[A 3T fMRI study of cortical projection of visual scotomas: preliminary results]

We used functional magnetic resonance imaging (fMRI) to precisely measure both the localization and size of the cortical projections of artificial scotomas in healthy subjects as well as the size of a reversible retinal scotoma in a patient with central serous chorioretinopathy (CSCR).

Using a 3T MRI scanner, anatomical and functional data were acquired on two healthy subjects and a patient with CSCR. Retinotopic maps were first reconstructed using phase mapping techniques. Next, a block paradigm consisting of a grey background alternating with a full-field, flickering checkerboard was used to stimulate the complete central (19.5 degrees) visual field. A condition with artificial peri-foveal scotomas of different sizes and eccentricities was interleaved in healthy subjects. Differential maps were computed to obtain the cortical representation (size and location) of artificial scotomas. Full-field functional data were also acquired in the CSRC patient, at the acute stage and after recovery.

Cortical projections of each scotoma were identified using differential maps and carefully characterized with quantitative analysis: the measured cortical positions of the inactivated cortical zones were compared with the known radius and eccentricity values in the scotomas in the visual field. We also compared the size of the inactivated cortical zones to the known size of scotomas. However, we found a consistent relationship between the size of the scotomas and their cortical projections, albeit with the absolute size smaller than expected from known cortical magnification factors. The cortical deactivation zone was also observed in the CRSC patient, which disappeared at recovery stage.

Cortical retinotopic mapping can be performed successfully for both artificial and retinal scotomas. This study can serve as a basis for the future investigation of cortical plasticity in the visual cortex.