David B. D'Ambrosio and Kenneth O. Stanley (2007)
A Novel Generative Encoding for Exploiting Neural Network Sensor and Output Geometry
In: Proceedings of the Genetic and Evolutionary Computation Conference (GECCO 2007). New York, NY: ACM, 2007 (8 pages)

Nominated for Best Paper Award in Generative and Developmental Systems at GECCO-2007

A significant problem for evolving artificial neural networks is that the physical arrangement of sensors and effectors is invisible to the evolutionary  algorithm. For example, in this paper, directional sensors and effectors are placed around the circumference of a robot in analogous arrangements.  This configuration ensures that there is a useful geometric correspondence between sensors and effectors. However, if sensors are mapped to a single input layer and the effectors to a single output layer (as is typical), evolution has no means to exploit this fortuitous arrangement. To address this problem, this paper presents a novel generative encoding called connective Compositional Pattern Producing Networks (connective CPPNs) that can effectively detect and capitalize on geometric relationships among sensors and effectors.  The key insight is that sensors and effectors with consistent geometric relationships can be exploited by a repeating motif in the neural architecture. Thus, by employing an encoding that can discover such motifs as a function of network geometry, it becomes possible to exploit it. In this paper, a method for evolving connective CPPNs called Hypercube-based Neuroevolution of Augmenting Topologies (HyperNEAT) discovers sensible repeating motifs that take advantage of two different placement schemes, demonstrating the utility of such an approach.