In primary visual cortex fMRI responses to chromatic and achromatic stimuli are interdependent and predict contrast detection thresholds

Chromatic and achromatic signals in primary visual cortex have historically been considered independent of each other but have since shown evidence of interdependence. Here, we investigated the combination of two components of a stimulus; an achromatic dynamically changing check background and a chromatic (L-M or S cone) target grating. We found that combinations of chromatic and achromatic signals in primary visual cortex were interdependent, with the dynamic range of responses to chromatic contrast decreasing as achromatic contrast increased. A contrast detection threshold study also revealed interdependence of background and target, with increasing chromatic contrast detection thresholds as achromatic background contrast increased. A model that incorporated a normalising effect of achromatic contrast on chromatic responses, but not vice versa, best predicted our V1 data as well as behavioural thresholds. Further along the visual hierarchy, the dynamic range of chromatic responses was maintained when compared to achromatic responses, which became increasingly compressive.

Improved structure and function in early detected second eye neovascular age-related macular degeneration; FASBAT/EDNA report 1

PURPOSE

Visual Acuity (VA) and structural biomarker assessment before and at 24-months after early detection and routine treatment of second eye involvement with neovascular age-related macular degeneration (nAMD) and additional comparison with the first eye affected.

DESIGN

Prospective, 22-centre observational study of participants with unilateral nAMD in the Early Detection of Neovascular AMD (EDNA) study, co-enrolled into the Observing fibrosis, macular atrophy and subretinal highly reflective material, before and after intervention with anti-VEGF treatment (FASBAT) study for an additional 2-year follow-up.

PARTICIPANTS

Older adults (>50 years) with new onset nAMD in the first eye.

METHODS

Assessment of both eyes with optical coherence tomography (OCT), colour fundus photography (CFP), clinic-measured visual acuity (VA) and quality-of-life (QoL).

MAIN OUTCOME MEASURES

Prevalence of Atrophy, Subretinal Hyperreflective Material (SHRM), Intraretinal fluid (IRF), Subretinal fluid (SRF) and changes in VA over the study duration in both the first and second eyes affected with nAMD. Composite QoL scores over time.

RESULTS

Of 431 participants recruited to the FASBAT study, the second eye converted to nAMD in 100 participants at a mean of 18.9 months. VA was 18 letters better at the time of early diagnosis in the second eye compared with conventional diagnosis in the first eye (72.9 vs 55.6 letters). 24.9-months post-conversion in the second eye, VA was 69.5 letters compared with at a similar matched time point in the first eye (59.7 letters; 18.9 months). A greater proportion of participants had vision >70 letters in the second eye versus the first eye, 24.9-months post-conversion (61 vs 38). Prevalence of SHRM and IRF was lower in the second eye compared with the first eye at 24.9-months post-conversion to nAMD. However, SRF prevalence was greater in the second eye at 24.9-months post-conversion. The development and progression of total area of atrophy appears similar in both eyes. Mean composite QoL scores increased over time, with a significant correlation between VA for the second eye only 24.9 months post-conversion.

CONCLUSION

This study has shown that early detection of exudative AMD in the second eye is associated with reduced prevalence of SHRM and IRF and greater visual acuity which is significantly correlated with maintained quality-of-life.

Poster Session II: SSVEP measurements of color and spatial frequency response in V1

INTRO

Recent work from our group (Segala et al, eLife, 2023) shows that the rules for binocular luminance signal combination depend on spatial frequency (SF). Structured patterns show strong interocular suppression while unstructured inputs (mean field disks) do not. Here, we used SSVEPs to ask if SF dependence is also found in chromatic pathways.

METHODS

SSVEPs were recorded from 12 subjects using a canonical V1 template (Poncet & Ales, 2023). Eyes were targeted using shutter goggles and stimuli were contrast-reversing gratings or disks at 5Hz (left eye) and 7Hz (right eye). Experimental factors were stimulus SF (disk, grating 1cpd), chromaticity (LMS, L-M or S-cone isolating) and ocularity (left, right or both).

RESULTS

Monocular conditions generated large responses at 2F. In binocular conditions, all 2F responses showed suppression, and significant intermodulation (IM) terms (sums and differences of the inputs - e.g., 2Hz) were present. The magnitude of both suppression and IM in the binocular condition depended on SF and chromaticity; IM amplitudes were higher for gratings compared to disks in the luminance condition, but higher for disks compared to gratings in the chromatic conditions. Overall we found significant differences in the spectral response signatures across all stimulus combinations.

CONCLUSION

All inputs undergo binocular combination in V1 but the rules governing the combination appear to depend on both chromaticity and SF.

Achromatopsia-Visual Cortex Stability and Plasticity in the Absence of Functional Cones

Purpose

Achromatopsia is a rare inherited disorder rendering retinal cone photoreceptors nonfunctional. As a consequence, the sizable foveal representation in the visual cortex is congenitally deprived of visual input, which prompts a fundamental question: is the cortical representation of the central visual field in patients with achromatopsia remapped to take up processing of paracentral inputs? Such remapping might interfere with gene therapeutic treatments aimed at restoring cone function.

Methods

We conducted a multicenter study to explore the nature and plasticity of vision in the absence of functional cones in a cohort of 17 individuals affected by autosomal recessive achromatopsia and confirmed biallelic disease-causing CNGA3 or CNGB3 mutations. Specifically, we tested the hypothesis of foveal remapping in human achromatopsia. For this purpose, we applied two independent functional magnetic resonance imaging (fMRI)-based mapping approaches, i.e. conventional phase-encoded eccentricity and population receptive field mapping, to separate data sets.

Results

Both fMRI approaches produced the same result in the group comparison of achromatopsia versus healthy controls: sizable remapping of the representation of the central visual field in the primary visual cortex was not apparent.

Conclusions

Remapping of the cortical representation of the central visual field is not a general feature in achromatopsia. It is concluded that plasticity of the human primary visual cortex is less pronounced than previously assumed. A pretherapeutic imaging workup is proposed to optimize interventions.

The emergence of tuning to global shape properties of radial frequency patterns in the ventral visual pathway

Radial frequency patterns - created by sinusoidal modulations of a circle's radius - are processed globally when radial frequency is low. These closed shapes therefore offer a useful way to interrogate the human visual system for global processing of curvature. Radial frequency patterns elicit greater responses than those to radial gratings in V4 and more anterior face selective regions of the ventral visual pathway. This is largely consistent with work on non-human primates showing curvature processing emerges in V4, but is evident also higher up the ventral visual stream. Rather than contrasting radial frequency patterns with other stimuli, we presented them at varied frequencies in a regimen that allowed tunings to radial frequency to be derived from 8 human participants (3 female). We found tuning to low radial frequency in lateral occipital areas and to some extent in V4. In a control experiment we added a high frequency ripple to the stimuli disrupting the local contour. Low frequency tuning to these stimuli remained in the ventral visual stream underscoring its role in global processing of shape curvature. We then used representational similarity analysis to show that in lateral occipital areas the neural representation was related to stimulus similarity, when it was computed with a model that captured how stimuli are perceived. We show therefore that global processing of shape curvature emerges in the ventral visual stream as early as V4, but is found more strongly in lateral occipital regions, which exhibit responses and representations that relate well to perception.Significance StatementWe show tuning to low radial frequencies, known to engage global shape processing mechanisms, was localised to lateral occipital regions. When low level stimulus properties were accounted for such tuning emerged in V4 and LO2 in addition to the object selective region LO. We also documented representations of global shape properties in lateral occipital regions and these representations were predicted well by a proxy of the perceptual difference between the stimuli.

