In order to distinguish eye- and head-centred coding, subjects had to perform the visual search task just described at three eye-gaze orientations, namely straight ahead, 10° left and 10° right, realized by shifting the fixation spot accordingly. The three eye-gaze orientations were tested in separate blocks of trials whose order was pseudo-randomized between subjects (Fig. 1A). In order to assess the BOLD activity contributed by the preparation and execution of the indicative saccades and PD0332991 in vivo the shifts of eye-gaze, subjects had to perform
a ‘control’ task, which had the same visual and oculomotor requirements as the main task, while lacking the need for visual search. In this control tasks, subjects saw the same sequence of visual stimuli. However, rather than
deciding on the direction of the indicative saccade based on the presence or absence of the target item, subjects were asked to ignore the search target and to saccade to the upper response target on the first trial. And, thereafter, MAPK Inhibitor Library solubility dmso they had to alternate between the upper and the lower one. Each subject completed three-five fMRI sessions, each session containing four blocks, with each containing one search condition defined by the specific location of the search set relative to the eyes and the head. Within each block, both the occurrence and the position of the target item were pseudo-randomized. Each block contained 12 search and 12 control trials matched with respect to eye-gaze direction, with trial-to-trial intervals varying from 5 s to 7 s. Thus, each Thalidomide session always contained 12 × 2 × 4 = 96 trials. To ensure that the subjects were able to perform the task and to collect additional behavioural data, we trained most (11) of them outside the scanner. Subjects were scanned in a 3-Tesla Siemens Tim Trio whole-body MRI system with an eight-channel head
coil. The head was immobilized with foam rubber placed between the head and the head coil. BOLD echo-planar functional images were acquired in 44 transverse slices (TR = 3 s, matrix size = 64 × 64, in-plane voxel dimensions = 3 × 3 mm, TE = 35 ms, flip angle = 90°, slice thickness = 2.5 mm). Anatomical images were acquired using a magnetization-prepared, rapid acquisition gradient echo (MP-RAGE) T1-weighted structural MRI sequence (number of slices = 176, matrix size = 256 × 256, in-plane voxel dimensions = 1 × 1 mm, TE = 2.92 ms, flip angle = 8°, TR = 2300 ms, slice thickness = 1 mm). Images of each subject were preprocessed using the statistical parametric mapping program package SPM2 (Wellcome Department of Cognitive Neurology, London, UK, www.fil.ion.ucl.ac.uk/spm). Functional images were first spatially realigned and slice time corrected. Structural images were co-registered to the mean volume of the functional images and normalized to the Montreal Neurological Institute space. Normalized functional data were then spatially smoothed using an isotropical Gaussian filter (10 mm full-width-at-half-maximum).