89 research outputs found
Increased visual sensitivity and occipital activity in patients with hemianopia following vision rehabilitation
Acknowledgements We would like to thank the participants for giving up their time to engage with the research.Peer reviewedPublisher PD
No Effect of Anodal Transcranial Direct Current Stimulation (tDCS) Over hMT+ on Motion Perception Learning
Background: Human visual cortical area hMT+, like its homolog MT in the macaque monkey, has been shown to be particularly selective to visual motion. After damage to the primary visual cortex (V1), patients often exhibit preserved ability to detect moving stimuli, which is associated with neural activity in area hMT+. As an anatomical substrate that underlies residual function in the absence of V1, promoting functional plasticity within hMT+ could potentially boost visual performance despite primary visual cortical damage.Objective: To establish in healthy participants whether it is possible to use transcranial direct current stimulation (tDCS) over hMT+ to potentiate learning of visual motion direction discrimination.Methods: Twenty-one participants were trained daily for 5 days on a visual motion direction discrimination task. Task difficulty was increased as performance improved, by decreasing the proportion of coherently moving dots, such that participants were always performing at psychophysical threshold. tDCS, either anodal or sham, was applied daily during 20 min of training. Task performance was assessed at baseline and at the end of the training period. Performance was also compared with a third group of 10 participants from an earlier study who had undergone the same procedures but without tDCS.Results: All participants showed improved task performance both during and after training. Contrary to our hypothesis, anodal tDCS did not further improve performance compared to sham stimulation or no stimulation. Bayesian statistics indicated weak evidence in favor of the null hypothesis.Conclusion: This study found no evidence for a robust effect of anodal tDCS over hMT+ on visual motion direction discrimination learning in the young healthy visual system, although more subtle effects may have been missed in the relatively small sample size
Neural Modulation by Binocular Disparity Greatest in Human Dorsal Visual Stream
Although cortical
activation to binocular disparity can be demonstrated throughout
occipital and parietal cortices, the relative contributions to depth
perception made by different human cortical areas have not been
established. To investigate whether different regions are optimized for
specific disparity ranges, we have measured the responses of occipital
and parietal areas to different magnitudes of binocular disparity.
Using stimuli consisting of sinusoidal depth modulations, we measured
cortical activation when the stimuli were located at pedestal
disparities of 0, 0.1, 0.35, and 0.7° from fixation. Across all areas,
occipital and parietal, there was an increase in BOLD signal with
increasing pedestal disparity, compared with a plane at zero disparity.
However, the greatest modulation of response by the different pedestals
was found in the dorsal visual areas and the parietal areas. These
differences contrast with the response to the zero disparity plane,
compared with fixation, which is greatest in the early visual areas,
smaller in the ventral and dorsal visual areas, and absent in parietal
areas. Using the simultaneously acquired psychophysical data we also
measured a greater response to correct than to incorrect trials, an
effect that increased with rising pedestal disparity and was greatest in
dorsal visual and parietal areas. These results illustrate that the dorsal
stream, along both its occipital and parietal branches, can reliably
discriminate a large range of disparities
Responses to interocular disparity correlation in the human cerebral cortex
Purpose: Perceiving binocular depth relies on the ability of our visual system to precisely match corresponding features in the left and right eyes. Yet how the
human brain extracts interocular disparity correlation is poorly understood.
Methods: We used functional magnetic resonance imaging (fMRI) to characterize brain regions involved in processing interocular disparity correlation. By varying
the amount of interocular correlation of a disparity-defined random-dot-stereogram, we concomitantly controlled the perception of binocular depth and measured the percent Blood-Oxygenation-Level-Dependent (%BOLD)-signal in
multiple regions-of-interest in the human occipital cortex and along the intraparietal sulcus.
Results: A linear support vector machine classification analysis applied to cortical responses showed patterns of activation that represented different disparity correlation levels within regions-of-interest in the visual cortex. These also revealed a positive trend between the difference in disparity correlation and classification accuracy in V1, V3 and lateral occipital cortex. Classifier performance was significantly related to behavioural performance in dorsal visual area V3. Cortical responses to random-dot-stereogram stimuli were greater in the right compared to the left hemisphere.
Conclusions: Our results show that multiple regions in the cerebral cortex are sensitive to changes in interocular disparity correlation, and that dorsal area V3
may play an important role in the early transformation of binocular disparity to depth perception
Assessment of Scotopic Function in Rod–Cone Inherited Retinal Degeneration With the Scotopic Macular Integrity Assessment
Purpose: The scotopic macular integrity assessment (S-MAIA) can perform scotopic assessment to detect localized changes to scotopic rod and cone function. This study is an exploratory investigation of the feasibility of using the S-MAIA in a rod–cone dystrophy population to identify the pattern of loss in scotopic photoreceptor function.
