The Corollary Discharge

Many, many years ago I did a PhD on whether an area of the brain called the frontal eye fields was involved in something called the “corollary discharge”.

A corollary discharge, or “efference copy”, was first postulated by Hermann von Helmholtz in the 19th century to account for the fact that when we move our eyes we don’t experience the visual world moving, even though the image moving over our retina would be expected to generate just such a sensation.  Helmholtz’s elegant explanation was that the motor command in the brain to move the eyes is copied as a corollary discharge and used to cancel out the apparent sensory signal coming from the eyes.  You can prove that this is what happens by gently pressing on one side of your eyeball – the visual world does appear to move in this case (sensory input but no corollary discharge to cancel it out).  I don’t recommend you try the next stage of Helmholtz’s proof which was to immobilise his eye then try and move it, which also caused a sensation of movement in the opposite direction (corollary discharge but no sensory input)!


My research provided zero evidence that the frontal eyefields were involved in this mechanism!  However, this area of the brain was clearly involved in selective attention in some way and this is what I concluded: “The function of the frontal eyefields is one of organising co-ordinated shifts of attention  … the mechanism underlying this function is that of a repeated testing of an internalised schematic map of the visual environment against afferent sensory information”.  This claim was criticised at the time for going beyond the evidence (these were the dark days of behaviourism) but forty years on I still reckon I was on to something.  Here’s how.

If instead of the “internalised schematic map of the visual environment” we generalise and call it a comprehensive “mental model”, and similarly generalise the “corollary discharge mechanism” to a fundamental process of “testing hypotheses about the outside world generated by the model against what happens when we interact with the outside world” then I think we have a valuable insight into how the brain might work.  Now let’s generalise a bit further to an internal model of the self interacting with an internal model of the outside world and only really paying attention when the sensory evidence from the real outside world is different from what the model predicts.  According to this interpretation we normally only need to process a very small fraction of our sensory input – the rest of what we perceive is internally generated.  But as soon as something unexpected happens (not predicted by the corollary discharge) we immediately pay attention and concentrate on finding out what’s new.  Generalise even further and I can start to envisage a mental model which includes not just a model of me but models of other people I’m interacting with, and once again, I think it’s helpful to think of myself continually testing out the “corollary discharge” of these interactions generated by the model inside my head with what actually happens in the outside world, helping me to plan accordingly and behave in a socially acceptable manner.

Whether any of this is anything to do with the frontal eyefields is besides the point, although, interestingly, the prefrontal cortex does seem to be very much involved with attention, perception, planning, personality, and all the other qualities we associate with higher consciousness.

Finally, what happens in dreams, where we only have the corollary discharge, so to speak, without any sensory feedback?  According to the theory, this should result in the internal representation of the outside world acting strangely in some way, by analogy with what Helmholtz saw when he immobilised his eyeball.  My personal experience is that my dream self’s interaction with its dreamed environment does indeed tend to be “difficult” in some way.  The clearest example of this would be those dreams where I’m trying to move – usually running away from something dangerous – and I find that I’m too weak, or bogged down in quicksand or otherwise constrained.  Familiar?  So what I think is happening here is that the “corollary discharge” of my action is not cancelled out by what I’d expect in the real world – the visual world streaming past me as I ran through it – so the model then tries to rationalise this by fabricating a narrative about me (weakness) or the world (quicksand) or whatever.  With a little imagination (ie going beyond the evidence big time!) it seems to me that a lot of the strange things which happen to me in my dreams make more sense in these terms – failing to complete some goal, getting lost in large cities, being interrupted just as I’m about to have sex (unfair!), people I know acting out of character, a general clumsiness of execution on the part of my dream self.  But maybe I’m just weird!


Hermann von Helmholtz 1821 – 1894

2 Responses to “The Corollary Discharge”

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