Assessing the structure of the posterior visual pathway in bilateral macular degeneration

Macular degeneration (MD) embodies a collection of disorders causing a progressive loss of central vision. Cross-sectional MRI studies have revealed structural changes in the grey and white matter in the posterior visual pathway in MD but there remains a need to understand how such changes progress over time. To that end we assessed the posterior pathway, characterising the visual cortex and optic radiations over a ~ 2-year period in MD patients and controls. We performed cross-sectional and longitudinal analysis of the former. Reduced cortical thickness and white matter integrity were observed in patients compared to controls, replicating previous findings. While faster, neither the rate of thinning in visual cortex nor the reduction in white matter integrity during the ~ 2-year period reached significance. We also measured cortical myelin density; cross-sectional data showed this was higher in patients than controls, likely as a result of greater thinning of non-myelinated tissue in patients. However, we also found evidence of a greater rate of loss of myelin density in the occipital pole in the patient group indicating that the posterior visual pathway is at risk in established MD. Taken together, our results revealed a broad decline in grey and white matter in the posterior visual pathway in bilateral MD; cortical thickness and fractional anisotropy show hints of an accelerated rate of loss also, with larger effects emerging in the occipital pole.

Cortical Atrophy Predicts Visual Performance in Long-Term Central Retinal Disease; GCL, pRNFL and Cortical Thickness Are Key Biomarkers

Purpose

The aim of this study was to assess both retinal and cortical structure in a cohort of patients with long-term acquired central retinal disease in order to identify potential disease biomarkers and to explore the relationship between the anterior and posterior visual pathways.

Methods

Fourteen participants diagnosed with long-term central retinal disease underwent structural assessments of the retina using spectral-domain optical coherence tomography, including macular ganglion cell layer (GCL) and peripapillary retinal nerve fiber layer (pRNFL) thickness. Structural magnetic resonance imaging was used to measure visual cortex, including cortical volume of the entire occipital lobe and cortical thickness of the occipital pole and calcarine sulcus, representing the central and peripheral retina, respectively.

Results

Mean thickness was significantly reduced in both the macular GCL and the inferior temporal pRNFL across patients. Cortical thickness was significantly reduced in both the occipital pole and calcarine sulcus, representing the central and peripheral retina, respectively. Disease duration significantly correlated with GCL thickness with a large effect size, whereas a medium effect size suggests the possibility that cortical thickness in the occipital pole may correlate with visual acuity.

Conclusions

Long-term central retinal disease is associated with significant structural changes to both the retina and the brain. Exploratory analysis suggests that monitoring GCL thickness may be a sensitive biomarker of disease progression and reductions in visual cortical thickness may be associated with reduced visual acuity. Although this study is limited by its heterogeneous population, larger cohort studies would be needed to better establish some of the relationships detected between disease dependent structural properties of the anterior and posterior visual pathway given the effect sizes reported in our exploratory analysis.

Structural changes to primary visual cortex in the congenital absence of cone input in achromatopsia

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Anatomy of primary visual cortex (V1) assessed with surface-based morphmetry in those with congenital achromatopsia (ACHM).

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Reduction in cortical surface area in foveal, parafoveal and paracentral representations of V1 in those with ACHM.

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In ACHM a localized thickening in the area of V1 that represents the region of retina occupied solely by cones.

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V1 changes in ACHM may limit its ability to take on normal properties if retinal function were to be restored.

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Early intervention, before the development plastic period is over, may offer better restoration of vision in ACHM.

Autosomal recessive Achromatopsia (ACHM) is a rare inherited disorder associated with dysfunctional cone photoreceptors resulting in a congenital absence of cone input to visual cortex. This might lead to distinct changes in cortical architecture with a negative impact on the success of gene augmentation therapies. To investigate the status of the visual cortex in these patients, we performed a multi-centre study focusing on the cortical structure of regions that normally receive predominantly cone input. Using high-resolution T1-weighted MRI scans and surface-based morphometry, we compared cortical thickness, surface area and grey matter volume in foveal, parafoveal and paracentral representations of primary visual cortex in 15 individuals with ACHM and 42 normally sighted, healthy controls (HC). In ACHM, surface area was reduced in all tested representations, while thickening of the cortex was found highly localized to the most central representation. These results were comparable to more widespread changes in brain structure reported in congenitally blind individuals, suggesting similar developmental processes, i.e., irrespective of the underlying cause and extent of vision loss. The cortical differences we report here could limit the success of treatment of ACHM in adulthood. Interventions earlier in life when cortical structure is not different from normal would likely offer better visual outcomes for those with ACHM.

Structural Differences Across Multiple Visual Cortical Regions in the Absence of Cone Function in Congenital Achromatopsia

Most individuals with congenital achromatopsia (ACHM) carry mutations that affect the retinal phototransduction pathway of cone photoreceptors, fundamental to both high acuity vision and colour perception. As the central fovea is occupied solely by cones, achromats have an absence of retinal input to the visual cortex and a small central area of blindness. Additionally, those with complete ACHM have no colour perception, and colour processing regions of the ventral cortex also lack typical chromatic signals from the cones. This study examined the cortical morphology (grey matter volume, cortical thickness, and cortical surface area) of multiple visual cortical regions in ACHM (n = 15) compared to normally sighted controls (n = 42) to determine the cortical changes that are associated with the retinal characteristics of ACHM. Surface-based morphometry was applied to T1-weighted MRI in atlas-defined early, ventral and dorsal visual regions of interest. Reduced grey matter volume in V1, V2, V3, and V4 was found in ACHM compared to controls, driven by a reduction in cortical surface area as there was no significant reduction in cortical thickness. Cortical surface area (but not thickness) was reduced in a wide range of areas (V1, V2, V3, TO1, V4, and LO1). Reduction in early visual areas with large foveal representations (V1, V2, and V3) suggests that the lack of foveal input to the visual cortex was a major driving factor in morphological changes in ACHM. However, the significant reduction in ventral area V4 coupled with the lack of difference in dorsal areas V3a and V3b suggest that deprivation of chromatic signals to visual cortex in ACHM may also contribute to changes in cortical morphology. This research shows that the congenital lack of cone input to the visual cortex can lead to widespread structural changes across multiple visual areas.

