaps. This poor traffic gap judgment ability in the PFL subjects raises important concerns for their safety when attempting to cross the street.

Quantification of stato-kinetic dissociation by semi-automated perimetry.

The difference in threshold sensitivities that are found when examining the visual field (VF) with static versus kinetic perimetric methods is called stato-kinetic dissociation (SKD). In this pilot study, we describe a semi-automated procedure for quantifying SKD. Fifteen patients with VF defects were examined with kinetic and static perimetry. SKD values were defined as positive when the static scotoma was larger than the kinetic one. We found significant local variations of SKD along scotoma borders with the individual reaction time as an important criterion when determining kinetic thresholds. There was a verifiable SKD in all patients with locally negative values in eight subjects.

Artificial scotoma-induced perceptual distortions are orientation dependent and short lived.

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.

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 degrees ), 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.