Add the parameter R to the LIF model (#224)

Add the parameter R to the LIF model

Author: c-xy17

brainpy/dyn/neurons/reduced_models.py

@@ -31,7 +31,7 @@ class LIF(NeuGroup):
 
   .. math::
 
-      \tau \frac{dV}{dt} = - (V(t) - V_{rest}) + I(t) \\
+      \tau \frac{dV}{dt} = - (V(t) - V_{rest}) + RI(t) \\
       \text{after} \quad V(t) \gt V_{th}, V(t) = V_{reset} \quad
       \text{last} \quad \tau_{ref} \quad  \text{ms}
 
@@ -56,6 +56,8 @@ class LIF(NeuGroup):
     Reset potential after spike.
   V_th: float, JaxArray, ndarray, Initializer, callable
     Threshold potential of spike.
+  R: float, JaxArray, ndarray, Initializer, callable
+    Membrane resistance.
   tau: float, JaxArray, ndarray, Initializer, callable
     Membrane time constant.
   tau_ref: float, JaxArray, ndarray, Initializer, callable
@@ -84,6 +86,7 @@ def __init__(
       V_rest: Union[float, Tensor, Initializer, Callable] = 0.,
       V_reset: Union[float, Tensor, Initializer, Callable] = -5.,
       V_th: Union[float, Tensor, Initializer, Callable] = 20.,
+      R: Union[float, Tensor, Initializer, Callable] = 1.,
       tau: Union[float, Tensor, Initializer, Callable] = 10.,
       tau_ref: Union[float, Tensor, Initializer, Callable] = 1.,
       V_initializer: Union[Initializer, Callable, Tensor] = ZeroInit(),
@@ -103,6 +106,7 @@ def __init__(
     self.V_rest = init_param(V_rest, size, allow_none=False)
     self.V_reset = init_param(V_reset, size, allow_none=False)
     self.V_th = init_param(V_th, size, allow_none=False)
+    self.R = init_param(R, size, allow_none=False)
     self.tau = init_param(tau, size, allow_none=False)
     self.tau_ref = init_param(tau_ref, size, allow_none=False)
     if noise_type not in ['func', 'value']:
@@ -121,7 +125,7 @@ def __init__(
     self.refractory = bm.Variable(bm.zeros(size, dtype=bool))
 
     # integral
-    f = lambda V, t, I_ext: (-V + self.V_rest + I_ext) / self.tau
+    f = lambda V, t, I_ext: (-V + self.V_rest + self.R * I_ext) / self.tau
     if self.noise is not None:
       g = noise if (noise_type == 'func') else (lambda V, t, I_ext: self.noise / bm.sqrt(self.tau))
       self.integral = sdeint(method=method, f=f, g=g)