Assessing functional reorganization in visual cortex with simulated retinal lesions

Macular degeneration (MD) causes central vision loss, removing input to corresponding representations in the primary visual cortex. There is disagreement concerning whether the cortical regions deprived of input can remain responsive, and the source of reported cortical responses is still debated. To simulate MD in controls, normally sighted participants viewed a bright central disk to adapt the retina, creating a transient ‘retinal lesion’ during a functional MRI experiment. Participants viewed blocks of faces, scrambled faces and uniform grey stimuli, either passively or whilst performing a one-back task. To assess the impact of the simulated lesion, participants repeated the paradigm using a more conventional mean luminance simulated scotoma without adaptation. Our results suggest our attempt to create a more realistic simulation of a lesion did not impact on responses in the representation of the simulated lesion. While most participants showed no evidence of stimulus-driven activation within the lesion representation, a few individuals (22%) exhibited responses similar to a participant with juvenile MD who completed the same paradigm (without adaptation). Reliability analysis showed that responses in the representation of the lesion were generally consistent irrespective of whether positive or negative. We provide some evidence that peripheral visual stimulation can also produce responses in central representations in controls while performing a task. This suggests that the ‘signature of reorganization of visual processing’, is not found solely in patients with retinal lesions, consistent with the idea that activity may be driven by unmasked top–down feedback.

Electronic retinal prosthesis for severe loss of vision in geographic atrophy in age-related macular degeneration: First-in-human use

BACKGROUND

To date there are yet no available approved therapies for Geographic Atrophy (GA) secondary to age-related macular degeneration (AMD).

METHODS

Single site, non-randomized safety and efficacy study presenting the preliminary results in a cohort of five late stage AMD (GA) patients successfully implanted with the Argus II Retinal Prosthesis System (Second Sight Medical Products Inc., Sylmar, CA, USA). Extensive fundus imaging including retinal photographs from which the GA area was measured. A combination of custom and traditional tests designed for very low vision subjects assessed visual function in study subjects. A Functional Low-Vision Observer Rated Assessment was carried out to evaluate the impact of the system on the subject's daily life. In addition, a study to evaluate structural characteristics of the visual cortex of the brain was performed in one subject using magnetic resonance imaging.

RESULTS

Seven device-related adverse events were reported, four of which were classed as serious adverse events. Retinal detachment was reported in three patients and was successfully treated within 12 months of onset. Testing showed an improvement in visual function in three of five patients with the system turned on. Magnetic resonance imaging assessed in one patient after implantation indicates a selective increase in cortical myelin and thickness in visual brain regions 1 year post implantation.

CONCLUSIONS

Epiretinal prostheses can successfully be implanted in those affected by GA secondary to late-stage AMD and can elicit visual percepts by electrical stimulation of residual neuroretinal elements and improve basic visual function in those affected.

Cortical Reorganization: Reallocated Responses without Rewiring

Is the brain able to reorganise following loss of sensory input? New work on individuals with sight loss shows that, while brain areas normally allocated to vision respond to other sensory stimuli, those responses are unlikely to mean the brain has rewired.

Triple visual hemifield maps in a case of optic chiasm hypoplasia

In humans, each hemisphere comprises an overlay of two visuotopic maps of the contralateral visual field, one from each eye. Is the capacity of the visual cortex limited to these two maps or are plastic mechanisms available to host more maps? We determined the cortical organization of the visual field maps in a rare individual with chiasma hypoplasia, where visual cortex plasticity is challenged to accommodate three hemifield maps. Using high-resolution fMRI at 7T and diffusion-weighted MRI at 3T, we found three hemiretinal inputs, instead of the normal two, to converge onto the left hemisphere. fMRI-based population receptive field mapping of the left V1-V3 at 3T revealed three superimposed hemifield representations in the left visual cortex, i.e. two representations of opposing visual hemifields from the left eye and one right hemifield representation from the right eye. We conclude that developmental plasticity including the re-wiring of local intra- and cortico-cortical connections is pivotal to support the coexistence and functioning of three hemifield maps within one hemisphere.

Quantifying nerve decussation abnormalities in the optic chiasm

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Diffusion MRI is capable of detecting structural abnormalities of the optic chiasm.

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Quantification of crossing strength in optic chiasm is of promise for albinism diagnostics.

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Optic chiasm is a powerful test model for neuroimaging methods resolving crossing fibers.

Objective

The human optic chiasm comprises partially crossing optic nerve fibers. Here we used diffusion MRI (dMRI) for the in-vivo identification of the abnormally high proportion of crossing fibers found in the optic chiasm of people with albinism.

Methods

In 9 individuals with albinism and 8 controls high-resolution 3T dMRI data was acquired and analyzed with a set of methods for signal modeling [Diffusion Tensor (DT) and Constrained Spherical Deconvolution (CSD)], tractography, and streamline filtering (LiFE, COMMIT, and SIFT2). The number of crossing and non-crossing streamlines and their weights after filtering entered ROC-analyses to compare the discriminative power of the methods based on the area under the curve (AUC). The dMRI results were cross-validated with fMRI estimates of misrouting in a subset of 6 albinotic individuals.

Results

We detected significant group differences in chiasmal crossing for both unfiltered DT (p = 0.014) and CSD tractograms (p = 0.0009) also reflected by AUC measures (for DT and CSD: 0.61 and 0.75, respectively), underlining the discriminative power of the approach. Estimates of crossing strengths obtained with dMRI and fMRI were significantly correlated for CSD (R² = 0.83, p = 0.012). The results show that streamline filtering methods in combination with probabilistic tracking, both optimized for the data at hand, can improve the detection of crossing in the human optic chiasm.

Conclusions

Especially CSD-based tractography provides an efficient approach to detect structural abnormalities in the optic chiasm. The most realistic results were obtained with filtering methods with parameters optimized for the data at hand.

Significance

Our findings demonstrate a novel anatomy-driven approach for the individualized diagnostics of optic chiasm abnormalities.

Asymmetries between achromatic and chromatic extraction of 3D motion signals

Motion in depth (MID) can be cued by high-resolution changes in binocular disparity over time (CD), and low-resolution interocular velocity differences (IOVD). Computational differences between these two mechanisms suggest that they may be implemented in visual pathways with different spatial and temporal resolutions. Here, we used fMRI to examine how achromatic and S-cone signals contribute to human MID perception. Both CD and IOVD stimuli evoked responses in a widespread network that included early visual areas, parts of the dorsal and ventral streams, and motion-selective area hMT+. Crucially, however, we measured an interaction between MID type and chromaticity. fMRI CD responses were largely driven by achromatic stimuli, but IOVD responses were better driven by isoluminant S-cone inputs. In our psychophysical experiments, when S-cone and achromatic stimuli were matched for perceived contrast, participants were equally sensitive to the MID in achromatic and S-cone IOVD stimuli. In comparison, they were relatively insensitive to S-cone CD. These findings provide evidence that MID mechanisms asymmetrically draw on information in precortical pathways. An early opponent motion signal optimally conveyed by the S-cone pathway may provide a substantial contribution to the IOVD mechanism.

