Experimental Design

Figure 1: Example stimuli for main experiment

Figure 1: Example stimuli for main experiment

In developing these tutorials, we wanted an experiment that used at least one brain area that shows robust activation and one brain area that is less well explored. As such we chose to study faces and hands. Images of faces reliably activate a right-hemisphere lateralized network of regions 1,2 that includes the well-studied Fusiform Face Area (FFA) 3 Images of hands activate (though perhaps less reliably) a scarcely explored region of the lateral occipitotemporal cortex (LOTChand) 4 .

We wanted a main experiment that would be amenable to various designs (e.g., block vs. event-related designs), analyses (e.g., univariate subtractions vs. multivariate pattern analysis), and strategies (e.g., region-of-interest vs. voxelwise approaches).

We decided to test whether activation levels (and perhaps activation patterns and functional connectivity) differ between faces and hands that are directed toward the observer vs. away. Behavioral evidence shows that participants are faster to respond to a target when primed by a face looking toward vs. away from the target 5 .

Given brain-activation differences between faces that differ in gaze direction (direct vs. sideways; left vs. right)6, we expected that we could replicate these differences and explore a more novel question of whether activation differences existed for images of pointing hands (direct vs. sideways; left vs. right), which have also shown behavioral differences 7 .

The main experiment was a 2x3 factorial design that crossed stimulus category (faces vs. hands) with stimulus orientation (left, centre, right), as shown in example stimuli in Figure 1.

Figure 2: Example stimuli for localizer

Figure 2: Example stimuli for localizer

Localizer

To provide the opportunity for conducting region-of-interest analyses using independent data, we also collected localizer runs for images of faces, hands, bodies, scrambled images compared to a fixation baseline.

Participants

Data from a group ( 14 participants ) will be utilized for the core analyses in the exercises. Additional data were collected from three additional participants but were discarded because one showed excessive/abrupt motion and two had only 5 main experiment scans available (fewer than the 6 used for most analysis).

Participant age, sex, and handedness have not yet been compiled. All participants had normal or corrected-to-normal vision and were financially compensated. Informed consent was obtained prior to scanning. All experimental procedures were approved by the University of Western Ontario’s Health Sciences Research Ethics Board.

Experimental Design and Timing

Localizers

Figure 3: Localizer Stimuli

Figure 3: Localizer Stimuli

Localizer stimuli enabled identification of brain regions selective to three categories of visual stimuli: faces, hands, and bodies 8. These categories could be compared against a fourth stimulus type -- scrambled versions of the same images – to identify brain regions that respond to coherent visual stimuli.

Figure 4: Protocol for Localizer 1

Figure 4: Protocol for Localizer 1

Participants were instructed to maintain their gaze on a central fixation point, which was also presented on a blank screen during baseline periods. Participants monitored the stream of visual stimuli for repetitions (a one-back task) to maintain attention.

Images were presented to subjects according to a conventional block design, with a block duration of 16 s. Each block consisted of 16 stimuli, each presented for 0.8 s with a 0.2-s intertrial interval.

Four cycles of four blocks (faces, hands, bodies, scrambled images) were presented in each run. Baseline blocks (also 16 s) occurred at the beginning and end of each run and between cycles. The total run duration was 336 s (21 blocks x 16 s/block). Each stimulus block consisted of 16 stimuli, each presented for 0.8 s with a 0.2 s intertrial interval. The order of blocks within cycles was balanced such that for a run, each category was presented once in each of the first, second, third and fourth part of the cycle. Two runs with two different orders were collected. The order for Localizer 1 is visualized by Figure 1.

Main Experiment

As shown in Figure 1, stimuli in the main experiment formed a 2 x 3 factorial design with image category (faces/hands) and orientation (left/centre/right). Orientation indicated direction of eye gaze and direction of pointed index finger, for faces and hands, respectively. A static fixation point and a one-back task were also used in the main experiment.

