A review of the literature on the impact of acute and chronic stress upon brain waves

Background: The biological responses associated with stress originate in the brain and involve different physiological and physical effects. The direct effect of stress on cortical responses can be visualized by recording the brain’s electrical waves using an encephalograph. These waves are recorded by means of an electroencephalogram (EEG). EEG is the most commonly used neuroimaging technique to study the patterns of brainwaves and functioning of the brain. It also measures the variation of the electric field produced by neuronal activity a millisecond at a time. To systematically analyze published studies on the difference between brain wave patterns in terms of their frequencies among subjects with acute stress, chronic stress, and normal individuals. Methodology: The data from published studies was arranged quantitatively and qualitatively by producing a planned summary measure. Studies that focused on brain wave analysis of the EEG of healthy adult subjects with no history of mental illness or head injury were included in the review. The selected literature included many types of stressors that are acute or chronic, and that affected the neuronal electrical activity. The only electronic database utilized to identify relevant studies was PubMed. Result: Fifteen studies were included that were based on a variety of acute stressors to observe alterations in brain wave activity between stress-free and stressed states. These studies showed that stressors could be a causative factor to generate fluctuations in neuronal oscillations that also leads to significant psychological, physiological and neurobiological deteriorations to some extent. An additional sixteen studies were included, which showed the effect of chronic stress on the asymmetry of the amplitude in the frequencies of brain waves. Conclusion: The most common change observed was in the alpha frequency (8-13Hz), followed by changes in beta waves (13-30 Hz) and theta (4-8Hz). Though, there is not always the same resultant pattern of waves explored with even the same type of stressors due to interpersonal differences in response to a stressful situation.


Introduction
Everyone, at least once in their lifetime, faces a stressful situation. This stress can be beneficial if it motivates a person to do challenging tasks to achieve their goals. But it can be harmful when it begins to affect the physical or mental health of the subject. The term stress is defined as; when the equilibrium between internal and external environment is disturbed, it alters the bodily mechanisms 1 . This scenario casts a bad impact on the central, and peripheral regulatory systems that leads to deprived health and mental wellbeing 2 . Stress is also responsible for the progression of chronic disorders. Its long-term exposure is linked with several health problems, including obesity, peripheral vascular disease, diabetes, and depression. Therefore it is essential to evaluate stress levels at the early stage before they start to interfere with everyday routine. Stress can be evaluated based on physiological and behavioral responses. Traditionally, physicians tend to assess stress by using critically designed questionnaires 3 , i.e. a subjective method. Stress can also be measured by assessing different biological indicators like cortisol 4 , alpha-amylase levels, body vitals namely blood pressure 5 and skin conductivity 6 . The direct effect of stress on cortical responses can be obtained by using a neuroimaging technique, electroencephalogram (EEG) 7 . EEG is the electrophysiological technique used to assess the electrical activity of the brain 8 . It also measures the variation of the electric field produced by neuronal activity at the millisecond resolution.

Assessment of Acute and Chronic Stress Based on EEG Features
Biological responses associated with stress originate in the brain and involve different types of physiological and physical effects. Previous studies have investigated the variations in EEG signals during stressful conditions. The alpha frequency band ranges between 8-13 Hz 9 . Alpha waves are usually recorded in a relaxed, calm and tension-free condition 10 , when a subject is exposed to a distress in a controlled laboratory setting a distinct reduction in power is observed 11 . During the stressful condition, the right hemisphere shows more frontal altered alpha waves then left hemisphere 12,13 . Other studies discuss the relationship between negative emotions, stress or depression with alpha frequency 12,14 . Marshall and Lopez Duran suggest a decline in the power of alpha frequency in the prefrontal cortex during a stressful situation 15 . As well, there is an increase in alpha power in the frontal cortex during fatigue 16

Study Characteristics
This systematic review protocol is based on PRISMA guidelines 18 . The only electronic database involved in this study is PubMed and the records and data throughout the review is managed by M.S. Word.

Data Synthesis
The data was arranged quantitatively and qualitatively by producing a planned summary measure, reviewing original articles in the same aspects, extracting and screening the citation and studies, handling the studies, screening them and combining them according to the methods given by the following PRISMA flow diagram (Figure 1   This method was used as a medium for extractions and simplification of the combined data and rate it in its quality and quantity.  Table 1 summarizes the sample size, gender, included brain frequencies, altered oscillations, and type of chronic stressor measured. The stressors mentioned in Table 1 discuss the disturbance in neuronal oscillations. Many studies discuss several reasons for the asymmetry in brain waves that can be a result of continuous exposure to a stressful situation. Different stressors have been reported; one of the stressors is sleep deprivation, which increases alpha waves 19 . Another stressor is isolation; during 520 days, isolation shows the increase in beta waves at the frontal region while alpha and delta remain unaffected 20 . Loganovsky et al. found an increase in alpha at the temporal and frontal area, delta at the anterior brain, and theta at the anterior brain and right temporal region and beta increase at anterior brain while decrease at the temporal region due to workload 21 . Jacubowski et al. considers two stressors, one is isolation and the other is exercise, and both stressors resulted in high alpha and beta waves 22 . Hu concluded the influence of different stressors caused by unemployment, and the frequent examination on students and mothers of disabled children 23 . They found alpha, beta and theta frequencies increase anteriorly. Luijcks et al. used electro-shocker as a chronic stressor and reported an increase in alpha and slow beta waves frequency at central and parietal-temporal areas, spectrum of gamma wave's decrease at frontal, central and occipital regions 24 . Vanneste et al. assessed chronic tinnitus and reported that alpha 1 and beta are altered 25 . However, in this condition, alpha 1 at the subgenual anterior cingulate cortex, beta 3 at dorsal anterior cingulate cortex, delta, theta, alpha 2, beta 1 and 2 remain unaffected. When extreme isolation was studied by Yi et al. they found a decrease in alpha and beta frequency 17 .  Table 2. Summarizes the sample size, their gender, included brain waves, altered oscillations, and type of acute stressor. The stressors mentioned in Table 2 discuss the disturbance in neuronal oscillations. Acute stressors can be beneficial as these stressors make the body able to adapt according to their surroundings. Alonso et al., applied two psychological and physical stressors, a Stroop test and sleep deprivation in which the Stroop test resulted in an increase in alpha 1 and beta whereas in sleep deprivation, theta increased and there was a decline in alpha 1, finally high alpha decreases and high beta increases in stress responses 34 . Zambotti et al., applied the Trier social stress test on insomniac patients and compared them with a control group, beta 1 increases in the control group showed no change in brain wave symmetry 35 . Acute mental arithmetic tasks cause a decrease in alpha and increase in beta and delta and theta stay unchanged 36 . Allen et al., used a socially evaluated cold presser test (S.E.C.P.T.) and found an increase in theta at the frontal midline and that alpha1, 2 and beta 1, 2 and delta did not responded to the stressor 37 . Banis et al., used the Distressing Video and Monetary Incentive Delay Task and found alpha power increase in reward cues, which was unaffected during a stressful situation. They also found that theta increased in on reward signal 38

