Stressors Length and the Habituation Effect-An EEG Study

Abstract

The research described in this paper aimed to determine whether people respond differently to short and long stimuli and whether stress stimuli repeated over time evoke a habituation effect. To meet this goal, we performed a cognitive experiment with eight subjects. During this experiment, the subjects were presented with two trays of stress-inducing stimuli (different in length) interlaced with the main tasks. The mean beta power calculated from the EEG signal recorded from the two prefrontal electrodes (Fp1 and Fp2) was used as a stress index. The main results are as follows: (i) we confirmed the previous finding that beta power assessed from the EEG signal recorded from prefrontal electrodes is significantly higher for the STRESS condition compared to NON-STRESS condition; (ii) we found a significant difference in beta power between STRESS conditions that differed in length-the beta power was four times higher for short, compared to long, stress-inducing stimuli; (iii) we did not find enough evidence to confirm (or reject) the hypothesis that stress stimuli repeated over time evoke the habituation effect; although the general trends aggregated over subjects and stressors were negative, their slopes were not statistically significant; moreover, there was no agreement among subjects with respect to the slope of individual trends.

Keywords: EEG signal; habituation; repeated stimuli; stimulus length; stress.

Figure 1

The order of events in Task A.

Figure 2

The order of events in Task B.

Figure 3

The preprocessing procedure via signal recorded from subject S1 performing Task B; left column of plots—Fp1 channel, right column of plots—Fp2 channel; (a) raw signal, (b) signal after temporal filtering, (c) signal after spatial filtering, (d) signal after median filtering, (e) signal divided into epochs, each vertical line corresponds to the epoch onset (the epochs’ order is presented in Figure 2).

Figure 4

STRESS vs. NON-STRESS condition—difference in medians for Task A and Task B.

Figure 5

STRESS vs. NON-STRESS condition—difference in medians across events in Task A (a), and Task B (b); Q: Question.

Figure 6

STRESS vs. NON-STRESS condition—difference in medians across subjects for Task A (a), and Task B (b); S: Subject.

Figure 7

Task A vs. Task B—difference in medians across subjects for the STRESS condition; S: Subject.

Figure 8

The comparison of medians for RELAX and STROOP events arranged in time (a), arranged according to tasks (b).

Figure 9

The stress-inducing events over time; (a) Task A, (b) Task B; Q: Question; blue line: beta power for each question, dotted line: linear trend.

Figure 10

The stress-inducing events from Task A over time; upward trends are marked in red, downward trends are marked in blue, dotted lines present linear trends, asterisks (*) denote linear trends with a significant slope (p-value < 0.05); Q: Question, S: Subject.

Figure 11

The stress-inducing events from Task B over time; upward trends are marked in red, downward trends are marked in blue, dotted lines present linear trends; Q: Question, S: Subject.