Biomedical Electronics Bioinstrumentation Fundamental of Electroencephalogram EEG Contents

Biomedical Electronics & Bioinstrumentation Fundamental of Electroencephalogram (EEG)

Contents Introduction to EEG Neuronal Electrical Reactions EEG Electrodes & 10 -20 System EEG Amplitude & Frequency Bands The EEG System Preamplifiers & EEG System Specifications

Introduction to EEG Electroencephalogram (EEG) is a representation (writing on paper or display on CRT) of the electrical activity of the brain.

Introduction to EEG The i. technique involves: Biopotential pickup Cranial or cerebral surface transducer electrodes. ii. EEG signal conditioning Transducer output amplification and filtering. iii. EEC signal recording Signal displayed on graphic recorder or CRT. iv. EEG signal analysis Visual or computer interpretation of resulting EEG.

Introduction to EEG The EEG record obtained in item 3 in the preceding list is used primarily for diagnosis, including the following: i. Help detect and localize brain lesions (assymetry / irregularity in EEG tracings). ii. Aid in studying epilepsy (recurrent, transient attacks of disturbed brain function with irregular sensory and motor activity such as convulsions). iii. Assist in diagnosing mental disorders. iv. Assist in studying sleep patterns. v. Allow observations and analysis of brain responses to sensory stimuli.

Introduction to EEG The EEG record obtained in item 3 in the preceding list is used primarily for diagnosis, including the following: i. Help detect and localize brain lesions (assymetry / irregularity in EEG tracings). ii. Aid in studying epilepsy (recurrent, transient attacks of disturbed brain function with irregular sensory and motor activity such as convulsions). iii. Assist in diagnosing mental disorders. iv. Assist in studying sleep patterns. v. Allow observations and analysis of brain responses to sensory stimuli.

Introduction to EEG Many physicians and neurologists view EEG signals as interesting artifacts but confess that they are not certain of the signal origins. In fact, until recently, EEG waveforms were originally thought to be a summation of action potentials of neurons as they made their way to the cranial surface. Later ideas reflect stimulation associated by diverse neurons.

Introduction to EEG Modern interpretation of EEG origin rests with knowledge of basic neuronal electrochemical processes. The action potential (AP) from neurons has been recorded with microelectrodes at the cellular level. Essentially, the synaptic fibers, terminal boutons, neuronal membrane, and axon contribute the distinguishable response characteristics.

Neuronal Electrical Reactions Electrical reactions of neurons includes the following potentials: i. iii. iv. v. Presynaptic spike potential (rapid 1 ms positive event resulting from presynaptic depolarization). Excitatory postsynaptic potential (prolonged 2 ms graded positive potential). Spike potential (high voltage, sudden 2 ms positive discharge of 10 to 30 m. V). After hyperpolarization (prolonged positive potential). Inhibitory postsynaptic potential (negative potential associated with neuronal inhibition).

Neuronal Electrical Reactions

The EEG Signal The EEG is composed of electrical rhythms and transient discharges which are distinguished by: i. iii. iv. v. vi. Location Frequency Amplitude Form Periodicity Functional properties Synchronization appears in the EEG, and the resulting slow activity is evident.

The EEG Signal

EEG Electrodes EEG electrodes transform ionic currents from cerebral tissue into electrical currents used in EEG preamplifiers. The electrical characteristics are determined primarily by the type of metal used. Silver-silver chloride (Ag/Ag. Cl) is commonly found in electrode discs.

EEG Electrodes Essentially, i. five types of electrodes: Scalp: Silver pads, discs, or cups, stainless steel rods and chlorided silver wires. ii. Sphenoidal: Alternating insulated silver and bare wire and chlorided tip inserted through muscle tissue by a needle. iii. Nasopharyngeal: Silver rod with silver ball at the tip inserted through the nostrils.

EEG Electrodes Essentially, i. ii. five types of electrodes: Electrocorticographic: Cotton wicks soaked in saline solution that rests on the brain surface (removes artifacts generated in the cerebrum by each heartbeat). Intracerebral: Sheaves of Teflon-coated gold or platinum wires cut at various distances from the sheaf tip and used to electrically stimulate the brain.

EEG Electrodes Reusable scalp disc or cup electrodes (most common in the clinic) are placed on the head using a conductive cream (similar consistency to body fluids or electrolytes). The area is first cleaned with alcohol or acetone to remove skin oils. It is good practice (using conductive paste) to lower the resistance below 10 kΩ to ensure good EEG signal recording.

10 -20 System The amplitude, phase and frequency of EEG signals depend on electrode placement. This placement is based on the frontal, parietal, temporal and occipital cranial areas. One of the most popular schemes is the 10 -20 electrode placement system established by the International Federation of EEG Societies.

10 -20 System In this setup, the head is mapped by four standard points: i. iii. iv. The nasion (nose) The inion (external occipital proturberence) Left preauricular point Right preauricular point Nineteen electrodes, plus one for grounding the subject is used.

