15-883 Homework #2
Computational Models of Neural Systems

Issued: January 19, 2005. You do not have to hand anything in for this assignment.

How to Run the Simple Matrix Demo

This Matlab script demonstrates creation of a matrix memory using the outer product rule. The memory stores three items correctly using orthogonal keys; it fails catastrophically when a fourth item is added whose key is not orthogonal to the others. However, the four keys are linearly independent, so a new memory is constructed using the matrix inverse technique and is verified to work correctly.
  1. Go to the matlab/simplemat directory.
  2. Execute the file script1.
  3. Each time you see the word pause, press the space bar to proceed.
The file script1.log shows the output you should see if you execute the script correctly.

How to Run the Graphical Matrix Memory Demo

You should cd to the directory matlab/matmem, or download the file matmem.zip and unzip it. Read the README file before proceeding further.

When you're ready to begin, type "matlab" to start up Matlab. Then type "matmem" to start the matrix memory demo.

Experiment with the Matmem Demo

  1. Click on the Random button to generate a set of random memory items. Then start clicking on memory item buttons. As you click on a button, it turns pink, and the result box to its right should also turn pink to show you that the stored item is successfull retrieved. As you click on additional memory items, the memory fills up, and eventually one of the corresponding result boxes will turn blue, indicating that retrieval of that item is not completely correct.

  2. How many items can you store before the first retrieval error occurs? What percentage of memory bits are active when the first retrieval error occurs? Hit the New button and then Random to generate another set of data items. Try the experiment several times.

  3. Switch from a dense to a sparse representation by clicking on the Dense button. The button label will change from "Dense" to "Sparse". Now instead of using a 5-bit binary code, each letter is stored using a 2-of-8 bit code. This requires 24 input units instead of 15.

  4. Re-run the previous experiment a few times. How many items can you store using the sparse representation? What percentage of memory bits are turned on when the first retrieval error occurs?
Dave Touretzky
Last modified: Wed Jan 22 03:37:14 EST 2003