A Chip That Focuses an Image on Itself

In the modeling of neural systems, time is often treated as a sequencer, rather than as an expresser of information. We believe that this point of view is restricted, and that in biological neural systems, time is used throughout as one of the fundamental representational dimensions. We have developed this conviction partially because we model neural circuitry in analog VLSI, where time is a natural dimension to work with, and we believe there are deep similarities between the technology we use and the one nature has chosen for us. Neurobiologists are beginning to explore neural control systems that selfgenerate sensory input. The focus chip we report here models the focusing system of our eye. The human focusing mechanism is a one-dimensional control system in which experimenters have access to both visual input and motor output signals. For our model, the primary hypothesis about this system is that control signals are generated actively, by the motor system in the course of control. We have built and partially characterized a model system, using analog VLSI circuit primitives already developed for other purposes, that incorporates this hypothesis. This chip focuses an image on itself, using time domain information about the quality of the optical image and the motion of the lens.