Methods: Twenty patients diagnosed with a rod–cone dystrophy underwent visual acuity testing, full-field stimulus threshold assessment, and multiple S-MAIA tests after dark adaptation periods of 20 minutes and 45 minutes performed separately. Only right eyes were tested. Three tests were performed following a learning test. A Bland–Altman analysis was used to assess repeatability and agreement between tests after the two time periods. Spatial interpolation maps were created from the group plots to display the pattern of rod and cone loss.
Results: Learning effects took place between testing sessions 1 and 2 but not 2 and 3. Limits of agreement were larger in the patient eyes than control eyes, but within previously reported values. Using longer adaptation time of 45 minutes did not offer a significant advantage over 20 minutes. Patterns for the cyan and red sensitivities were different, indicating different patterns of loss for rods and cones.
Conclusions: A dark adaptation time of 20 minutes before testing is sufficient for thresholding. The S-MAIA is suitable for use in patients with a logarithm of the minimum angle of resolution vision of at least 0.7 and provides a viable outcome measure for patients with rod–cone dystrophies and preserved central vision. The spatial information about scotopic function from the S-MAIA provides information about disease processes and progression.
Translational Relevance: There is a need for scotopic measures for use in clinical trials. Scotopic microperimetry works well in patients with early disease, allowing the extension of recruitment criteria for novel therapies of rod–cone dystrophies
Structural and Functional Changes across the Visual Cortex of a Patient with Visual Form Agnosia
Loss of shape recognition in visual-form agnosia occurs without equivalent losses in the use of vision to guide actions, providing support
for the hypothesis of two visual systems (for “perception” and “action”). The human individual DF received a toxic exposure to carbon
monoxide some years ago, which resulted in a persisting visual-form agnosia that has been extensively characterized at the behavioral
level.We conducted a detailed high-resolution MRI study of DF’s cortex, combining structural andfunctional measurements.We present
the first accurate quantification of the changes in thickness across DF’s occipital cortex, finding the most substantial loss in the lateral
occipital cortex (LOC). There are reduced white matter connections between LOC and other areas. Functional measures show pockets of
activity that survive within structurally damaged areas. The topographic mapping of visual areas showed that ordered retinotopic maps
were evident for DF in the ventral portions of visual cortical areas V1, V2, V3, and hV4. Although V1 shows evidence of topographic order
in its dorsal portion, such maps could not befound inthe dorsal parts of V2 and V3. We concludethat it is not possibleto understandfully
the deficits in object perception in visual-form agnosia without the exploitation of both structural and functional measurements. Our
results also highlight for DF the cortical routes through which visual information is able to pass to support her well-documented abilities
to use visual information to guide actions
Ventral extra-striate cortical areas are required for optimal orientation averaging
We examined the ability of a previously well-studied patient with visual agnosia to compute the average orientation of elements in visual displays. In a structural MRI study, we show that the lesion is likely to involve a variety of ventral extra-striate areas, including V2, V3 and V4; however, the lesion does not extend dorsally. Subsequently we show that some ability to compute average orientation is spared, though there are limitations on the ability to scale the averaging process as a function of the numbers of elements. The results suggest that some aspects of orientation averaging can be accomplished in spared regions of V1 but flexible averaging requires ventral extra-striate cortex
Vivid visual mental imagery in the absence of the primary visual cortex
The role of the primary visual cortex in visual mental imagery has provided significant debate in the imagery literature. Functional neuroimaging studies show considerable variation depending on task and technique. Patient studies can be difficult to interpret due to the diverse nature of cortical damage. The type of cortical damage in patient SBR is exceedingly rare as it is restricted to the gray matter of the calcarine sulcus. In this study, we show that in spite of his near-complete cortical blindness, SBR exhibits vivid visual mental imagery both behaviorally and when measured with functional magnetic resonance imaging. The pattern of cortical activation to visual mental imagery in SBR is indistinguishable from individual sighted subjects, in contrast to the visual perceptual responses, which are greatly attenuated
Structural and Functional Characteristics of Color Vision Changes in Choroideremia
Color vision is considered a marker of cone function and its assessment in patients with retinal pathology is complementary to the assessments of spatial vision [best-corrected visual acuity (BCVA)] and contrast detection (perimetry). Rod-cone and chorioretinal dystrophies—such as choroideremia—typically cause alterations to color vision, making its assessment a potential outcome measure in clinical trials. However, clinical evaluation of color vision may be compromised by pathological changes to spatial vision and the visual field. The low vision Cambridge Color Test (lvCCT) was developed specifically to address these latter issues. We used the trivector version of the lvCCT to quantify color discrimination in a cohort of 53 patients with choroideremia. This test enables rapid and precise characterization of color discrimination along protan, deutan, and tritan axes more reliably than the historically preferred test for clinical trials, namely the Farnsworth Munsell 100 Hue test. The lvCCT demonstrates that color vision defects—particularly along the tritan axis—are seen early in choroideremia, and that this occurs independent of changes in visual acuity, pattern electroretinography and ellipsoid zone area on optical coherence tomography (OCT). We argue that the selective loss of tritan color discrimination can be explained by our current understanding of the machinery of color vision and the pathophysiology of choroideremia
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