I agree with Russ that this is a loaded theoretical issue.
One thing to keep in mind is that the term deactivation implies that there is some stable baseline that signal is either above (activation) or below (deactivation). BOLD however is a RELATIVE signal--its simply the net difference between conditions and a 'deactivation' could just as easily be interpreted as an activation to the comparison task. There is no absolute baseline with BOLD despite us having names for comparison conditions like "rest" or cross-hair fixation or "null events", etc.
my 2cents
best wishes,
Sterling
>>> Russell Poldrack <[log in to unmask]> 8/31/2006 1:01 PM >>>
Using the term "deactivation" to describe negative signal changes in
fMRI is theoretically loaded, because it implies that the area has
been somehow functionally "turned off." However, all we can conclude
from decreased signal in fMRI is that there is a reduction in net
synaptic activity. It is clear from a range of work that fMRI signals
can be dissociated from neural firing; both glutamatergic and
GABAergic synaptic activity appear to increase blood flow (see the
work by Martin Lauritzen) , and some recent work (Gsell et al., J.
Neuroscience, 2006) also shows that blockade of either AMPA or NMDA
glutamate receptors reduces imaging signals, whereas only blockade of
AMPA substantially reduced evoked potentials.
There are lots of ways in which negative signal change might be
produced which could have very different interpretations. Jiansong
mentioned two of them (engagement of default mode or resting state
processes, and top-down suppression). In addition, negative signal
change may occur downstream from sites of synaptic inhibition; for
example, Hershey et al (2003, Neurology) showed that stimulation in
the STN resulted in increased signal in GP (due to excitation from
STN) and thalamus (due to inhibition from GP) but decreased signal in
cortex (due to decreased excitation from thalamus).
cheers
russ
On Aug 31, 2006, at 8:36 AM, Zhang Zhiqiang wrote:
> But In the 1st chapter of the book "Functional MRI" (edited by
> C.T.W Moonen et al, Springer Press2000), the author said the BOLD
> activation is the the result of neurophysiology processing, either
> excitatory or inhihibitory synaptic activity can result in BOLD
> activation, not deactivation.
> " It is known that the afferents from the ipsilateral ear are
> excitatory while the afferents from the contralateral ear are
> inhibitory. stimulation, however, led to increase dexo-glucose
> uptake n both, in the ipsilateral as well as in the contralateral
> auditory system, ...it seems likely that these inhibitory and
> excitatory synaptic events also produce a blood flow change in the
> same direction." page 5
>
>
> Jiansong Xu <[log in to unmask]> **
> There are a lot of publications discussing “deactivation” now. The
> most interesting ones are “default brain areas” concept. Another
> major area of deactivation is “top-down attentional control”.
>
>
> From: Zhang Zhiqiang <[log in to unmask]>
> Reply-To: Zhang Zhiqiang <[log in to unmask]>
> Date: Thu, 31 Aug 2006 17:26:42 +0800
> To: <[log in to unmask]>
> Subject: [SPM] How to understand the "deactivation" of SPM in
> neurophysiology
>
> Hi Dear SPMer:
>
> In the spm conditions contrast using t-test, for example, I
> specified the "1" is hand movement task, the "0" (or -1)is the
> resting control state, then the activation of result will appear
> in M1 etc. area, and if I use "-1 0"(or -1 1), there will be
> "deactivation" in some brain area. Though I know it is owing to the
> difference of task order when analyzed, however, how to
> understand the "deactivation" in neurophysiology, it's the neuron
> whose function is to depress the hand movement?
>
>
>
> Appreciating any reply of yours!
>
>
>
> Zhang Zhi-qiang
>
> Nanjing Jinling hospital
>
> Medical School, Nanjing University, P.R. China.
>
> Mp3**-****** <http://music.yahoo.com.cn/?
> source=mail_mailbox_footer>
>
> ******-3.5G***20M***
>
>
> *****-3.5G***20M**
---
Russell A. Poldrack, Ph.d.
Associate Professor
Wendell Jeffrey and Bernice Wenzel Term Chair in Behavioral Neuroscience
UCLA Department of Psychology
Franz Hall, Box 951563
Los Angeles, CA 90095-1563
phone: 310-794-1224
fax: 310-206-5895
email: [log in to unmask]
web: www.poldracklab.org
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