Functional Dynamics of Deafferented Early Visual Cortex in Glaucoma

Abstract

In advanced retinitis pigmentosa with retinal lesions, the lesion projection zone (LPZ) in the early visual cortex can be driven during visual tasks, while it remains unresponsive during passive viewing. We tested whether this finding translates to advanced glaucoma, a major cause of acquired blindness. During visual stimulation, 3T fMRI scans were acquired for participants with advanced glaucoma (n = 4; age range: 51–72) and compared to two reference groups, i.e., advanced retinitis pigmentosa (n = 3; age range: 46–78) and age-matched healthy controls with simulated defects (n = 7). The participants viewed grating patterns drifting in 8 directions (12 s) alternating with uniform gray (12 s), either during passive viewing (PV), i.e., central fixation, or during a one-back task (OBT), i.e., reports of succeeding identical motion directions. As another reference, a fixation-dot task condition was included. Only in glaucoma and retinitis pigmentosa but not in controls, fMRI-responses in the lesion projection zone (LPZ) of V1 shifted from negative for PV to positive for OBT (p = 0.024 and p = 0.012, respectively). In glaucoma, these effects also reached significance in V3 (p = 0.006), while in V2 there was a non-significant trend (p = 0.069). The general absence of positive responses in the LPZ during PV underscores the lack of early visual cortex bottom-up plasticity for acquired visual field defects in humans. Trends in our exploratory analysis suggesting the task-dependent LPZ responses to be inversely related to visual field loss, indicate the benefit of patient stratification strategies in future studies with greater sample sizes. We conclude that top-down mechanisms associated with task-elicited demands rather than visual cortex remapping appear to shape LPZ responses not only in retinitis pigmentosa, but also in glaucoma. These insights are of critical importance for the development of schemes for treatment and rehabilitation in glaucoma and beyond.

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

“…For each participant, we collected in two separate sessions, fMRI data for (1) pRF mapping and (2) block-design full-field stimulation. The block-design data had been analyzed for a previous publication ( Prabhakaran et al, 2021 ), which provided the extraction criteria for the selection of stimulation-driven voxels in our present analysis.…”

“…In each 1 s trial, the stimulus was presented for 750 ms followed by a 250 ms mean luminance gray. This fMRI data-set was analyzed previously for the assessment of task-dependences of the fMRI responses ( Prabhakaran et al, 2021 ). Since, we use the results from these data exclusively for the selection of voxels for the VF-reconstruction analysis of the present study, we refer to the publication for details on preprocessing and analysis steps.…”

“…We tested the informative value of the atlas-based VFreconstruction approach and the three different performance measures (accuracy, sensitivity, and specificity) further. For this purpose, we applied the approach, in addition to the PV condition of the full-field stimulus, to the experimental condition OBT, which renders the cortical signature of the VF-defects in patients, but not controls, less specific (Masuda et al, 2010(Masuda et al, , 2008Prabhakaran et al, 2021). For a meaningful measure, it is expected that the correspondence measures between SAP and atlas-based VF-reconstruction change for OBT compared to PV in patients, but not in controls.…”