Neonatal Amygdalectomy and Visual Scanning Behavior in Monkeys: An Investigation of Scanpath Abnormalities and the Effects of Fixation Definition Open Access

Abstract

Previous case studies have shown that a patient with bilateral amygdala damage, S.M., habitually
fails to spontaneously fixate the eye region of human faces under free-viewing conditions. Such
findings have led researchers to hypothesize that the amygdala is critically involved in directing
visual attention to salient, social stimuli in the environment which might contain valuable social
information. However, these effects have yet to be demonstrated in others with similar damage.
Two rhesus monkeys with extensive bilateral amygdala damage (sustained at one month of age)
viewed images of unfamiliar monkey faces as their eye movements were recorded. Contrary to
our predictions, subjects showed no obvious disturbances in their visual scanning behavior
during the task. Further research is necessary to investigate the possibility that a difference in
the timing of the lesions led to the discrepancy in the findings between the current study and
previous work with S.M. The current data were also used in order to compare the effects of the
application of two different temporal fixation definitions on the outcome(s) of eye tracking data
analysis. Because the current subjects represent a special population (i.e., monkeys with brain
lesions), the use of a minimum fixation duration lower than the de facto standard in the field was
warranted. Lowering the duration minimum from the customary 100ms to 55ms increased the
number of fixations included in the data set by 41%. Yet, no differences as a function of
duration threshold were seen in the percentage of fixations made within and outside of regions of
interest (ROIs), the percentage of fixations made within each ROI, the percentage of time spent
viewing within and outside of ROIs, or the percentage of time spent viewing each ROI. This
suggests that the use of a lower temporal fixation threshold, when justifiable, increases the total
amount of data included in analyses, thereby increasing the power of those analyses as well as
sensitivity to between-group differences when special populations are being compared to healthy
ones. At the same time, use of the lower threshold will not significantly alter the spatiotemporal
patterns of fixation on face stimuli.

Table of Contents

Table of Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
A Role for the Amygdala in Visual Scanning of Social Stimuli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Defining Fixations: Temporal Criteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Subjects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Stimuli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Apparatus and Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Measures and Analyses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Visual Scanning Patterns of Amygdalectomized Subjects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Defining a Fixation: Standard Duration Threshold versus Lower Threshold for Special Populations . . . . 12
Comparative Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Description of Visual Scanning Patterns of Amygdalectomized Subjects . . . . . . . . . . . . . . . . . . . . . . . . . 14
Fq12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Ht12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
100ms versus 55ms Fixation Duration Threshold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Effects on Data Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Effects on Descriptive Statistics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Visual Scanning Behavior of Amygdalectomized Nonhuman Primates . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Visual Scanning of Face versus Object Stimuli . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Individual Differences in Scanning Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Comparison with Previous Literature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Temporal Fixation Definition: Standard versus Study-Specific Thresholds . . . . . . . . . . . . . . . . . . . . . . . 26

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Tables and Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

About this Master's Thesis

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School	Laney Graduate School
Department	Psychology
Subfield / Discipline	Neuroscience and Animal Behavior
Degree	MA
Submission	Master's Thesis
Language	English
Research Field	Psychology, Psychobiology Psychology, General Psychology, Cognitive
Keyword	face scanning visual scanning rhesus monkey eye tracking amygdala
Committee Chair / Thesis Advisor	Rodman, Hillary R, Emory University
Committee Members	Wallen, Kim, Emory University Bachevalier, Jocelyne, Emory University Nygaard, Lynne, Emory University

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