Figure 5: Example protocol for experimental run (Order 3)

Figure 5: Example protocol for experimental run (Order 3)

Images were presented in a rapid event-related design with a total run duration of 280 s. Baseline blocks (16 s) occurred at the beginning and end of each run. The remaining 248 s were divided into 62 trials (4 s/trial). According to 10 different, shuffled run orders, a balanced subset of the visual stimuli was presented over 51 trials (including 3 one-back stimuli). The remaining 11 trials were randomly interspersed non-consecutive null stimuli. Six runs with six different orders were collected. For each subject, a different combination of run orders (210 possible) was used. An example of one experimental run order is visualized by Figure 3. Examples of stimuli used in experimental runs are shown in Figure 4.

MRI Acquisition

All scans were collected using a Siemens Magnetom Prisma 3-Tesla MRI scanner at the Centre for Functional and Metabolic Mapping at the Robarts Research Institute of Western University. Functional scans utilized 2.5-mm isotropic resolution with a matrix size of 84×84 over 52 slices, repetition time (TR) = 1000ms, multi-band gradient-echo echoplanar pulse sequence. Anatomical scans were collected using T1-weighted 3D MPRAGE sequence with a matrix size of 248×256 over 176 1-mm slices.

Data Preprocessing

Data were preprocessed in Brain Voyager 20.6 software. Standard preprocessing steps for the primary pipeline consisted of functional-anatomical alignment, three-dimensional motion correction, temporal high-pass filtering, transformation into 2-mm isotropic resolution, and warping into stereotaxic space using the MNI-152 template.

Variant Scan Descriptions

Variants

Several variants of the localizer protocols were used to collect additional data. These variants provide concrete examples to explore the analytical effects of varying experimental design and conditions.

Variant 1 – Alternative (Suboptimal) Block Designs

The block design protocols for this variant use alternating blocks of faces and hands. In all cases, stimuli are presented for 0.8 s with an intertrial interval of 0.2 s. No baselines are used. Block duration ranges from 4 s to 64 s, and all seven run orders share a run duration of 336 s. All block design run orders are described by the following legend and figures.

Order 1 – alternating 16 s blocks of hands and faces.

Order 2 – alternating 16 s blocks of faces and hands.

Order 3 – alternating 4 s blocks of hands and faces.

Order 4 – alternating 4 s blocks of hands and faces with starting and ending hand baselines.

Order 5 – alternating blocks of hands and faces with different block durations: hands (4 s) and faces (8 s) with starting and ending hand baselines.

Order 6 - alternating blocks of hands and faces with different block durations: hands (4 s) and faces (16 s) with starting and ending hand baselines.

Order 7 – alternating blocks of hands and faces with long block duration (64 s) with ending hand baseline.

Variant 2 – Block and Slow Event-Related Designs with Baselines

This variant integrates baselines (stimulus-free periods) into four block design orders and three slow event related orders. Once again, stimuli are presented for 0.8 s with an intertrial interval of 0.2 s. Order durations vary.

image8.png

Order 1 – alternating blocks of baseline (16 s) and stimulus (16 s), where stimulus alternates between faces and hands, ending with additional baseline (16 s). Order duration is 336 s.

Order 2 - alternating blocks of baseline (16 s) and stimulus (16 s), where stimulus alternates between hands and faces, ending with additional baseline (16 s). Order duration is 336 s.

image8.png

Order 3 – repeating blocks of baseline (16 s), faces (16 s), and hands (16 s), ending with additional baseline (16 s). Order duration is 304 s.

Order 4 – repeating blocks of baseline (16 s), hands (16 s), and faces (16 s), ending with additional baseline (16 s). Order duration is 304 s.

Order 5 – Slow Event Related.

Order 6 – Slow Event Related.

Order 7 – Slow Event Related.

 

References

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  4. Bracci, S., Ietswaart, M., Peelen, M. V & Cavina-Pratesi, C. Dissociable neural responses to hands and non-hand body parts in human left extrastriate visual cortex. J. Neurophysiol. 103, 3389–97 (2010).
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  8. Downing, P. E. et al. A cortical area selective for visual processing of the human body. Science 293, 2470–3 (2001).