[Eeglablist] p300 convention

[Achim, André]

I do not know about actual evidence, but consider that the intracellular contents of neurons are isolated from the extracellular space (by the neuron membranes) and normally maintain a difference of some -70 mV with respect to extracellular space.  Neurons fire when this level drops to about -50 mV (mathematically, this is increasing the negative value toward zero; I do not remember the exact levels, but that does not matter in the reasoning). Inhibition can be accomplished by increasing the gradient (say to -75 mV) away from the firing threshold and facilitation (e.g. due to selective attention) by lowering the gradient toward the threshold. A preparation to respond, as in a CNV situation, may be interpreted as partial depolarization, so that incoming excitatory action potentials may more readily bring the temporary resting level to the firing threshold. To bring the resting potential of a population of neurons from -70 to, say, -65 mV requires either bringing negative charges (ions) from inside to outside or positive charges from outside to inside the cells, making the extracellular space more negative overall. Our scalp electrodes are "insulated" from the intra-neuronal space by the neuron membranes but are in relatively good contact with the extracellular space. Consequently, pre-depolarizing a population of neurons drives the nearby extracellular space negatively while hyperpolarizing it brings the nearby extracellular space more positive, which is what the surface electrode recordings express. This reasoning applies at least to relatively tonic physiological adaptation. I am not sure whether it completely extends to phasic processes, like those underlying the P300.

 

In support of the above interpretation, in Birbaumer's lab, they train epileptic patients to be aware of the (general) depolarization level of their brain (responsible for the "aura") and to acquire some control over it. By shifting their scalp polarization level positively, they prevent the seizure announced by the aura. I remember prof. Birbaumer mentioning that a ( non epileptic) colleague of his, who trained on DC shift control, became so good at it that he fell off his chair during a particularly successful negative shift. He had induced an epileptic seizure.

 

André Achim

Département de Psychologie

Université du Québec à Montréal

 

 

De : eeglablist-bounces at sccn.ucsd.edu [mailto:eeglablist-bounces at sccn.ucsd.edu] De la part de Teresa Hawkes
Envoyé : 17 juin 2011 16:04
À : Geert van Boxtel
Cc : eeglablist at sccn.ucsd.edu; Steve Luck
Objet : Re: [Eeglablist] p300 convention

 

Dear List,

 

I was under the impression that positive deflections reflect dipoles moving toward an electrode and negative deflections reflect dipoles moving away from an electrode. I would be interested in seeing the evidence correlating negative potentials with cortical activation and positive potentials with cortical inhibition.

 

Cheers,

Teresa Hawkes

University of Oregon

On Thu, Jun 16, 2011 at 6:55 PM, Geert van Boxtel <G.J.M.vBoxtel at uvt.nl> wrote:

Dear list,

The historical "accidents" that Steve describes actually had a sound basis in neurophysiology in that negativity measured at the scalp was supposed to reflect cortical activation, whereas positivity measured at the scalp was thought to reflect cortical inhibition, both at the level of postsynaptic potentials (EPSPs and IPSPs, respectively). Much research in the 1960s focused on three types of ERPs: the negative Contingent Negative Variation (CNV) and Readiness Potential (RP), and the positive P300. For the CNV and RP, an increase in negativity meant an increase in cortical activity, so it made perfect sense to plot these potentials negative-up.

Nowadays, the convention originating from physics to plot positivity upwards is becoming more and more current. Of course, polarity depends on the reference used in the recording, so it should be considered good science to be able to think both negative-up and positive-up.

Best regards,
Geert



Op 16-6-2011 21:42, Steve Luck schreef: 

Mehmet-

 

The polarity conventions in ERP research are arbitrary and partially reflect historical accidents.  You can plot negative upward if you mainly want to communicate your results to people who are used to looking at ERPs with negative upward.  You should plot positive upward if you mainly want to communicate your results to the 99.999999% of scientists who use the Cartesian convention of plotting positive upward.

 

Steve

 





 

From: mehmet ali <theretinaguy at gmail.com>

Date: June 16, 2011 5:29:38 AM PDT

To: eeglablist at sccn.ucsd.edu

Subject: [Eeglablist] p300 convention



Hi all,

p300 is conventionally plotted on an inverted axis( positive potentials to down), is there a reason to end up with this convention?

Best,




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-- 
Teresa D. Hawkes, B.F.A.
Ph.D. Candidate
Department of Human Physiology
University of Oregon
348 Gerlinger Hall,
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Phone: 541-337-9443 (cell) or 541-346-0275 (laboratory)
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