An enhanced role for right hV5/MT+ in the analysis of motion in the contra- and ipsi-lateral visual hemi-fields

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TMS applied to MT/TO-1 and MST/TO-2 disrupts translational motion.

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In the right hemisphere, disruption affects contra-and ipsi-lateral hemi-fields.

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In the left hemisphere, disruption is restricted to the contra-lateral hemi-field.

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Suggests enhanced role for right hemisphere in full-field motion perception.

Previous experiments have demonstrated that transcranial magnetic stimulation (TMS) of human V5/MT+, in either the left or right cerebral hemisphere, can induce deficits in visual motion perception in their respective contra- and ipsi-lateral visual hemi-fields. However, motion deficits in the ipsi-lateral hemi-field are greater when TMS is applied to V5/MT + in the right hemisphere relative to the left hemisphere. One possible explanation for this asymmetry might lie in differential stimulation of sub-divisions within V5/MT + across the two hemispheres. V5/MT + has two major sub-divisions; MT/TO-1 and MST/TO-2, the latter area contains neurons with large receptive fields (RFs) that extend up to 15° further into the ipsi-lateral hemi-field than the former. We wanted to examine whether applying TMS to MT/TO-1 and MST/TO-2 separately could explain the previously reported functional asymmetries for ipsi-lateral motion processing in V5/MT + across right and left cerebral hemispheres. MT/TO-1 and MST/TO-2 were identified in seven subjects using fMRI localisers. In psychophysical experiments subjects identified the translational direction (up/down) of coherently moving dots presented in either the left or right visual field whilst repetitive TMS (25 Hz; 70%) was applied synchronously with stimulus presentation. Application of TMS to MT/TO-1 and MST/TO-2 in the right hemisphere affected translational direction discrimination in both contra-lateral and ipsi-lateral visual fields. In contrast, deficits of motion perception following application of TMS to MT/TO-1 and MST/TO-2 in the left hemisphere were restricted to the contra-lateral visual field. This result suggests an enhanced role for the right hemisphere in processing translational motion across the full visual field.

Emergence of symmetry selectivity in the visual areas of the human brain: fMRI responses to symmetry presented in both frontoparallel and slanted planes

Symmetry is effortlessly perceived by humans across changes in viewing geometry. Here, we re-examined the network subserving symmetry processing in the context of up-to-date retinotopic definitions of visual areas. Responses in object selective cortex, as defined by functional localizers, were also examined. We further examined responses to both frontoparallel and slanted symmetry while manipulating attention both toward and away from symmetry. Symmetry-specific responses first emerge in V3 and continue across all downstream areas examined. Of the retinotopic areas, ventral occipital VO1 showed the strongest symmetry response, which was similar in magnitude to the responses observed in object selective cortex. Neural responses were found to increase with both the coherence and folds of symmetry. Compared to passive viewing, drawing attention to symmetry generally increased neural responses and the correspondence of these neural responses with psychophysical performance. Examining symmetry on the slanted plane found responses to again emerge in V3, continue through downstream visual cortex, and be strongest in VO1 and LOB. Both slanted and frontoparallel symmetry evoked similar activity when participants performed a symmetry-related task. However, when a symmetry-unrelated task was performed, fMRI responses to slanted symmetry were reduced relative to their frontoparallel counterparts. These task-related changes provide a neural signature that suggests slant has to be computed ahead of symmetry being appropriately extracted, known as the "normalization" account of symmetry processing. Specifically, our results suggest that normalization occurs naturally when attention is directed toward symmetry and orientation, but becomes interrupted when attention is directed away from these features.

A Direct Demonstration of Functional Differences between Subdivisions of Human V5/MT

Two subdivisions of human V5/MT+: one located posteriorly (MT/TO-1) and the other more anteriorly (MST/TO-2) were identified in human participants using functional magnetic resonance imaging on the basis of their representations of the ipsilateral versus contralateral visual field. These subdivisions were then targeted for disruption by the application of repetitive transcranial magnetic stimulation (rTMS). The rTMS was delivered to cortical areas while participants performed direction discrimination tasks involving 3 different types of moving stimuli defined by the translational, radial, or rotational motion of dot patterns. For translational motion, performance was significantly reduced relative to baseline when rTMS was applied to both MT/TO-1 and MST/TO-2. For radial motion, there was a differential effect between MT/TO-1 and MST/TO-2, with only disruption of the latter area affecting performance. The rTMS failed to reveal a complete dissociation between MT/TO-1 and MST/TO-2 in terms of their contribution to the perception of rotational motion. On the basis of these results, MT/TO-1 and MST/TO-2 appear to be functionally distinct subdivisions of hV5/MT+. While both areas appear to be implicated in the processing of translational motion, only the anterior region (MST/TO-2) makes a causal contribution to the perception of radial motion.

Using magnetic resonance imaging to assess visual deficits: a review

Purpose

Over the last two decades, magnetic resonance imaging (MRI) has been widely used in neuroscience research to assess both structure and function in the brain in health and disease. With regard to vision research, prior to the advent of MRI, researchers relied on animal physiology and human post‐mortem work to assess the impact of eye disease on visual cortex and connecting structures. Using MRI, researchers can non‐invasively examine the effects of eye disease on the whole visual pathway, including the lateral geniculate nucleus, striate and extrastriate cortex. This review aims to summarise research using MRI to investigate structural, chemical and functional effects of eye diseases, including: macular degeneration, retinitis pigmentosa, glaucoma, albinism, and amblyopia.

Recent Findings

Structural MRI has demonstrated significant abnormalities within both grey and white matter densities across both visual and non‐visual areas. Functional MRI studies have also provided extensive evidence of functional changes throughout the whole of the visual pathway following visual loss, particularly in amblyopia. MR spectroscopy techniques have also revealed several abnormalities in metabolite concentrations in both glaucoma and age‐related macular degeneration. GABA‐edited MR spectroscopy on the other hand has identified possible evidence of plasticity within visual cortex.

Summary

Collectively, using MRI to investigate the effects on the visual pathway following disease and dysfunction has revealed a rich pattern of results allowing for better characterisation of disease. In the future MRI will likely play an important role in assessing the impact of eye disease on the visual pathway and how it progresses over time.

