Up until now in this paper, I have been guilty of a small deception. I have spoken as if the only reason that human designers work within a constrained space of circuits -- for instance, synchronous digital logic circuits -- is to make the design process simple. In fact, precluding the detailed properties of the medium from contributing to the system behaviour not only supports design abstractions, but also gives robustness. Those properties that have been excluded from the designer's model (and prevented from influencing the system's behaviour by means of constraining its structure and dynamics) can vary greatly without causing the system to malfunction. Such changes typically arise from process variations between nominally identical silicon chips, ageing, and temperature and power-supply fluctuations.
Once evolution is allowed to explore the full spectrum of possible behaviours that the medium can support, this `automatic' robustness is lost. A trade-off needs to be found between exploiting the properties of the medium, and being tolerant to variations in them. Tolerance to variation in a property does not necessarily imply that the property is not used at all: several aspects of the medium which vary in different ways can be balanced against each-other to give stable overall system behaviour, or different mechanisms can be called into play for different conditions.
As in the case of spatial interactions, the evolution of adequate robustness is important for all physical `nervous systems', whether electronic or biological. Nature will be a rich source of inspiration for techniques in the evolution of robustness. Rather than precluding large swathes of the natural behaviour of the medium from ever being put to use (as does conventional design), the natural approach in an evolutionary framework is to provide a selection pressure for robustness, and to allow evolution to build robustness into the overall system behaviour using the full set of resources available. This selection pressure may be provided by evaluating the circuits in the presence of those variations with which they are required to cope, so that to be fit they must operate well under a wide set of conditions.
An especially promising idea from biology is the notion of an external `timegiver' which can stabilise the timescales of the system's internal dynamics . This timegiver could be inherent in the system's ongoing interaction with the environment: for example, in the tone decoder example of the previous section, the fact that the input waveform is always either 1kHz or 10kHz could be used as a time-reference. In addition, the circuit could interact with an external timegiver more explicitly. In analogy to the daily light/dark cycles that entrain circadian rhythms in animals, a stable oscillation could be applied to the evolving circuits as an extra input, at the same time as a selection pressure towards robustness is maintained. This `clock' is not a constraint on the system's dynamics -- evolution could choose to ignore it altogether -- but instead enriches the spectrum of possible dynamical behaviours with stability, which can be incorporated in subtle ways.
Preliminary experiments on this `unconstrained' approach to robustness for the tone-discrimination task are encouraging, but not yet conclusive.