fMRI experiment

Data were acquired using a Philips Achieva 3 Tesla scanner. T1-weighted structural MRI data were acquired in each session at 1 mm isotropic resolution. Functional MRI data (gradient echo EPI) were acquired with 3 × 3 mm inplane resolution. 30 oblique-axial slices were obtained (3 mm slice thickness, separated by a 0.5 mm gap). Other scan parameters: 2 s TR, 25 ms TE, 79° flip angle, A-P phase-encode direction. At the start of each session, an opposite phase-encode direction (P-A) scan (1 TR) was acquired for distortion compensation. Each scanning session lasted about 1 hour.

Stimuli were displayed via projector (Epson Powerlite 7250 or Eiki LCXL100A, following a hardware failure), operating at 60 Hz using Presentation software (Neurobehavioral Systems, Berkeley, CA) on a PC running Windows XP. Images were projected on a semicircular screen at the rear of the scanner, and viewed through a mirror on the head coil at a distance of 66 cm. Projector luminance was linearized using custom software.

The experiment was designed to measure the amplitude of the response in human MT+ and early visual cortex (EVC) to two stimulus contrast levels. Sixteen gratings were presented at the center of the screen during each block; to prevent adaptation, gratings moved in one of eight possible directions in a randomized and counter-balanced order for 400ms. We used longer stimulus durations than typical psychophysical duration thresholds to ensure robust BOLD responses to moving stimuli. Twenty-five blocks were presented during each run (10 s each, 125 TRs total). Stimulus diameter was 2°, and contrast varied across blocks. Each run began with a blank block (0% contrast), in which only the fixation task was presented on a blank background. Blocks of drifting gratings at 3% and 98% contrast were then presented centrally in an alternating order, each followed by a blank block (6 low contrast, 6 high contrast, and 13 blank blocks per run). The blank block served as baseline. A 10 s baseline period was chosen to balance between the competing needs of accurately estimating response amplitudes – for which a long (20-30 s) baseline period would afford full recovery of the hemodynamic response – and increasing the number of trials in the experiment to maximize signal-to-noise. To functionally localize MT+ we used a separate localizer scan. Drifting and static 2° gratings (15% contrast) were presented centrally in alternating 10 s blocks (13 static and 12 drifting blocks, 125 TRs total). Another localizer scan was used to functionally identify EVC. Here, contrast-reversing checkerboard stimuli (2° diameter, 100% contrast, 8 Hz) were presented at the center of the screen, and alternated with blank backgrounds across a total of 16 blocks (8 stimulus & blank blocks, 10 s per block, 80 TRs total). Subjects completed two runs per session and all but one subject completed two fMRI sessions (other experiments, not presented here, were included in the fMRI sessions).

24 = 0.01, p = 0.99). During all scans, subjects performed a color & shape conjunction detection task, responding to a green circle within a series of colored shapes presented briefly at fixation ( Figure 2 A). A total of 8 shapes were presented in each 10 s block, and were selected randomly from 8 possible shapes (including the green circle target). Hit rate in this task (averaged across all scans within each subject) did not differ between males (mean = 95%, SD = 6.7%) and females (mean = 95%, SD = 7.6%; t= 0.01, p = 0.99).

FMRI data were processed in BrainVoyager (Brain Innovation, Maastricht, the Netherlands). The following steps were performed in order: motion correction, distortion compensation, high-pass filtering (cutoff = 2 cycles/scan), and alignment to the T1 anatomy. No spatial smoothing or normalization were performed for ROI-based, within-subjects analyses. ROIs were identified from the functional localizer data using correlational analyses, with an initial significance threshold of p < 0.05 (Bonferroni corrected). The top 20 most-significant voxels within the ROI were then selected for further analyses. In cases where there were not 20 voxels that satisfied the above threshold, the threshold was relaxed until 20 voxels were included. ROIs were defined for each hemisphere in 2 anatomical regions: motion-selective MT+ in the lateral occipital lobe, and the region of EVC representing the center stimulus (near the occipital pole). ROI position was verified by visualization on an inflated cortical surface.

Average fMRI time courses were extracted from each ROI for further analyses in MATLAB using BVQXTools. Time course data were divided into epochs from 4 s before to 4 s after each block. For each block, response baseline was calculated as the average signal across all epochs between 0-4 s prior to block onset. The time course in each block was then converted to percent signal change. Time courses were then averaged across hemispheres in each run, and across runs in each subject. The response peak, defined as the average signal change from 8-12 s after the block onset, served as the measure of the fMRI response. A full-brain, voxel-wise mixed ANOVA (sex X contrast) was also performed on spatially (Talairach) and temporally (%-transform) normalized data.