10 -20 System

10 -20 System

10 -20 System Electrode arrangements may be either unipolar or bipolar. A unipolar arrangement: Composed of a number of scalp leads connected to a common indifference point such as an earlobe. Hence one electrode is common to all channels. A bipolar arrangement: Achieved by the interconnection of scalp electrodes.

10 -20 System Unipolar Arrangement

10 -20 System Averaging Arrangement

10 -20 System Bipolar Arrangement

EEG Amplitude EEG signal voltage amplitudes range from about 1 to 100µVpp at low frequencies (0. 5 to 100 Hz) at the cranial surface. At the surface of the cereberum, signals may be 10 times stronger. Also, brain-stem signals measured at the cranial surface are often no larger than 0. 25µVpp(100 to 3000 Hz). Weak EEG signals require input preamplifiers (differential type) that have high gain and internal or external noise rejection.

EEG Frequency Bands EEG frequency bands are normally classified into five categories: i. iii. iv. v. Delta ( ): 0. 5 -4 Hz Theta ( ): 4 -8 Hz Alpha ( ): 8 -13 Hz Beta ( ): 13 -22 Hz Gamma ( ): 22 -30 Hz and higher The meaning of these frequencies is not completely known.

EEG Frequency Bands Alpha activity i. Less than 10µVpp and are reasonably stable. ii. These signal arises from the posterior brain in a waking person with eyes closed. iii. Opening and focusing attention greatly reduces alpha waves. Beta i. activity Less than 20µVpp over the entire brain but is predominant over the central region at rest. ii. High states of wakefulness and desynchronized alpha patterns produce beta waves.

EEG Frequency Bands Gamma activity i. Less than 2µVpp and consists of low-amplitude, high frequency waves that. ii. Result from attention or sensory stimulation. Theta and Delta activity i. Less than 100µVpp and are the strongest over the central region. ii. Indications of sleep. The usable bandwidth is not much beyond 50 Hz.

The EEG System

The EEG System Twenty electrodes are placed on the patient’s scalp, and these are switch-selected to the input of eight differential preamplifiers. The eight outputs are further amplified and presented to eight driver or power amplifiers that supply sufficient current to drive the pen deflectors. A calibration signal, usually in the form of a pulse, is generated by a separate circuit and applied to the diff-amp inputs. It is advantageous to connect the calibration signal to the switch selector box to check the system operation.

The EEG System Calibration signal amplitude gives an indication of correct sensitivity settings. If the reading is not within specifications, the amplifier system must be adjusted. Calibration signal waveform gives an indication of frequency response. The low-voltage power supply design and operation is very important in EEG systems because the low-level input signals (as small as 5µVpp), can easily pick up extraneous 50 Hz internal as well as external noise.

Preamplifiers EEG preampifiers are perhaps the most important link in the EEG system. They are usually differential amplifier and have the following characteristics: i. iii. iv. Low internal noise High gain (5, 000 -10, 000 x) High CMRR (100 d. B) Low-frequency ac-coupled operation (1 Hz and below) v. Low dc drift vi. High input impedance (10 MΩ and above)

Preamplifiers EEG preampifiers are perhaps the most important link in the EEG system. They are usually differential amplifier and have the following characteristics: i. iii. iv. Low internal noise High gain (5, 000 -10, 000 x) High CMRR (100 d. B) Low-frequency ac-coupled operation (1 Hz and below) v. Low dc drift vi. High input impedance (10 MΩ and above)

Preamplifiers Differential Input

Preamplifiers Single-Ended Input

EEG System Specifications EEG machine specifications include: i. Input impedance: 12 MΩ minimum at 10 Hz. ii. Sensitivity: 0. 5µV/mm maximum. iii. Sensitivity controls: 10 -position master (2 to 75µV/mm), six-position individual channel (X 20 to X 0. 25), and individual-channel gain equalizer. iv. Calibration voltages: 5 to 1000µV. v. CMRR: 2000 or 66 d. B minimum at 60 Hz ant 10, 000 or 80 d. B minimum at 10 Hz. vi. Noise: 1µVrms (equivalent referred to input) with input shorted.

EEG System Specifications EEG machine specifications include: vii. Low frequency (time constant): 30% attenuation, 0. 16 through 5. 3 Hz, at time constants of one through 0. 03 s, respectively. viii. High frequency response: 30% attenuation at 1 to 1000 Hz. ix. 50 Hz filter: 50 d. B down at 50 Hz. x. Chart speeds: 20 to 60 mm/s.

Further Reading… Webster, J. G. (2010). Medical Instrumentation: Application and Design. 4 th Ed. , Wiley. ü Chapter 4 2. Carr, J. J. (2000). Introduction to Biomedical Equipment Technology. 4 th Ed. Prentice Hall. ü Chapter 13 1.