Chapter III: Face Preference Decision-Making And Visual Behavior
3.3 Gaze Cascade Study in Amygdala Lesion Patients
3.3.3 Discussion
We tested three patients with rare amygdala lesions to examine whether social processing impairments would affect preference decisions for faces and the temporal evolution of those decisions. Interestingly, amygdala lesions patients demonstrated an intact feedback loop linking gaze and preference for faces and made similar preference choices as controls. There were, however, hints in the fixation behavior of the amygdala group, suggesting that there are underlying differences in the pattern of gaze towards faces.
Due to the small sample size, group comparisons lacked the statistical power to allow us to draw quantitative conclusions. There were, however, notable qualitative trends in the data worthy of discussion that I will review briefly in this section.
At the individual level, the gaze cascade effect was observed for each of the amygdala subjects in all four conditions, indicating that the feedback loop between preference and foveation is normal and intact in the amygdala lesions subjects. At the group level, the sigmoid curves fit to the raw data were generally similar between groups. With the exception of the Open Eyes condition, inspection of the confidence bounds did not reveal any significant differences in the four parameters describing each group’s sigmoids. In the Open Eyes condition, the sigmoid for the amygdala group rose more rapidly than it did for controls, and also reached a higher plateau. This suggests there may be an underlying difference in the time course of face preference formation in the amygdala group. Moreover, the sigmoid curves appeared similar across the four conditions in the amygdala group whereas the sigmoid curves are notably different across conditions for healthy controls, indicating that the amygdala group demonstrated relatively invariant gaze during decision-making, regardless of task or stimulus type. Preference decisions were also similar between groups, at least in the low difficulty trials, as indicated by the relatively similar beta weights resulting from the linear regression in the low difficulty trials. While between-group correlations for preference choice in the low and high difficulty trials did not reach significance in the preference decision tasks, this is likely due to an issue of the statistics being underpowered.
Analysis of gaze behavior indicated that fixation patterns towards faces were likely different in patients with amygdala lesions relative to controls. First, fixation durations were longer for the amygdala group than the control group, particularly for preference decisions regarding faces.
In both the Open Eyes condition and the Timed condition, the amygdala subjects made longer fixations relative to controls, while in the Nature Scenes condition, they made shorter fixations than controls. Moreover, when we compared fixation rates in each condition (fixations per second), there was a trend for noticeably slower fixation rates in the Open Eyes condition and Timed conditions, marginally slower rates in the Roundness conditions, and no difference in the Nature Scenes condition. In other words, the amygdala group made fewer and longer
fixations for decisions involving face preference, but not nature scenes or objective decisions for faces. Longer fixations are generally believed to be an indication of difficulty extracting information (Fitts, Jones, & Milton, 1950; Goldberg & Kotval, 1998; Rayner et al., 2007). On the other hand, they could also indicate increased engagement with the stimuli (Poole & Ball, 2014). Moreover, fixation rates might also simply reflect the speed of processing in mental tasks (Kahneman, 1973). Hence, there is at least weak (however, non-significant) evidence for preference decisions involving faces to be associated with amygdala-functioning.
There was also a trend for longer latency to first fixation on a stimulus in the amygdala group compared to controls. This effect was observed in all four conditions, which could be associated with slower mental processing or could suggest a lack of interest in initially engaging with the task. Though there were no differences in basic motor response times, the longer reaction times in the Open Eyes and Timed conditions suggest that the amygdala subjects were slower to make preference decisions for faces but not for nature scenes or face roundness.
The lack of systematic group differences in the gaze cascade curves and the relatively similar preferences in the amygdala lesion group relative to controls suggests that the amygdala is not required for forming preferences for social stimuli. While the amygdala plays a crucial role in processing faces and emotions, lesion effects might be stronger for tasks involving emotional processing rather than general face processing, since general face processing relies more on the fusiform gyrus and superior temporal sulcus (Haxby, Hoffman, & Gobbini, 2002).
Alternatively, it may be the case that other brain regions have compensated for the processes impaired by amygdala lesions. Indeed, there is evidence suggesting that some amygdalar functions can be partially compensated for, such that certain aspects of social processing, such as theory of mind (Shaw et al., 2004) and fear recognition (Becker et al., 2012) appear to remain intact or only moderately impaired (Brierley, 2004).
In sum, while there were qualitative trends in the data suggesting there are underlying differences in patterns of face gaze in amygdala participants, the comparisons of gaze behavior and reaction time did not yield significant group differences with the exception of the gaze
cascade curve for the Open Eyes condition. The absence of significant effects in our study may have been due to small sample sizes. Only significantly large effects would be detectable in such a small group comparison and it may be the case that differences would be observed with a larger sample size. Thus, while our findings suggest that the amygdala does not play a critical role in face preference formation, future studies using larger sample sizes will be necessary to elucidate if this conclusion holds true.
3.4 Familiarity versus Novelty Study in Autism