Differential processing of the direction and focus of expansion of optic flow stimuli in areas MST and V3A of the human visual cortex

Human neuropsychological and neuroimaging studies have raised the possibility that different attributes of optic flow stimuli, namely radial direction and the position of the focus of expansion (FOE), are processed within separate cortical areas. In the human brain, visual areas V5/MT+ and V3A have been proposed as integral to the analysis of these different attributes of optic flow stimuli. To establish direct causal relationships between neural activity in human (h)V5/MT+ and V3A and the perception of radial motion direction and FOE position, we used transcranial magnetic stimulation (TMS) to disrupt cortical activity in these areas while participants performed behavioral tasks dependent on these different aspects of optic flow stimuli. The cortical regions of interest were identified in seven human participants using standard functional MRI retinotopic mapping techniques and functional localizers. TMS to area V3A was found to disrupt FOE positional judgments but not radial direction discrimination, whereas the application of TMS to an anterior subdivision of hV5/MT+, MST/TO-2 produced the reverse effects, disrupting radial direction discrimination but eliciting no effect on the FOE positional judgment task. This double dissociation demonstrates that FOE position and radial direction of optic flow stimuli are signaled independently by neural activity in areas hV5/MT+ and V3A.NEW & NOTEWORTHY Optic flow constitutes a biologically relevant visual cue as we move through any environment. With the use of neuroimaging and brain-stimulation techniques, this study demonstrates that separate human brain areas are involved in the analysis of the direction of radial motion and the focus of expansion in optic flow. This dissociation reveals the existence of separate processing pathways for the analysis of different attributes of optic flow that are important for the guidance of self-locomotion and object avoidance.

An Orientation Dependent Size Illusion Is Underpinned by Processing in the Extrastriate Visual Area, LO1

We use the simple, but prominent Helmholtz's squares illusion in which a vertically striped square appears wider than a horizontally striped square of identical physical dimensions to determine whether functional magnetic resonance imaging (fMRI) BOLD responses in V1 underpin illusions of size. We report that these simple stimuli which differ in only one parameter, orientation, to which V1 neurons are highly selective elicited activity in V1 that followed their physical, not perceived size. To further probe the role of V1 in the illusion and investigate plausible extrastriate visual areas responsible for eliciting the Helmholtz squares illusion, we performed a follow-up transcranial magnetic stimulation (TMS) experiment in which we compared perceptual judgments about the aspect ratio of perceptually identical Helmholtz squares when no TMS was applied against selective stimulation of V1, LO1, or LO2. In agreement with fMRI results, we report that TMS of area V1 does not compromise the strength of the illusion. Only stimulation of area LO1, and not LO2, compromised significantly the strength of the illusion, consistent with previous research that LO1 plays a role in the processing of orientation information. These results demonstrate the involvement of a specific extrastriate area in an illusory percept of size.

Preserved retinotopic brain connectivity in macular degeneration

PURPOSE

The eye disease macular degeneration (MD) is a leading cause of blindness worldwide. There is no cure for MD, but several promising treatments aimed at restoring vision at the level of the retina are currently under investigation. These treatments assume that the patient's brain can still process appropriately the retinal input once it is restored, but whether this assumption is correct has yet to be determined.

METHODS

We used functional magnetic resonance imaging (fMRI) and connective field modelling to determine whether the functional connectivity between the input-deprived portions of primary visual cortex (V1) and early extrastriate areas (V2/3) is still retinotopically organised. Specifically, in both patients with juvenile macular degeneration and age-matched controls with simulated retinal lesions, we assessed the extent to which the V1-referred connective fields of extrastriate voxels, as estimated on the basis of spontaneous fMRI signal fluctuations, adhered to retinotopic organisation.

RESULTS

We found that functional connectivity between the input-deprived portions of visual areas V1 and extrastriate cortex is still largely retinotopically organised in MD, although on average less so than in controls. Patients with stable fixation exhibited normal retinotopic connectivity, however, suggesting that for the patients with unstable fixation, eye-movements resulted in spurious, homogeneous signal modulations across the entire input-deprived cortex, which would have hampered our ability to assess their spatial structure of connectivity.

CONCLUSIONS

Despite the prolonged loss of visual input due to MD, the cortico-cortical connections of input-deprived visual cortex remain largely intact. This suggests that the restoration of sight in macular degeneration can rely on a largely unchanged retinotopic representation in early visual cortex following loss of central retinal function.

Surface-Based Analyses of Anatomical Properties of the Visual Cortex in Macular Degeneration

Introduction

Macular degeneration (MD) can cause a central visual field defect. In a previous study, we found volumetric reductions along the entire visual pathways of MD patients, possibly indicating degeneration of inactive neuronal tissue. This may have important implications. In particular, new therapeutic strategies to restore retinal function rely on intact visual pathways and cortex to reestablish visual function. Here we reanalyze the data of our previous study using surface-based morphometry (SBM) rather than voxel-based morphometry (VBM). This can help determine the robustness of the findings and will lead to a better understanding of the nature of neuroanatomical changes associated with MD.

Methods

The metrics of interest were acquired by performing SBM analysis on T1-weighted MRI data acquired from 113 subjects: patients with juvenile MD (JMD; n = 34), patients with age-related MD (AMD; n = 24) and healthy age-matched controls (HC; n = 55).

Results

Relative to age-matched controls, JMD patients showed a thinner cortex, a smaller cortical surface area and a lower grey matter volume in V1 and V2, while AMD patients showed thinning of the cortex in V2. Neither patient group showed a significant difference in mean curvature of the visual cortex.

Discussion

The thinner cortex, smaller surface area and lower grey matter volume in the visual cortex of JMD patients are consistent with our previous results showing a volumetric reduction in their visual cortex. Finding comparable results using two rather different analysis techniques suggests the presence of marked cortical degeneration in the JMD patients. In the AMD patients, we found a thinner cortex in V2 but not in V1. In contrast to our previous VBM analysis, SBM revealed no volumetric reductions of the visual cortex. This suggests that the cortical changes in AMD patients are relatively subtle, as they apparently can be missed by one of the methods.

Organization of the Central Visual Pathways Following Field Defects Arising from Congenital, Inherited, and Acquired Eye Disease

Visual field defects that arise from eye disease are increasing as human life spans lengthen. The consequences of visual field defects on the central visual pathways are important to assess, particularly in light of potential treatments of eye disease that restore function to the retina. For individuals with field defects arising from congenital eye disease, primary visual cortex (V1) appears to remap, whereas this form of reorganization is not present in individuals with field defects that arise later in life as a result of inherited or acquired eye disease. However, research has revealed that the areas of V1 that normally map the visual field defect are active under specific circumstances. This review attempts to resolve whether or not this activity reflects reorganization of the central visual pathways. Alongside the measures of function are measures of anatomical properties of the human visual pathway, which demonstrate transneuronal degeneration in individuals with eye disease. These results are concerning because degeneration of the central visual pathways may ultimately limit the success of sight-restoring treatments of eye disease.

