ALPHA RHYTHM

The frequency of EEG waves is divided into 4 frequency bands
1. Delta frequency band- under 4 Hz
2. Theta frequency band- 4 to 7 Hz
3. Alpha frequency band- 8 to 13 Hz
4. Beta frequency band- over 13 Hz


The activities seen in the EEGs of awake adults consist of frequencies in the alpha and beta ranges, with the alpha rhythm constituting the predominant background activity.

It is important to understand the difference b/w alpha activity and alpha rhythm. Alpha activity refers to any activity in the range b/w 8 and 13 Hz. While alpha rhythm is a specific rhythm consisting of alpha activity having following properties:

• Rhythm at 8-13 Hz occurring during wakefulness over the posterior region of the head, generally with higher voltage over the occipital areas.


• The amplitude of alpha waves often waxes and wanes, but is mostly below 50 µv in adults.


• Alpha rhythm is best seen with eyes closed and under condition of physical and mental relaxation.


• Alpha rhythm is blocked or attenuated by attention especially visual or mental effort (alpha reactivity).
  
  

Reference:

1. A glossary of terms commonly used by clinical electroencephalographers. Elcetroencephalogr. Clin. Neurogphysiol. 37: 538-548
2. Electroencephalography: General Principles and Adult Electroencephalograms in: Jasper R. Daube. Clinical Neurophysiology, Philadelphia: F. A. Davis Company
   

FILTERS

Normal and pathological waveforms arising in the brain are rarely less than 0.5 Hz or more than 100 Hz. Filters are used to attenuate or exclude waveforms of relatively high or low frequency from the EEG so that waveforms in the most important range can be recorded clearly, without significant attenuation or distortion.

Two types of filters are commonly used - low frequency filters and high frequency filters.

LOW FREQUENCY FILTERS (also called high-pass filters)
These filters control the response of the instrument to lower frequencies while the response to higher frequencies remains unaffected. The low filter frequency setting specifies the cutoff frequency at which sine waves are reduced in amplitude by a set percentage. This percentage varies with different EEG machines.

In Grass models, the low frequency filter settings are denominated as 0.1, 0.3, 1, 3 and 10.. These numbers denote the frequencies in Hz at which the amplitude has dropped by 20%. Thus a sine wave of 0.3 Hz is nearly abolished by a filter setting of 3 Hz, severely reduced by a setting of 1 Hz and reduced by 20% by a setting of 0.3 Hz.

HIGH FREQUENCY FILTERS (also called low-pass filters)
The high frequency response of the EEG instrument is controlled by high frequency filters.

In Grass instruments these filters switches are denominated 12, 17, 35 and 70 Hz. These are the values of the frequencies for which the response has declined by 20%.

CLINICAL IMPLICATIONS
During the recording, the low frequency filter should be routinely set at 1 Hz and the high frequency filter at 70 Hz.

• A low-frequency filter setting higher than 1 Hz should not be used routinely to attenuate slow-wave artifacts in the record. Vital information may be lost when pathologic activity in the delta range is present.
• Setting the high frequency filter at a lower frequency than usual may give high frequency artifact (e.g. muscle artifact) the misleading appearance of cerebral potentials such as epileptiform spikes or fast background activity. It can also distort or attenuate spikes and other pathologic discharges into unrecognizable forms.
  

Reference:

1. Electricity and Electronics in Clinical Neurophysiology, in: Jasper R. Daube. Clinical Neurophysiology, Philadelphia: F. A. Davis Company
2. Fisch BJ. Spehlmann’s EEG primer, Amsterdam: Elsevier, 3rd edition
3. User Manual – Version 3.5 : Grass-Telefactor TWIN: Recording and analysis software 2005
4. Guideline One: Minimum Technical Requirements for Performing Clinical Electroencephalography. J. Clin. Neurophysiol. 11 (1) 2-5, Raven Press Ltd. New York