Auditory Middle-Latency Response (AMLR) Flashcards
(59 cards)
What is AMLR or MLR?
Voltage oscillations occurring between 10 to 80 ms from stimulus onset
Believed to be generated by the thalamocortical pathway with some input from the inferior colliculus
How do you label positive and negative peaks?
Positive = P
Negative = N
Is it very difficult to measure AMLR and MLR?
Yes
Technology is a big limitation for these responses
Do CANS alterations affect more the amplitude than the latency for middle and late responses?
Yes, in contrast to ABR
Latency is not that tight in these responses - large variations even among normal individuals (because they arise from multiple and overlapping subcortical and cortical neural generators)
Because there is little to no high frequency energy in the mid and late responses, precise latency resolution is less important (only low frequency responses) - SD will be much larger; still look at them because they are the only norms we have
But standard amplitude norms are lacking limiting clinical use
Not very good amplitude standards - no norms
The difference between normal and abnormal is not tight
Does AMLR require a slower recording rate compared to early AERs?
Yes
Because they are generated by larger and slower subcortical and cortical neurons rather than smaller stimulus-onset VIII N neurons
These responses take a little longer
Why we use lower freq stimulus
Why do we study AMLR
Provides information about the functionality at higher systems in the auditory system
Is AMLR more affected by neuromaturation than ECochG and ABR?
Yes
Typically, they become adult-like by about early adolescence to 10-12 years of age
Not useful for newborn hearing - but they can be done
Also affected by the state of the patient, which are not with ABR and ECochG
Prefrontal cortex not fully developed until 25 years
Do AMLRs have greater low frequency content?
Yes, and therefore are more affected by low frequency non-stimulus artifacts
Often result in poorer morphology than ECochG and ABR
Are early and middle latency responses exogenous?
Yes, they are obligatory responses to sound and arise from neural components that depend on the physical nature of the stimulus eliciting a response
Does not require internal processing of the stimulus
Provides information about functional integrity of peripheral and central auditory pathways
Are most late auditory responses endogenous?
Yes
They arise from neural components that depend on the psychologic or cognitive processing of stimulus information by the listener
Attention is the primary cognitive process by which interpretation of the sensory signal occurs
Attention usually changes the auditory late responses without affecting other auditory evoked responses
Endogenous potentials provide clues to the nature of higher cognitive function
Sometimes describes at event-related potentials (ERPs)
Are AMLRs exogenous?
Yes, but ALRs are endogenous (especially the later responses of the ALR)
Amplitude changes with attention
Are the AMLR responses between the ABR and the ALR?
Yes
They are the middle latency responses
What do AMLRs consist of?
A series of biphasic (positive and negative phase peaks) waveforms occurring between 10 to 80 ms following acoustic onset
Primarily represents responses sensory specific to the auditory stimulus
What is the clinical usefulness of AMLR?
Neurodiagnostics
Not using it for estimating thresholds
What are the characteristic waveforms for AMLR?
Po - positive wave occurring at about 10 ms
Na - negative wave occurring at onset of AMLR at about 20 ms (18-20 ms)
Pa - largest positive wave occurring at 30 ms (15-30 ms)
Nb - negative wave occurring at 40 ms (least consistently recorded)
Pb - positive wave occurring at about 50 ms (sometimes identified as the P1 or P50 component of late-latency AER
Are Po and Pb present in less than 50% of the time under normal conditions?
Yes
Need to be very careful about recording parameters if you want to view these - has to do with ANSD
When do Na and Pa components become mature or adult like?
By 8 to 12 years of age
Detection of Pa increases from birth to adolescence (20% detection at birth (detectable in 20% of patients), 90% at 12 years)
To obtain reliable recordings in infants, does subject sleep state needs to be controlled?
Yes - hard to tell what state the child is in and if they are actually asleep
Can the Pa be detected in children as young as 4 years?
Yes, but a very small response
The child can be awake or in certain sleep stages (stage 1, REM, alpha stage (EEG waves observed during periods of relaxation but still awake)
AMLR is absent in the same children during sleep stages 2, 3, and 4 (deep sleep)
This is a limitation of testing - gives us a lot of information of how the auditory system functions (not always aware of sounds)
Do sleep stages also affect adults responses to AMLR?
Yes, not affected by neuromaturation
Where are AMLR responses generated?
Inferior colliculus, thalamus, and the auditory cortex
The exact neural generator of each waveform is, however, controversial and difficult to tease out
The Na is associated with neural activity from the midbrain, thalamus, and thalamocortical radiation
The Pa in humans is believed to be generated from the primary auditory cortex
*Not universally agreed by researchers
Is human variability the highest at the level of the CNS?
Yes
Is the AMLR is affected more than other responses by muscle artifacts?
Yes
What are the muscles that affect AMLR responses?
Postauricular muscle (right by mastoid) - large negative peak at 12 ms, positive peak at 16 ms (why we shouldn’t put the electrode on the mastoid)
Temporalis muscle - large negative peak at 17 ms, positive peak at 23 ms (easily recordable from subjects with clenched teeth)
Neck muscles - multiple and complex components; negative peaks at 11 and 25 ms, positive peaks at 17 adn 34 ms
Frontalis muscle - positive peak at 30 ms (highly variable)
