Writing synapse models

NEST has a very flexible system to allow users to write their own synapse types. Synapses in NEST can either have static parameters or apply some dynamics on them. Each connection needs to have at least the following parameters:

• The connection delay

• The connection weight

• The target node of the connection

• The receiver port, which identifies the connection on the postsynaptic side

The source node of a connection is implicitly stored by the position in the data structure that is used internally. These parameters are implemented in the StaticConnection synapse type, which can be used as a base class for more advanced synapse types.

Writing a synapse type is basically very simple. You can directly derive your new connection type from StaticConnection, which provides all mechanisms to register and send a connection. A synapse type consist of two files, a header and an implementation. Skeletons for both of them are shown shown in the following listings:

NESTSRC/models/my_synapse.h

#ifndef MY_SYNAPSE
#define MY_SYNAPSE

#include "static_synapse.h"
#include "generic_connector.h"

namespace nest
{
  class my_synapse : public static_synapse
  {
    public:
      my_synapse () {}
      my_synapse (const my_synapse &) {}
      ~my_synapse () {}

      update_dynamics ();
      void send (Event & e, double_t t_lastspike, const CommonSynapseProperties & cp);
  };

  inline void my_synapse::send (Event & e, double_t t_lastspike, const CommonSynapseProperties &)
  {
    update_dynamics();

    e.set_receiver(*target_);
    e.set_weight(weight_);
    e.set_delay(delay_);
    e.set_rport(rport_);
    e();
  }
} // namespace nest

#endif /* #ifndef MY_SYNAPSE */

The first thing we do is include the header files of our base class, StaticConnection. It already defines funtions for registering the connection with the ConnectionManager of NEST, for storing the mandatory parameters weight and delay and functions to set and retrieve these parameters from within the SLI interpreter.

NESTSRC/models/my_connection.cpp

#include "my_synapse.h"

void nest::my_synapse::update_dynamics ()
{
  /* Do fancy stuff with weights here! */
}

To apply some (activity dependent) dynamics on the weight of the connection you simply have to override the method send(). It is the one that is called each time an event flows over the connection. Except for the call to update_dynamics() in which the synaptic weight is calculated, the function MyConnection::send() is a copy of the implementation from StaticConnection. It fills in the rest of the parameters of the event and sends the event to the target.

Registering the new synapse type

After your files are written, you have to add their names to the CMakeLists.txt file in NESTSRC/models to have it be compiled and linked to NEST.

To make the synapse type available inside of NEST scripts, you have to include and register it with the module. Add the following line to the beginning of NESTSRC/models/modelmodule.cpp:

#include "my_synapse.h"

And the following line to the end of the file:

register_connection_model< my_synapse >( "my_synapse" );