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I have two questions regarding the fact that each sensory receptor has its specific ion channels that open in response to a specific stimulus (for example, ion channel that opens when streching for somatosensation), and is connected to correspondent sensory cortex.

  1. Did our sensory system developed perfectly so that each ion channels in the sensory receptors are connecting to the corespondent sensory cortex? If not, can you elaborate and give examples?

  2. Theoretically, what would happen if they weren't corresponding? And specific, if for example, we had sound receptor that was connected to the visual cortex?

Thanks!

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    $\begingroup$ Welcome. The two questions are related, but are different enough to warrant two separate questions. This will greatly enhance their answerability. Secondly, I would recommend to add some prior research to narrow the scope. I'm saying, because the first question contains a faulty supposition -- ion channels in sensory receptors are not connected to the brain at all. Answering the questions therefore will need a basic outline of neuroscience (what is an ion channel, a receptor, a neuron, an axon, projections to the brain etc etc) before we can even get to the answer. $\endgroup$
    – AliceD ♦
    Jun 13, 2020 at 19:26
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    $\begingroup$ Questions need to be focused. As of now we need to write down an entire Introduction to Neurosci for you. I recommend Khan Academy, which has great introductory material in this area that is freely accessible . $\endgroup$
    – AliceD ♦
    Jun 13, 2020 at 19:29

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Without going into the business of ion channels, sensory receptors connect to specific brain areas (although there are relays in-between). For vision it is V1 in the occipital cortex, for audition A1 in the temporal cortex, for somato-perception S1 in the parietal cortex. Olfaction, gustation and the vestibular systems don't seem to have a clear "primary" area like audition, vision and somatoperception. The orbito-frontal cortex seems to be quite involved in olfaction/gustation, but it's also involved in a lot of different stuff. For the vestibular system some areas of the parietal cortex are also quite involved, but also in other stuff.

Primary areas receive direct connections from the sensory organs (again, with relays in-between) and are organized topically. For example, the visual cortex has a retinotopic organization where adjacent cells in the cortex encode information from adjacent cells on the retina. A1 has a (somewhat looser) tonotopic organization, and S1 has a "body-topic" organization (called the homunculus). Those pathways a quite fixed and quite distinct. There is no way to have a "mix-up" between them, and you never see that. There can be some "mix-up" within one pathway. For example amblyopia in vision where the visual cortex does not develop as expected and results in some visual loss (less than 20/20 vision in one eye).

So yes, the sensory organs develop well (with some exceptions) and project to very distinct areas all over the brain (frontal, parietal, occipital and temporal lobes). What would happen if you rewired the brain is unclear. There was one famous experiment where optical nerves in ferrets were connected to the auditory cortex. Experimenters observed a cortical map similar to what they would have expected in visual areas. So it suggests that at least the visual and auditory cortices are "interchangeable". But then these areas have their own connections to other areas, so what actually happens is anyone guess (do you "hear" visual stimuli?). Those are deep philosophical questions, similar to "do we all see colors similarly?", that we are far to have a good handle on scientifically.

https://en.wikipedia.org/wiki/Visual_cortex

https://en.wikipedia.org/wiki/Auditory_cortex

https://en.wikipedia.org/wiki/Primary_somatosensory_cortex

https://en.wikipedia.org/wiki/Cortical_homunculus

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  • $\begingroup$ I don't think this really answers the question that was asked (although, the question could use some improvement before it gets an answer, per AliceD's comments). They already know that there are different sensory cortical areas for different stimuli, and are wondering how they are connected that way, whereas this answer just says "it happens". And for the last bit, there is zero evidence and no theory of consciousness that I am aware of that would suggest you would "hear" things if you hooked up visual inputs to the auditory cortex in absence of auditory input. $\endgroup$
    – Bryan Krause ♦
    Jun 15, 2020 at 0:21
  • $\begingroup$ I agree. Still, it looks like you could replace the visual cortex by the auditory cortex. I have no idea what an observer would perceive. Let's take the opposite situation and assume you could successfully connect auditory nerves to the visual cortex. The information being processed would then flow to areas V2, MT etc. Would you think you could train that "rewired" animal to successfully discriminate between high and low pitch tones? Assuming V1 would now makes the same computation A1 usually does, I would assume so. In principle that is a similar question to sensory replacement. $\endgroup$
    – user17122
    Jun 15, 2020 at 0:59
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    $\begingroup$ I would expect that it depends on when in development the rewiring occurred. It's not really plausible besides thought experiment to do it very late; V2, MT etc would not exist as visual areas if visual cortex was getting auditory input. Consider those regions in congenitally deaf folks, for example. $\endgroup$
    – Bryan Krause ♦
    Jun 15, 2020 at 16:42
  • $\begingroup$ I meant prenatally. It's a pure thought experiment, but I would assume this animal would react to sounds. Yet I doubt the resulting percept would be the same as "true" hearing. Just like in sensory replacement you can react to an artificial sense (let's say magnetic fields), but that does not necessarily mean you have magneto-perception in the same sense pigeons do. A pure behaviorist approach would say the distinction doesn't matter as long as you produce a response, but as you seem to be aware there are more nuanced approaches. $\endgroup$
    – user17122
    Jun 17, 2020 at 3:03

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