**8. Problems of visual representation of a multichannel EEG study**

When trying to visually and phenomenologically analyze a multichannel EEG recording, it is difficult to identify individual diagnostically significant patterns, since

#### **Figure 2.**

*Spatial localization of 128 electrodes of a multichannel EEG system using a spherical head model (EEGLAB®, author's observation).*

**Figure 3.** *Visual representation of native recording of 128 channels of EEG recording (EEGLAB®, author's observation).*

a large amount of linear data is located in a limited space, so attempts to identify dynamic changes in bioelectrical activity under these conditions are doomed to failure in advance (**Figure 3**).

It is more rational to use mathematical processing of primary data with the presentation of results in the form of frequency graphs and amplitude-frequency maps (**Figure 4**) [13, 14].

This approach makes it possible to identify functional changes in the bioelectrical activity of the brain by the type of potentials associated with the event, the use of a large number of electrodes that create a dense network allows not only to fix the response to the event, but also to see the reaction of adjacent cortical zones.

#### **Figure 4.**

*Mathematical analysis of the native recording, which allows to determine the area of desynchronization of the sensorimotor zone (EEGLAB®, author's observation).*

*Practical Recommendations for Conducting an EEG Study in a Neurophysiological Laboratory DOI: http://dx.doi.org/10.5772/intechopen.108879*

### **9. The functional study**

In the most acute and early recovery period, the patient's condition is still unstable, because the cerebral blood supply system is being restructured, damaged as a result of occlusion of the cerebral arteries and the formation of a compensating collateral blood supply system.

Under these conditions, the determination of changes in the bioelectrical activity of the brain is directly related to disorders of cerebral microcirculation. Since the nerve cell does not have in its structure organelles containing a supply of nutrients, any change in the conditions of its blood supply leads to disturbances in the formation of action potential (AP) and postsynaptic potential (PSP), which is manifested by the appearance of slow-wave activity in the structure of the EEG recording.

To determine the change in the characteristics of bioelectrical activity, we will compare the state of passive relaxed wakefulness and physical activity in the form of rhythmic squeezing—unclenching the hands (**Figure 5**).

In a state of passive relaxed wakefulness, a zone of slow-wave activity of theta range is recorded in the left frontotemporal leads. A preserved motor area in the right hemisphere of the brain and a shift in the focus of alpha activity to the undamaged hemisphere are determined (**Figures 6** and **7**).

At the beginning of the load, a change in bioelectrical activity is observed, which is characterized by the appearance of a section of slow waves in the frontal regions of the right (intact) hemisphere of the brain, which is a reflection of the formation of a collateral flow along the system of the anterior communicating artery and the steal syndrome of the frontal regions of the healthy hemisphere.

At the third minute of the exercise test in the affected hemisphere, a pronounced increase in the area of slow-wave activity is observed, which characterizes

#### **Figure 5.**

*The state of passive relaxed wakefulness in a patient with ischemic stroke in the basin of the left middle cerebral artery (EEGLAB®, author's observation).*

#### **Figure 6.**

*Formation of areas of low-frequency pathological activity in a patient who started the exercise test (EEGLAB®, author's observation).*

#### **Figure 7.**

*Continued changes in the characteristics of bioelectrical activity by the end of 3 minutes of functional exercise testing (EEGLAB®, authors observation).*

pronounced microcirculation disorders due to decompensation of the collateral blood supply system. At the same time, the patient does not complain about the deterioration of the general condition.

*Practical Recommendations for Conducting an EEG Study in a Neurophysiological Laboratory DOI: http://dx.doi.org/10.5772/intechopen.108879*

This observation is important for the development of exercise programs at the early stages of medical rehabilitation, since, as shown above, even slight physical activity can cause decompensation of the emerging collateral blood supply system in such patients.
