Physical units
Brian includes a system for physical units. The base units are defined by their
standard SI unit names: amp
/ampere
, kilogram
/kilogramme
,
second
, metre
/meter
, mole
/mol
, kelvin
, and candela
.
In addition to these base units, Brian defines a set of derived units:
coulomb
, farad
, gram
/gramme
, hertz
, joule
, liter
/
litre
, molar
, pascal
, ohm
, siemens
, volt
, watt
,
together with prefixed versions (e.g. msiemens = 0.001*siemens
) using the
prefixes p, n, u, m, k, M, G, T
(two exceptions to this rule: kilogram
is not defined with any additional prefixes, and metre
and meter
are
additionaly defined with the “centi” prefix, i.e. cmetre
/cmeter
).
For convenience, a couple of additional useful standard abbreviations such as
cm
(instead of cmetre
/cmeter
), nS
(instead of nsiemens
),
ms
(instead of msecond
), Hz
(instead of hertz
), mM
(instead of mmolar
) are included. To avoid clashes with common variable
names, no oneletter abbreviations are provided (e.g. you can use mV
or
nS
, but not V
or S
).
Using units
You can generate a physical quantity by multiplying a scalar or vector value with its physical unit:
>>> tau = 20*ms
>>> print(tau)
20. ms
>>> rates = [10, 20, 30]*Hz
>>> print(rates)
[ 10. 20. 30.] Hz
Brian will check the consistency of operations on units and raise an error for dimensionality mismatches:
>>> tau += 1 # ms? second?
Traceback (most recent call last):
...
DimensionMismatchError: Cannot calculate ... += 1, units do not match (units are second and 1).
>>> 3*kgram + 3*amp
Traceback (most recent call last):
...
DimensionMismatchError: Cannot calculate 3. kg + 3. A, units do not match (units are kilogram and amp).
Most Brian functions will also complain about nonspecified or incorrect units:
>>> G = NeuronGroup(10, 'dv/dt = v/tau: volt', dt=0.5)
Traceback (most recent call last):
...
DimensionMismatchError: Function "__init__" expected a quantitity with unit second for argument "dt" but got 0.5 (unit is 1).
Numpy functions have been overwritten to correctly work with units (see the developer documentation for more details):
>>> print(mean(rates))
20. Hz
>>> print(rates.repeat(2))
[ 10. 10. 20. 20. 30. 30.] Hz
Removing units
There are various options to remove the units from a value (e.g. to use it with analysis functions that do not correctly work with units)
Divide the value by its unit (most of the time the recommended option because it is clear about the scale)
Transform it to a pure numpy array in the base unit by calling
asarray
(no copy) orarray
(copy)Directly get the unitless value of a state variable by appending an underscore to the name
>>> tau/ms
20.0
>>> asarray(rates)
array([ 10., 20., 30.])
>>> G = NeuronGroup(5, 'dv/dt = v/tau: volt')
>>> print(G.v_[:])
[ 0. 0. 0. 0. 0.]
Temperatures
Brian only supports temperatures defined in °K, using the provided kelvin
unit object. Other conventions such as °C, or °F are not compatible with Brian’s
unit system, because they cannot be expressed as a multiplicative scaling of the
SI base unit kelvin (their zero point is different). However, in biological
experiments and modeling, temperatures are typically reported in °C. How to use
such temperatures depends on whether they are used as temperature differences
or as absolute temperatures:
 temperature differences
Their major use case is the correction of time constants for differences in temperatures based on the Q10 temperature coefficient. In this case, all temperatures can directly use
kelvin
even though the temperatures are reported in Celsius, since temperature differences in Celsius and Kelvin are identical. absolute temperatures
Equations such as the Goldman–Hodgkin–Katz voltage equation have a factor that depends on the absolute temperature measured in Kelvin. To get this temperature from a temperature reported in °C, you can use the
zero_celsius
constant from thebrian2.units.constants
package (see below):from brian2.units.constants import zero_celsius celsius_temp = 27 abs_temp = celsius_temp*kelvin + zero_celsius
Note
Earlier versions of Brian had a celsius
unit which was in fact
identical to kelvin
. While this gave the correct results for
temperature differences, it did not correctly work for absolute
temperatures. To avoid confusion and possible misinterpretation,
the celsius
unit has therefore been removed.
Constants
The brian2.units.constants
package provides a range of physical constants that
can be useful for detailed biological models. Brian provides the following
constants:
Constant 
Symbol(s) 
Brian name 
Value 

Avogadro constant 
\(N_A, L\) 

\(6.022140857\times 10^{23}\,\mathrm{mol}^{1}\) 
Boltzmann constant 
\(k\) 

\(1.38064852\times 10^{23}\,\mathrm{J}\,\mathrm{K}^{1}\) 
Electric constant 
\(\epsilon_0\) 

\(8.854187817\times 10^{12}\,\mathrm{F}\,\mathrm{m}^{1}\) 
Electron mass 
\(m_e\) 

\(9.10938356\times 10^{31}\,\mathrm{kg}\) 
Elementary charge 
\(e\) 

\(1.6021766208\times 10^{19}\,\mathrm{C}\) 
Faraday constant 
\(F\) 

\(96485.33289\,\mathrm{C}\,\mathrm{mol}^{1}\) 
Gas constant 
\(R\) 

\(8.3144598\,\mathrm{J}\,\mathrm{mol}^{1}\,\mathrm{K}^{1}\) 
Magnetic constant 
\(\mu_0\) 

\(12.566370614\times 10^{7}\,\mathrm{N}\,\mathrm{A}^{2}\) 
Molar mass constant 
\(M_u\) 

\(1\times 10^{3}\,\mathrm{kg}\,\mathrm{mol}^{1}\) 
0°C 

\(273.15\,\mathrm{K}\) 
Note that these constants are not imported by default, you will have to
explicitly import them from brian2.units.constants
. During the import, you
can also give them shorter names using Python’s from ... import ... as ...
syntax. For example, to calculate the \(\frac{RT}{F}\) factor that appears
in the Goldman–Hodgkin–Katz voltage equation
you can use:
from brian2 import *
from brian2.units.constants import zero_celsius, gas_constant as R, faraday_constant as F
celsius_temp = 27
T = celsius_temp*kelvin + zero_celsius
factor = R*T/F
The following topics are not essential for beginners.
Importing units
Brian generates standard names for units, combining the unit name (e.g.
“siemens”) with a prefixes (e.g. “m”), and also generates squared and cubed
versions by appending a number. For example, the units “msiemens”, “siemens2”,
“usiemens3” are all predefined. You can import these units from the package
brian2.units.allunits
– accordingly, an
from brian2.units.allunits import *
will result in everything from
Ylumen3
(cubed yotta lumen) to ymol
(yocto mole) being imported.
A better choice is normally to do from brian2.units import *
or import
everything from brian2 import *
which only imports the units mentioned in
the introductory paragraph (base units, derived units, and some standard
abbreviations).
Inplace operations on quantities
Inplace operations on quantity arrays change the underlying array, in the same way as for standard numpy arrays. This means, that any other variables referencing the same object will be affected as well:
>>> q = [1, 2] * mV
>>> r = q
>>> q += 1*mV
>>> q
array([ 2., 3.]) * mvolt
>>> r
array([ 2., 3.]) * mvolt
In contrast, scalar quantities will never change the underlying value but instead return a new value (in the same way as standard Python scalars):
>>> x = 1*mV
>>> y = x
>>> x *= 2
>>> x
2. * mvolt
>>> y
1. * mvolt