Electroencephalograph
(EEG)
The Electroencephalograph or EEG signal is an extremely small, roughly sinusoidal, fairly low frequency signal generated by the human or mammal brain during its normal functioning. The EEG signal is rarely constant, and often shows up as bursts of signals at various frequency ranges. Both sensation and cognitive processes can effect the EEG.
On humans, the EEG is normally sensed using skin surface electrodes placed at various locations on the head. Since the actual EEG signals have a very low amplitude, a substantial amount of amplification is required. As a result, obtaining an extremely low applied electrode contact impedance is a crucial prerequisite for obtaining a clean EEG.
It is not uncommon to record the EEG from multiple sites simultaneously. The development of standardized locations for the electrode sites has led to the use of EEG electrode caps. Often made of an elastic material, these caps are easily pulled down over the head, and have the electrodes pre-positioned at the correct locations. The electrodes are usually Ag-AgCl, but the gel needs to be applied after careful skin preparation at each electrode site. Multi-channel EEG amplifier and data collection systems are used with these caps to supply the data from the resulting field of EEG electrodes. The EEG field data can then be processed by complex mapping and visualization algorithms on a computer.
Various sensation processes can show up in the EEG signal as well. Evoked Responses refers to the process of deliberately presenting a specific stimulus, and then looking for the response, the result of the sensation process, in the EEG signal. Such responses are usually much smaller than the extremely small EEG. As a result, the stimulus is carefully controlled, and the resulting evoked responses in the EEG are carefully compiled, or summed over multiple Stimulus and Response recordings. The noise will cancel out, and the response signal will slowly emerge. Generally light and sound are used, and this process can be used to assess the function of the sensory and neural pathways involved.
The various members of the model 1089 Checktrode family can be very useful in evaluating EEG electrode contact impedance. For one or two channels, our standard 1089 or lower cost 1089e can be used to check the electrodes two at a time. We developed the 1089-ES to bring easy contact impedance testing to larger arrays of EEG electrodes. And the 1089-NP is designed to supply rapid cap electrode check-out for the popular 'qEEG' and similar caps used by many multi-channel EEG equipment suppliers.
We can also recommend our 3991 or 3992 Biolog as a good platform for ambulatory EEG recording. The Biolog can provide fairly high speed sampling of the EEG for a number of EEG channels (up to 7, depending on the platform, desired sample rate, and other channels). The Biolog can also be used to record EEG and other subject measures at the same time.
Our Baercom is a specialized, portable, veterinary BAER hearing testing system. This system records the EEG response to an audio stimulus, which is repeatedly sampled and summed in order to compile the BAER response. All of the circuitry required for basic Baer testing is packed into this small, 9V battery powered device.
Our 2283FTi Fetrode amplifier can be used to supply the amplified EEG to your existing data collection system. Also, the SC2000 Simple Scope PC based data collection system can be configured to record the EEG signal as well. EEG can also be included as a measure in our Multi-Subject Data Collection System installations.
If you still have questions, or if you don't see what you need, drop us a line, and we will see how we can help you!