# Getting started with Brian¶

If you are familiar with Brian 1 and you’re looking to get started with Brian 2, see the section Brian 1 users below. For everyone else, read on.

## New users¶

In this section, we will give a very brief overview of how Brian works. We recommend that after reading this you read through the User’s guide and try out some of the Examples.

from brian2 import *


Once this has been imported, you will have access to all the Brian objects and functions. This includes the unit system, designed to specify values with physical dimensions like volts, amps, etc. For example, try this in an IPython console:

>>> from brian2 import *
>>> print 1000*mV
1.0 V
>>> print 0.001*volt
1.0 mV


This system is designed to stop you from inadvertently making errors either with the scale of units (e.g. entering a value in mV when it should have been in volts) as well as writing dimensionally inconsistent statements, e.g. the following raises an error:

>>> from brian2 import *
>>> print 1*volt+1*amp
...
DimensionMismatchError: Addition, dimensions were (m^2 kg s^-3 A^-1) (A)


The two core concepts in Brian are as follows:

• Groups of neurons are defined by a NeuronGroup, which consists of differential equations defining the evolution of the model, as well as equations specifying the “threshold condition” for a spike and the “reset statement” defining what happens after a spike.
• Synapses are defined by a Synapses object. This consists of (1) equations defining the evolution of the variables - same as for neurons, (2) statements defining what happens when a presynaptic or postsynaptic neuron fires a spike, (3) the pattern/structure of the synaptic connectivity (i.e. which neurons are connected via a synapse to which other neurons).

In addition, there are objects for putting input stimuli into a simulation, as well as objects for recording the activity of a network (e.g. the spikes produced or the time evolution of a particular variable).

A simple example that demonstrates this:

from brian2 import *
N = 1000
tau = 10*ms
vr = -70*mV
vt = -60*mV
eqs = '''
dv/dt = -v/tau : volt
'''
G = NeuronGroup(N, eqs, threshold='v>vt', reset='v=vr')


In this example we have defined a group of N = 1000 neurons which behave as leaky integrate and fire neurons with instantaneous firing if the membrane potential increases over vr = -60 mV followed by a reset to vt = -70 mV. The equations string dv/dt = -v/tau : volt gives the group G a single variable v that evolves according to the differential equation, and specifies that the physical unit of the variable is the volt.

A more detailed example that actually shows some interesting behaviour can be seen in the CUBA example. For more information, see the User’s guide.

## Brian 1 users¶

In most cases, Brian 2 works in a very similar way to Brian 1 but there are some important differences to be aware of. The major distinction is that in Brian 2 you need to be more explicit about the definition of your simulation in order to avoid inadvertent errors. For example, the equations defining thresholds, resets and refractoriness have to be fully explicitly specified strings. In addition, some cases where you could use the ‘magic network’ system in Brian 1 won’t work in Brian 2 and you’ll get an error telling you that you need to create an explicit Network object.

The old system of Connection and related synaptic objects such as STDP and STP have been removed and replaced with the new Synapses class.

A slightly technical change that might have a significant impact on your code is that the way ‘namespaces’ are handled has changed. You can now change the value of parameters specified outside of equations between simulation runs, as well as changing the dt value of the simulation between runs.

The units system has also been modified so that now arrays have a unit instead of just single values. Finally, a number of objects and classes have been removed or simplified.

For a full list of changes see Changes from Brian 1.