Lightning is always seen, thunder always heard


An old suggestion that crossing the visual and auditory pathways to the brain would lead to light being experienced as sound, and vice versa, has been tested and found to be false.

Nicholas Swindale, in Current Biology, 2000

Okay, so this isn’t new news, but it was new to me and too good a story not to share.

If, from birth, the information from the eyes is routed to the auditory cortex then the brain learns to see like normal – at least in ferrets, with whom they’ve done these experiments. The cortex has the potential to cope with whatever information it is provided with during development. So, it seems, the regional specialisations of the brain aren’t genetically predetermined. But a question remains: if your auditory cortex is processing visual stimuli, how are they actually experienced? The brain might be processing the information well enough to guide behaviour, but how do the stimuli actually feel? Are they experienced as visions or as sounds? Or, as Swindale puts it:

are the types of sensory processing that ultimately give rise to qualia innately determined properties of different cortical areas, or are they the secondary outcome of a general purpose learning algorithm applied to sensory inputs which have a different information content?

And, crucially, is there any way of working this out in a ferret? Is there a way of telling what a ferret’s experience is really like? Well, there is, and it involves rewiring just half of the brain – so that visual inputs to one side go to the ‘auditory cortex’ and visual inputs from the other go to the visual cortex as normal. Now if you train the animal to go left to visual inputs on the intact side and right to sounds, which way will it go to a visual input presented to the rewired side? If it experiences the visual input as most like a sound it will go right, but if it experiences it as most like a light it will go left. The animals go left – so visual stimuli are experienced as visual whereever in the brain they are initially processed.

Swindale’s review
The original research von Melchner L, Pallas SL, Sur M: Visual behaviour mediated by retinal projections directed to the auditory pathway. Nature 2000, 404:871-876.

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