Hi,
I like the discussion about the association between neuronal activation
and blood flow changes.
The qustion of association between neural activity and blood flow
is probably one of the most puzzling issues of fMRI. Several points
should be regarded before claiming close association between neuronal
activation and blood flow (or BOLD) changes, especially in humans.
1. The magnetic field strength is important. The standard 1.5-T can
hardly disentangle blood flow changes in capillaries and veins, and the
BOLD
signal is probably more sensitive on deoxy/oxyHb changes on the venous
side for this field
strength. A comparison of the early "negative dip" of BOLD signal
(corresponding
to increased deoxyHb concentration) with electrical events may be much
fruitful than a comparison of
electrical changes with positive BOLD because the negative dip is a
short-latency response
with a much focused topography compared to subsequent BOLD increase (e.g
Duong et al., Nature Neurosci. 2000). However, negative dip has been
observed even with 1.5
Tesla magnet (Yacoub et al., Magn. Res. Med., 1999)
2. The time scale is important. Most of the studies which compare the
cortical electrical activity (evoked potentials) and BOLD changes try to
correlate events
which are on millisecond scale with events having much longer time
constant. However, the
electrical responses of the cortex comprise a cascade of changes and
components of different latencies, and
it is not clear which of these would best correlate with BOLD signal.
Cheer,
Andrej Stancak
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