Emotion Category and Face Perception Task Optimized for Multivariate Pattern Analysis

## Sample

We used a cross-sectional group of 16 volunteers from both sexes (8
female, 8 male) with an average age of 25 years, recruited at UNAM
campus Juriquilla from October 2019 to June 2020. Participants were
briefly interviewed to exclude those previously diagnosed with
neurological or psychiatric conditions. With the exception of one male
subject, all of them reported having right-handed phenotype. Prior to
the study, subjects formally consented to participating after being
informed of its aims, risks and procedures — in accordance with the
1964 Declaration of Helsinki.

## Image acquisition

Images were obtained from a 3-Tesla General Electric Discovery MR750
scanner at the MR Unit at UNAM's Institute of Neurobiology, during a
single session per participant. The protocol included 5 echo-planar
imaging (EPI) blood-oxygen level-dependent (BOLD) sequences for fMRI,
185 volumes each. A T1-weighted scan of head anatomy was also
acquired, in addition to a 10-minutes long _resting state_ fMRI before
the task. Sequence parameters are described in the table.
Echos were registered using a head-mounted 32-channel coil.

| Parameter | EPI BOLD | T1w FSPGR | T2w FSE |
|----------------|--------------:|-----------------:|------------:|
| Slice orientation | Axial | Axial or sagital | Axial |
| Slices | 35 | 176 | 35 |
| Field of view | 64×64 | 256×256 | 512x512 |
| Voxel size | (4 mm)^3 | (1 mm)^3 | (.5x.5x.4 mm^3) |
| Flip angle | pi / 2 | 3*pi / 45 | 7*pi / 9 |
| TR (ms) | 2000 | 8.18 | 6255 |
| TE (ms) | 30 | 3.19 | 101.4 |
| TInv (ms) | | 450 | |

## Stimuli and task

Each of the 5 fMRI sequences was temporally coupled to a psychological
block-based task implemented in PsychoPy 3.0.1. All 5
tasks were identical, save for the pseudo-random order in which their
30 s blocks were administered. A total of 6 block classes were used:
happy faces, sad faces, angry faces, neutral faces, pseudo (scrambled)
faces and low-stimulation (dim).

Pseudo-faces and dim blocks were introduced so as to buttress and
diagnose the analysis pipeline, by way of more trivial contrasts (like
pseudo-faces vs low-stimulation and faces vs pseudo-faces).

Each block in turn comprises 10 randomly-presented images belonging to
that class, each one shown for about 3 seconds and without possibility
of reinstantiation during the same block. Each block occurs twice per
sequence, yielding a total of 12 of them (360 s = 6 min). After their
presentation, participants had to wait for 10 seconds before
concluding the sequence, in order to capture the hemodynamic response
(HR) elicited by the last stimuli. A selection of 10 grayscale
photographs per category of frontal human faces (male and female)
served as stimuli. These were chosen from the classical "Pictures of
Facial Affect" database (Ekman, 1976). As for the low-stimulation
(a.k.a. "dim") blocks, a small but visible fixation cross was made
fluctuate from quadrant to quadrant at random every 3 seconds.

Additionally, behavioral responses were recorded throughout the task
in order to measure performance and thus evaluate the suitability of
physiological data for further analysis. Participants were instructed
at the beginning of every sequence to indicate whether faces belonged
to a man or a woman as soon as they were perceived. The response was
submitted with the press of a button — one at each hand. Analogously,
for scrambled and dim blocks (when no faces should have been
perceived), the instruction was to simply report image change,
alternating between buttons.

Task files and responses available [here](https://github.com/isacdaavid/emotional-faces-psychopy-task).