This is the "negation" problem introduced on page 346 of Parallel Distributed Processing, Explorations in the Microstructure of Cognition, Vol. 1, edited by David Rumelhart and James McClelland.

If the first bit of the input is off, the output should be the same as the last three bits of the input. But if the first bit is on, the output should be the complement of the first three bits.

Two networks are provided, negation and negation2. The first network has full connectivity between the inputs and the hidden and output layers. Try training it on the set "all", which has all 16 patterns. It should master the set pretty easily.

Now open the Link Viewer and examine the link structure. If you have a copy of the PDP book, read the description of how their network solved the problem. Did yours solve it in a similar way? Probably not. You might try using weight decay to keep the weights reasonable.

Now let's try something harder. Switch to the "train" training set. This has only 14 of the 16 patterns. Train on that and then check the performance on the two witheld patterns in the testing set. I'll venture to guess that it didn't learn them. As with the XOR problem, holding something out here changes the problem enough that the network can't generalize.

So now lets tip the scales back in the network's favor. Change to the "negation2" network. Examine the link structure in the Link Viewer. This network has fewer connections. Specifically, it has just the links it needs to solve the problem. Now train again on the "train" set and check the performance on the testing set. It should generalize well.

Is this cheating? Probably. Can you figure out how to get a network to generalize from 14/16 of the examples without a gerrymandered architecture?

Douglas Rohde Last modified: Thu Nov 16 00:04:46 EST 2000