## 4 EEG Normal Rhythms and Modification in Diabetes  
The latest studies shown that VN stimulation induces EEG changes [84]. Therefore, it is plausible that changes in the microbiota composition profile due to obesity and diabetes are modulated by the VN and can be captured through EEG signaling. An integrative understanding of the mechanism that subserves the relationship between the gut, the brain and also the metabolic diseases, in this case, diabetes, will need to take into consideration. One mechanism that is very intensively used to investigate the abnormalities of brain functioning is electroencephalography (EEG) [85]. EEG is based on low costs and in addition, it is fully noninvasive. To work with EEG signals and to extract useful information and features that help in the early diagnosis of neurological diseases related to diabetes, the rhythmic activity of the EEG should be understood. A clinical EEG wave has an amplitude between 10 and 100 \(\upmu\)V and a frequency range of 1 Hz to 100 Hz [86]. EEG has five rhythms with different frequency bands. The most clinically relevant bands are: alpha (8-12 Hz), beta (13-30 Hz),theta (3.5-7.5 Hz) delta (<3 Hz) and gamma (31-44 Hz) [87]. Alpha rhythm consists of continuous oscillations, modeled in amplitude and it appears in the waking period, in physical and mental relaxation and is present in people beginning at 2 years old. It is best detected when the subject has their eyes closed. The alpha rhythm attenuates with the opening of the eyes and is best observed in the visual cortex (occipital) [88]. Alpha activity normally disappears in attention states (stress, open eyes, mental operations). Beta occurs in normal adult subjects, especially in the precentral regions. Low amplitude (1-10 uV, rarely up to 20 uV) with multiple and varied frequencies is often associated with intense activities and active concentration. Beta rhythm is also associated with various pathologies or effects of barbiturate drugs [89]. Delta occurs in young children or some neurological diseases. They normally occur in deep sleep at adults as well as at children. This waveform can be recorded frontally in adults and posteriorly in children [90]. Theta is associated with the waking-sleep transition state in children, adolescents, and young adults. It can be produced by hyperventilation. This rhythm can also occur during trance or hyponsis state. They normally appear in sleep at any age. In awake adults, they are considered abnormal if they occur in excess. Theta wave is recorded across the temporal and parietal region of the scalp [89]. Gamma rhythm seems to be involved in higher mental activities including perception, fear, solving problems, etc. Until late adulthood, the activities of delta and theta rhythms are reduced with age, whereas those of alpha and beta waves increase linearly [91]. An inverse relationship has been observed between delta and glucose metabolism in the case of neurological diseases within the prefrontal cortex as a consequence of dementia cognitive impairment (DCI) [92]. Studies on patients with T2D vs healthy subjects, described in Table 1, use basic signal processing methods such as spectral analysis or Fourier transform combined with statistical analysis revealed significantly decreased in delta and theta bands of the T2D group [93] or decrease in resting EEG beta activity over the central and posterior region [94]. [PERSON] et al. [95] observed a decreased EEG power in the delta (1-4 Hz) and gamma (30-45 Hz) bands in diabetic patients. Topographic analysis revealed that these changes were confirmed to the frontal region for the delta band and to central cortical areas for the gamma band. [PERSON] et al. [96] used absolute power estimates to compute the spectral power ratio and statistical analysis (ANOVA, MANOVA) and observed that T2D is specifically associated with a reduction of alpha power in the temporal regions, with no significant differences observed in other brain regions. In addition, a review of [PERSON] et al.