Figure 4.5-11 illustrates the patterned bursts produced by RINGOSCl.t. Note that the N4 burst is not a full 180° from the N1 burst. This is essentially 4/5 of a five phase oscillator. By adding one RPFM neuron outside the ring, driven from N4, it is possible to realize the missing phase lag. Neurons 1 and 5 become 180° out of phase (Figure 4.5-12). The program is the same as above with the addition of one RPFM neuron and one synapse. (The additional parameters are a5 = 0.25, b5 = 0.25, c5 = 0.3, and phi5 = 1.)
Another ring oscillator is shown in Figure 4.5-13. In this case, the four neurons are modeled more simply with IPFM SGLs, and the synapses are represented by single time-constant low-pass filters generating psps. This alpha model is used to test the hypothesis that a ring oscillator must have negative feedback to produce stable bursts. It is instructive to examine the equivalent loop gain of the four-neuron system. Each synaptic ballistic filter (including the inhibitory one) is of the single time-constant form:
Laplace transforming, yields the transfer function:
FIGURE 4.5-11 Steady-state bursting activity around the four-neuron ring oscillator. Neu rons from the bottom: Nj, N. listing in text.
2, N3, and N4 at the top. Time in ms. Parameter used in program
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