Overview Flashcards
Is there spontaneous and random neuroelectric activity in the CNS in the absence of sensory stimulation?
Yes
This activity can be recorded as a potential difference using scalp electrodes and is the bases of the electroencephalogram (EEG)
Can neural activity in response to specific types of sensory stimulation also be extracted from the EEG?
Yes
These responses (especially the early responses such as the ECochG and ABR) are typically much smaller than the EEG
Signal averaging and amplification is required to view all evoked responses
What do evoked responses or potentials measure?
The electrophysiologic responses of the nervous system to variety of stimuli
In theory, almost any sensory modality can be tested
What are the evoked responses that are most frequently measured in clinical practice?
Visual evoked responses (VERs)
Short-latency somatosensory evoked responses (SERs)
Short-latency brainstem auditory evoked responses (BAERs)
Cortical evoked responses (includes late latency responses and the P300)
What are somatosensory evoked responses (SERs)?
Generated by stimulation of afferent peripheral nerve fibers by either physiological or electrical means
Typical stimulation sites: median nerve (wrist), common peroneal (nerve at the knee), and posterior tibial (nerve at the ankle)
Recording electrodes are placed over the scalp, spine, and peripheral nerve proximal to stimulation sites
Waveforms are described in terms of morphology (how it looks), amplitude (how big the responses are), and dispersion (latency)
What conditions are somatosensory evoked responses (SERs) useful in diagnosing?
Neurologic disease
Prognosis of comatose patients especially after postanoxic coma (no oxygen)
Intraoperative monitoring of “at risk” somatosensory pathways
What are cortical late evoked responses?
Generally used for studying higher cortical functions (the P300 in Alzheimer’s disease)
Clinical usefulness for audiology is limited, they are generally considered experimental and not used routinely
Show promise and may prove to be useful for clinical practice in the future
Has the clinical use of evoked responses changed over time?
Yes
Advances in imaging technology (MRI) have decreased their use in clinical practice
Most clinical questions of the involved neurologic structures are better answered by MRI (but in some cases, MRI and ERs may be complementary)
What is the basic difference between evoked response and MRI studies?
MRI is an imaging/structural/anatomic test and it provides more accurate information about structural problems
ER assesses functionality and supplies information about the physiology of a certain anatomic pathway, providing much less spatial or localizing information than an MRI
What are the major categories of AERs?
ECochG
ABR
AMLR
Auditory Late Response (ALR)
P300 Response
Mismatch negativity (MMN) Response
What are auditory evoked responses (AERs)?
Brain waves or electrical responses that are generated when the auditory system is stimulated by sound (picked up by electrodes places on the vertex/high forehead or near the ears such as on the lobes or mastoid)
Sounds used to elicit AERs: clicks (abrupt onset, very short, broad band), tone bursts (short duration, frequency specific sounds), speech sounds (/ba/ and /da/)
Sounds are presented via some acoustic transducer (typically an earphone - usually an insert)
As a general rule, the louder the stimulus intensity, the larger the AER
What does a typical electrode consist of?
A wire with a metal disc or adhesive patch at one end that makes contact with the skin
The other end has a DIN in that plugs into an electrode box or preamplifier
What is a DIN PIN?
An electrical connector that was originally standardized by the Deutsches Institut fur Normung (DIN), the German national standards organization
Commonly refers to a member of a family of circular connectors that were initially standardized by DIN for analog audio signals
Now DIN mini connectors that can connect to computer keyboard and mouse
The original DIN standards for these connectors are no longer in print and have been replaced with the equivalent IEC standard
What does it mean if someone has no EEGs?
They are brain dead
Does the activity evoked by the sound occur from structures at a distance from the electrodes?
Yes
Far-field recording (on the surface of the skin, needs to go through barriers to reach electrode)
This stimulus evoked sensory and neural activity is conveyed from the auditory structures through body tissues and fluids to the surface electrodes
From the electrode, what does this activity need to go through the be analyzed?
A pre-amplifier/amplifier
Filters
Analog-to-digital converter
A computer, where it is analyzed and visualized
If the electrodes are so far away from the generator site how do we know the response is coming from the auditory central nervous system (CANS)?
Although the specific source within the CANS is often difficult and sometimes impossible to pinpoint
The stimulus evoking the response is sound so the response is from within the auditory system
How long does it take for an AER to occur post-stimulus?
About 1 second
By analyzing the pattern of the responses and calculating the time of the occurrence of the AER, it is possible to determine the region, and sometimes specific sites, in the CANS generating the response
The AERs are described in milliseconds (1/1000 of a second)
Where do the earliest responses with the shortest latencies of AERs come from?
Generated by the inner ear and auditory nerve
Responses occurring a few ms later reflect brainstem activity
Responses occurring still later reflect activity in the higher structures of the CANS such as the cerebral cortex
When responses are evoked from the higher centers of the CANS, are their neural generators less specifically identified?
Yes
Does the brain activity that makes up the AERs result in a very small voltage?
Yes
Measured in microvolts
A µ volt is one-millionth of a volt or one-thousandth of a millivolt
Why is the activity measured from the higher regions of the CANS (cerebral cortex, etc.) larger in size than activity from the cochlea and the auditory nerve?
This activity involves possibly millions of neurons
The electrodes also are closer to the source of activity
Activity generated by the brainstem (ABR) in contrast involves way fewer neurons and is recorded farther away from the source
