Saussan, Darren, and All,
>
> 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)?
If by "firing" it is meant post-synaptic firing, then
my understanding from my (rather meager) reading of the literature is that
this is not the case. In particular, it has been demonstrated that
inhibition of spiking in (cerebellar) Purkinje cells by parallel fibers
was associated with an increase in CBF, and so presumably BOLD signal
(Mathiesen, C., K. Caesar, et al. (1998). ÒModification of
activity-dependent increases of cerebral blood flow by excitatory
synaptic activity and spikes in rat cerebellar cortex.Ó Journal of
Physiology 512: 555-566).
> >By the same token, deactivation means decreased stimulation, or possibly
> >increased inhibition.
> >
By above, it would seem that inhibition of soma can lead to increases
in CBF.
> >-Are there any references that i can look , to find an answer for that.
An excellent review (albeit from a few years back) of the coupling of
CBF and neural activity is
Jueptner, M. and C. Weiller (1995). ÒReview: Does measurement of regional
cerebral blood flow reflect synaptic activity?-Implications for PET and
fMRI.Ó Neuroimage 2: 148-156.
>
> ______
>
> 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.
This is perhaps subtle, but I think it is important to realize that
results like that of Mathiesen and colleagues support that inhibition in
region A can lead to CBF increases in region A itself (and not
necessarily only in a region X which projects to region A).
Sincerely,
Eric
> 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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