If anyone is interested to read more on this topic, there is another recent, closely related and perhaps slightly more accessible paper ( "High-Level Invariant Features with Scalable Clustering Algorithms" http://bit.ly/KDuN04 ) from Stanford that also learns face neurons from unsupervised collection of images (disclaimer: I'm co-author). It uses a slightly different model based on layers of k-means clustering and linking, but the computation in the end is very similar.

I'm familiar with both models so I can also try to answer any questions.