22.interacting populations and continuum models
network of networks and continuum network
interacting populations
Every heterogeneous network could be viewed as a combination of different homogeneous network, or the interaction among different populations. The simplest case is the balanced random network: with excitatory and inhibitory neurons.
Let’s look at a fully connected network with excitatory and inhibitory neurons. Excitatory neurons belong to population $n$ and inhibitory neurons belong to population $m$.
So the activity of population $n$ or population $m$ is:
$$ A_n(t)=\frac{1}{N_n}\sum_{j\in \Gamma_n}\sum\delta(t-t_j^{(f)}) $$
So for each single neuron in population $m$, which receives the inputs from all neurons in population $n$, the membrane potential is changing accordingly:
$$
\begin{align}
h_i(t|\hat{t_i}) & = \sum_j\sum_fw_{ij}\eta(t-\hat{t_i},t-t_j^{(f)})\
& = \sum_mJ_{nm}\int_0^{\infty}\eta(t-\hat{t_i},s)\sum_{j\in \Gamma_m}\sum_f\frac{\delta(t-t_j^{f}-s)}{N_m}\
& = \sum_mJ_{nm}\int_0^{\infty}\eta(t-\hat{t},s)A_m(t-s)ds
\end{align} $$
That is just an example population with SRM0 neuron model. For a general population, the population activity evolution could be rewrite as:
$$ A_n(t) = \int_{-\infty}^t P_n(t|\hat{t})A_n(\hat{t})d\hat{t} $$
$P$ is the interval distribution, it could also be viewed as a kernel for population activity, which is determined by the post-synaptic potential (PSP).
Stationary states and gain function of interacting populations
When the network reaches stationary state, the activity is stable, so the inputs from one sub-population ($N$ for example) to another sub-population is more or less constant.
So the inputs could be viewed as $I_m$ (to population $M$), and the activity of this population is: $$ A_m=g_m(I_m), $$
with
$$ I_m = \sum_nJ_{mn}A_n, $$
we have
$$ A_m = g_m(\sum_n J_{mn}A_n) $$
spatial continuum network
Features: the auditory cortex is a good example, with neurons organized along an axis with continuously changing preferred frequency.
So the principle of analyzing this network is \emph{binning} the cortex with a binsize $d$. Each sub-population could be labeled according to its spatial location. For instance, population $N$ is the neurons between $[nd,(n+1)d]$, and in each bin, the neuron number is $N_n=\rho d$, where $\rho$ is the spatial density.
Then this network is a special case of interacting populations, with $d$ approaching zero.
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Current Ref:
- snm/22.interacting-populations-and-continuum-models.md