Routine Name: planarweight
Description: Sets the weight fields on groups of synapses between
specified lists of elements. Most often used to set
weights on groups of synapses that have been set up
by calling the "planarconnect" command. This function
can assign groups of synapses to a fixed weight, can
assign weights in proportion to the distances between
pre- and postsynaptic elements, and can add various
types of randomness to weight values.
Usage: planarweight sourcepath [destination_path] \
-fixed weight \
-decay decay_rate max_weight min_weight \
-uniform scale \
-gaussian stdev maxdev \
-exponential mid max \
-absoluterandom
sourcepath A wildcarded list of elements which are the
sources of the SPIKE messages connecting the
pre- and postsynaptic elements (i.e. the presynaptic
elements). These must be of class "spiking" and are
usually spikegen or randomspike objects.
destination_path A wildcarded list of elements which must be
synchans or objects derived from synchan. If this
(optional) argument is given, only the weights between
the given set of pre- and postsynaptic elements will
be set by this command. If this argument is not
given, then all the synapses receiving SPIKE messages
from the presynaptic elements will have their weights
set by this command. NOTE: this optional argument is
new and is not documented in the Book of GENESIS.
-fixed weight -- This option sets all the synaptic weights in
question to be equal to "weight".
-decay decay_rate max_weight min_weight -- This option sets
the synaptic weights in question to be proportional to
the distance between the source and destination
elements according to the equation:
weight = (max_weight - min_weight) *
exp(-decay_rate * distance) + min_weight
For planarweight, the distance is measured as:
distance =
sqrt((x_src - x_dest)^2 + (y_src - y_dest)^2)
where x_src is the x component of the source element,
x_dest is the x component of the destination element,
and so on. Note that the z component is not taken
into account, which is unrealistic. volumedelay
uses the z component as well.
The next four options are used to add random components to the
weights established using the -fixed or -decay options. How
these random components are added to the weights is explained
below.
-uniform scale -- This option gives a random number taken
from a uniform distribution in the range
{-scale, scale}.
-gaussian stdev maxdev -- This option gives a random number
taken from a gaussian distribution centered on zero,
with a standard deviation equal to "stdev" and with
a maximum value of "maxdev". The maximum value is
used to limit the random component to a given range.
-exponential mid max -- This option gives a random number
taken from an exponential distribution with a
minimum value of zero, a 1/e point of "mid" and a
maximum value of "max". This is mainly for backwards
compatibility with genesis 1.4.
-absoluterandom This option alters the way the random number
is combined with the nominal weight to give the actual
weight, as described below.
Once a random component has been created for a given weight,
it is used to set the weight as follows. If the
-absoluterandom option has not been selected the weight is set
to be:
final_weight = weight + (weight * random_number)
Whereas if the -absoluterandom option has been selected then
we have
final_weight = weight + random_number
Thus the default is to have the amount of randomness as a
constant proportion of the weight value.
Example: [modified from the Orient_tut simulation:]
planarweight /retina/recplane/rec[]/input \
-decay 0.5 10.0 0.1 \
-gaussian 0.1 0.3
This command will set the size of the weights of synapses
that are receiving their inputs from
/retina/recplane/rec[]/input. It gives exponentially decaying
weights with a maximum size of 10.0, a minimum size of 0.1,
and a decay rate of 0.5. It also specifies that gaussian
noise be added to the weights with a mean value of 0.1
(which represents 10% of the original weight, since
-absoluterandom has not been selected) and a maximum value of
0.3 (which is 30% of the original weight value).
Notes: The "destination_path" optional argument is new and is not
documented in the Book of Genesis.
This routine calculates distance using only the x and y
coordinates of the element positions. It is convenient for
objects laid out in planar arrays but ignoring the z
direction is somewhat unrealistic. volumeweight is identical
to planarweight except that it uses the positions of elements
in three dimensions to calculate distances and is thus more
realistic.
The weights are never allowed to go negative even if a large
negative random component is added. Negative weights are set
to zero.
The options -fixed and -decay are mutually exclusive. The
different random options -uniform, -gaussian, and -exponential
are also mutually exclusive.
See also: volumeweight, planarconnect, planardelay; Chapter 18
of the Book of GENESIS (2nd ed.) has a lengthy discussion on
this and related commands.