Nov. 30 (Thursday) Malcolm MacIver (Northwestern)
Embodiment, control, and cognition
Abstract:
Animals diverged from plants more than 1.5 billion years ago. This split has resulted in two quite different strategies for obtaining the energy needed for life: for a plant, it is "stay in place and absorb," and for an animal, it is "move around and grab." As soon as motion enters the scene, so do two quite different regimes under which motion can occur: the first is the "viscous" regime, where an animal will stop in its tracks as soon as it ceases generating locomotory forces, and the second is the "dynamic" regime, where an animal will keep moving along even after it ceases generating such forces. For our fluid-bound ancestors, this transition occurred with the dawn of the metazoans around 0.6 billion years ago. Control of motion is much more difficult in the dynamic regime, a fact well known in the engineering of robotic systems. This sets the fundamental problem for nervous systems to solve: the transformation of sensory signals into motor signals in a manner that accounts for the animal's dynamic constraints.
We have been studying this problem using weakly electric fish from the Amazon Basin. These animals are "active sensing systems" because they emit a weak electrical field to detect their prey while hunting in the dark. We couple computational models of how this animal's nervous system is activated by the prey it hunts, from the basic physics of the signals to spike trains on individual nerve fibers. We utilize robotics and computational fluid dynamics to analyze the dynamics of the fish's swimming. Results we've obtained suggest that the sensing range is titrated to the stopping distance of the animal, a function of its dynamics. This tight coupling may be a unique feature of active sensing systems, which emit energy in order to perceive their dark environment. We hypothesize that the relationship between these two spaces may provide an index into whether an animal needs to use a reactive behavioral control system, or can utilize a deliberative control system. Vision on land exploits a biophysical sweet spot that allows very long range sensing systems to develop. This may be necessary for the evolution of deliberative control systems.