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In-Depth Information
To identify the retinotopic areas of the visual cortex, we carried out fMRI scans
while subjects viewed phase-encoding stimuli [8]. A high-contrast, black-and-white,
radial checkerboard pattern (mean luminance 110cd/m^2, contrast 97%) reversed
contrast at a frequency of 8 Hz [9], with eccentricity ranging from a 0° to 60° visual
angles. Two types of stimulus were used for locating visual area boundaries and esti-
mating eccentricity. The stimulus for locating boundaries was a 22.5° wedge that
rotated slowly counterclockwise about a red fixation spot at the center of the stimuli
(As Fig.1 Polar). The wedge rotated in steps of 22.5°, remaining in each position for 4
s before instantaneously rotating to the next position. The stimulus for estimating
eccentricity was an expanding checkered annulus. The flickering radial checkerboard
was moved from the center to the periphery in discrete steps (each step 7.5°, with a
total of eight steps, As Fig.1 Eccentricity), remaining at each position for 8s before
instantaneously expanding to the next position.
2.2 MR Data Acquisition
The fMRI experiment was performed using a 1.5 T Philips clinical scanner (Intera
Achieva; Best, The Netherlands). All images were acquired using a standard radio-
frequency head coil. We acquired 23 slices approximately orthogonal to the calcarine
sulcus to cover most of the cortical visual areas. The T2*-weighted gradient echo-
planner imaging sequence was used with the following parameters: TR/TE = 2000/50
ms; FA = 90°; matrix size = 64 × 64; and voxel size = 3 × 3 × 3 mm. Before acquiring
the functional images, T2-weighted anatomical images were obtained in the same
planes as the functional images, using the spin echo sequence. A T1-weighted high-
resolution image was also acquired after each functional experiment.
2.3 Data Analysis
The functional and anatomical data were processed using BrainVoyager software
package (Brain Innovation, Masstricht, Netherlands). After preprocessing the func-
tional data, anatomical data was processed. The recorded high-resolution T1-weighted
three-dimensional (3-D) recordings were used for surface reconstruction. The gray
and white matter was segmented using a region-growing method, and the white matter
cortical surface was reconstructed. Prior to surface flattening, the cortical surface was
inflated and cut along the calcarine from the occipital pole to slightly anterior of the
POS [10].
The functional data was aligned onto the 3-D anatomic image using the image co-
ordinates. To identify boundaries (wedge stimuli), maps were created based on cross-
correlation values for each voxel, determined by a standard hemodynamic box-car
function (r ≥ 0.25). We identified the boundaries of the V1 by hand based on the hori-
zontal and vertical meridians and knowledge of the retinotopic organization of the
visual cortex.
All volume measurements were made on the 3-D anatomical image. Under each
eccentricity condition (0°-7.5°, 7.5°-15°, 15°-22.5°, 22.5°-30°, 30°-37.5°,
37.5°-45°, 45°-52.5°, 52.5°-60°), each strongest-response voxel in the V1 was
