eeg recording and generator in sulcus

crosley at tcp.co.uk crosley at tcp.co.uk
Sun Sep 15 11:43:35 EST 1996


In Article<514scf$qsi at vixen.cso.uiuc.edu>, 
<kspencer at s.psych.uiuc.edu> write:
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> From: kspencer at s.psych.uiuc.edu (Kevin Spencer)
> Newsgroups: bionet.neuroscience
> Subject: Re: eeg recording
> Date: 10 Sep 1996 23:07:59 GMT
> Organization: UIUC Department of Psychology
> Lines: 46
> Message-ID: <514scf$qsi at vixen.cso.uiuc.edu>
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> 
> Marcus <73370.2242 at CompuServe.COM> writes:
> 
> >Since the brain has many folds & invaginations, many of the 
> >apical dendrites would not be at right angles to the surface of 
> >the brain.
> 
> That's correct -- the fields coming from a gyrus project radially,
> or perpendicularly to the surface of the scalp.  But the fields 
from
> the bank of a sulcus project tangentially to the scalp surface.
> 
> Now let's say we have a focal source, say an evoked potential.  If 
that
> source is being generated in a gyrus, the field will be maximal on 
the
> scalp directly above the source.  But if the source is in a bank 
of a
> sulcus, the maximum of the field will be some distance away from 
the
> source -- perhaps even in the other hemisphere.  And if both sides 
of
> the sulcus are active, the fields from the two sides will cancel 
each
> other out.
> 
> > Why isn't this a problem in assuming the valididty of 
> >EEG recordings ?
> 
> The EEG is what it is... but the conclusions you draw from it may 
or
> may not be valid.  It wouldn't be valid to conclude that a 
particular
> source is directly under the recording electrode, for instance, 
without
> using other measures.  This is the "inverse problem".  It can be 
approached
> with measures such as current source density, and by dipole 
modeling.
> 
> BTW, magnetoencephalography (MEG) is sensitive to tangential 
sources, not
> radial sources.  So you could get a more complete picture of 
electromagnetic
> fields of the brain by recording EEG and MEG simultaneously.
> 
> > Would local field potentials recorded from one 
> >laminellar layer be a more suitable means for determining the 
> >activity of a brain area and perhaps for investigating the 
> >relationship between say, midbrain structures and the cortex ?
> 
> Yes, I think so...
> 
> Regards,
> Kevin
> -----------------------------------------------------------
> Kevin Spencer
> Cognitive Psychophysiology Laboratory and Beckman Institute
> University of Illinois at Urbana-Champaign
> kspencer at s.psych.uiuc.edu
> -----------------------------------------------------------
     Re potential recorded from source in sulcus.
     The potential measured by a monopolar electrode 
     at a distance from a dipole generator is not only 
     dependent on the equivalent dipole voltage, but 
     also on the solid angle subtended by the dipole 
     at the electrode. The dipole is considered to be
     generated by a cortical layer of pyramidal cells
     discharging synchronously. The solid angle concept 
     is used to demonstrate the difficulty of recording
     the potential generated within a sulcus lying at 
     right angles to the scalp surface.

     See Gloor P. (1985) Neuronal generators and the 
     problem of localization in electroencephalography:
     application of volume conduction theory to 
     electroencephalography. J. Clin. Neurophysiol.,
     2(4): 327-354. John Shaw.




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