Biomedical Engineering Reference
In-Depth Information
the first event that occurred 200 ms earlier. This suggested that
the processing activity with paradigm III included not only those
induced by the two sequential inputs but more by additional
inputs beyond the two. It is known that MT receives inputs not
only from V1 path but also directly from LGN. The latency of
activation of neuronal population of MT is about 50-90 ms and
is only just few milliseconds longer than V1 on an average
(7)
.
There, one would expect the time delay between the inputs to
MT from the two different channels, the input from LGN being
sooner than the input from V1 channel. The reduction of signal in
the paired presentation beyond the signal by the single presenta-
tion did not occur when the activation at V1 by low contrast pre-
sentation was very weak and the input to MT from V1 path was
expected to be minimal. Therefore, the regular refractory sup-
pression was observed for the straight MT input pair. When such
extra suppression of signal as above is observed with the paired
stimulus presentation, one can suspect that there are multi-inputs
to the site beyond the sequential two inputs. Further experiments
by varying ISI or changing the type of stimulus will elucidate the
timing of these events.
4. Functional
Activation and
Interaction at
Object Processing
Areas
It is known that visual objects are processed along the ventral
visual pathway in the human brain. With fMRI, functional areas
involved in the processing of visual object information have been
identified, but it is still difficult to assess how the neuronal pro-
cessing progresses from lower to higher functional areas. It is
widely accepted that object stimuli of the same or similar attribute
are processed in a specialized functional area or in a network. Dif-
ferent stimuli of the same attribute are represented on different
areas in the early visual cortices and the representation converges
to specific sites for the processing of the common information
in the stimuli. In progressing, the interaction between the inputs
generated by the stimuli of a stimulus pair can be different from
area to area, and the suppression is most pronounced when the
inputs are identical. The identical input cases are well demon-
strated in V1 area. The ISI for suppression and the strength of
the suppression can be also different in those areas on the path.
The extent of the suppression would be also dependent on con-
tents of stimuli in a stimulation pair.
As a simple case, we will show an experiment in which two
stimuli in a stimulation pair have spatial and temporal proximity.
Two rectangular frames were presented in close spatial proximity
with no partial overlap and with ISI (inter-frame) at 0, 50 and
240 ms (
Fig. 11.7
). Each trial consisted of five paired stimuli.
