Auditory Evoked Potentials (AEPs) - Introduction, Classification, and Instrumentation

Auditory Evoked Potentials (AEPs) Auditory evoked potentials  The activity of the nervous system produces electrical signals that can be picked up by the electrodes placed on the head, and can then be displayed on a screen of a recording device and/or plotted on a paper (EEG- Ele...

Auditory Evoked Potentials (AEPs) Auditory evoked potentials  The activity of the nervous system produces electrical signals that can be picked up by the electrodes placed on the head, and can then be displayed on a screen of a recording device and/or plotted on a paper (EEG- Electro encephalo gram).  A change in the activity of the nervous system occurs when it reacts to a stimulus. (eg; sound)  This change in neural activity also produces a change in the electrical signals picked up by the electrodes.  As a result, the nervous system’s reaction to a stimulus can be seen as a change in the electrical signals that are displayed on the recording device.  These electrical responses of the nervous system that are elicited by a stimulus are called as evoked potentials.  When the stimulus used is sound, then they are called as Auditory evoked potentials (AEPs). Advantages of AEPs  These auditory evoked potentials can be used  1. to check the integrity of the auditory system  2. to make inferences about hearing  3. they are usually non invasive – almost always being measured from outside of the body with electrodes on the surface of the skin. Classification of AEPs  There have been several approaches to classifying and naming AEP, but none has yet to be completely standardized.  Classification of AEPs are generally based on the following aspects.  1. Latency epoch  2. Anatomic origin  3. Stimulus – response relationship  4. Electrode placement AEPs based on Latency epoch  Latency epoch is the time domain within which the response occurs after stimulus onset.  AEPs are classified into 3 categories based on this 1. Early or short latency response (SLR)  AEP occurring within the first 10 -15ms following stimulus onset are generally referred as SLR or early latency response.  ECochG, ABR, SN10 (scalp negative response at 10ms), and FFR(frequency following response) are considered as SLRs.  The clinical use of SN10 and FFR are overshadowed by other AEPs. 2. Middle latency response (MLR)  These are other components of AEPs which are seen in the latency epoch of 10 – 50ms.  Most prominent of this response is the 40Hz steady state potential (SSP) 3. Long latency response (LLR)  These components of AEPs are generated beyond 50 – 80ms post stimulus onset.  These are also called as slow or late or long latency responses.  These includes N1 – P2 complex and P300 AEPs based on anatomic origin  Under this category AEPs are classified as 1)Brainstem and 2) cortical  Short latency responses comes under brainstem origin category and LLR comes under cortical origin.  But the use of this method has often been misleading  For example, ABR comes under brainstem category but the first peak of ABR is originated from the AN.  However, it is generally agreed that the latency epoch of an AEP is reflective of the level of the auditory system contributing the bulk of the electrical activity for a given response.  Thus the SLR arise from the auditory periphery and pontine to mid brain level brainstem.  LLR are cortically generated  MLR appears to arise from structures beyond he inferior colliculus to and including the primary auditory cortex.  AEPs based on stimulus – response relationship  Under this system responses are classified as  1. Transient or Sustained potentials  2. Endogenous or exogenous potentials Transient or sustained potential  ABR or other AEPs evoked by clicks or tones with fast rise/fall time is identified as transient.  Because the response is dependent on a rapid change in the stimulus.  Prolonged stimulation, such as that from the long tone bursts or quasi steady state signals may produce a sustained response that lasts as long as the stimulus.  Transient potential – eg: ABR  Sustained potential – eg: EcochG responses – Cochlear microphonics, summating potentials for tone burst stimuli  Exogenous or endogenous potentials  Exogenous potentials are predominantly depending on the physical features of the stimulus. And these potentials are not dependant on the attention or state of arousal of the patient.  Eg: SLR, MLR, N1 – P2 complex of LLR  Endogenous or perceptual potentials are largely independent of the physical features of the stimulus.  But these are sensitive to the “context” within which the stimuli are presented and the ability of the subject to recognize or attach meaning to this context.  These potentials require patients attention during the test  For eg: P300 and CNV (contingent negative variation)  This is elicited using an oddball paradigm.  Which means two different stimuli are presented together in a rare – frequent paradigm(oddball) AEPs based on electrode placement  This classification is depending upon the location of the recording electrodes relative to the site of the response generators.  Which is classified into 2 categories 1. Near filed recordings:  These are obtained when the electrodes are near or even on the neural generators.  Under this condition, slight changes in the location of the electrodes can have profound effects on the morphology of the resultant waveform.  