Tutorial for Constructing Simulations in PDP - Computational Explorations in Cognitive Neuroscience - page 427

the number of units, you need to remove the existing
ones, reshape it, and then build it again. You can do
this in one of two ways — we will do the procedure that
should be used if you only want to reshape one layer.

Then, just type in Input , which will overwrite the high-
lighted text. Then, press the Tab key, which will go to
the next field, and use Ctrl-u again and type Hidden ,
and then label the last layer Output .Press Ok when
done.
An alternative way of editing layer names is to click
with the right mouse button on the layer name, which
will automatically bring up the edit dialog for that layer
name.

Press the Select button, and then click twice on the
hidden layer, so that the Rmv UnitGp(s) button shows
up as one of the options on the left hand side of the
window. Then, you just click on this, and it will remove
the entire group of units that were selected (i.e., all the
units in the layer).
Alternatively, you could have selected
Actions/Remove Units from the network
window menu, and this would have removed all units
in the network, enabling you to reshape any layer.

, !

B.2.4 Creating Projections and Connections

The connectivity between layers is specified in terms of
a Projection , which must be created first before indi-
vidual units get connected to each other.

Now, ReShape the layer to be 4 x 4 , and then hit
Build All again.
It is also possible to set the number of units in a layer
to be different than the full geometry of the layer (e.g.,
if you wanted 7 units in a 4 x 2 layer shape). To do this,
you must select an empty layer that has been shaped to
the desired geometry, and then hit the New Unit(s)
button, where you can enter the specific number of units
you want within the layer.

To create a projection, select the receiving layer
first (i.e., the Hidden layer), using the left mouse button.
Then extend the selection as before to the sendinglayer
(i.e., the Input layer) using either the middle button or
shift plus the left button.
You should see the New Prjn(s) button get high-
lighted.

, !

Press the New Prjn(s) button.
You will see that an arrow going from the input to
the hidden was created. This arrow has an empty ar-
rowhead, indicating that the actual connections between
individual units have not yet been created — we have
merely specified how to create such connections, with-
out actually doing it yet. This is similar to the way we
first reshaped the layer to a desired size, and then used
the build command to fill in the actual units. As you can
see, the Connect All button is highlighted, which
would create the actual connections for all projections.
Let's create some more projections before we do this,
though.

We will not do this here,

though.
Now let's give the layers names.

Click on the Select tool, and then click twice on the
layer name (i.e., until there is a box around this name,
and the Edit Name(s) button shows up on the left).
Note that you have to click twice because the first
time you click you select the entire layer, the second
time you select the lower-level object within the layer
that the pointer is over (i.e., the name in this case, but
the group of units in the previous case), and the next
time you get the next level down, if applicable (i.e., an
individual unit, if you were over a unit). After the first
layer name is selected, you can extend the selection to
include the second and third layer's names.

, !

Select the output layer, and then extend the se-
lection to the hidden layer. This time, we will press
the New BiPrjns button (to the left of New Prjn(s) ),
which will create bidirectional connectivity between the
two selected layers.
This bidirectional connectivity is essential for the
GeneRec error driven learning to work (only the input
layer can get by with only feedforward projections).

, !

Either use the middle mouse button or hold down
shift and use the left mouse button, and click twice on
the second and third layer's names.

Then, click on

Edit Name(s) .
You should get an edit dialog with all three names in

it.

Now you can press Connect All , and see that the
arrowheads have been filled in.

Click into the first name, and press Ctrl-u (the con-
trol (ctrl) key plus u), which highlights the entire name.