Basic Instrumentation, Acquisition and Recording Considerations
Key Points
- computer, amplifier, electrodes and transducers
- transducers present a variety of stimuli
- electrodes act like an antenna
- amplifier brings small amplitude into the voltage range of the computer
- computer analog/digital conversion, filtering unwanted signal frequencies and performing signal averaging to extract EP
Signal versus Noise
- EEG is a biologic wave that is somewhat noiselike in appearance and can exhibit voltage as high as 50 to 100uV
- in a relaxed person the EEG can be as low as 10 to 20uV
- when a person becomes drowsy the EEG drops to ~10 to 3 Hz
Digital Signal Processing
- A-D conversion is a two step process involving sampling and quantization
- sampling is breaking a signal down into a limited number of manageable units, called samples, each having equal time duration
- ABR is commonly recorded in a window of 10msec and fixed number of sampling points 256
- means the ABR will be divided into 256 pieces with each piece (sample) having 0.0390625 msec
- quantization is breaking down a continuous signal into manageable amplitude units, called steps
- 16-bit computer has the ability to quantize into 65, 536 steps
- Sampling rate determines the maximum signal frequency that can be digitized
- nyquist theorem states that the sampling rate should be at least two times the highest frequency in the signal of interest
- aliasing can occur in the A-D process if the nyquist is not satisfied and signal will be misrepresented
- nyquist theorem states that the sampling rate should be at least two times the highest frequency in the signal of interest
Time and Frequency Domain
- A time domain analysis evaluates the amplitude of a signal over time and the signal appears as a waveform with alternating positive and negative values
- In contrast, a frequency domain analysis removes the element of time to reveal the spectral energies of the signal as the waveform is translated to its respective amplitude values across frequencies
- An AEP is nothing more than a complex waveform combination of many frequencies or sine wave components
Instrumentation
Stimulus Generator
Transducers
Trigger
Acquisition Parameters
Differential Amplification
Filtering
Signal Averaging
Electrodes
Electrode Impedance
Electrode Placement
Number of Electrodes versus Number of Channels
Time Window
Sampling Rate
Number of Sweeps
Stimulation Rate
Filter Settings
Amplification
Artifact Rejection
General Subject Factors
Muscle Activity
Attention
Temperature
Stimulus Generator
- most commonly used stimuli include 100 usec clicks and short-duration tone bursts
- ASSR auditory steady state responses, signals are typically amplitude or frequency modulated tones or they may use a combination of both, referred to as mixed modulation
Transducers
- ER-3A tubal-style insert earphones
Trigger
- the trigger is the key to the recording of all AEPs and works in tandem with signal averaging
- the trigger is a digital pulse or word that lets the averaging computer know precisely when each stimulus is being presented
- once the recording time window is defined, the trigger and stimulus onset are essentially synonymous with time zero in the analysis time window
Acquisition Parameters
- With few exceptions, AEPs are smaller in amplitude than the background EEG and thus they have a poor signal to noise ration (SNR)
- To extract the smaller AEP signal and attenuate noise (improve SNR) several techniques are necessary to condition the AEP signal for later analysis: amplification, filtering and averaging
Differential Amplification
- to reduce background noise
- bring the signal of interest into the range of the A-D converter
- a minimum of three electrodes is required
- noninverting
- inverting
- ground
- Common Mode Rejection (CMR) is signal canceling common to both inputs
- greatly improves SNR
- In addition to differential amplification, there is additional amplification, referred to as gain, that can be applied to the recorded response
- the AEP software will include gain as a parameter than can be modified within the acquisition parameters
Filtering
- Improves SNR
- attempts to pass signals of interest while rejecting noise
- Analog and Digital
Signal Averaging
- basis for the SNR improvement by signal averaging is due to noise not being time locked to the external stimulus
Electrodes
Electrode Impedance
- for optimum recording, electrical impedances should not exceed 5kΩ
Electrode Placement
- 10-20 System
- F - Frontal
- P - Parietal
- O - Occipital
- T - Temporal
- C - Central
Number of Electrodes versus Number of Channels
- Cz or Fz (non-inverting)
- Ai (inverting)
- Ac (ground)
- 1-channel ABR (3 electrodes)
- Ch1 Input 1 (+) Fpz (non-inverting/active)
- Ch1 Input 2 (-) A1/A2 (inverting/reference)
- Ground (A1/A2)
- 2-channel ABR (4 electrodes)
- Ch1 Input 1 (+) Fpz (non-inverting)
- Ch1 Input 2 (A1/A2)
- Ch2 Input 2 (A1/A2)
- Ground at Fpz/Forehead
- *Jumper cable used to join Ch1 Input 1 (+) and Ch2 Input 1 (+) together
- 2-Channel MLR (5 electrodes)
- Ch1 Input 1 (+)
- Ch1 Input 2
- Ch2 Input 1 (+)
- Ch2 Input 2 (-)
- Ground @ Forehead
Time Window
- window that is sufficiently long enough to capture the entire AEP while not including much response information beyond the components of interest
- example: ABR window of 10msec is often chosen to capture typical recordings of 6 msec after the stimulus
Sampling Rate
- 256, 512 and sometimes 1024
- time window is 10msec or 0.01s
- if we choose 256 sample points; 256/0.01 = 25,600Hz
- If nyquist frequency in the ABR is around 1000Hz, it must be sampled at least 2000Hz which means that a sampling rate of 25, 600Hz is more than plenty to sample the ABR with high resolution
- each sample duration is ~0.039 msec
Number of Sweeps
- Inversely proportional to the SNR and the amplitude of the AEP of interest
- As the SNR improves and the amplitude of the AEP increases, the number of sweeps required for testing decreases
- In general, the longer the latency of the response, the greater the amplitude
- the higher up in the central auditory nervous system being tested, the fewer the number of sweeps that will be necessary to adequately view the AEP response
- If the SNR is good, the clinician may stop averaging
- If the SNR is poor, more sweeps are necessary
Stimulation Rate
- Dependent on the time window and duration of the stimulus
- Example: during ABR testing, the time window is usually 10 msec and the stimuli are very brief 100usec click
Filter Settings
- employed to eliminate spectral energy not contained within the AEP of interest in an effort to maximize the SNR of the recording
- filter settings employed are based on the frequencies that constitute the AEP
- ABR 100 - 3000Hz
Amplification
- Gain is the AEP acquisition feature used to amplify and in turn visualize the recorded response
Artifact Rejection
- Artifact can still make a visualization even with aforementioned precautions
- "artifact reject" is a parameter which can be set to eliminate any epochs in which the electrical activity exceeds a predetermined criteria
General Subject Factors
- Age
- central auditory system is not fully mature until adolescence
- ABR is not adult like until 18-24 months
- longer latencies are typical of this time frame which would require a longer time window
- immature or aging auditory systems or degenerating are often negatively affected by faster stimulation rates
- latency increases slightly with age
- Gender
- Women have shorter latencies and larger amplitude responses than men
- basilar membrane is longer in men
Muscle Activity
- muscle activity can obscure the response
Attention
- relax with eyes closed during ABR recordings
Temperature
- lower temperature can prolong latencies where a longer time window will be necessary