okay, I'll bite. Anyone for Mag Stim?

Tristan Davies tbd at neuro
Mon Aug 5 18:43:26 EST 1991

In article <1991Aug5.134013.19105 at njitgw.njit.edu> jfa0522 at hertz.njit.edu (john f andrews ece) writes:
>I have become involved in an experimental project involving transcranial
>magnetic stimulation of the motor cortex.
>Naturally, this is a frontier area, and is not well understood as of yet. 
>Has anyone any information regarding this?
>FYI, a high-field (2T) biphasic
>magnetic pulse is sent through a coil which has been postioned on the scalp,
>over the motor cortex. The coil design is such that the field gradient
>is high, with a theoretical field focal point a few centimeters beneath
>the scalp. The focal "point" is less than a square cm, and may be 
>even sharper (depending on your definition of focus, the threshold for
>stimulation, and the true-ness of the theoretical field mapping).
>It is supposed that the field is at the very least concentrated within
>a cortical column.

I think that "cortical columns," whatever they are, are smaller than
a square cm.  Ocular dominance columns in visual cortex, which is as
close as anyone's come to defining a cortical column I think, are on the
order of 1 mm in width.  If you choose to define a "column" as an
orientation-selective piece of brain, then the column gets even smaller,
maybe even 0.1 mm square.  Tread carefully on such thin ice...

Maybe columns would be an interesting subject to discuss in more depth in 
this group, although it _is_ a fairly common subject for debate, at least
around my department.

>The changing field induces an electric current in the cortex, regional to the
>field gradient at pulse time. In theory then, this is electrical stim without
>the need to burn a path from the scalp down to the area of the cortex to be
>stimulated. The current densities are comparable to those recorded with 
>selectrical stimulation. The effects are also similar, i.e. motor response
>elicited through selective cortical stimulation.

This *is* a clever idea, seeing as how it seems to stimulate neurons to fire.
It might even have some clinical possibilities...how about detecting and
thwarting seizures???

In the world of experimental neuroscience, though, I can think of a problem.
If you want to understand in any detail how the firing of neurons generates
a behavior, I think it's necessary to know in detail where the site of
stimulation was.  Recording electrodes leave tracks, which can help you
find the recording site, and if you record through a pipette, you can then
eject markers (e.g. HRP) to mark the recording site with a level of resolution
and certainty that don't seem to be possible with transcranial stimulation.

>Is it "safe"? Is it useful? It is (is it?) non-invasive... 

Given the recent evidence linking electromagnetic fields to some diseases,
I wonder (as you do) how non-invasive this really is...
Of course, X-rays cause cancer, so I guess nothing's perfect.

>It has been around a few years, there are a few companies building stimulators,
>it has FDA approval for peripheral stimulation, and several researchers have
>been doing transcranial stuff for a few years (mainly NIH researchers).
>							-=john

Regardless, it sure is nice to see some life in this old group!

Tristan Davies

e-mail: tbd at neuro.duke.edu
Department of Neurobiology, Duke University Medical Center

"grblb blabt unt mipt speeb!! oot piffoo blaboo..." -- Opus

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