# noise_generator – Generate a Gaussian white noise current¶

## Description¶

The noise_generator can be used to inject a Gaussian “white” noise current into a node.

The current is not truly white, but a piecewise constant current with a Gaussian distributed amplitude with mean \(\mu\) and standard deviation \(\sigma\). The current changes at a user-defined interval \(\delta\) and is given by

where \(N_j\) are Gaussian random numbers with unit standard deviation and \(t_0\) is the device onset time.

Additionally a sinusodially modulated term can be added to the standard deviation of the noise:

The effect of the noise current on a neuron depends on the switching interval \(\delta\). For a leaky integrate-and-fire neuron with time constant \(\tau_m\) and capacitance \(C_m\), the variance of the membrane potential is given by

for \(\delta \ll \tau_m\). For details, see the noise generator notebook.

All targets of a noise generator receive different currents, but the currents for all targets change at the same points in time. The interval \(\delta\) between changes must be a multiple of the time step.

Recording the generated current

You can use a multimeter to record the average current sent to all targets for each time step if simulating on a single thread; multiple MPI processes with one thread each also work. In this case, the recording interval of the multimeter should be the equal to the simulation resolution to avoid confusing effects due to offset or drift between the recording times of the multimeter and the switching times of the noise generator. In multi-threaded mode, recording of noise currents is prohibited for technical reasons.

All stimulation devices share the parameters `start`

and `stop`

,
which control the stimulation period. The property `origin`

is a
global offset that shifts the stimulation period. All three values are
set as times in ms.

For spike-emitting devices, only spikes with times t that fulfill start < t <= stop are emitted. Spikes that have timestamp of t = start are not emitted.

For current-emitting devices, the current is activated and deactivated such that the current first affects the target dynamics during the update step (start, start+h], i.e., an effect can be recorded at the earliest at time start+h. The last interval during which the current affects the target’s dynamics is (stop-h, stop].

The property `stimulus_source`

defaults to an empty string. It can
be set to the name of a stimulation backend, in which case it will
take its parameters from the configured backend instead of from the
internally stored values. More details on available backends and their
properties can be found in the guide to stimulating the network.

## Parameters¶

- label
A string (default: “”) specifying an arbitrary textual label for the device. Stimulation backends might use the label to generate device specific identifiers like filenames and such.

- origin
A positive floating point number (default : 0.0) used as the reference time in ms for start and stop.

- start
A positive floating point number (default: 0.0) specifying the activation time in ms, relative to origin.

- stimulus_source
A string (default: “”) specifying the name of the stimulation backend from which to get the data for updating the stimulus parameters of the device. By default the device uses its internally stored parameters for updating the stimulus.

- stop
A floating point number (default: infinity) specifying the deactivation time in ms, relative to origin. The value of stop must be greater than or equal to start.

- mean
The mean value \(\mu\) of the noise current (pA)

- std
The standard deviation \(\sigma\) of the noise current (pA)

- dt
The interval \(\delta\) between changes in current (ms; default: 10 * resolution)

- std_mod
The modulation \(\sigma_{\text{mod}}\) of the standard deviation of the noise current (pA)

- frequency
The frequency of the sine modulation (Hz)

- phase
The phase of sine modulation (0–360 deg)

### Setting parameters from a stimulation backend¶

The parameters in this stimulation device can be updated with input coming from a stimulation backend. The data structure used for the update holds one value for each of the parameters mentioned above. The indexing is as follows:

mean

std

std_mod

frequency

phase

## Sends¶

CurrentEvent