Basic principles and interpretations of electroencephalography [Workshop 1]

At the end of the workshop, the participant will be able to: 1. Understand the principles of EEG recording. 2. Identify commonly encountered EEG artifacts. 3. Recognize normal awake and sleep EEGs in children and adults. 4. Interpret some common abnormal EEG patterns. Electroencephalography [EEG] is the science relating to the electrical activity of the brain. The background electrical activity of the human brain was first analyzed in a systematic fashion by the German Psychiatrist Hans Berger [1929]. Since then, EEG has been used in clinical practice to disclose non-epileptiform and epileptiform cortical dysfunction. EEG recording. EEG is recorded with scalp electrodes. The recordings may be bipolar or unipolar. Bipolar records show fluctuations in potentials between 2 electrodes and the unipolar record show potential difference between cortical electrodes and theoretically in-different electrodes. A digital EEG machine allows any section of the record to be completely reformatted namely, viewed with any Montage, Gain, Filter or Timebase. The standard placement recommended by the American EEG society for use in the International 10-20% system is for 21 electrodes [Figure 1]. The standard numbering system in the 10-20 system places odd-numbered electrodes on the left and even numbered electrodes on the right, with the letter designating the anatomic area. Montage. The term montage refers to the particular combination of electrodes examined at a particular point of time. In most instances, multiple montages are more useful than a single montage for long periods. The function of the montage is to record from all areas of the scalp and also to record activity in such a manner that it is easily perceived by the reader. The principal montages are longitudinal bipolar "Double Banana", coronal bipolar, circumferential bipolar, laplacian, common average referencial and ear [A1, A2] referential. It should be mentioned here that bipolar runs provide better localized focal or regional features, while the morphology of the widespread phenomena appear best on referential montages. Physiological basis of EEG [EEG generator]. The activity recorded in the EEG is mostly that of the most superficial layers of the cortical gray matter. The potential changes in the cortical EEG are due to current flow in the fluctuating dipoles formed on the dendrites of the cortical cells and the cell bodies [Figure 2] namely current to flow through the volume conductor between "source" at the soma and basal dendrites and the "sink" at the apical dendrites sustaining excitatory postsynaptic potential [EPSPs]. Figure 2 illustrates current paths taking increasingly remote curving routes. The zero potential surface is located halfway between the positive and negative poles of the dipole. To understand how electrical potentials are recordable on the scalp generated by populations of the pyramidal neurons, it can be easily understandable by the solid angle concept of the volume conduction theory. In this, the potential generated by a dipole layer in a volume conductor [brain and its environs] is proportional to the solid angle subtended by the dipole layer at the point of the measurement [Figure 3]. EEG interpretations. EEG interpretation requires a structured approach to ensure that important information is not lost or data misinterpreted. Prior to the interpretation, the only clinical data known to the reader should be the age of the patient. In evaluating an EEG pattern or event, one should answer the following: Is it an artifact

effects of common carotid artery occlusion on the somato-sensory evoked potentials [SSEP's] in Newzealand white rabbits

Somatosensory evoked potentials [SSEP's] study was made by placing surface electrode directly on the exposed cortical surfaces [post- central gyrus] of both the cerebral hemispheres in anesthetized New Zealand White Rabbits. The positive component P1 of NPN complex potential showed a significant delay [P < 0.01] in the affected side [right] 48 hours after the occlusion of the right common carotid artery. The amplitude of P1 is also reduced. The absolute peak latenceis of the other two negative components [N1 and N2] showed similar changes as P1 after the occlusion. The P1 of the un-affected hemisphere] showed a non-significant prolongaion