Functional Dynamics of Deafferented Early Visual Cortex in Glaucoma

Glaucoma-associated abnormalities in cortical activity during a visuocognitive task

OBJECTIVE

To investigate neurophysiological dynamics during a visuocognitive task in glaucoma patients vs. healthy controls.

METHODS

Fifteen patients with early-stage primary open-angle glaucoma (POAG) and fifteen age-matched healthy participants underwent a "go/no-go" task, monitored with EEG. Participants had to semantically categorize visual objects in central vision, with animal or furniture as targets according to the experimental block.

RESULTS

Early visual processing was delayed by 50 ms in patients with POAG compared to controls. The patients displayed a smaller difference between animal and furniture categorization during higher-level cognitive processing (at 400-600 ms). Regarding behavioral data, the groups differed in accuracy performance and decision criterion. As opposed to the control group, patients did not display facilitation and a higher accuracy rate for animal stimuli. However, patients maintained a consistent decision criterion throughout the experiment, whereas controls displayed a shift towards worse decision criteria in furniture trials, with higher error rate.

CONCLUSIONS

The comparative analysis of behavioral and neurophysiological data revealed in POAG patients a delay in early visual processing, and potential high-level cognitive compensation during late, task-dependent activations.

SIGNIFICANCE

To our knowledge, our findings provide the first evidence of modification in cognitive brain dynamics associated with POAG.

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.

Mapping Visual Field Defects With fMRI - Impact of Approach and Experimental Conditions

Current initiatives to restore vision emphasize the need for objective assessments of visual field (VF) defects as pursued with functional magnetic resonance imaging (fMRI) approaches. Here, we compared population receptive field (pRF) mapping-based VF reconstructions to an fMRI method that uses more robust visual stimulation (on-off block design) in combination with individualized anatomy-driven retinotopic atlas-information (atlas-based VF). We investigated participants with sizable peripheral VF-deficits due to advanced glaucoma (n = 4) or retinitis pigmentosa (RP; n = 2) and controls (n = 6) with simulated scotoma. We obtained (1) standard automated perimetry (SAP) data as reference VFs and 3T fMRI data for (2) pRF-mapping [8-direction bar stimulus, fixation color change task] and (3) block-design full-field stimulation [8-direction drifting contrast patterns during (a) passive viewing (PV) and (b) one-back-task (OBT; reporting successions of identical motion directions) to probe the impact of previously reported task-related unspecific visual cortex activations]. Correspondence measures between the SAP and fMRI-based VFs were accuracy, assisted by sensitivity and specificity. We found an accuracy of pRF-based VF from V1 in patients [median: 0.62] that was similar to previous reports and increased by adding V2 and V3 to the analysis [0.74]. In comparison to the pRF-based VF, equivalent accuracies were obtained for the atlas-based VF for both PV [0.67] and, unexpectedly, the OBT [0.59], where, however, unspecific cortical activations were reflected by a reduction in sensitivity [0.71 (PV) and 0.35 (OBT)]. In conclusion, in patients with peripheral VF-defects, we demonstrate that previous fMRI procedures to obtain VF-estimates might be enhanced by: (1) pooling V1-V3 to enhance accuracy; (2) reporting sensitivity and specificity measures to increase transparency of the VF-reconstruction metric; (3) applying atlas-based procedures, if pRF-based VFs are not available or difficult to obtain; and (4) giving, counter-intuitively, preference to PV. These findings are expected to provide guidance to overcome current limitations of translating fMRI-based methods to a clinical work-up.