Abstract
Abstract: :
Purpose: To determine the effect of glaucoma on the cortical organization of primary visual cortex (V1) in humans. Methods: Five patients with asymmetric POAG participated in the study. Visual field defects were measured using SITA–SAP. The blood oxygen level dependent (BOLD) fMRI signal was measured using a General Electric 3 Tesla scanner (TR = 2000 ms, Flip angle = 90 deg.). Each scan lasted 240 seconds, and each patient participated in three sessions with 6–8 scans per session. Visual stimuli were generated on a computer and presented via fiber optic goggles. In the first scanning session, a retinotopic map of the visual world was obtained for occipital cortex using standard stimuli (100% contrast, 8 Hz flickering checkerboard patterns in the shape of expanding rings or rotating wedges). The BOLD response was projected onto a computationally flattened representation of the visual cortex, and V1 was distinguished from other visual areas. In the second session, a 16–degree isopter in the hemifield with the scotoma was stimulated using a stationary flickering arc. The BOLD responses from sessions 1 and 2 were fit simultaneously with a template, which was used to project the scotoma onto the flattened cortex. In the third session, glaucomatous and fellow eyes were compared in the same observer by alternately presenting each eye with a full–field flickering checkerboard pattern in 20–second intervals. For each patient, we assessed whether the spatial pattern, the sign, and the mean amplitude of the BOLD activity on the flattened cortical representation agreed with individual predictions based on that patient’s visual field defect. Results: The spatial pattern of activity observed in the flattened representation qualitatively agreed with the pattern of visual field loss measured with SITA. Within the region of V1 defined by the projected scotoma, the sign of the BOLD fMRI signal was predicted by the difference in mean SITA thresholds between the glaucomatous and fellow eyes. The mean amplitude of the BOLD fMRI response was significant for 4 out of 5 patients tested (one–way t–test (d.f. = 7); all p < .05). Conclusions: The effects of glaucoma can be quantified in V1 of humans using fMRI, and these techniques may serve as a valuable tool for understanding the long–term effects of treatment on post–retinal function.
Keywords: imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • visual fields • plasticity