Effect of Visual Scotomas on the Retinotopic Organization of the Human Visual Cortex: A Quantitative fMRI Study

Abstract

Abstract: : Purpose: We used functional magnetic resonance imaging (fMRI) to map the retinotopic organization of human cortical areas and to measure the localization and surface of the cortical projections of peri–foveal visual scotomas of different sizes. Our goal is to apply these quantitative techniques to investigate the cortical consequences of retinal scotomas in patients. Methods: Two sets of experiments were run on 8 healthy subjects. Retinotopic maps were first reconstructed using classical mapping techniques. Second, a block paradigm consisting of a grey background alternating with a fullfield, flickering checkerboard was used to stimulate the complete central (19.5°) visual field. In both experiments, conditions with 4 peri–foveal scotomas of different sizes and centered at different eccentricities were interleaved. Subjects were asked to fixate a central cross and to report transient colour changes of the latter at random intervals, ensuring that they maintained fixation and attention at the center of the visual field. A Brucker 3T scanner equiped with head coil and custom optical system was used to acquire sets of echo–planar images of 20 occipital coronal slices within a RT of 2111msec and an 8 mm³ voxel resolution. Surface models of each subject’s occipital lobes were constructed using Brainvisa software from a sagital T1 weighted image with a 1mm³ voxel resolution. The cortical models were then inflated to get unfolded surfaces. Statistical analyses of the functional data were made through General Linear Models under SPM99, and the responses amplitudes were finally assigned to the cortical reconstructed surfaces. Results: We identified boundaries between retinotopically organized visual areas by the use of eccentricity and polar angle retinotopic maps. We also measured the relationships linking cortical positions and visual field coordinates within area V1. We identified the cortical projections of each artificial scotoma and confirmed their relations by quantitative analysis: the measured cortical positions and surfaces of the inactivated cortical zones were compared with the known values of radius, eccentricity and surface of scotomas in the visual field. We confirmed the quality and reproducibility of our results by quantitative factors. Conclusions: We developed a quick retinotopic cortical areas mapping method, which allows us to measure the cortical surface and localization of visual scotoma cortical projections.

Keywords: visual cortex • imaging methods (CT, FA, ICG, MRI, OCT, RTA, SLO, ultrasound) • neuro-ophthalmology: cortical function/rehabilitation