For example: recording of AEP made during intra operative, direct nerve monitoring and ECochG 2. Far filed recordings:  the distance between the electrodes and the generators is comparatively large and placement site becomes a less critical variable.  All AEP recordings from the scalp may be considered as far filed recording  Eg: ABR, MLR, LLR etc Instrumentation of AEPs  Wide range of sophisticated computer based systems is available for clinical recordings of evoked potentials.  The instrumentation of AEPs can be broadly divided into two categories.  Stimulus system and recording system.  The components common to most of these systems are  Stimulus generator  Electrodes  Amplifiers  Filters  Signal averager with artifact rejection  Response display unit  Response processing and means to print the results Stimulus generator  The most effective and widely used stimulus for neurological applications is clicks.  This has essentially instantaneous onset and is of brief duration (0.1ms or 100micro seconds)  These pulses or clicks are broad band stimuli which are shaped by the frequency response of the earphones/insert receivers.  This provides maximal stimulation in the 2000Hz to 4000Hz range.  Thus brief pulses/clicks reflect activity primarily from basal portions of the cochlea.  Stimuli can be presented to independent ears or binaurally too.  Level of the stimulus is controlled by attenuators  When using AEPs to estimate hearing sensitivity, it is critical to have equipment capable of generating tone bursts where the frequency, duration, rise and fall time can be specifically selected. Transducers 1. Insert earphones  ER 3A are recommended when acquiring AEPs. Due to the following reasons  Separation of stimulus artifact from the onset of the response through 0.9ms delay makes wave I of the ABR more visible than other headphones like TDH 49.  Insert receivers prevents ear canal collapse,  it has more Inter aural attenuation which reduces he signal cross over to the NTE  It provide comfort while using it for long duration  It attenuates surrounding environmental noise than other earphones. Fig 3.4, linda hood, page 37  More precise placement  Flatter frequency response : for both spectral and temporal features.  Aural hygiene : use and throw ear tips are available  Option for TIPtrode use: which enhances the amplitude of wave I 2. Supra Aural earphones  TDH 39 or 40 earphones are suitable for ABR recording purpose.  The advantages of insert receivers are the most common disadvantages of headphones.  Common disadvantages are  Headband of supra aural earphones may be too big for infants  Coupling of the earphone cushion to the pinna may create ear canal collapse which reduces the stimulus intensity  There is no way to disinfectant or sterilize the supra aural ear cushions  Temporal and spectral characteristics are more reliable for insert receivers than earphones. 2. Bone vibrators  When AEP is used for threshold estimation or to estimate hearing sensitivity bone conduction transducer is also necessary.  Most commonly Radio ear B70 or B71 are used for this purpose  Different transducers used in AEPs Trigger  To extract a time locked waveform from background noise, a computer must know when the stimulus occurs and when to begin a recording epoch.  A trigger pulse that marks the beginning of the recording epoch can be set to occur in conjunction with the stimulus.  Usually the recording epoch begins with the onset of the stimulus. But in certain circumstances trigger may place few ms before the stimulus to record the pre stimulus period (eg: 0.9ms in case of ER 3 A)which may be useful in evaluating the noise levels. Electrodes  AEPs are recorded by attaching the electrodes on the surface of the scalp.  Montages of either 3 or 4 electrodes are generally used for recording either one or two channel ABRs respectively. Two channel recordings:  Using a 4 electrode montage in order to obtain ipsi lateral and contra lateral recordings.  One electrode is positioned at vertex, which has to be equidistant between two ear canals and equidistant between nasion and inion  2 other electrodes are placed on both mastoids.  4th electrode is the ground electrode which is placed usually on the forehead.  Based on the international 10 -20 system of electrode montages the vertex electrode position is referred as Cz, left ear as A1(left mastoid as M1), Right ear A2 (Right mastoid as M2) and forehead is Fpz. One (single) channel recordings:  Here, 3 electrodes are used with attachment at the vertex and each ear. (Cz, A1/M1 and Fpz)  Recordings are obtained between the electrodes placed at vertex and one ear.  The other ear electrode acts as ground.  Different types of electrodes used in AEP recording Amplifier and Pre amplifiers  This is the second stage of the recording system and the attachment point of the electrodes.  The preamplifiers are either placed near the attachment point of the electrodes or connected to the electrodes through an electrode input box and cable.  Because of low amplitude of the ABR (0.1 to 1.0micro volts) or other AEPs a companion amplifier and a preamplifier is necessary to provide amplification of the signal by about 100000 times. Display responses and Print  Visual displays are provided of the waveforms as it is either averaged or summed.  Viewing the ongoing activity allows the operator to identify periods of excessive activity such as when the patient moves or external electrical artifacts interfere the recordings.  Hence the clinician can decide whether to stop the run or to discard the run etc  Finally the resultant waveform of any AEP can be printed. Thank you..