Tutorial for Constructing Simulations in PDP - Computational Explorations in Cognitive Neuroscience - page 430

which control the configuration of the events. Pressing
Edit Events would take you back to the previous
mode of editing the events. The event specs determine
the layout and action of the events that they are asso-
ciated with — each event points to its associated event
spec and obtains its configuration and the way it inter-
acts with the network from this spec. The event spec
that was automatically created when we created events
is shown as EventSpec_0 on the lower left side of
the window.

Set dist to 8, and the metric to HAMMING .
This ensures that all the patterns will overlap by no
more than 1 active unit (a hamming distance of 8 means
there are 4 units that are different between the 2 pat-
terns, meaning that there is 1 out of the 5 in common).
Don't worry about the remaining two parameters, which
just affect the way that the distance is computed, and
aren't relevant for hamming distance.

Press Ok .
You should now see that the input patterns for all the
events contain random activity patterns.
Let's repeat this process for the output layer.

Select EventSpec_0 to edit.
You should see the PatternSpec s that define the
input and output patterns of the events. The lower left
hand corner of each pattern spec shows some of the crit-
ical parameters associated with each pattern. One thing
you should notice is that the to_layer of the Output
pattern spec is set to LAST — because we created a new
layer in the network, this means that this pattern will au-
tomatically go to this new last layer, which is no longer
appropriate.

Select Permuted Binary Min Dist again, and
set pat_no to 1, n_on to 6, and dist to 10 (again en-
suring only 1 bit of overlap).
This will provide a simple input-output mapping task
for the network to learn.
As you can see, configuring the environment for a
simple input-output learning task is relatively simple.
To see how to configure a more complex example, we
can create a second output layer in the network, and
then configure the environment to provide a target for
this new layer.

To fix this, click with the right mouse button on the
to_layer text until an edit dialog appears (three times
— the first click selects the event spec and the second
the pattern spec), and then set the to_layer for this
pattern to LAY_NAME (and hit OK ).
This will ensure that the pattern is presented to the
layer specified in the layer_name field, which should
be Output .
Now, we need to create the new pattern spec for the
new output layer.

In the network window, do New Layer(s) and cre-
ate one new layer. Reshape this layer to be 5 x 1 units,
and do Build All .
Then, we can move this new layer down to the same
level as the Output layer, and to its right.

Do Move and drag the layer into that position.
Note that because the network geometry is three-
dimensional, and the screen is only two-dimensional,
we can only move layers in two dimensions at one time.
By default, movement is in the X (horizontal) and Z
(vertical) dimensions. Using the right mouse button
moves within the X and Y (depth, into the screen) di-
mensions. We can rename this layer as before.

, !

Select the event spec by clicking once on a
grid in the display, and then hit the New Pattern
button. Then, select this new pattern (at the top of
the display) by clicking twice on it, and then hit the
Set To Layer button, and select the Output2 layer
(ignore the error message about not being able to find
the PatternSpec_2 layer in the network).
This will automatically configure the pattern to fit the
Output2 layer, including placing it in the same loca-
tion as the corresponding layer in the network. If this
automatic positioning is not appropriate, the Move but-
ton can be used to move patterns around (simply click
on the pattern and drag it around — patterns can sim-
ilarly be ReShape d).

Select and edit the label (use the right-mouse-button
shortcut) and call it “Output2”. Then, bidirectionally con-
nect this layer to the hidden layer ( New BiPrjns ), and
do Connect All .
Now, we return to the environment window to create
a pattern corresponding to this new layer.

Press the Edit Specs button.
This switches the environment to editing the
EventSpec and associated PatternSpec objects,

Return to Select if you did

move.