Electrophysiology Flashcards
Auditory Development: 0-4 months
- Startle, and blink in response to loud sounds
- Physiological measures (e.g. 80 dB)
Auditory Development: 4-7 months
- Head turn towards sounds
- OAE and ABR tests
Auditory Development: 7-24 months
- Head turn at soft levels, approaching threshold
- VRA tests
Auditory Development: 24 months
- Possible to engage children in play
- Playtests
- Worst case scenario if you NEED to get the thresholds but children don’t cooperate use sedated ABR
How can we differentiate between SNHL and CHL with young babies using ABR?
- By using AC ABR vs. BC ABR
- Always do it twice
In ABR Threshold Estimation, we always base our findings on wave ___
V
Why do we always base our ABR threshold estimation on wave V? (3)
robust with level change (closest to threshold)
robust with high rates (easy to record)
distinct pattern and well-defined Latency-Intensity function (easy to see)
When we measure electrical brain activity using stimuli, we are generating _____________________
Evoked Potentials
What are the three components of Electricity?
I = current flows between + to -
V = voltage
R = resistance
What is Voltage? (2)
- a difference in potential
- electrical current flows down the potential gradient (e.g., like water flows down a height gradient)
Explain how the the Electric Neuron works:
- When an excitatory neuron is active, it releases neurotransmitters that cross the synaptic cleft and bind to the target neuron, opening its Na+ channels
- The inflow of Na+ into the dendrite makes that part of the cell more positive in potential, and the extracellular fluid more negative in potential
- This creates a sink in the extracellular space (a low potential), and a source in the intracellular space
When we measure ABRs, we basically measure what is occurring in __________________
Extracellular Space
How can we measure potential differences using electrodes?
When electrodes are attached to the scalp, these potential differences can be recorded
(Blue vs Red)
What are the characteristics of recording in the Near-field and Far-field?
Far-Field Requires highly synchronized activity (3cm or farther away from the source)
Near-Field (2-3 cm max away from +/- dipole source)
Thus, the ABR is a test of __________________
Neural Synchrony
How can we differentiate between SNHL and CHL with young babies using ABR?
- By using AC ABR vs. BC ABR
- Always do it twice
What is the layout of the cortex between layers?
3-5 mm thick
* 6 layers
* Large pyramid cells, aligned in the same direction perpendicular which is perfect for us, provides better neuro synchrony
roughly:
100 000 in 1 mm2
What is the Equivalent Dipole?
Spatial Summation of Neurons
Lots of Neurons doing the same thing together
What is the volume conductor?
The volume conductor is the extracellular space—the brain ‘bathtub’
What is the relationship between potential difference and volume conductor? (2)
- A potential difference (e.g., due to excitation or inhibition at some point) is like a BATTERY, with current flowing through the volume conductor from the high point (+) to the low (-)
- When many of these are active in the SAME DIRECTION at the SAME TIME, they spatially sum… it’s as if there is one large battery
In order to measure an ABR, you need: (3)
spatial summation
synchrony
open field (same direction)
How do we record ABR?
We attach electrodes to the head with conductive paste after first removing the outer layer of skin (exfoliating))
How do we conduct evoked response and averaging?
What are transient potentials?
Transient = Only happens once
Log scale X-axis
ABR is Separated into Three Responses:
Auditory Brainstem Response (looking for V threshold)
Middle Latency Response (looking for Pa)
Slow Auditory Response (looking for N1)
ABR important generators to remember are:
l ANF
ll End of ANF
lll CN in the CNS
V LL
Middle latency response important generator to remember is:
Pa first response from A1 (25ms)
Slow latency response important generator to remember is:
N1 largest simple response from playing a beep or click
Give information about the Amplitude and Latency of the ABR responses: (4)
- Amplitudes are variable, latencies are stable
- Waves VI and VII are difficult to see
- I-V can usually be identified (IV-V are fused)
- I, III and V are usually best!
Give the time of occurrence of each wave:
V could be 5.5 or 5.7 in exam
What are the interpeak latencies?
What are characteristics of Vestibular Schwannomas?
- Will usually result in delayed or absent ABR peaks (~95% of the time for tumours ≥ 1 cm)
- unilateral in 95% of cases
- Will usually produce an asymmetric sensorineural hearing loss (this is the primary symptom of vestibular schwanomma!)
