Before we describe the model, we present the neuroscience framework of the problem.
1.1. The Neuroscience Framework
The activity of one neuron is described by the evolution of its membrane potential. This evolution presents from time to time a brief and high-amplitude depolarization called an action potential or spike. The spiking probability or rate of a given neuron depends on the value of its membrane potential. These spikes are the only perturbations of the membrane potential that can be transmitted from one neuron to another through chemical synapses. When a neuron i spikes, its membrane potential is reset to 0 while the so-called “post-synaptic neurons” influenced by neuron i receive an additional amount of membrane potential.
From a probabilistic point of view, this activity can be described by a simple point process since the whole activity is characterized by the jump times. In the literature, Hawkes processes are often used to describe systems of interacting neurons, see [
1,
2,
3,
4,
5,
6] for example. The reset to 0 of the spiking neuron provides a variable length memory for the dynamic and therefore point processes describing these systems are non-Markovian.
On the other hand, it is possible to describe the activity of the network with a process modeling not only the jump times but the whole evolution of the membrane potential of each neuron. This evolution needs then to be specified between the jumps. In [
7] the process describing this evolution follows a deterministic drift between the jumps, more precisely the membrane potential of each neuron is attracted with exponential speed towards an equilibrium potential. This process is then Markovian and belongs to the family of Piecewise Deterministic Markov Processes introduced by Davis ([
8,
9]). Such processes are widely used in probability modeling of e.g., biological or chemical phenomena (see e.g., [
10] or [
11], see [
12] for an overview). The point of view we adopt here is close to this framework, but we work without drift between the jumps. We therefore consider a pure jump Markov process and will make use of the abbreviation PJMP in the rest of the present work.
We consider a process where N is the number of neurons in the network and where for each neuron and each time each variable represents the membrane potential of neuron i at time . Each membrane potential takes value in . A neuron with membrane potential x “spikes” with intensity where is a given intensity function. When a neuron i fires, its membrane potential is reset to interpreted as resting potential, while the membrane potential of any post-synaptic neuron j is increased by . Between two jumps of the system, the membrane potential of each neuron is constant.
Working with Hawkes processes allows consideration of systems with infinitely many neurons, as in [
1] or [
6]. For our purpose, we need to work in a Markovian framework and therefore our process represents the membrane potentials of the neurons, considering a finite number
N of neurons.
1.2. The Model
Let
be fixed and
be a family of
i.i.d. Poisson random measures on
with intensity measure
. We study the Markov process
taking values in
and solving, for
, for
,
In the above equation for each is the synaptic weight describing the influence of neuron j on neuron . Finally, the function is the intensity function.
This can be seen in the following way. The first term is the starting point of the process at time . The second term corresponds to the reset to 0 of neuron i when it spikes. The point process gives the times where a spike of neuron i can occur which actually happens with rate leading to a reset to 0 of neuron i due to the term . The third term corresponds to the modification of membrane potential of post-synaptic neurons influenced by neuron The modification value is given by the synaptic weight provided the new value is smaller than the maximum potential which is ensured by the second indicatrix function.
The generator of the process
X is given for any test function
and
is described by
where
for some
. With this definition the process remains inside the compact set
Furthermore, we also assume the following conditions about the intensity function:
for some strictly positive constants
c and
.
The probability for a neuron to spike grows with its membrane potential so it is natural to think of the function
as an increasing function. Condition (
5) implies that this growth is at least linear, and models the spontaneous activity of the system: whatever the configuration
x is, the system will always have a positive spiking rate.
1.3. Poincaré-Type Inequalities
Our purpose is to show Poincaré-type inequalities for our PJMP, whose dynamic is similar to the model introduced in [
1]. The main difference between our framework and the one of [
1] relies in the fact that as in [
7], we study a process modeling the membrane potential of each neuron instead of a point process focusing only on the spiking events. Focusing on the spike train is sufficient to describe the network activity since it contains all the relevant information. However, the membrane potential integrates each relevant spike that occurred in the past and this gives us a Markovian framework. Regardless of the point of view, the notable difference in dynamics with [
1] is the absence of drift between jumps. We assume here that there is no loss of memory between spikes. Therefore, our conclusions cannot directly apply to the model studied in [
1].
We will investigate Poincaré-type inequalities at first for the semigroup and then for the invariant measure . Concerning the semigroup inequality, we will study two different cases. The first, the general one, where the system starts from any possible initial configuration. Then, we restrict to initial configurations that belong to the domain of the invariant measure.