Plasticity, and Its Limits, in Adult Human Primary Visual Cortex

There is an ongoing debate about whether adult human primary visual cortex (V1) is capable of large-scale cortical reorganization in response to bilateral retinal lesions. Animal models suggest that the visual neural circuitry maintains some plasticity through adulthood, and there are also a few human imaging studies in support this notion. However, the interpretation of these data has been brought into question, because there are factors besides cortical reorganization, such as the presence of sampling bias and/or the unmasking of task-dependent feedback signals from higher level visual areas, that could also explain the results. How reasonable would it be to accept that adult human V1 does not reorganize itself in the face of disease? Here, we discuss new evidence for the hypothesis that adult human V1 is not as capable of reorganization as in animals and juveniles, because in adult humans, cortical reorganization would come with costs that outweigh its benefits. These costs are likely functional and visible in recent experiments on adaptation--a rapid, short-term form of neural plasticity--where they prevent reorganization from being sustained over the long-term.

The distribution of unique green wavelengths and its relationship to macular pigment density

Monochromatic unique green (UG) is more variable across the population than any other unique hue. Some researchers have reported that this broad distribution of UG settings is bimodal and that the distribution results from the superposition of two or more subpopulations. We have investigated this claim using a Wright colorimeter to measure the unique green wavelength of 58 participants and we have analyzed previous unique green literature by applying a rigorous statistical test to historical datasets. We have also explored the possibility that individual differences in macular pigment density may be responsible for the variation in unique green wavelength. Our results indicate that unique green wavelengths in our population are distributed unimodally and are correlated positively with macular pigment density; individuals with a higher density of macular pigment select longer wavelengths of light as unique green than individuals with a lower density of macular pigment. We model this effect using simulations of monochromatic unique green matching to broadband illuminations and show that matches in the region at approximately 500 nm exhibit particularly high variance both with respect to macular pigment density and also with respect to the precise shape of the broadband reference exemplar spectrum.

Population receptive field dynamics in human visual cortex

Seminal work in the early nineties revealed that the visual receptive field of neurons in cat primary visual cortex can change in location and size when artificial scotomas are applied. Recent work now suggests that these single neuron receptive field dynamics also pertain to the neuronal population receptive field (pRF) that can be measured in humans with functional magnetic resonance imaging (fMRI). To examine this further, we estimated the pRF in twelve healthy participants while masking the central portion of the visual field. We found that the pRF changes in location and size for two differently sized artificial scotomas, and that these pRF dynamics are most likely due to a combination of the neuronal receptive field position and size scatter as well as modulatory feedback signals from extrastriate visual areas.

The noninvasive dissection of the human visual cortex: using FMRI and TMS to study the organization of the visual brain

The development of brain imaging techniques, such as fMRI, has given modern neuroscientists unparalleled access to the inner workings of the living human brain. Visual processing in particular has proven to be particularly amenable to study with fMRI. Studies using this technique have revealed the existence of multiple representations of visual space with differing functional roles across many cortical locations. Yet, although fMRI provides an excellent means by which we can localize and map different areas across the visual brain, it is less well suited to providing information as to whether activation within a particular cortical region is directly related to perception or behavior. These kinds of causal links can be made, however, when fMRI is combined with transcranial magnetic stimulation (TMS). TMS is a noninvasive technique that can bring about localized, transient disruption of cortical function and can induce functional impairments in the performance of specific tasks. When guided by the detailed localizing and mapping capabilities of fMRI, TMS can be used as a means by which the functional roles of different visual areas can be investigated. This review highlights recent insights that the techniques of fMRI and TMS have given us with regard to the function and contributions of the many different visual areas to human visual perception.

The contribution of human cortical area V3A to the perception of chromatic motion: a transcranial magnetic stimulation study

Area V3A was identified in five human subjects on both a functional and retinotopic basis using functional magnetic resonance imaging techniques. V3A, along with other visual areas responsive to motion, was then targeted for disruption by repetitive transcranial magnetic stimulation (rTMS) whilst the participants performed a delayed speed matching task. The stimuli used for this task included chromatic, isoluminant motion stimuli that activated either the L-M or S-(L+M) cone-opponent mechanisms, in addition to moving stimuli that contained only luminance contrast (L+M). The speed matching task was performed for chromatic and luminance stimuli that moved at slow (2 degrees/s) or faster (8 degrees/s) speeds. The application of rTMS to area V3A produced a perceived slowing of all chromatic and luminance stimuli at both slow and fast speeds. Similar deficits were found when rTMS was applied to V5/MT+. No deficits in performance were found when areas V3B and V3d were targeted by rTMS. These results provide evidence of a causal link between neural activity in human area V3A and the perception of chromatic isoluminant motion. They establish area V3A, alongside V5/MT+, as a key area in a cortical network that underpins the analysis of not only luminance but also chromatically-defined motion.

The effect of age and fixation instability on retinotopic mapping of primary visual cortex

PURPOSE

Functional magnetic resonance imaging (fMRI) experiments determining the retinotopic structure of visual cortex have commonly been performed on young adults, who are assumed to be able to maintain steady fixation throughout the trial duration. The authors quantified the effects of age and fixation stability on the quality of retinotopic maps of primary visual cortex.

METHODS

With the use of a 3T fMRI scanner, the authors measured cortical activity in six older and six younger normally sighted participants observing an expanding flickering checkerboard stimulus of 30 degrees diameter. The area of flattened primary visual cortex (V1) showing any blood oxygen level-dependent (BOLD) activity to the visual stimulus and the area responding to the central 3.75 degrees of the stimulus (relating to the central ring of our target) were recorded. Fixation stability was measured while participants observed the same stimuli outside the scanner using an infrared gazetracker.

RESULTS

There were no age-related changes in the area of V1. However, the proportion of V1 active to our visual stimulus was lower for the older observers than for the younger observers (overall activity: 89.8% of V1 area for older observers, 98.6% for younger observers; P < 0.05). This effect was more pronounced for the central 3.75 degrees of the target (older subjects, 26.4%; younger subjects, 40.7%; P < 0.02). No significant relationship existed between fixation stability and age or the magnitude of activity in the primary visual cortex.

CONCLUSIONS

Although the cortical area remains unchanged, healthy older persons show less BOLD activity in V1 than do younger persons. Normal variations in fixation stability do not have a significant effect on the accuracy of experiments to determine the retinotopic structure of the visual cortex.

The fovea regulates symmetrical development of the visual cortex

The foveal region contains the highest cell density in the human retina; consequently a disproportionately large area of the visual cortex is dedicated to its representation. In aniridia and albinism the fovea does not develop, and the corresponding cortical representation shows a reduction in gray matter volume. In albinos there are chiasmatic irregularities in the hemispheric projections, which are not found in aniridics. Here, we ask whether the anomalies in central retinal development, present in albinism and aniridia, have a wider impact on the architecture of the visual cortex. The length, depth, and topology of the calcarine fissure is analyzed in albino, aniridic, and normal subjects. These measures are compared between groups and between the cortical hemispheres within each subject. We show that the calcarine fissure, where the primary visual cortex is represented, is abnormally short in those lacking a fovea. Moreover, surface reconstructions of the calcarine fissure revealed marked interhemispheric asymmetries. The two groups could not be distinguished on the basis of their cortical features, and we therefore interpret the abnormalities in cortical architecture in terms of the absence of the fovea, the common retinal feature found in both groups.

