Discussion and conclusion:

The possible dependency of emotional activities on inter-hemispheric correlation has been studied to classify healthy controls and patients diagnosed with FEP. The results show that patients provide the high level of inter-hemispheric correlation in response to neutral pictures, whereas controls provided the high level of inter-hemispheric correlation in response to pleasant pictures at commonly anterio-frontal and central lobes in Gamma.

The best classification performance was obtained in Gamma. Although, the useful results could be observed in other high fba (Alpha, Beta) to classify the data mediated by neutral and un-pleasant pictures, combination of the separate features extracted from each sub-band produced the poor performance.

In conclusion, emotional functions of the brain could be observed in relatively higher fba (16−3216−32 and 32−64Hz32−64Hz). The lower fba (0.5−40.5−4 and 4.5−8Hz4.5−8Hz) do not reflect the emotional functions in detail. Visual and static pictures activated mostly anterio-frontal and central lobes in Gamma. The largest level of inter-hemispheric correlation was observed at mostly frontal lobe in Gamma in response to pleasant pictures in controls, while the largest level of inter-hemispheric correlation was observed at mostly frontal lobe in Gamma in response to neutral pictures in patients. In Alpha, the lowest level of inter-hemispheric correlation in addition to the narrower interval of correlations (mean∓stdmean∓std) was observed at temporal lobes (T3–T4, T5–T6) in response to un-pleasant and neutral pictures in controls and patients, respectively. In Gamma, the lowest level of inter-hemispheric correlation was observed at parieto-central lobes (P3–P4, O1–O2) in response to un-pleasant pictures in both controls and patients.

Our results are compatible with the previous findings, including three main statements as follows: decreased hippocampal volume closely linked with emotional dysfunctions [79], functional insufficiency at mostly the right hemisphere in depression [80], and increased emotional coherence in controls in Gamma (low Gamma: 30−50Hz30−50Hz and high Gamma: 50−80Hz50−80Hz) [80]. In addition, the relatively decreased cortical activities at right and left hemispheres were found to be related to withdrawal and approach motivations, respectively, in MDD [81]. In detail, the relatively higher EEG asymmetry was reported as associated with motivational system and affect at anterio-frontal (F3–F4) and posterio-frontal (F7–F8) regions in Alpha in depression before treatment [82]. Depression was also reported as connected with not only increased resting-state activities at left hemisphere but also decreased resting-state activities at right hemisphere in Alpha [83–85]. Commonly, depressive inter-hemispheric emotional dysfunctions were correlated with relatively higher fba in the literature.

The amygdala having the main role in existing an emotion, includes perceptual pathways (from primary visual cortex to inferior temporal cortex) and reciprocal connections (between prefrontal cortex and orbito-frontal cortex) in humans. The orbito-frontal cortex, located at the base of the frontal lobes, receives direct neural inputs such as emotional stimuli from medial thalamus. Then, sensory information is received and strengthen by amygdala. In summary, once the brain is mediated by an affective picture, the thalamus and cortex interact to each other through not only firing of individual neurons but also transient functional integrations of local neuronal assemblies across right and left-brain regions. Therefore, our results support that cortical EEG series can be analyzed by means of neuronal inter-hemispheric correlation in wavelet domain in high fba (16−32Hz16−32Hz and 32−64Hz32−64Hz) for early detection of FEP.

In future work, both POMS scale [86] and theta coherence analysis [87] would be examined to re-analyze the emotional data explained in the present study.