# What is the brain power devoted to vision and haptics?

I heard a talk by Vincent Hayward on the sense of touch as a multi-modal system, where he claimed that the brain power devoted to haptics is at least as big as the one devoted to vision. I have found some answers with respect to vision, which claim the percentage to be between 25%-33% of the neo-cortex. The cortical homunculus shows a sensory map of the body, where I guess the eye depicted is more related to the haptic part of the eye region, rather than the vision.

• Can you confirm the percentage for vision?
• What is the percentage for haptics?

Quotable sources would be fantastic of-course, but just a rough number would also do. I am mostly interested in the rough relative relation between the two modalities.

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would it be more appropriate to quantify the surface area or volume of the cortex for which the activity of it is modulated by the sensory modality? "Brain power" sounds like an ill-defined concept. – Memming Sep 29 '12 at 22:02
Agreed, "brain power" is probably too vague to quantify. But since I am looking for vague numbers any of the immediate quantifications that you mentioned would do. – Jakob Oct 1 '12 at 7:03
Either way, I think this is a very vague, hence less scientific question you are asking, and not likely to get a meaningful answer given our current understanding of the brain. – Memming Oct 1 '12 at 16:44

I'd argue that "brain power" is dangerously vague. Are you looking at weight, volume, area (including or excluding cortical folding), are you looking at just the cortex, or the whole brain?

At a lower level, are you counting just cell bodies, or do you count the processes that join them; do you just weigh the whole lot and try to figure out "power" that way? Do you count long axons projecting from the peripheral nervous system? Because touch would have a massive edge over vision if you counted the weight of those long, thick, myelinated axons in the spinal column.

None of those straight physical measures will give you an estimate of "power" because power implies other more cognitive factors; speed and capacity notably. Myelination gives speed of response to neurons, as does thickness. But a shorter neuron will be faster (more powerful?) than a longer one, and will also weigh less - clearly weight isn't a good estimator of speed, in fact the further information travels through axons the less "powerful" the whole system will be.

But that's just talking about speed - what about capacity? The overall number of indivudal points of input might relate to capacity, but the brain does plenty of post processing on visual input to allow it to process far more information than the eyes actually give it at any one time. Is capacity related to the number of synapses each neuron makes? This probably has something do to more with "depth" of processing than capacity, as it implies that each percept is sent to more places.

And beyond even speed, depth and capacity, what about learning? Which system has a greater ability to adaptively learn and pass learned information back into the decision making parts of the brain? How do you measure that? By the number of NMDA receptors in each system?

Also, what's this "touch" thing anyway? Are you talking about perception of heat, cold, pain (which sort, a$\delta$, perhaps C fibres)? Are you talking about sensations of pressure, like that felt when pushing a heavy object? Those sort of sensations are related more to proprioception than touch - they are all about sensing the environment's reaction to the body based on stretch receptors in the muscles, this is another type of input that makes up the heterogeneous sense labelled "touch".

Each of these types of pressure, superficial touch, pain, heat and cold have different afferent projections, and each has it's own substrate in the brain, with different architectures, different strengths and weaknesses - there is no straightforward "power" to compare any of those components of touch, much less to compare touch to something as remote as vision. This sort of question and attempts to answer it dangerously obscure the complex science behind both systems, giving a false sense of understanding where most of the above questions are unanswered, and possibly unanswerable.

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