Appendix 3: Vocabulary

Vocabulary

Appendix 3: Vocabulary

The terms listed are defined here to minimize confusion over the loose use of previous terminology. Terms are not alphabetically ordered.

client- who is being treated

clinician - the treating person

electrode - the flat or cupped piece of metal used to sense electrical energy;

also has a wire on it to attach to an amplifier or preamplifier

signal - a voltage (measurement) varying with time; always made with respect to something (differentially);

ProComp preamplifier leads are called Signal+ (yellow) and Signal- (blue);

Picciottino amplifier leads are called Signal (yellow) and Reference (blue);

also Exploring electrode and Referential electrode in classical EEG work

reference - Depends (see signal above)

base - A term defining the electrode used as a zero-voltage level for signal and reference measurements. This lead is also called "ground" due to a history of use with instruments connected to power lines.

ground - Depends but note that the Picciottino "Ground" (black) lead is not really GROUND.

The "ground" is an isolated current-limited input used to provide a "zero voltage" point with respect to the signal being measured against its reference.

impedance - opposition to the flow of a cyclic (alternating current) current through the interface between an electrode and the scalp. If measuring, must be at least 500 ohms and less than 10,000 ohms between any two electrodes.

Hz - Hertz, the measurement unit of cyclic activity

amplifier - a device to increase the amplitude of a signal

preamplifier - a device used to increase the amplitude of a small signal

A/D - Analog (voltage) to digital converter

Channel- each physical input measurement is one. It may take several 'connections' to a client for each physical input (such as 3 electrodes). A channel is comprised of the electrode(s), preamplifiers, and amplifiers that make up a measurement path.

Stream - each processed entity is one; may be made up from one or more channels

trace - a graphical display (strip chart display) of a stream

band - range of frequencies; sometimes with a name (e.g. theta) but preferably just the explicit range of frequencies (e.g. 2-7 Hz).

Layout - arrangement of traces with implied connections of data to game display

class - a general grouping of treatment modalities (such as eyes-closed, eyes-open, etc.)

session - portion of client process where the clinician/client interacts with the computer

stage - one piece of the session process, usually a particular action/operation.

peak- Highest (voltage) amplitude of a signal

P-P - (peak-to-peak) difference between the most positive and negative values of a cycle of a (sine) wave; all EEG amplitudes are reported in peak-peak voltage

phase - angular relationship of a particular signal from one or more channels

smoothing - a method of limiting the effect of short term excursions of data samples. The technical term is Exponentially-Weighted-Moving-Average filtering.

All our smoothing is :

SMOOTHED=CURRENT_SAMPLE*(k) + SMOOTHED*(1-k)

where k is something like

k= 1/(time_constant*samples_per_second)

average - peak-to-peak equivalent of rectified signal value, smoothed

integral - peak-to-peak equivalent of signal value, smoothed

~~goal~~/threshold - peak-to-peak voltage amplitude which is compared to integral value when deciding if the smoothed p-p signal is above/equal a point

% over threshold - smoothed ratio of samples over threshold to total samples

artifact - when an input channel threshold is exceeded;

used to prevent large muscle (EMG) signals from affecting the smaller (EEG) signal processing; usually from extracerebral sources or ambient electrical signals.

fft - Fast Fourier Transform. A mathematical method of taking a time-varying set of data points (i.e. signal voltages for a period of time) and transforming the data into a frequency-varying set of data. Note that over a set of time measurements (e.g. 256 points), the frequency result corresponds to the time at the center of the time measurements. At a 256 Hz sample rate, a 256 point fft result always refers to 1/2 second before the last sample.

spectral - A frequency versus (power) amplitude display of data. The frequency data is generated from a moving fft computation.

DC level - The average value of the zero-crossing points of a periodic wave. If not zero, this means that there is a steady-state offset of the signal from its "ground" reference point. Computed by summing each sample. Used to move the incoming signal to a point where positive and negative excursions occur around a "zero voltage" point.

filter- a mathematical method of attenuating/excluding cyclic activity (waves) of specific frequency ranges.

FIR - Finite Impulse Response filters. Characterized by flat phase response throughout filter range. A typical FIR filter (12-15 Hz) requires about 200 "taps" or computations.

IIR - Infinite Impulse Response filters. Characterized by relatively flat amplitude response throughout the filter range. A typical IIR filter (12-15 Hz) requires about 3 "taps" or computations. This is the filter type used. Phase errors are not an issue since the same (apparent) phase shift occurs for all samples at the same frequency.

lowpass filter - passes low frequencies but attenuate high frequencies.

bandpass filter - attenuates frequencies below/above the bandpass limits

center frequency- frequency at midpoint between lower and upper limits of a filter

edge frequency - beginning of transition region where attenuation starts decreasing amplitudes; sometimes called "cutoff" frequency.

ripple - variation in amplitude of frequencies passed through a filter

rolloff - rate at which attenuation occurs at frequencies outside a filter's pass band. The higher the rolloff value (measured in decibels), the narrower/sharper the attenuation is outside the pass band and the more computation required.

Examples:

Shown is a typical filter diagram with frequency as the horizontal scale and relative amplitude as the vertical scale (adjusted to unity).

Ripple is the amplitude variation between points A,B, and C of a pass band.

Rolloff is how fast the attenuation takes place (as frequency changes) measured to the 30 db

(50% ) point (D) from point C. There is also rolloff on the low edge of the pass band.

Edge frequency is point A or C.

Center frequency is 4.5 Hz (midway between A at 2 Hz and C at 7 Hz).

Frequency band names commonly used

delta .1 to 4 Hz

theta 4-7.5 Hz

alpha 7.5 - 13 Hz

beta 13-30 Hz

gamma 30-50 Hz

smr (low beta) 12-15 Hz

beta(mid beta) 15-18 Hz

hi beta > 18 Hz