[Eeglablist] p300 convention

Tue Jun 21 14:50:23 PDT 2011

Dear all,

Please take care to separate two distinct issues:

1) Convention in plotting positive voltages upwards or downwards in the EEG/ERP
community -- I think previous contributes to this thread have tackled this one.

2) Physiological meaning of positive vs negative voltages as measured on the
scalp -- short answer, they reflect input to cortical pyramidal cells but we
cannot distinguish if it is produced by inhibitory or excitatory synaptic activity.

Long(er) answer:
When measuring post-synaptic membrane potentials, positive is inhibitory and
negative is excitatory, but this is only true in the extracellular proximity of
the synaptic gap. If we measure voltage further away from the synaptic gap then
this relation no longer holds due to the extracellular current flows, and these
currents can be represented by a dipole. For excitatory synapses we get a
negative pole at the synapse and a positive pole elsewhere; for inhibitory
synapses we get a positive pole at the synapse and a positive pole elsewhere.

Because of this, the polarity of the voltage measured at the EEG sensors on the
scalp will depend not only on whether there are mostly inhibitory or excitatory
synapses being activated (as EEG activity is a sum of a high number of
extracellular field potentials), but also on their position relative to the
sensors (let's leave electrode reference out of the picture for simplicity):
superficial excitatory and deep inhibitory synapses will contribute to negative
scalp voltage, and deep excitatory and superficial inhibitory synapses will
contribute to positive scalp voltages.

Add volume conduction to the picture and it becomes clear why the physiological
meaning of EEG measures is not clear at all. For example, the N170
face-sensitive ERP component consists of a bilateral parieto-occipital
negativity accompanied by a positivity at the vertex, which is very likely
simply the projection of the electrical activity of the same cortical sources --
again, this means that the same physiological event produces bot positive and
negative voltages, depending on where you are looking.

Longest answer -- see Niedermeyer & Lopes da Silva's (2005) book
"Encephalography" for a detailed account.

I believe this is the standard take on the physiology of EEG and hope it
clarifies a few questions raised in this thread.

Best,
Fernando Ferreira-Santos

> 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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> --------------------------------------------------------------------
>
> Steven J. Luck, Ph.D.
>
> Director, Center for Mind & Brain
>
> Professor, Department of Psychology
>
> University of California, Davis
>
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>
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>
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>
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> --
> Teresa D. Hawkes, B.F.A.
> Ph.D. Candidate
> Department of Human Physiology
> University of Oregon
> 348 Gerlinger Hall,
> 1240 University of Oregon
> Eugene, Oregon 97403-1240
> Phone: 541-337-9443 (cell) or 541-346-0275 (laboratory)
> thawkes at uoregon.edu
>
>
>
>

-- 
Fernando Ferreira-Santos

Laboratory of Neuropsychophysiology
Faculty of Psychology and Education Sciences
University of Porto
Rua Dr. Manuel Pereira da Silva
4200-392 Porto (Portugal)
Tel. +351 226079700 (ext. 301)
Site: http://www.fpce.up.pt/labpsi/
Email: frsantos at fpce.up.pt

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