This one will be short because the alternative is that it be very, very long. The short version is that our brain does a lot of work we are not consciously aware of to make sensory input make sense. More than that, different input takes a varying amount of work, and the areas of the brain that do that work can become exhausted to the point they cannot function, just like a muscle. The tests for sensory integration issues involve stimuli specifically designed to overwork these regions and observing what fails first (each region producing different characteristics mistake).
[As an aside, if you are getting tested for sensory processing issues, don’t plan on doing much afterwords. The whole point of the test is not only to mentally exhaust you, but to do so in a way your conscious brain is blind to. I am convinced hardened spies would reveal anything if you interrogated them after a sensory integration test.]
People with sensory integration issues find this processing more taxing- either because they’re fed more or noisier input, their brains have a lower capacity, or their brains are attempting to do more processing on the data. My most obvious symptom is some noise sensitivity, but there’s also a school of thought that sensory and digestive issues are linked. I wish I could tell you more on this, and I will, eventually, but I don’t have very good sources. My (now ex-) sensory integration therapist gave me a book that referred to thimerosal causing autism as if it was an undisputed fact.* I’d burn it if the exercises weren’t so helpful. So we’re going to put a pin in this and I’ll tell you more when I know it.
I mentioned sensory issues and digestive issues appear to be linked. Some of it is that a lot of sensory issues seem to relate to the trigeminal nerve, which is also involved in chewing. If the issues make chewing unpleasant, people won’t do it, which has a lot of gastrointestinal consequences. But there’s more than that.
I’m reading a book called The Second Brain, which I will undoubtedly have more to say about when I’m more than five pages in. But already it’s discussing the enteric nervous system and how it appears to perform complex, coordinated behaviors without any input from the brain or spine.** This tickled something I found in my research on the gastric system: the vagus nerve.
I have only minimal neurology, so take this description with a grain of salt, but: the Vagus nerve is a nerve collection that goes straight from the brain to certain parts of the body, without going through the spinal cord. Those parts? The digestive system, the heart, some reproductive and sexual organs, and the trigeminal nerves. This is a long way from conclusive, but it is does suggest that time spent studying the vagus nerve could benefit me.
**It implies that the enteric system is unique in this. I have a vague sense it’s not, or at least that I’m misrepresenting what is unique about it, but I’d need to research it to be sure.