Abstract
fMRI was used to investigate human visual cortex responses to higher-order motion stimuli. Acquisition was on a Siemens 1.5 T scanner (T2*, gradient-recalled EPI, TR 3000 ms, TE 84 ms, flip angle 90°, 2 mm × 2 mm voxels, 256 mm FOV, 10 4-mm slices, 54 acquisitions per run). The measured volume included occipital and posterior parietal cortex. T1 scouts and, in some subjects, high resolution T1 volume images were also acquired. Visual stimuli were gamma-corrected movies (480 × 480 pixels), presented by a PowerMac via an LCD projector, shown through the rear of the scanner onto an adjustable mirror fixed above the subject's eyes. Three types of stimuli were used: (1) first-order motion, (2) second-order motion (both radial sine waves on random-dot backgrounds), (3) structure-from-motion consisting of two rotating circular patches (5 deg diameter) within which dots moved in a constant (centripetal) direction, superimposed on randomly moving dots. Three interleaved comparisons were made: stimulus vs blank, first-order vs second-order, and random motion vs structure-from-motion (27 s each phase, 3 repeats).
Analysis was based on a correlation coefficient method, after head-motion correction. Initial correlation was with the stimulus profile vector, then with an average BOLD response vector. Voxels with a correlation >0.5 (p<0.0003) were accepted as significant. In all subjects (seven- teen normals), all stimuli evoked bilateral activity in V1/V2 (BA17/18), and in extrastriate area V5/MT (BA37/19). Bilateral activation was also found in areas V3/V3a (BA19) and BA7. A more pronounced activation of area MST/V5a (BA37/39) was found in response to the structure-from-motion stimulus, compared with random motion.[Supported by: Wellcome Trust, Schilling Foundation.]
Get full access to this article
View all access options for this article.