Identifying human albinism: a comparison of VEP and fMRI

PURPOSE

To compare VEP and fMRI as a means of detecting the abnormal visual projections in albinism in different stimulation conditions.

METHODS

Cortical response to monocular full-field pattern-onset and hemifield pattern-onset and -reversal stimulation of 18 subjects with a known diagnosis of albinism, 17 control subjects, and 6 control subjects with infantile nystagmus syndrome (INS) was determined by VEP and fMRI. An asymmetry index was used to quantify the extent of response lateralization as measured by both VEP and fMRI. The extent to which each method and stimulus combination differentiated participant groups was summarized with a receiver operating characteristic (ROC) analysis, where A(A-C) and A(A-N) refer to areas under the ROC curve for albino versus control and albino versus nystagmus comparisons.

RESULTS

Cortical response to full-field monocular stimulation conditions offered robust detection of the abnormal response lateralization in albinism, with fMRI (A(A-C) = 1.00; A(A-N) = 0.91) being slightly more robust than the VEP under these conditions (A(A-C) = 0.91; A(A-N) = 0.79). Hemifield stimulation paradigms were somewhat poorer at differentiating between groups, particularly when VEP was used in combination with pattern-reversal stimulation (pattern-onset fMRI A(A-C) = 0.94, A(A-N) = 0.84, and VEP A(A-C) = 0.86, A(A-N) = 0.86; pattern-reversal fMRI A(A-C) = 0.90, A(A-N) = 0.88, and VEP A(A-C) = 0.69, A(A-N) = 0.64). However, when only the most posterior aspects of the occipital lobe were considered with hemifield stimulation, fMRI achieved the best differentiation between the subject groups, most notably with hemifield pattern-reversal stimulation (A(A-C) = 1.00; A(A-N) = 1.00).

CONCLUSIONS

An interocular comparison between the lateralization of cortical responses elicited by full-field stimulation reliably distinguished between those with albinism and control groups, when both fMRI and VEP were used to assess cortical responses. Hemifield stimulation of one eye offers an alternative method for assessing misrouting associated with albinism and is highly effective when cortical signals are assessed with fMRI, but less so when VEP is used.

Abnormale Repräsentationen im visuellen Kortex von Albinismus-Patienten

A characteristic feature of patients with albinism is the misrouting of the optic nerves, which causes the visual cortex to receive an abnormal input. This report details how the detection of misrouting using visual evoked potentials assists the clinical diagnosis of albinism. Further, it shows how the projection abnormality observed in patients with albinism provides a model for investigating the self-organisation of the human visual cortex. This is highlighted by recent findings that were obtained using functional magnetic resonance imaging, visual evoked potentials, and static visual field perimetry.

Pigmentation predicts the shift in the line of decussation in humans with albinism

In albinism a large proportion of nerve fibres originating in temporal retina cross the midline at the chiasm and project to the contralateral hemisphere. Studies in rodents with albinism have suggested that the extent of this misrouting at the chiasm is inversely related to pigmentation levels. Here, we examine whether there is evidence for a similar relationship in humans with albinism. Functional MRI was performed on 18 subjects with albinism, 17 control subjects and six controls with nystagmus as they underwent hemifield visual stimulation of nasal or temporal retina. Functional activation in 16 coronal slices beginning at the posterior occipital lobes were analysed and the extent of hemispheric response lateralization at each slice position was determined. During temporal retina stimulation, the control response was lateralized to the hemisphere ipsilateral to the stimulated eye for all slices. In albinos, the response in posterior slices was predominantly in the contralateral hemisphere, consistent with misrouting of temporal retina fibres. However, as slice location became progressively anterior, response lateralization reverted to the ipsilateral hemisphere. The slice location at which the transition from contra- to ipsilateralization occurred provided an estimate of the extent of fibre misrouting in the individual. The slice transition location correlated negatively with pigmentation level, providing the first evidence for a relationship between pigmentation and the extent of misrouting in humans with albinism.

Perceptual distortions of speed at low luminance: Evidence inconsistent with a Bayesian account of speed encoding

Our perception of speed has been shown to be distorted under a number of viewing conditions. Recently the well-known reduction of perceived speed at low contrast has led to Bayesian models of speed perception that account for these distortions with a slow speed 'prior'. To test the predictive, rather than the descriptive, power of the Bayesian approach we have investigated perceived speed at low luminance. Our results indicate that, for the mesopic and photopic range (0.13-30 cd m(-2)) the perceived speed of lower luminance patterns is virtually unaffected at low speeds (<4 deg s(-1)) but is over-estimated at higher speeds (>4 deg s(-1)). We show here that the results can be accounted for by an extension to a simple ratio model of speed encoding [Hammett, S. T., Champion, R. A., Morland, A. & Thompson, P. G. (2005). A ratio model of perceived speed in the human visual system. Proceedings of Royal Society B, 262, 2351-2356.] that takes account of known changes in neural responses as a function of luminance, contrast and temporal frequency. The results are not consistent with current Bayesian approaches to modelling speed encoding that postulate a slow speed prior.

Retinal abnormalities in human albinism translate into a reduction of grey matter in the occipital cortex

Albinism is a genetic condition associated with abnormalities of the visual system. Defects in melanin production cause underdevelopment of the fovea, reduced retinal cell numbers and abnormal routing of ganglion cell nerve fibres at the optic chiasm. We examined 19 subjects with albinism and 26 control subjects to determine whether retinal abnormalities affect the structure of the visual cortex. Whole-brain, high-resolution anatomical magnetic resonance imaging volumes from each subject were obtained on a 1.5-T scanner and segmented into grey and white matter. A voxel-wise statistical comparison of grey and white matter volumes in the occipital lobes between the two groups was performed using voxel-based morphometry. Our analysis revealed a regionally specific decrease in grey matter volume at the occipital poles in albinism. The location of the decrease in grey matter corresponds to the cortical representation of the central visual field. This reduction is likely to be a direct result of decreased ganglion cell numbers in central retina in albinism.

A ratio model of perceived speed in the human visual system

The perceived speed of moving images changes over time. Prolonged viewing of a pattern (adaptation) leads to an exponential decrease in its perceived speed. Similarly, responses of neurones tuned to motion reduce exponentially over time. It is tempting to link these phenomena. However, under certain conditions, perceived speed increases after adaptation and the time course of these perceptual effects varies widely. We propose a model that comprises two temporally tuned mechanisms whose sensitivities reduce exponentially over time. Perceived speed is taken as the ratio of these filters' outputs. The model captures increases and decreases in perceived speed following adaptation and describes our data well with just four free parameters. Whilst the model captures perceptual time courses that vary widely, parameter estimates for the time constants of the underlying filters are in good agreement with estimates of the time course of adaptation of direction selective neurones in the mammalian visual system.

