## Using CSA with Topology layers

This example shows a brute-force way of specifying connections between NEST Topology layers using Connection Set Algebra instead of the built-in connection routines.

Using the CSA requires NEST to be compiled with support for libneurosim. For details, see Djurfeldt M, Davison AP and Eppler JM (2014) Efficient generation of connectivity in neuronal networks from simulator-independent descriptions, Front. Neuroinform. http://dx.doi.org/10.3389/fninf.2014.00043

For a related example, see csa_example.py

This example uses the function GetLeaves, which is deprecated. A deprecation warning is therefore issued. For details about deprecated functions, see documentation.

First, we import all necessary modules.

import nest
import nest.topology as topo

Next, we check for the availability of the CSA Python module. If it does not import, we exit with an error message.

try:
import csa
haveCSA = True
except ImportError:
print("This example requires CSA to be installed in order to run.\n" +
"Please make sure you compiled NEST using\n" +
"  -Dwith-libneurosim=[OFF|ON|</path/to/libneurosim>]\n" +
"and CSA and libneurosim are available from PYTHONPATH.")
import sys
sys.exit()

We define a factory that returns a CSA-style geometry function for the given layer. The function returned will return for each CSA-index the position in space of the given neuron as a 2- or 3-element list.

This function stores a copy of the neuron positions internally, entailing memory overhead.

def geometryFunction(topologyLayer):

positions = topo.GetPosition(nest.GetLeaves(topologyLayer)[0])

def geometry_function(idx):
return positions[idx]

return geometry_function

We create two layers that have 20x20 neurons of type iaf_psc_alpha.

pop1 = topo.CreateLayer({'elements': 'iaf_psc_alpha',
'rows': 20, 'columns': 20})
pop2 = topo.CreateLayer({'elements': 'iaf_psc_alpha',
'rows': 20, 'columns': 20})

For each layer, we create a CSA-style geometry function and a CSA metric based on them.

g1 = geometryFunction(pop1)
g2 = geometryFunction(pop2)
d = csa.euclidMetric2d(g1, g2)

The connection set cs describes a Gaussian connectivity profile with sigma = 0.2 and cutoff at 0.5, and two values (10000.0 and 1.0) used as weight and delay, respectively.

cs = csa.cset(csa.random * (csa.gaussian(0.2, 0.5) * d), 10000.0, 1.0)

We can now connect the populations using the CGConnect function. It takes the IDs of pre- and postsynaptic neurons (pop1 and pop2), the connection set (cs) and a dictionary that maps the parameters weight and delay to positions in the value set associated with the connection set.

pop1_gids = nest.GetLeaves(pop1)[0]
pop2_gids = nest.GetLeaves(pop2)[0]

nest.CGConnect(pop1_gids, pop2_gids, cs, {"weight": 0, "delay": 1})

Finally, we use the PlotTargets function to show all targets in pop2 starting at the center neuron of pop1.

topo.PlotTargets(topo.FindCenterElement(pop1), pop2)