Seven participants contributed to the study; all were undergraduates or postgraduates at Royal Holloway. All had normal or corrected vision. The experiments were conducted in accordance with the Declaration of Helsinki, approved by a local ethics committee at Royal Holloway, University of London, and written informed consent was obtained. Standard MRI screening procedures were followed and volunteers were paid for their participation. Participants were scanned on two occasions, usually separated by approximately a week. Additional scanning runs were also performed on other occasions to define regions of interest (ROIs; see below for details).
MRI images were acquired with a 3-Tesla Siemens Magnetom Trio scanner with an 8-channel array head coil. Anatomical (T1-weighted) images were obtained at the start of each scanning session (MP-RAGE, 160 axial slices, in-plane resolution 256 x 256, 1 mm isotropic voxels, TR = 1830 ms, TE = 4.43 ms, flip angle = 11°, bandwidth = 130 Hz/pixel). This was followed by six scanning runs of functional data acquisition with a gradient echo, echoplanar sequence (TR = 2s, 28 contiguous axial slices, interleaved acquisition order, 3 mm isotropic voxels, inplane resolution of 64 x 64 voxels, FOV = 192 x 192 mm, flip angle = 90°, TE = 30 ms, bandwidth = 1396 Hz/pixel). Functional scanning runs consisted of 224 volumes and therefore lasted seven minutes 28 seconds.
All stimuli were back-projected onto a screen mounted in the rear of the scanner bore by a computer-controlled LCD projector. Participants viewed the stimuli at a distance of 65cm via a mirror mounted on the head coil which provides a horizontal viewing angle of approximately 30 degrees of visual angle.
The stimuli consisted of a single large sinusoidal grating, presented in the centre of the screen, and approximately 20 degrees in diameter. The central one degree of the stimulus was coloured medium gray (as was the background) and contained a small black fixation cross which was present throughout the experiment. The spatial frequency of the grating was 1 cycle/degree, and the grating drifted upwards throughout each trial. There were four different speeds used for the drifting grating, which defined the four conditions: 2, 4, 8 and 12 degrees/second. The luminance of the stimulus was controlled by the use of two cross-polarising filters mounted on the end of the projector waveguide which extended into the scanner room, behind the back-projection screen. By rotating one filter with respect to the other a continuous modulation of the output of the projector could be achieved, which enabled very precise adjustment. Average luminance levels across the stimulus area were measured using a photometer to set the levels used in the experimental sessions. The high luminance condition’s average luminance was 30 cd/m2, and the low luminance condition was set at 1.5 cd/m2. These levels were based on those used in Hammett et al. (2007). ...