Chapter III: Face Preference Decision-Making And Visual Behavior
3.3 Gaze Cascade Study in Amygdala Lesion Patients
3.3.1 Materials and methods
Figure 3.6. Anatomical scans of the amygdala lesion patients. Red arrows indicate location of the amygdala calcification damage. (Source: Mike Tyszka, Caltech Brain Imaging Center).
The comparison group consisted of 3 healthy female controls (Mage = 34.0 years, SD =4.6, age range = 29-38), group-matched for age and IQ (as measured by the Wechsler Abbreviated Scale of Intelligence; (Wechsler, 1999), with no family history of psychiatric illness. Table 3.7 summarizes demographic and diagnostic information for participants.
Independent samples t-tests showed that the groups did not significantly differ in terms of age, t(4) = 0.07, p = .945) and IQ (t(4) = -2.30, p = .083. All participants gave written informed consent to participate under a protocol approved by the Institutional Review Board of the California Institute of Technology.
Table 3.7. Demographic information for amygdala lesion and control participants in the gaze cascade study.
a. Verbal IQ and full-scale IQ from the Wechsler Abbreviated Scale of Intelligence;
AQ: Autism Quotient.
3.3.1.2 Stimuli and apparatus
Stimuli were identical to the stimuli described in the Methods section of the Gaze Cascade study in autism participants (for details, see section 3.2.3.2 Stimuli and apparatus). Stimuli consisted of either pairs of social stimuli (computer-generated human faces) or pairs of non- social stimuli (nature scenes sourced from a google image search for “desert” and
“mountain”). Face images, generated using Facegen (Singular Inversions, Vancouver, Canada), were front-facing with neutral emotional expression and direct eye contact. Stimuli were paired using the same procedures outlined in the previous study with autism
participants.
Amygdala participants Age Verbal
IQ
Full scale
IQ AQ
AP 27 92 98 20
AM 38 94 96 21
BG 38 99 101 18
Mean 34.3 95.0 98.3
SD 6.4 3.6 2.5
Control participants Age Verbal
IQ
Full scale
IQ AQ
1 35 108 107 -
2 29 116 116 9
3 38 104 102 11
Mean 34.0 109.3 108.3
SD 4.6 6.1 7.1
Images were presented on a 23” TFT monitor with a pixel resolution of 1920 x 1080. The stimuli in each test pair were presented simultaneously on the left and right side of the screen.
At a viewing distance of approximately 62 cm, each stimulus pair had an overall size of 36.5 (width) x 14.4 (height) degrees of visual angle.
A desk-mounted Tobii TX300 eye tracker (Tobii Technology, Falls Church, VA, USA) was used to collect gaze data. Stimuli were presented using Matlab (Mathworks, Natick, MA) and the Psychophysics toolbox (Brainard, 1997), and the T2T-Talk2Tobii toolbox (Deligianni, Senju, Gergely, & Csibra, 2011). Corneal and pupil reflection were recorded at a sampling rate of 300 Hz. At the beginning of each block, a 9-point calibration was performed. Each trial began by requiring subjects to fixate on a central drift correction dot. After the eye-tracker registered a successful fixation, participants pressed the space bar to start the trial.
3.3.1.3 Procedure
Experiment procedure was identical to the procedure described in the Methods section of the Gaze Cascade study in autism participants (for details, see section 3.2.3.3 Procedure).
Subjects performed various 2-alternative forced-choice (2AFC) tasks while eye-gaze was tracked (see Figure 3.7 for summary of experimental conditions and example stimuli). The same conditions were tested as in the study with autism participants, with the exception of Faces with Closed Eyes, which we omitted here. Each amygdala participant was tested twice on two different days to increase the likelihood of collecting reliable data.
Condition Name
Stimuli Description
Example Stimuli
Time per Trial
Decision Type
Open Eyes Faces with Open Eyes
Self-paced (2 x 40 trials)
Preference Judgment
Timed Faces with
Open Eyes
1.5 seconds (40 trials)
Preference Judgment
Roundness Faces with Open Eyes
Self-paced (40 trials)
Objective Judgment
Nature Scenes Nature Scenes Self-paced
(40 trials)
Preference Judgment
Figure 3.7. Summary of experimental conditions and example stimuli.
3.3.1.4 Analyses
Analyses were identical to the analyses described in the Methods section of the Gaze Cascade study in autism participants (see section 3.2.3.4 Analyses), unless noted here. The general methods of the analyses are described below again for convenience, but for further detail, please refer to Chapter 3.2.
Data from the amygdala group and from the control group were each fit with a four-
parameter sigmoid regression curve for each condition, with four parameters representing the following: (1) bottom plateau – baseline comparison probability between the two stimuli, (2) top plateau – gaze bias at which the participant made the conscious behavioral choice, and (3
& 4) point of inflection and slope at point of inflection – timescale indicating the quickness of the decision. Finally, 95% confidence intervals were calculated for each of the four parameters estimated.
Due to the small sample size in this study, the permutations test that was performed in the ASD subjects was determined to lack power and therefore was not performed. For the comparisons presented here, differences between groups were considered significant when confidence intervals did not overlap.
Analysis of accuracy was limited to low difficulty trials because only low difficulty trials had an objectively correct (i.e., higher-rated) image for the preference tasks, allowing us to define accuracy.
Since the distributions for reaction times (RT) were positively skewed, log-transformations were conducted prior to statistical analysis. Raw values are reported in the text and figures.
Trials were excluded if reaction times were greater than 3 SD outside the group mean, if no valid button press was registered, or if more than 20% of the eyetracking data was invalid (<
4% of the data).