Mike Worden mworden at neurocog.lrdc.pitt.edu
Sat Dec 6 19:15:49 EST 1997

Neal Prakash (nprakash at rigel.oac.uci.edu) wrote:
: On 6 Dec 1997, Mike Worden wrote:
: |ablab at usa.net wrote:
: |: I've seen there are some physicists out there also. Can someone explain
: |: how Functional Magnetic Resonance (hope it's correct) works? It should
: |: be one of the methods used in the brain function scanning. 
: |
: |...and it is.
: |
: |In a nutshell:
: |Local increases in neural activity lead to
: |local increases in local blood flow (outpacing demand for O2), leads to
:                                       ^^^^^^^^^^^^^^^^^^^^^^^^
: Mike, What do you mean by this? My understanding of the situation now
: (Malonek & Grinvald, 1996; and several others I don't have in front of me
: right now) is that there is an initial increase in O2 demand, as seen
: clearly by imaging spectroscopy studies, and now also by fMRI (with higher
: temporal resolution than most previous work). So the "classic" BOLD signal
: does indeed reflect local decreases in deoxyHg, but there is an initial
: dip in the signal (with the proper temporal resolution) that reflects the
: initial increase in deoxyHg.

Agreed.  The current party line is that there is probably an initial dip 
in signal which is realted to the intial usage of O2 before the 
corresponding increase in blood flow "floods out" the local region.  
"Watering the whole garden for the sake of one thirsty flower," to quote 
Grinvald (don't ya just love that analogy?).  The initial dip has be 
somewhat controversial, I think, in that it is quite difficult to see (at 
least with fMRI).  This signal is both smaller in size and shorter in 
duration the later increase and, so far, seeing it seems to require lots 
of signal averaging, cardiac gating, etc.

: Why do I bring this up??? Because this O2-demand signal is probably a
: better spatial indicator of neuronal activity (i.e. from the capillary 
: beds) that is uncontaminated by blood vessel signals (i.e. arteries and
: veins).

This would seem to be the case, based primarily on the work of Grinvald 
an associates.  I think the usefulness of this particular characteristic 
of the singal probably depends quite a bit on the specific type of 
question which you are trying to ask.  Off hand, I don't know of any 
published study which makes use of this signal specifically (other than 
demonstrations of its existence).  I'm ready to be corrected on this 
point and I'm sure that the story is likely to change.  I think that most 
studies at this point are better off using the more robust singnal 
increase associated with the local blood flow increases.

    __Mike Worden mworden at neurocog.lrdc.pitt.edu
  o/  630 LRDC  University of Pittsburgh 
 <\__,Pittsburgh, PA 15260  412 624-5279
  ">  http://neurocog.lrdc.pitt.edu/~mworden
   `  -climb

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