Biomedical Engineering Reference
In-Depth Information
Figure 8
. Time course of activity, synaptic strength (
s
), and effective connectivity (
n
#
s
)
produced by the
s
-model before ("control") and after ("reduced connectivity")
n
was de-
creased by 25% (from 1.2 to 0.9). Activity stops after
n
was decreased, because the effective
connectivity becomes too small to support network activity. This allows
s
to increase beyond
its control level, until
n
#
s
reaches its control level. Activity then reoccurs, although with
larger inter-episode intervals. Modified from Tabak et al. (36).
duration was markedly reduced for the recovered activity (not shown). This is an
important qualitative difference between the
s
- and R-models. This difference in
how the models react to a decrease in connectivity suggests that the episodic
nature of the spontaneous activity in the chick cord is due mostly to synaptic
depression, not cellular adaptation.
In order to understand the recovery of activity, we plotted again the activity
generated by the
s
-model in Figure 8, together with the slow depression variable
(
s
) and the effective connectivity (
n
#
s
). As described previously,
s
and therefore
n
#
s
decrease during the episodes and increase during the inter-episode intervals.
When the connectivity (
n
) is suddenly decreased by 25% (from 1.2 to 0.9), this
causes a corresponding decrease of
n
#
s
, the effective connectivity in the net-
work, i.e., the effective gain of the positive feedback due to excitatory connec-
tions. Activity is therefore blocked until this gain can reach back to its "control"