Let us first describe the general framework and define the Poincaré inequalities on a discrete setting (see also [
13,
14,
15,
16,
17]). At first we should note a convention we will widely use. For a function
f and measure
we will write
for the expectation of the function
f with respect to the measure
that is
We consider a Markov process
which is described by the infinitesimal generator
and the associated Markov semigroup
. For a semigroup and its associated infinitesimal generator we will need the following well know relationships:
(see for example [
18]).
We define
to be the invariant measure for the semigroup
if and only if
Furthermore, we define the “carré du champ” operator (see [
19]) by:
For more details on this important operator and the inequalities that relate to it one can look at [
18,
19,
20]. For the PJMP process defined above with the specific generator
given by (
2) a simple calculation shows that the carré du champ takes the following form.
We say that a measure
satisfies a Poincaré inequality if there exists constant
independent of
f, such that
where the variance of a function
f with respect to a measure
is defined with the usual way as:
. It should be noted that in the case where the measure
is the semigroup
, then the constant
C may depend on
t,
.
For the Poincaré inequality for continuous time Markov chains one can look in [
16,
21]. In [
13,
14,
17], the Poincaré inequality (SG) for
has been shown for some point processes, for a constant that depends on time
t, while the stronger log-Sobolev inequality, has been disproved. The general method used in these papers that will be followed also in the current work, is based on the so-called semigroup method which shows the inequality for the semigroup
.
The main difficulty here is that for the pure jump Markov process that we examine in the current paper, the translation property
used in [
13,
17] does not hold here. This appears to be important because the translation property is a key element in these papers, since it allows to bound the carré du champ by comparing the mean
where the process starts from position
x with the mean
where it starts from
the jump-neighbor of
However, we can still obtain Poincaré-type inequalities, but with a constant
which is a polynomial of order higher than one. This power is higher than the constant
, the optimal obtained in [
17] for a path space of Poisson point processes.
It should be noted that the aforementioned translation property relates with the
criterion (see [
22,
23]) for the Poincaré inequality, which states that if
then the Poincaré inequality is satisfied. A more detailed discussion on this criterion follows later in
Section 2.2. Since this is not satisfied in our case we obtain a Poincaré-type inequality instead.
Before we present the results of the paper it is important to highlight an important distinction on the nature of the initial configuration from which the process can start. We can classify the initial configurations according to the return probability to them. Recall that the membrane potential of every neuron i takes positive values within a compact set and that whenever a neuron j different than i spikes, the neuron i jumps positions up, while the only other movement it does is jumping to zero when it spikes. That means that every variable can jump down only to zero while after the first jump, can only pass from a finite number of possible positions since the state-space is then discrete inside a compact due to the imposed maximum potential Since the neurons stay still between spikes, that implies that there is a finite number of possible configurations to which can return after every neuron has spiked for the first time. This is the domain of the invariant measure of the semigroup , and we will denote it as . Thus, if the initial configuration does not belong to , after the process enters , it will never return to this initial configuration.
It should be noted that it is easy to find initial configurations . For example one can consider any x such that at least one of the s is not a sum of synaptic weights , or any x with for every i and j.
Below we present the Poincaré inequality for the semigroup for general starting configurations.
Theorem 1. Assume the PJMP as described in (2)–(5). Then, for every , the following Poincaré-type inequality holds.with a second order polynomial of the time t that does not depend on the function f and β a constant. One notices that since the coefficient is a polynomial of second order and is a constant, the first term dominates over the second for long time t, as shown in the next corollary.
Corollary 1. Assume the PJMP as described in (2)–(5). Then, for every , and t sufficiently large, i.e., where is a constant depending only on One should notice that although the lower value depends on the function f, the coefficient of the inequality does not depend on the function f.
The proof of the Poincaré inequality for the general initial configuration is presented in
Section 2.
In the special case where x is on the domain of the invariant measure we obtain the stronger inequality.
Theorem 2. Assume the PJMP as described in (2)–(5). Then, there exists a such that for every and for every , the following Poincaré-type inequality holds.with a third order polynomial of the time t that do not depend on the function f. As in the general case, for t large enough we have the following corollary.
Corollary 2. Assume the PJMP as described in (2)–(5). Then, there exists a , such that for every , and t sufficiently large, i.e., where is a constant depending only on , We conclude this section with the Poincaré inequality for the invariant measure presented on the next theorem.
Theorem 3. Assume the PJMP as described in (2)–(5). Then π satisfies a Poincaré inequalityfor some constant .