ERP help

[toby jones]

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Hey. Not sure where to post this...

So... A P wave is a `positive' wave and an N wave is a `negative' wave (just stop me where I go wrong). And a P wave shows dendrites are being active (more likely to result in action potentials) and an N wave shows dendrites are being inactive (less likely to result in action potentials).

So a P100 looks pretty interesting because STUFF SEEMS TO BE BEING DONE. And then when there is an N300 with the SAME PATTERN in SIMILAR AREAS except it is an N wave instead of a P wave then...

People get all excited.

So... Why do we care about N waves again? Because inactivity in some areas might result in greater activity in others? If so then how come we don't just focus on which areas are active rather than which areas are inactive.

If I look at whats not going on at N100 (On the other side of the brain) then there seems to be a mirror image of that (but positive) round P300.

Is it that the brains metabolism is limited so there can't be a P wave everyplace at the same time (so the N300 is due to activation in linguistic / object processing in the other hemisphere)? Or... Could the later N wave be due to the refractory period? (does that show up on EEG's or is my timing way out)?

Little help.

Ta.

 

[toby jones]

maybe i should say that i'm not trying to be smart or anything. i'm probably being stupid. i really don't know very much...

it is just that i need to write something on `re-entrant processes' of which the N300 is supposed to be one and i don't understand how it is an instance of a `re-entrant process' because.... not much is being done...

but maybe an N wave signifies something more than the lack of activity in dendrites? i don't know...

Is my thought that the more of an N wave the less activity that is going on simply false?

(I am trying to find out by the way but finding it hard to find something comprehensible)

 

[masterofmonkeys]

it has been years and years since I was a neurosci major, but I will give it my best shot. I might be completely wrong however.

First off, N300 is not 'less activity'. There is baseline, there is positive deflection and there is negative deflection. A positive deflection indicates excitation. A negative deflection indicates inhibition. N300 would thus be an active property rather than a passive one. The postsynaptic nerve is being forcibly MADE less responsive.

As far as re-entrant processes go, here's the basic idea.

You present a stimulus. Then you measure latency. Since you are STIMULATING the brain you would expect an early p-wave in a certain area of the brain. This would be your P100. Since we have a P100, we know that the stimulus caused this brain region to be activated more strongly than baseline. Now, all of a sudden (well all of 200 msec later), that same area is now LESS active than baseline. This is the N300.

The argument goes that since this area has already responded to the stimulus positively, something else must be modulating it in a negative fashion. Something internal. A higher level processing/filtering structure. I.e. feedback. So people get excited about N300 because a higher-level structure is changing the way this lower level structure will react to continued input.

A practical analogy might be you're half-consciously reading Kaplan and Sadock near your window when a guy starts jack hammering the sidewalk apart. Your auditory cortex immediately is stimulated and relays this ifnormation to the attentional centers of your brain (P-wave) and you jolt upright, realizing you can't remember anything of hte last 50 pages you read. The jack hammering continues, but it doesn't keep jolting you, because your attentional centers have already processed this information and decided that since it's stable you don't need to pay any more attention to it, so it tells your auditory cortex to calm the heck down (N300).

I really hope that made sense.

From what I can remember, ADHD kids and meth psychotics can show decreased amplitude of N-waves in various regions of the brain. The implication being that they have a reduced ability to tune out external stimuli.

In contrast, schizophrenics can have markedly reduced P-wave amplitude in auditory areas of the brain, implying they have trouble processing external auditory stimuli.

I hope that helped.