Stimuli and design

For the EOO experiment, stimuli were designed to excite the receptive fields of neurons across a range of regions of interest (ROIs), including early visual areas V1, V2 and V3, and motion-sensitive areas hMT and hMST. They were spatially and temporally broadband, to accommodate the differences in preferred spatial frequency of monocular and binocular neurons (Schwarzkopf et al., 2010), variations in velocity tuning (Rodman & Albright, 1987), and different receptive field sizes. Stimuli consisted of randomly positioned isotropic

Laplacian-of-Gaussian (LoG; Dakin & Mareschal, 2000) elements (dot density = 1.5 dots/deg2_{, dot sigma = 0.05º), presented at 50% contrast on a mean grey background.}

Elements were generated by convolving the x and y point positions with a LoG function given by

∇2𝐺(𝑥, 𝑦, 𝜎) = 1 𝜎2(1 − 𝑥2+ 𝑦2 𝜎2 ) exp (− (𝑥2+ 𝑦2₎ 2𝜎2 ),

where 𝜎 = 0.05 (example shown in Figure 4.2). Dots moved in two cardinal directions

(up/down or left/right) during different instances of stimulus presentation, and at four different speeds within each display (0.20, 0.68, 2.34 and 8.00º/s). Because of the balance of contrast polarity in each individual dot, LoG profiles avoid artefactual “motion streaks” (Apthorp, Cass, & Alais, 2011; Geisler, 1999) that can confound the decoding of motion direction (Clifford, Mannion, & McDonald, 2009; Maloney, Watson, & Clifford, 2014).

Figure 4.2 Stimulus conditions for the EOO (panels A and B) and stimulus localiser (panel C)

fMRI scans. A zoomed-in binocular view of the EOO stimulus is shown in panel A. Elements moved at 4 different speeds, either leftward and rightward, or upwards and downwards

(indicated by blue arrows). Each eyes’ views during different stimulation conditions are shown in panel B. From top to bottom: left eye stimulated, right eye stimulated, binocular stimulation, baseline fixation. Panel C shows the stimuli used to map the retinotopic extent of the EOO stimulus (bottom – same size as the stimulus in panel A), and the unstimulated periphery (top). The whole field of view is shown, giving an estimate of the size of the EOO stimulus in context.

Dot fields were viewed through a circular aperture (0.25º inner and 1.5º outer radius, edges ramped with a cosine envelope of 0.4º). This scale ensured visual stimulation well away from the blind spot (at an eccentricity of roughly 15º from the fovea, with a monocular representation in V1; Tong & Engel, 2001; Tootell, Hadjikhani, Vanduffel, et al., 1998). Stimuli were presented to the left eye, right eye, or both eyes simultaneously. In the monocular conditions, the unstimulated eye viewed the mean grey background with the same fixation point that was also present during stimulus conditions. This fixation point (diameter 0.22º) was presented at the centre of the display to control the eye position of participants. To ensure participants were fixating, and to control the allocation of spatial attention, participants

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pressed a button when the centre of the fixation point changed subtly in luminance.

Luminance changes occurred randomly over the course of a scan, where the time between changes varied between 1.5s and 9s.

Stimuli were presented for 3s – at onset and offset, stimulus contrast was ramped over 300ms with a raised cosine envelope, and stimuli were at maximum contrast for 2.4s. They were presented in a dense, rapid event-related design, where the inter-stimulus interval (ISI) varied between 3s (the length of 1TR) and 12s. Stimulus onsets were determined separately for each scan using Optseq2 (https://surfer.nmr.mgh.harvard.edu/optseq). Stimulation conditions were: left eye stimulation, right eye stimulation, binocular stimulation, and a fixation-only baseline condition. Each condition occurred 12 times in each scan – of these, motion in the stimulus could be up/down or left/right with a 50/50 split. The EOO stimulus and presentation conditions are depicted in Figure 4.2, panels A and B.

We also designed localiser stimuli to map the retinotopic extent of the EOO stimulus, and the unstimulated periphery. fMRI responses to the localisers allowed early visual areas (V1, V2 and V3) to be split along the eccentricity dimension, into stimulated ‘inner’ and unstimulated ‘outer’ ROIs. Localiser stimuli were contrast-reversing log-scaled radial checkerboard rings updating at 1Hz, where the ‘inner’ ring was the same size as the EOO stimulus (extending 0.25º to 1.5º from fixation) and the ‘outer’ stimulus mapped the

surrounding area (extending 2º to 11.75º from fixation, leaving a 0.5º gap between the ‘inner’ and ‘outer’ rings). Stimuli were presented at 50% contrast on a mean grey background, to the left eye, to the right eye, or to both eyes. A fixation point at the centre of the display stabilised eye gaze, and the centre of the fixation point changed in contrast in the same manner as for the EOO stimulus.

