Why doesn the lack of sensory input reclaim “processing power” for the brain, possibly boosting cognitive capacity?
I think there are a few complementary pieces to the answer: 1) The way the brain is wired up means that not every piece of it can be repurposed at the drop of a hat for a different task. A lot of this repurposing takes place but you can’t, for example, close your eyes and then dedicate your retina and optic nerve to doing speech recognition. 2) Reduced sensory input does make processing power available for other things. Most people who need to focus on a math problem are going to do better at it if they are not in the presence of many TVs, lots of conversation, flashing lights, etc. What you’re describing as a surge in cognitive capacity in absence of input is more usually described as a difficulty in concentrating in the presence of it. 3) There are two ways to not give your brain over to processing sensory input – one is to close your eyes (ears, etc.) and the other is to not pay attention. The latter works pretty well, which is why you don’t have to actually close your eyes to think
