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
-----------------------------------------------------------
