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

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

• 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