Biomedical Engineering Reference
In-Depth Information
Visual information delivered to the primary visual cortex (V1)
is passed onto the dorsal and/or the ventral pathways according
to the content of the information. Contents related to the shape
are processed on the ventral path and those related to motion
or to location are on the dorsal path. The neuronal processing
and the interaction among related areas proceed very fast. In the
next section, we will show that such fast neuronal processing can
be followed in some cases by a paired stimulus paradigm which
allows an fMRI signal to respond in amplitude change rather than
temporal change to these fast processes.
2. An fMRI
Technique for
Detecting Some
Fast Neuronal
Activity - A Paired
Stimulus
Paradigm
Information processing in the brain proceeds while many func-
tional units deliver the information to each other or make feed-
back and modulate the activity of others. These processes occur
at a fast temporal scale. It often happens that such activity-
modulating interaction is suppressive. The interaction process can
be probed by measuring the corresponding fMRI response if
we can control the suppression by an external factor. This can
be achieved by varying temporal relation between two stimulus
inputs (paired stimulus paradigm). In this section, we will show,
in experiments with the paired stimulus paradigm, the linkage of
fMRI signal to fast neuronal interaction in the rat somatosensory
cortex and also in the human visual cortex.
When one of the front paws of a rat was stimulated electrically,
the response of SEP (somatosensory evoked potential) was strong
in the contralateral area but was weak, if any, in the ipsilateral area
(
Fig. 11.1(a)
). The corresponding responses of fMRI were also
strong in the contralateral area and weak in the ipsilateral area
with a different response shape (
Fig. 11.1(b)
). When one paw
was stimulated and then the other side with a time delay, strong
suppression of SEP at the contralateral area of the side of the sec-
ond paw stimulation was observed. The suppression appeared at
40 ms inter stimulus interval (ISI) and the corresponding fMRI
response was suppressed too (
Fig. 11.2
). This indicates that fast
progressing of neuronal activities can be detected in a time scale of
40 ms as in this case. Furthermore, this suggests that the response
to the second stimulation can be affected by the neuronal activa-
tion occurring earlier at a different site. Such event is an example
of an interaction between functional units.
In an experiment to examine the suppressive response in the
human visual area, two successive visual stimuli of the same kind
were given with time delays (ISI) of about 100-1000 ms. The
response to the second input at 200 ms ISI was almost completely
suppressed and recovered at 1000 ms (
Fig. 11.3
). The state that