- Will lead to abnormal reflexes (elevated, absent or present with decay)
- Will lead to abnormally poor speech perception, often poorer at high levels (rollover)
- Usually grow internal auditory meatus in the petrous section
What would be the best peaks to measure in ABR if there would be 8th nerve tumor/Vestibular Schwannoma?
- I-III are the best measure to see if something is wrong in the ANF
- Normal: 2–2.1 ms (.2 ms SD)\
- Abnormal for VIIIth nerve tumours (about 85-100% of the time)
(Increase delay or Absent Wave)
Musiek (1986):
abnormal > 2.4 ms
Lightfoot (1992):
abnormal > 2.52 ms
What would be the best peaks to measure in ABR if there would be cerebellar/pontine/brainstem tumor/lesion?
lll-V (Increase delay or Absent Wave)
Normal: 1.9 ms (.2 ms SD)
Sometimes abnormal for VIIIth nerve tumours, but likely only when tumour in CPA
Musiek:
abnormal > 2.3 ms
Reflects problem in or above CN
What is the absolute wave V latency? (3)
- Abnormal if greater than 6.3 ms (to a click)
- Affected by hearing loss, so may be difficult to interpret in isolation
- Sometimes the only possible measure
What is the V/I Amplitude Ratio? (3)
- Abnormal if < .75 (i.e. if wave V is less than ¾ of the amplitude of wave I)
- Only interpret if amplitude is stable (varies less than 20% between runs)
- Cannot use if near-field recordings of wave I (or horizontal recordings!)
What are the Morphology: ABR Patterns in V?
All suggestive of tumors
What is Auditory Neuropathy Spectrum Disorder?
Auditory neuropathy spectrum disorder (ANSD) is a rare type of sensorineural hearing loss where sound is not transmitted properly from the cochlea (inner ear) to the hearing nerve or from the hearing nerve to the brain.
Absent ABR
Low WRS/Reflexes/Hearing
Normal OAE
What is (De)myelination?
Na+ channels at nodes of Ranvier - current jumps from node to node (saltatory)
if a demyelinated portion is encountered, conduction velocity is slowed
variable lengths of demyelinated portions will result in a loss of synchronization
What is the ANSD incidence in children?
- About 10% of childhood hearing loss appears to be due to auditory neuropathy (i.e. OAEs or CMs present)
- OAEs screenings will miss Auditory Neuropathy
What the applications of Middle-Larency Potentials testing in infants?
None
only appropriate for awake subjects :(
young children may not have MLR
response amplitude is highly variable
presentation rate must be very slow
1/s (long recording time)
adult like at 10 yrs
Explain the The P1-N1-P2-(N2): The Slow-Vertex Response:
Long-Latency Responses
P1
50-60 ms
N1 (the one we look at)
80-100 ms
P2
180-200 ms
Between P1-N1-P2, the response occurs primarily to:
the response occurs primarily to changes in energy
onsets, offsets, and changes
P1-N1-P2 Complex and Thresholds characteristics: (3)
- Less dependent on synchrony
- Can be recorded closest to the threshold
- Requires an awake and cooperative patient
What is the Mismatch Negativity?
An auditory event-related potential that occurs when a sequence of repetitive sounds is interrupted by an occasional “oddball” sound that differs in frequency or duration
- the difference between the deviant and standard (response is more negative for the deviant)
- the human brain constantly builds models of the world, the MMN indexes one of these models: auditory memory
What is a deviant?
A deviant is only a deviant if there is a standard—something that is more common
A deviant is only a deviant if there is a _________________________________________
—something that is more common
Mismatch Negativity vs. N1:
- N1 reflects Detection
- MMN reflects Discrimination
A less intense deviant elicits smaller N1 (because less intense), but larger MMN (since more deviant)
When does the MMN occur?
MMN occurs after object formation
All auditory stimuli /pa/
Visual standard /pa/
Visual deviant /ka/
N400 can vary and be measured during ________
Sleep
Meaning perception can occur without consciousness
What are the uses of Electrophysiology? Give 3!
- hearing screening (very important < ~ 7 mos for EHDI)
- hearing threshold estimation (very important < ~7 mos for EHDI)
- diagnosis of neuropathy/dys-synchrony
- detection of tumours
- objective measures of auditory abilities
- studying auditory neural processing
- language processing and understanding
Identify the Peaks:
