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>
> References: <50shbq$2vp at vixen.cso.uiuc.edu>
<513v8s$t1t$1 at mhade.production.compuserve.com>
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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.