In the stimulus localiser fMRI run, each stimulus condition (inner ring left eye, inner ring right eye, inner ring binocular, outer ring right eye, outer ring left eye, outer ring binocular, and a blank baseline fixation condition) was presented in 9s blocks with 3 repeats of each

4.3.4

MRI parameters

High-resolution T1-weighted anatomical scans (TR = 7.8ms; TE = 3.0ms; TI = 600ms; flip angle = 20°; FOV = 25.6 x 25.6 cm; matrix size = 256 x 256; voxel resolution = 1.0 x 1.0 x 1.0mm; 176 coronal slices to cover the whole head) were acquired on a 3T GE SIGNA HDx Excite MRI scanner for each participant in a separate scanning session, using an 8-channel whole-head phased-array coil (MRI Devices Corporation).

During the experimental session, data were collected using a 16-channel phased-array half-head coil (Novamed) to improve the signal-to-noise ratio in the occipital cortex. To co- register functional data with the high-resolution anatomical scan, a proton-density (PD) weighted reference scan with the same slice prescription as the EPI scans was collected for each participant (TR = 2700ms; TE = 38ms; flip angle = 90°; FOV = 19.2 x 19.2cm; matrix size = 512 x 512; voxel resolution = 0.38 x 0.38 x 2.5mm; 39 quasi-coronal, contiguous slices oriented along the calcarine sulcus and covering the occipital lobe). Standard gradient-echo EPI sequences (TR = 3000ms; TE = 30ms; flip angle = 90°; FOV = 19.2 x 19.2cm; matrix size = 96 x 96; voxel resolution = 2 x 2 x 2.5mm; 116 volumes including 4 dummy volumes, total scan time = 5 minutes 48 seconds) were used for one stimulus localiser scan and seven EOO scans.

Finally, each participant completed two motion localiser scans to identify motion- sensitive areas V3A/B, hMT and hMST. In addition, standard retinotopic mapping scans (typically five wedge and two ring scans, with eight stimulus cycles each) were carried out to delineate early visual areas. These data were collected in separate scan sessions, with fMRI parameters similar to those described above, and are described in more detail in Chapter 3.

4.3.5

Mapping regions of interest

ROIs (V1, V2, V3, V4, V3A/B, IPS-0, LO-1, LO-2, hMT and hMST) were defined in each subject, as described Chapter 3, using a combination of retinotopic mapping and motion localisers. Briefly, areas V1, V2, V3 (Dougherty et al., 2003; Schira, Tyler, Breakspear, & Spehar, 2009; Sereno et al., 1995), V4 (Brewer, Liu, Wade, & Wandell, 2005; Hansen, Kay, & Gallant, 2007; Wade, Brewer, Rieger, & Wandell, 2002; Winawer, Horiguchi, Sayres, Amano,

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& Wandell, 2010), LO-1, LO-2 (Larsson & Heeger, 2006), and IPS-0 (Press, Brewer, Dougherty, Wade, & Wandell, 2001; Swisher, Halko, Merabet, McMains, & Somers, 2007; Tootell, Hadjikhani, Hall, et al., 1998), were delineated using characteristic reversals in the polar angle phase map acquired using conventional retinotopic mapping methods. Motion- sensitive ROIs V3A/B, hMT and hMST (Amano, Wandell, & Dumoulin, 2009; Fischer, Bülthoff, Logothetis, & Bartels, 2012; Huk, Dougherty, & Heeger, 2002) were defined iteratively using both motion localisers and retinotopic mapping.

Early visual areas V1, V2 and V3 were restricted in the eccentricity dimension into six ROIs, using the stimulus localiser described above. ‘Inner’ ROIs corresponded to the cortical surface that was directly stimulated by the retinotopic extent of the EOO stimulus (from 0.25º to 1.5º from fixation). The ‘outer’ ROIs mapped the periphery (from 2º to 11.75º from fixation). Voxels in the ‘outer’ ROIs were not driven by the experimental stimulus, and some of the underlying receptive fields may be actively suppressed, resulting in a negative BOLD response (Shmuel et al., 2002). This dissociation allowed us to investigate whether the suppressive response also contains some monocular tuning.

Finally, the fusiform face area (FFA) was defined as a control ROI. This area is strongly driven by face stimuli. The FFA was chosen as it is visually responsive, but its high degree of category selectivity implies that it is unlikely to maintain any EOO tuning, given that this information is redundant for its functional specialism. The FFA was defined in each subject by centring a 5mm sphere on Talairach co-ordinates given in the original fMRI paper by

Kanwisher et al., where the amplitude of the BOLD response was compared between faces, objects and houses (Kanwisher, McDermott, & Chun, 1997). These co-ordinates were set at [-35 -63 -10] in the left hemisphere, and [40 -55 -10] in the right hemisphere. The size of the sphere was chosen because the mean size of the FFA is 5mm3 _{in the left hemisphere, and}

10mm3 _{in the right hemisphere (Kanwisher et al., 1997).}