Misrouting of the optic nerves in albinism: estimation of the extent with visual evoked potentials

PURPOSE

In human albinism a part of the temporal retina projects abnormally to the contralateral hemisphere. An objective VEP procedure to quantify the extent of the abnormality was devised.

METHODS

Monocular VEPs were recorded in 16 subjects with albinism and in 16 controls from occipital electrodes to pattern-onset stimulation in 1 of 10 adjacent rectangular apertures along the horizontal meridian covering a total of +/-27 degrees. For each eye interhemispheric difference potentials were calculated and correlated with each other to assess the lateralization of the responses: positive and negative correlations indicate lateralization on same or opposing hemispheres, respectively. Different stimulus conditions were compared to assess the sensitivity and specificity of the procedure for the detection of the misrouting of visual projections in albinism.

RESULTS

Locations that were affected by the projection abnormality were detected with a specificity of 100% and an average sensitivity of 97%. In the 16 subjects with albinism tested, the abnormal projection was confined to the central retina and varied in extent between subjects (2 degrees - 15 degrees; median, 8 degrees). The extent did not appear to be correlated with horizontal nystagmus amplitude or visual acuity.

CONCLUSIONS

Because of the great interindividual variability of the projection abnormality, studies of the contribution of the abnormally projecting retina to visual perception must be preceded by the localization of the abnormality. This VEP procedure allowed the authors to identify, with high sensitivity and specificity, visual field locations that are affected by the projection abnormality.

Macular pigments: their characteristics and putative role

The macular pigments (MP) absorb light in the blue-green region of the visible spectrum and comprise two carotenoids, lutein and zeaxanthin. In humans the concentration of MP varies widely across the normal population. There are two (not mutually exclusive) proposed roles for MP: to improve visual function and to act as an antioxidant and protect the macula from damage by oxidative stress. In this article we review the origin, spectral characteristics and ocular distribution of MP and also discuss the effect MP has on central visual function and the techniques available for measurement of MP optical density in vivo. Finally, we review the evidence for both proposed physiological roles of MP. Considering the first of these, we conclude that although MP might improve visual function in theory, to date there is no firm evidence that higher levels of MP are correlated with enhanced measures of visual performance. There is a growing body of evidence that has highlighted associations between macular disease and low levels of MP, most particularly with age-related macular degeneration (AMD) and with risk factors for AMD. However, all findings to date are associative only and there is no direct evidence for high MP levels conferring a protective effect. Increased dietary intake of MP gives rise to increased levels of serum and retinal MP. This, taken together with the associative evidence of low MP levels in disease, indicates that a potential, and perhaps serendipitous, therapeutic strategy for macular disease exists. We conclude, however, that the potential protective properties of MP will only be fully evaluated by undertaking longitudinal studies that follow initially healthy participants through to the development of macular disease.

The Role of Spared Calcarine Cortex and Lateral Occipital Cortex in the Responses of Human Hemianopes to Visual Motion

Some patients, who are rendered perimetrically blind in one hemifield by cortical lesions, nevertheless exhibit residual visual capacities within their field defects. The neural mechanism that mediates the residual visual responses has remained the topic of considerable debate. One explanation posits the subcortical visual pathways that bypass the primary visual cortex and innervate the extrastriate visual areas as the substrate that underlies the residual vision. The other explanation is that small islands of the primary visual cortex remain intact and provide the signals for residual vision. We have performed behavioral and functional magnetic resonance imaging experiments to investigate the validity of the two explanations of residual vision. Our behavioral experiments indicated that of the seven hemianopes tested, two had the ability to discriminate the direction of a drifting grating. This residual visual response was shown with fMRI to be the result of spared islands of calcarine cortical activity in one of the hemianopes, whereas only lateral occipital activity was documented in the other patient. These results indicate that the underlying neural correlates of residual vision can vary between patients. Moreover, our study emphasizes the necessity of ruling out the presence of islands of preserved function and primary visual cortex before assigning residual visual capacities to the properties of visual pathways that bypass the primary visual cortex.

Organization of the visual cortex in human albinism

In albinism there is an abnormal projection of part of the temporal retina to the visual cortex contralateral to the eye. This projection, together with the normally routed fibers from nasal retina, provides a cortical hemisphere with visual input from more than the normal hemifield of visual space. In many mammalian models of albinism, a possible sensory mismatch in the visual cortex is avoided either by reorganization of the thalamocortical connections to give the abnormal input an exclusive cortical representation, or by the abnormal input being substantially suppressed. In this study we examine, with fMRI, how the human visual cortex topographically maps its input in albinism. We find that the input from temporal retina is not substantially suppressed and forms a retinotopic mapping that is superimposed on the mapping of the nasal retina in striate and extrastriate areas. The abnormal routing of temporal fibers is not total, with the line of decussation shifting to between 6 and 14 degrees into temporal retina. Our results indicate that the abnormal input to visual cortex in human albinism does not undergo topographic reorganization between the thalamus and cortex. Furthermore, the abnormal input is not significantly suppressed in either striate or extrastriate areas. The topographic mapping that we report in human does not conform, therefore, to the commonly observed patterns in other mammals but takes the form of the "true albino" pattern that has been reported rarely in cat and in the only other individual primate studied.

Reorganization of human cortical maps caused by inherited photoreceptor abnormalities

We describe a compelling demonstration of large-scale developmental reorganization in the human visual pathways. The developmental reorganization was observed in rod monochromats, a rare group of congenitally colorblind individuals who virtually lack cone photoreceptor function. Normal controls had a cortical region, spanning several square centimeters, that responded to signals initiated in the all-cone foveola but was inactive under rod viewing conditions; in rod monochromats this cortical region responded powerfully to rod-initiated signals. The measurements trace a causal pathway that begins with a genetic anomaly that directly influences sensory cells and ultimately results in a substantial central reorganization.

Abnormal visual projection in a human albino studied with functional magnetic resonance imaging and visual evoked potentials

The albino visual pathway is abnormal in that many fibres from the temporal retina project to the contralateral visual cortex. The visual projections in a human albino and a control have been investigated with fMRI and VEP during independent visual stimulation of both hemifields. Activity in the occipital cortex in the normal was contralateral to the stimulated visual field, whereas it was contralateral to the stimulated eye in the albino, independent of the stimulated visual field. Thus, the albino visual cortex is activated not only by stimulation in the contralateral visual field, but also by abnormal input representing the ipsilateral visual field. These novel findings help elucidate the nature of albino misrouting.