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Figure 7 . Recovery of activity after partial block of excitatory connections. ( A ) Results from a
10-day-old chick, showing activity (upper trace) and inter-episode intervals in control and
after blockade of some glutamatergic connections (100 M APV). ( B ) Model results before
("control") and after n was decreased from 1.2 to 0.9 (-25%). The upper trace was obtained
with the s -model. On the interval plot, results from both the s -model (filled diamonds) and R-
model (open triangles) are shown. Modified from Tabak et al. (36).
(compared to the inter-episode intervals) but then recovers, as shown in Figure
7A (1,7). The activity is then thought to depend exclusively on gabaergic and
glycinergic synapses, which have, early in development, an excitatory role. It
must be noted that the recovery is not gradual, as the new inter-episode intervals
are not gradually decreasing to a new level but remain at a fixed level immedi-
ately after the activity has recovered (Figure 7A; note that after recovery the
intervals are larger than in control conditions). We could imagine an additional
process through which the lack of activity is detected and compensated by
slowly increasing network excitability until activity reaches its control level.
However, this slow homeostatic process may cause a progressive—not abrupt—
decrease of inter-episode intervals, unlike the experimental observation.
Surprisingly, the s -model can explain this recovery without introducing any
additional variable into the model. If we decrease the connectivity parameter ( n ),
in order to mimic the blockade of a fraction of the connections, the s -model re-
acts exactly like the chick spinal cord: activity first stops, then recovers with a
fixed inter-episode interval (Figure 7B; (37)). As for the experimental result, the
new inter-episode intervals were larger than before the blockade. In addition,
episode duration was only slightly affected, also in agreement with the experi-
ments. On the other hand, the R-model behaved in a different way: activity re-
covered quickly after the reduction in connectivity and the intervals were then
slightly lower than in "control" (Figure 7B, "R-model"). In addition, episode
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