The most positive peak between 40 and 75 msec, after the conditioning stimulus is P50. It can be elicited by either the paired click paradigm or the steady state paradigm. Paradigm has been adapted for testing sensory gating, or the reduced neurophysiological response to redundant stimuli.
1.1 Paired click test
In a paired click test, one auditory click sound will be presented, followed by a second click approximately 200 ms after the first one. The second sound is considered redundant, and so a typical control showing normal sensory gating will produce a reduced response to the second click.
Fig2: P50 in control vs schizophrenia
2 N100 or N1
A negative deflection peaking between 90 and 200 msec after onset of stimulus is observed when an unexpected stimulus is present. It is an orienting response or a matching process that is whenever a stimulus is present; it is matched with previously experienced stimuli. It is often referred as N1-P2 complex. Typically N100 is associated with auditory stimuli, but it is also seen in visual, olfactory, pain, balance and respiration.
Fig2: N100 wave
Fig3: N1-P1 complex
Auditory N100 is generated by network of neural populations in
- Superior temporal gyrus in Heschl’s gyrus
- Planum temporale
- Frontal and motor areas
N100 is pre-attentive and involved in perception. Its amplitude is strongly dependent upon pre-attentive perception of sound and also dependent on onset of sound and its loudness.
2.1 Clinical significance of N100
Cognitive and mental imparements are associated with N100 response.
Schizophrenia – N100 with paired clicks reduced
Tinnitus – small amplitude N100(less distressed) Large amplitude N100( more distressed)
Migraine – Increase in N100 with high intensity stimulation
Headache – more reactive N100
3 Mismatch Negativity
MMN is component of ERP to odd stimulus in a sequence of stimulus. MMN is evoked by deviant stimulus (changing frequency, intensity, duration of stimulus). MMN is elicited regardless of patient attention.
Fig4: Deviant stimulus
Latency of MMN determines the timing of behavioral responses to changes in auditory environment. MMN is component of chain of events in brain processing of changing auditory environmental response.
Various studies are reported on MMN impairments in
- Alzheimer’s disease
Fig5: MMN wave
Fig6: MMN wave in cognitively impaired vs cognitively unimpaired
Note:I dont own copyright of pictures.