Could a neuroscientist understand a microprocessor?

In a recent article, which has been widely commented (e.g., in a wordpress blog and in marginal revolution) Eric Jonas and Konrad Korning, from UC Berkeley and Northwestern universities, respectively, have described an interesting experiment.

They have applied the same techniques neuroscientists use to analyze the brain to the study of a microprocessor. More specifically, they used local field potentials, correlations between activities of different zones, the effects of single transistor lesions, together with other techniques inspired in state of the art brain sciences.

Microprocessors are complex systems, although they are much simpler than a human brain. A modern microprocessor could have several billion transistors, a number that compares poorly with the human brain, which has close to 100 billion neurons, and probably more than one quadrillion synapses. One could imagine that, by applying techniques similar to the ones used in neuroscience, one could obtain some understanding of the role of different functional units, how they are interconnected, and even how they work.

Castle_Chip_Layout

The authors conclude, not surprisingly, that no significant insights on the structure of the processor can be be gained by applying neuroscience techniques. The authors have indeed observed signals that are reminiscent of the signals obtained when applying NMR and other imaging techniques to live brains, and have observed significant correlations between these signals and the tasks the processor was doing, as in the following figure, extracted from the paper.

signals

However, the analysis of these signals did not provide any significant knowledge on the way the processor works, nor about the different functional units involved. They did, however, provide significant amounts of misleading information. For instance, the authors investigated how transistor damage affected three chip “behaviors”, specifically the execution of the games Donkey Kong, Space Invaders and Pitfall. They were able to find transistors which uniquely crash one of the games but not the others. A neuroscientist studying this chip might thus conclude a specific transistor is uniquely responsible for a specific game – leading to the possible conclusion that there may exist a “Space Invaders” transistor and a “Pitfall” transistor.

These may be bad news for neuroscientists. Reverse engineering the brain, by observing the telltales left by neurons working, may remain forever an impossible task. Fortunately, that still leaves open the possibility that we may be able to fully reconstruct the behavior of a brain, even without ever having a full understanding of its behavior.

First image: Chip layout of EnCore Castle processor, by Igor Bohem, available at Wikimedia commons.

Second image: Observed signals, in different parts of the chip.

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