Well, to a three year old question, I can give some inside info. The answer: ...as a grad student of Lettvin's I was privy to being present when some of these experiments were duplicated for other reasons in the '60's. In particular, we (about 5 of us) were playing with the "bug" detector. We were looking at the responses to one bug, two bugs...etc. up to 5 presented as a cluster moving in tandem. A single smooth movement of a single 'bug' produced a fine response. Interestingly, as the number increased, the response decreased until 5 or so did not elicit a response. The collective movement was smooth, not sporatic and stochastic. Certainly, a bit of jitter does improve the response. In an experiment I am now doing with Ed Gruberg who was then a post-doc in Lettvin's lab and is now at Temple Univ, Philadelphia...about 6 miles from where I am located, we use a stimulus that is deliberately jittered, but I want to emphasize that the jitter is not necessary for the response originally described.