Dear SPMers,
i aked Darren the following question, and decided to put the questions for
all.
>We know that the BOLD signal reflects hemodynamic changes as a result of
>neural activity changes.
>My questions are
>-Does positive activation necessarily mean increased stimulation (more
>firing)?
>By the same token, deactivation means decreased stimulation, or possibly
>increased inhibition.
>
>-Are there any references that i can look , to find an answer for that.
>
______
It is something we
have discussed and struggled with among our group at Northwestern
quite a bit.
For example sometimes you see curves that look like this
*** *** x
x * * xx
x * * * x
x x
x x
xx x
x
The point is that the asterisks and + curves are clearly separate
(and for arguments sake we'll say significantly different. Thus the
contrast 1 (*) vs.
-1 (x) would show "activation" of the areas corresponding to the *
task. BUT as you can see both curves go down meaning a decrease in
BOLD signal.
Physiologically does this decrease mean a reduction in neuronal
firing? Increased inhibition by the way might look at an activation
of the inhibiting area and then presumably reduced activation at the
site affected by the inhibition.
BOLD signal reflects change in local deoxyhemoglobin concentrations.
Are there other influences on this signal? Is it possible that blood
flow becomes uncoupled from neuronal firing even in the normal
individual during certain types of tasks? For example given that some
amount of vascular perfusion to an area appears to be a luxury what
if neuronal firing increased but other influences on local
hemodynamics such as acetylcholine decreased- would we see a decline
in signal despite the fact that neuronal firing had increased
somewhat.
Anyway- a very interesting question. I would encourage you to post
this on the SPM list and you can include my response, such as it is,
if you want. If you do gain more insight into this I'd like to know.
Darren
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