Examples: jets-n-sharks.in

This is an implementation of the Jets and Sharks interactive-activation model, which is discussed starting on page 25 of Parallel Distributed Processing, Explorations in the Microstructure of Cognition, Vol. 1, edited by David Rumelhart and James McClelland. It originally appeared in "Retrieving General and Specific Knowledge From Stored Knowledge of Specifics" by J. L. McClelland, 1981, Proceedings of the Third Annual Conference of the Cognitive Science Society, Berkeley, CA.

An interactive-activation model is generally a localist model with fixed-weight connections. Unit outputs are time averaged and tend to decay towards the initOutput value.

In this model, there are 27 gang members, each with a name, gang affiliation, age range, education level, marital status, and occupation. There is a group for each of these fields which has one unit for each distinct value in that field. For example, the marital status group has units for single, married, and divorced.

There is also a single instance unit for each person. There are excitatory connections between the instance unit and the property units of that individual. Within each group, there are inhibitory connections between each unit and every other unit.

The point of the network is to act as a sort of content-addressable memory. If an instance unit is activated, it should in turn activate all the properties of that person. If some properties that uniquely define a person are activated, we hope that the person's instance unit, name, and other properties will become activated.

I'm not going to go into great detail about the various features of this sort of model. You can learn about that in the PDP books or other tutorials, such as this one.

I will just say a few things about how the model is implemented in Lens. The groups all have the type "INPUT DOT_PRODUCT INCR_CLAMP INTERACT_INTEGR". They are INPUT groups because an external input can be specified for them in an example file to clamp their values. Each unit has an ordinary DOT_PRODUCT input function. Then there is an INCR_CLAMP input function. If an external input has been specified for a unit, the external input, scaled by the clampStrength will be added to the unit's input value. This is used to activate selected units.

The units have the INTERACT_INTEGR output function. It computes the unit outputs from the inputs as follows:

    in  = U->input;
    out = lastOutput[u];
    if (in > 0.0) out += dt * ((max - out) * in - (out - rest));
    else out += dt * ((out - min) * in - (out - rest));
    if (out > max) out = max;
    else if (out < 0.0) out = 0.0;
    U->output = lastOutput[u] = out;

max, min, and rest are taken from the group's maxOutput, minOutput, and initOutput, respectively. In this network, maxOutput is set to 1.0, minOutput to -0.2, and initOutput to 0.0.

This version of the interactive activation model differs a bit from the original formulation. Originally, units were allowed to have outputs less than 0. However, when using the output value to compute the input to other units, outputs were bounded at 0. Rather than having non-functional outputs below 0, Lens just bounds the output values at 0. This avoids the confusion of having to treat negative output values specially and the illogic of even having a negative activation. Also, the original formulation had a resting value of -0.1, but Lens uses 0.0 since that is the minimum activation. Otherwise, this simulation is identical to the original, as far as I can tell.

There is no training in the interactive activation model. All excitatory weights are fixed at 1.0 and all inhibitory weights at -1.0. In evaluating the model, you will simply be clicking on examples in the Unit Viewer to run the network on them.

Try opening the Unit Viewer and clicking on the example Lance-in. This drives the Lance instance unit. Click the triple-right arrow to go to the end of the example. Or use the single-right to step through all 60 ticks in the example.

You should find that in the end, the Lance name unit is the most active. Lance's other properties should also be the most active ones in their respective groups: Jets, 20's, J.H., Mar., and Burglar. You can see Lance's properties by looking at the Data section of the jets-n-sharks.in script.

Try clicking on the other *-in examples and checking the properties activated by the network with the correct properties. You should find that the network is pretty good on Lance, Art, Sam, Ralph, but it has a lot of trouble with Rick and Ken.

Let's graph what happend in the Rick case. Run this command:

lens> graphObject {Rick.output Ralph.output Fred.output Phil.output Sharks.output Jets.output} -u TICKS

Now click on the Rick-in example. You should see a bunch of curves appear on the graph. The Rick curve is in black. It dominates the other names early on, but then is overtaken by Ralph and Fred. Likewise, the correct gang, Sharks, is active early but is eventually overtaken by the Jets. Can you figure out why this happens?

Now click on the Lance, Art, Rick, Sam, Ralph, and Ken examples to activate their name units, rather than their instance units. The network will probably have more trouble in this case.

The "Sharks 20's" example drives the Sharks and 20's units. The only person in his 20's and in the Sharks is Ken. Is his the most active instance unit? How about the most active name? Similarly, the "Div. Pusher" example activates two units that are only consistent with Dave. Does the network correctly identify him?

In the "Lance-in on/off" example, the Lance instance unit is activated for thirty ticks and then the input is removed. Does the network maintain the proper activations once the input goes away? How about for "Art-in on/off"?

Read through the jets-n-sharks.in script. It automatically builds the network given the information in the Data array. This is why having a scripting language built into the simulator is so powerful. Can you figure out everything that goes on the script?

Try modifying the parameters of the model to get better performance. The parameters you might want to play with are the INHIB_WEIGHT, EXCIT_WEIGHT, and the groups' maxOutput, minOutput, initOutput, clampStrength, and dtScale. I had some luck by making the inhibitory weights more negative and the excitatory weights less positive.

Invent some more experiments you would like to run on the network and try to create an example file containing examples that implement your experiments. Taking a look at the jets-n-sharks.ex file should help.

Douglas Rohde
Last modified: Wed Nov 15 19:44:30 EST 2000