Event-related potentials (ERPs) are brain responses time-locked to some "event". This event may be a sensory stimulus (such as a visual flash or an auditory sound), a mental event (such as recognition of a specified target stimulus), or the omission of a stimulus (such as an increased time gap between stimuli).
Auditory evoked potentials (AEPs) are a subclass of ERPs. For AEPs, the "event" is a sound. AEPs (and ERPs) are very small electrical voltage potentials originating from the brain recorded from the scalp in response to an auditory stimulus (such as different tones, speech sounds, etc.). The AEPs that are recorded from the top of the head originate from structures within the brain (e.g., the auditory cortex, the auditory brainstem structures, the auditory VIIIth cranial nerve). They are very low in voltage: from 2-10 microvolts for cortical AEPs to much less than 1 microvolt from the deeper brainstem structures. Their low voltage combined with relatively high background electrical noise requires the use of highly sensitive amplifiers and computer averaging equipment
The figure to the left shows several auditory ERPs/AEPs (plotted with positivity upwards). To view/print a larger version, click right button with mouse pointer on figure, and "view image"... it may be a little slow.
The Auditory Brainstem Response ("ABR"; 1.5-15 ms post stimulus), which originates in the VIIIth cranial nerve (waves I and II) and brainstem auditory structures (wave V: region of lateral lemniscus and inferior colliculus).
The Middle Latency Response ("MLR", 25-50 ms poststimulus), includes waves Na (negative wave following ABR wave V, originates in upper brainstem and/or auditory cortex) and Pa (positive wave at about 30 ms, originates in the auditory cortex bilaterally).
The "Slow" cortical auditory ERPs, which include the P1-N1-P2 sequence (50-200 ms poststimulus; originating in auditory cortex). N1 is the large negative wave that occurs about 80-100 ms after the stimulus. It originates primarily in the auditory cortex bilaterally. In the figure to the left, it is the large negative wave seen both in the response to the "standard" (black line) and the "deviant" (or oddball; red line) stimuli.
The "Late" cortical auditory ERPs, especially the Mismatch
Negativity
("MMN"; beginning around the time of N1 and later). The MMN is
a response
reflecting detection by the brain of a change in the stimulus.
In
the figure to the left, the MMN is the increased negativity seen in the
response
to the deviant or change stimuli (red line), at about the time of N1
and
a little later. Other "late" ERPs, not present in these waves, include
"N2b"
and "P3b", which are cortical ERPs which are not specifically from
auditory
structures (see Cortical ERPs below).
A very different type of auditory evoked potential are the Auditory Steady-State Responses (ASSR), which are responses to stimuli presented at rates such that the brain response to one stimulus is overlapped with responses to other stimuli. Responses to slower modulation rates (<20 Hz) appear to originate largely in cortical structures; responses to faster rates (70 Hz and higher) appear to reflect brainstem processes. ASSRs to rates >70-Hz show great promise for rapid assessment of hearing infants. The multiple auditory steady-state evoked response (MASTER) technique provides a rapid and objective assessment of hearing. The technique is based on the statistical evaluation of the electrophysiological responses evoked by multiple auditory tones presented simultaneously. These auditory steady-state responses can be recorded from the human scalp intermixed with the other activity in the electroencephalogram (EEG). A combination of averaging and frequency-analysis can distinguish the responses from the background EEG. MASTER typically presents 8 continuous tones (4 to each ear) and each tone is sinusoidally modulated at a unique frequency [Click here to see how this works!] . The detection of the interwoven responses becomes possible after the electrophysiological data are transformed into the frequency domain [Click here to see how this works!] . MASTER evaluates the responsiveness of the auditory system to several tonal frequencies in the same time it would take to record one response if each stimulus was presented separately. Some of our our current research evaluates the clinical feasibility of MASTER for assessment of hearing in infants and in adults, as well as its possible tool for newborn hearing screening (click here for our recent papers on ASSRs).
The AEPs in humans are typically recorded using small disk-like electrodes
which are placed on various places on the head using either tape
or
water-soluble paste. The electrodes are non-invasive, they do not hurt,
and
they come off easily after completion of testing. The ABR and MLR and
the
ASSR are typically recorded using 3 or 4 electrodes; the SLOW and LATE
cortical
potentials often with many electrodes. In the HAPLAB, we currently
record
these later cortical responses using Neuroscan 64-channel "Quik-Cap"
electrode caps (see photo to left).
Currently, the brainstem responses are used for the clinical
identification ("screening") and assessment of infants and young
children with hearing loss
(see figure to the left), as well
as
in the neurophysiological assessment of pediatric and adult patients
who
may have lesions associated with their VIIIth nerve or brainstem
auditory pathways. Our research over the past 25+ years has been
instrumental in the
application of the tone-evoked auditory brainstem response for
estimation of the audiogram [for example, see our list
of recent
papers], and we continue this research in the HAPLAB. In the
HAPLAB,
we are also investigating the use of multiple auditory
steady-state
responses for screening and for diagnostic evaluation. See the "Clinical ABR Frequently Asked
Questions"
(FAQ) in
the
HAPLAB's website.
Currently, Province-wide "early hearing programs" (such as the British
Columbia Early Hearing Program and the Ontario
Infant Hearing Program) use the tone-evoked ABR to air-conducted
(and, when hearing loss is pressent, to bone-conducted) brief tones to
establish hearing thresholds for frequencies from 500 Hz to 4000 Hz,
and, when necessary, to determine the type of hearing loss.
Many clinics around the world have begun to use the ASSRs to assess
hearing in infants. Although ASSRs show great promise, there remain
important gaps in our clinical database. Thus, when hearing loss is
present, the clinical use of ASSRs must
be in conjunction with (not instead of) the tone-evoked ABR to
air- and bone-conducted brief tones.
The slow cortical AEPs are used by some audiological centres for the
objective
assessment of hearing threshold in adult compensation cases (for
example:
WorkSafe BC in British Columbia). There is current (2008) research (and
clinical) interest in using these responses in the assessment of
hearing using hearing aids or cochlear implants, though the evidence
base is still quite limited.
Later-occurring cortical ERPs (for example, see figure to the right) are
likely
to be a useful audiological tool in the near future. The cortical ERPs
shown
to the right are responses from individuals with normal hearing to the
speech
sound "/ba/". These responses were recorded in an active condition,
where
the subjects' task was to press a button whenever the stimulus changed
to
"/ba/". Responses seen are auditory N1, and the late responses N2b and
P3b.
Some of our current research evaluates the use of cortical ERPs to assess a hearing- impaired child's detection and discrimination of speech sounds with and without the use of their prescribed hearing aids.
The cortical ERPs have considerable utility for research. They
provide a window into the timing, sequence, strength, and
anatomic location of brain processes involved with the perception of
sounds. Current research underway concerns the use of cortical ERPs to
understand the brain processes underlying basic hearing percepts such
as loudness, pitch, and localisation, as well as those processes
involves with speech perception.
Current research in the HAPLAB involves all of these auditory evoked/event-related potentials.
HAPLAB research concerning frequency
selectivity/frequency
specificity and auditory evoked potentials
HAPLAB research
concerning
Auditory Steady-State
Responses
(ASSRs)