[Retinotopic representations of human visual cortex revealed by functional magnetic resonance imaging]

To establish a set of methodological standard of retinotopic mapping using functional magnetic resonance imaging (MRI), to discuss anatomical features of functionally delineated V1/V2 boundary based on retinotopic criteria.

Three adults aged 25-30 (1 male and 2 females), right-handed, without contraindications of MRI were studied. Visual acuity was 20/20 or corrected to 20/20. Visual stimuli subtended a field of view of approximately 12 degrees, consisted of high contrast, achromatic and drifting checkerboards. Two types of stimuli were used: the rotating wedge, either clockwise or counterclockwise, to map the polar angle and the annular ring, either contracting or expanding, to map eccentricity. MRI parameters: GE signa VH/i 3.0T scanner. Functional data: GRE-EPI sequence, 20 slices lay perpendicular to the calcarine sulcus, TR: 2 s, TE: 40 ms, FA: 90 degrees, Phase: 128, FOV: 19 cm, matrix: 64x64, slice: 3 mm, gap: 0, resolution: 2.9 mmx2.9 mmx3.0 mm. Anatomical data was obtained using 3D-SPGR sequence to acquire high resolution. Data was analyzed using AFNI. In order to avoid the disadvantages of restricted view due to highly convoluted occipital cortex, cortical surface was unfolded and then cut and flattened. Functional data was presented to this flattened surface and subsequently analyzed.

By using the phase-encoded visual stimuli, retinotopic organizations were exactly revealed both in the eccentricity and polar angle dimensions. Eccentricity maps showed that foveal representation lay in the occipital poles and the representation appeared further anterior as eccentricity increased. Polar angle maps showed that early retinotopically organized areas had either a mirror or non-mirror image representation of visual field. Boundaries of different visual areas were manually delineated based on retinotopic criteria called visual field sign, and the functionally delineated V1/V2 boundary located on the crests of gyrus.

Phase-encoded stimuli is validated to be an efficient way to reveal retinotopic representation of visual cortex, promising a solid foundation for the future research under pathological conditions. Anatomical features of functionally delineated V1/V2 boundary support the wiring optimization principle.