EEG feature of fear downregulation

How do you identify fear immediately? How do you classify fear from EEG? How would you quantify fear? These questions bothered me in the beginning when I started researching the intersection of emotions, affective physiological responses and EEG signal processing. I imagined a biofeedback computer game / therapy program that helps people conquer their fears by rewarding them in the process. As time went on, my empirical knowledge and experimental data were extended, which, though changed the questions, not the vision. Here I'd like to outline how it all happened, what are the results and prospects of the research.

Experiment

While reading papers on EEG response upon fear stimulation, I was quite annoyed that the stimuli used in previous experiments were mostly IAPS images or video clips from horror movies (yeah, The Shining) completely taken out of context. To this day I believe that such stimuli are not sufficient to evoke strong enough emotional responses in a time period higher than a couple of seconds, and thus unable to provide insight into the big picture of fear regulation.

The experimental design hasn't changed much since my previous measurements, but gameplay and webcam videos were also recorded this time, in addition to EEG, heart rate (HR), and galvanic skin response (GSR) vital signals. First open-, then closed-eye measurements were taken for Individual Alpha Frequency estimation. Participants then played the "daylight" version of a computer FPS game as the baseline measurement. Only the "night" version contained fear inducing stimuli.

40 participants (10 female) played with a horror game called Slender: The Eight Pages. I chose this game due to its simplistic approach to induce fear and no other emotion; no extra gore, no crying babies, just a shady guy following you in the woods. During the baseline gameplay, participants wondered around for about 2 minutes in daylight exploring the area. The night (normal) version of the horror game played after that lasted for a maximum of 15 minutes - depending on the time the player took until getting caught by Slender.

Means of self-report affective scores for different emotions at the beginning of the experiment (Baseline), after the daylight and after the night gameplay. Fear has a major significant increase after the night game compared to the other two situations. No significant difference was found in disgust, sadness or anger. It's funny to see how the perception of calmness and happiness diminished gradually.

Bit of statistics: the average age of participants was 23.05; all had finished or ongoing higher education; none of them had suffered from brain injuries, nor had been under psychological treatment. Only 5 of the participants had played any horror game before, 2 of them had tried Slender. Due to experimental difficulties (aka my sloppiness) data of 11 subjects became useless - taught me a lesson for life. HR and EEG data of 29 subjects were analyzed, 15 subjects had valid GSR data, 22 subjects had videos recorded in addition to vital signals. As the reaction mechanism of HR during receiving this complex stimuli is not trivial [1] GSR data was favored in the analysis - i.e., HR can increase all over the place, while GSR reacts to fear more selectively.

Analysis

EEG data preprocessing done using EEGLAB [2] plugins. FASTER [3] was applied in similar fashion for the most recent batch of recordings.

Scanning the literature of affective psychology and physiology of EEG, I came across the notion of frontal EEG asymmetry (FEA) [4]. FEA is a measure for the difference in activity when the two hemispheres are compared at the prefrontal cortex. It is also called frontal alpha asymmetry, as indeed, the spectral power between 7-13 Hz is measured for both hemispheres (requiring data from F3 and F4 sensors) before one is subtracted from the other. In short, the theory says that stronger right cortical activity suggests withdrawal oriented (~negative), stronger left activity suggests approach oriented (~positive) emotional state (and trait, but that's another story). As cortical activity is inversely proportional to alpha activity, the whole thing turns around and higher alpha on the right implies approach, higher alpha power on the left implies withdrawal. Now, this theory has been chewed and spit out a couple of times, but it seemed like a good place to start in order to look at the intensity of fear.

During the visual analysis of the first batch of data, I could not recognize any global change in frontal asymmetry that would follow GSR (or SCL/SCR in that matter), controversial to my expectation. Yet, I came across huge rapid drops of FEA around moments with high GSR / HR values. As the medial prefrontal cortex (mPFC) is well known area for emotion, including fear regulation [5], I began to fancy the idea that I found the sign of fear downregulation in EEG - in other words, the moments when one is calming oneself down.

Frontal EEG asymmetry (green) and galvanic skin response (blue) of a whole session - during open-, closed-eyed, baseline and fear inducing gameplay. Those downward spikes at the times when GSR starts decreasing made me wonder if they can be interpreted as fear downregulation.

Thus, the question got rephrased from "how to quantify fear", to "how to recognize moments of victory over fear". Why would there be short moments of ~positive emotional explosion right in the middle of intensified horror action? I guessed that it's the mechanism of the brain calming itself down. Moments of FEA drop were classified as FEA values 7 times the variance lower than the mean, where both the variance and mean were taken from the baseline gameplay - although, the results were not sensitive to this parameter 7 (significance always yielded with a parameter higher than 4.65), further investigation is necessary to determine if there's any meaning to the amplitude of the FEA drop.

Typical spectral head plot of a FEA drop at 9 Hz. Stronger right, opposed to left prefrontal alpha activity is shown, which indicates stronger left neuronal activity, which in turn implies approach oriented tendency.

Results

As I started looking at the mPFC for answers, I analyzed the alpha spectral difference between the AF3 and AF4 sensors (that are closer to mPFC) instead of F3 and F4, and indeed, my tests gave better, more significant results. I ran repeated-measures t-test comparing GSR and HR around FEA drops, and 5 sec after them, which revealed significant difference for GSR (t(419) = 3.05, p < .001) but not for heart rate. So it seems that following a frontal asymmetry change, GSR decreases, which indicates that this FEA phenomena has something to do with fear downregulation - at least after the FEA drop happens, the sweating of the hand (an indicator of fear) declines.

Discussion

Incrementally, I'm getting closer and closer to the neurofeedback horror game vision, the conquest over fear. Just take a second and imagine a gameplay, where you are confronted with your deepest fears, and get rewarded every time you beat them - or in the terms of fear conditioning, every time you unlearn them. Add a pinch of virtual reality and learn to love spiders, snakes, heights, or social interaction - whatever you're anxious about. What's left to prove is a more exact correlation between FEA drops and fear downregulation - seeing the results so far, I'm absolutely positive about it. The new experimental design is under construction - the question is, how do you make participants to intentionally calm themselves down during a horror gameplay.

To boost this research, I uploaded videos of gameplays, where I also show the FEA, HR and GSR values while the game goes on. From one perspective, these videos are fun to watch, but also presents data in a humanly digestible way, so connections between the vital signals and the videos could be revealed by subjective opinion: more eyes see more.

References

[1] S. D. Kreibig, “Autonomic nervous system activity in emotion: a review,” Biol Psychol, vol. 84, no. 3, pp. 394–421, Jul. 2010.

[2] J. A. Coan and J. J. B. Allen, “Frontal EEG asymmetry as a moderator and mediator of emotion,” Biological Psychology, vol. 67, no. 1–2, pp. 7–50, Oct. 2004.

[3] A. Delorme and S. Makeig, “EEGLAB: an open source toolbox for analysis of single-trial EEG dynamics including independent component analysis,” J. Neurosci. Methods, vol. 134, no. 1, pp. 9–21, Mar. 2004.

[4] H. Nolan, R. Whelan, and R. B. Reilly, “FASTER: Fully Automated Statistical Thresholding for EEG artifact Rejection,” J. Neurosci. Methods, vol. 192, no. 1, pp. 152–162, Sep. 2010.

[5] A. Guhn, T. Dresler, M. Andreatta, L. D. Müller, T. Hahn, S. V. Tupak, T. Polak, J. Deckert, and M. J. Herrmann, “Medial prefrontal cortex stimulation modulates the processing of conditioned fear,” Front Behav Neurosci, vol. 8, Feb. 2014.