Tutorial for Constructing Simulations in PDP - Computational Explorations in Cognitive Neuroscience - page 428

Now, you can do the usual inspection of the weights.

Now, you can just press Connect All again.
The network is now reconnected using the newly
specified connectivity pattern.

Select r.wt and click on various units.
Notice that the layers are fully interconnected — this
is the default type of connectivity pattern. We could
change this by creating a new ProjectionSpec ob-
ject in the .specs menu of the project, and then setting
a given projection to use this new spec. Let's do this.

Use r.wt to view the connectivity into the hidden
units — you need to click back on View at the top of the
display to pick the different units for viewing.
Notice how they only receive from a 2 x 2 square from
the input units. This is what the tessel prjn spec does!
Its many options allow you to determine how much the
input square moves in relation to the position of the out-
put units (here the input is directly proportional to the
output), and lots of other things. Tessel prjn specs were
used extensively in the object recognition simulations in
chapter 8 — use these simulations for further examples
of what they can do. Also, for something as complex
as a Tessel spec, it is useful to access the online Help,
which is available for all objects in the simulator.

, !

Go to .specs and first just click on Edit .
Notice that several Spec objects have automatically
been created. The LeabraUnitSpec_0 provides the
default specifications for the all the units in the network,
LeabraBiasSpec_0 specifies the parameters for the
bias weights on the units, LeabraLayerSpec_0
specifies applies to all the layers, LeabraConSpec_0
provides the specifications for all the connections
within the network, and FullPrjnSpec_0 specifies
that the projections have full connectivity.

, !

Go down to New in the .specs menu, then select
ProjectionSpec/TesselPrjnSpec .
This is one of the most useful forms of projection
specs, which allows you to specify repeated “tilings” or
receptive fields.

On the Tessel edit dialog, select Object/Help from
the menu at the top. A help browser (typically netscape)
should show up with detailed help on all the parame-
ters associated with the Tessel Prjn Spec. If the help
browser does not appear, check for error messages on
the terminal, and consult the platform-specific installa-
tion instructions to remedy the problem. After browsing,
you can then close the help window, and then hit Ok on
the TesselPrjnSpec dialog itself.

Be sure to press Ok using the right mouse button,
so you can edit this new spec.
There are many parameters that can be set in this ob-
ject. We will just leave these all at their defaults, and
create a rectangular receptive field for each unit.

Go to the Actions menu at the top of this edit
dialog, and select Make Rectangle .
The parameters for this function are shown in the next
window that comes up.

B.2.5 Specifying Layer Activity Levels

, !

The default parameters contained in the automatically
created unit, connection, and layer specs are probably
reasonable for most simulations. However, one thing
that usually must be specified on a layer-by-layer basis
is the kWTA activity level for each layer. We can do this
using the same technique as we did for the tessel prjn
spec, by creating a new spec object and then using the
Selections menu to apply that spec to the different
layers.
Usually, the only thing that differs about the differ-
ent layers is their activity level — all other parameters
in the layer spec should apply to all the layers. For-
tunately, PDP++ has a special mechanism for dealing
with just this kind of situation. The idea is that you can
create children of a parent spec, where the children
inherit parameters from the parent (except for specific

Enter a width and height of 2, and x and y center
values ( ctr_x and ctr_y ) of 0. Then press Ok .
We have now created a connectivity pattern, but we
have not yet specified which projection will use this new
connectivity pattern.

To do this, go to the network window and first click on
the Select button at the top ( View was active because
we were viewing network weights), and then click on the
projection arrow going from the input to the hidden layer.
Then, use the far right Selections menu in the net-
work window and choose Set Prjn Spec . You then
need to select which projection spec to use in the win-
dow that comes up — choose the TesselPrjnSpec_0 ,
which we just created.