The Physical Design of Biological Systems - Insights from the Fly Brain

Many different physical substrates can support complex computation. This is particularly apparent when considering human made and biological systems that perform similar functions, such as visually guided navigation. In common, however, is the need for good physical design, as such designs are smaller, faster, lighter, and lower power, factors in both the jungle and the marketplace. Although the physical design of man-made systems is relatively well understood, the physical design of biological computation has remained murky due to a lack of detailed information on their construction. The recent EM (electron microscope) reconstruction of the central brain of the fruit fly now allows us to start to examine these issues. Here we look at the physical design of the fly brain, including such factors as fan-in and fanout, logic depth, division into physical compartments and how this affects electrical response, pin to computation ratios (Rent's rule), and other physical characteristics of at least one biological computation substrate. From this we speculate on how physical design algorithms might change if the target implementation was a biological neural network.