Discussion
The conditions that are associated with stress produce significant psychological, physiological and neurobiological deteriorations. Distress affects neuronal circuits that further disturb the normal propagation of brain waves; these interruptions can be analyzed by EEG with precision and efficacy. This review reveals how the different stressors could be a causative factor in generating fluctuations in neuronal oscillations. It should be noted that gamma is the least observed wave in the aboveincluded studies. Above all, only Luijcks et al. reported a decrease in gamma waves at the frontal, central and occipital regions while others reported no change 9,24,25 . Slowwave delta least shows the deflection when influenced by chronic stressors; in many studies, delta waves remain unchanged before and after the stressor applied 20,[24][25][26] . Few studies mentioned the increase in delta waves during chronic stress 9, 12, 21 . However, Neylan et al. and Hall et al. suggested a decrease in delta waves 29,30 . Like delta, theta also in some studies reported to not be a respondent of a stressor 9,24,25,31 . While Begic et al. 26 , Brady et al. 28 and Hu et al. 23 observed a decrease in theta rhythms at central, frontal and anterior regions, respectively. However, Todder et al. 32 use QEEG and Low Resolution Electromagnetic Tomographis Analysis (LORETA) techniques and report that there is no difference observed. At the same time, LORETA reveals some other results; they distributed the theta band into higher and lower frequency band, both bands show low activity at different brain sites, the low band found at right temporal lobe while the higher band at the right and left frontal lobe. Now, beta waves are more involved in brain-specific tasks. Most of the studies reported an increase in beta oscillations at frontal, central, left occipital, anterior, right temporal and parietal temporal regions of the brain 18,20,[21][22][23]26,30 . Some detect a decline in this rhythm at the frontal and right hemispheric frontal site of the brain 8,9,17 . Also, Vanneste et al. observed alteration in frequency bands of beta 3 waves at the dorsal, anterior cingulate cortex 25 . Finally, the alpha wave remains unchanged in very few cases 20,26 . Mainly, the alpha oscillations were reported to be increased during or after chronic stressors were applied, and the regions indicated were frontal, right temporal, parietal temporal, central and anterior 18,22,25,28,29,31 .
On the contrary, Giannakakis et al. 9 and Yi et al. 17 suggested the decline in beta frequency in the frontal region of the brain. Additionally, Vanneste et al. found fluctuations in alpha 1 waves after the extensive exposure to stress 25 . Acute stressors are the second parameters in this review. Lithari et al. 46 included gamma wave in his study, but the rhythms remain un-deflected. Now, the delta waves mostly reported being unchanged during acute stress [34][35][36][37]46 . Whereas, Muttray et al. 40 and Rozhkov et al. 44 observed an increase in the frequency of the delta waves at temporooccipital and temporospatial regions of the brain. Theta oscillations are not respondents of acute stressors observed by Massimiliano de Zambotti 35 and Al Shargie et al. 36 But most of the studies mentioned an increase in theta wave and the regions are parietal and temporal, temporospatial, midline frontal and central part of the brain 34,40,44,46 . Allen et al., 37 suggested no change in the beta wave. Though, Muttray et al. 40 mentioned a decrease in beta rhythm at the temporoparietal occipital. On the other hand, most of the studies reported an increase in the beta wave at midline frontal and central regions [34][35][36]46 . Lastly, the alpha PP| Published By AEIRC| Volume 8 Issue 1 waves are the most considered wave to study stress. Some studies reported that alpha remains unchanged 27,35,37,47 . In acute stress, it is reported that alpha wave increases at frontal, midline frontal and central sites 34,38,43,46 , while other studies reported a decline in alpha waves at temporoparietal-occipital and frontal regions of the brain 41,42 .

Conclusion
The literature reviewed for this study shows the effect of numerous stressors on brain oscillations that change their frequency, affecting normal functions of the brain. These fluctuations in the power of brain waves could lead to some severe consequences if persisted for too long. Multiple interventions and therapies especially biofeedback techniques are making the mark and are now being tested and successfully applied to train the subject to revert the effect of stress. Most significant one amongst these were biofeedback and behavioral structuring techniques with high efficacy rates. Awareness in this regard is highly recommended.