There is loads of evidence from studying states that alter the brain.
For example, damage to certain parts of the brain produces hemispatial neglect. The person loses awareness of not just one half of the universe but the concept of that half. This is not consistent with an incorporeal soul, which would continue to make judgements about the full universe albeit with hemispatial blindness.
Then there are dissociative states, as with some migraine auras, where some aspect of self-awareness is lost, leaving the person with the sense that they are watching their own actions in the third person, but with otherwise intact behavior.
There are the dissociative anesthetics like nitrous oxide and ketamine, which create a solipsistic disconnection from the outside world without abolishing self awareness.
The memory function can become disconnected while consciousnes remains, as with some sleep disorders and drugs.
There are delirious states, seen in illness and with some drugs, where the person is conscious but experiencing imagination on the same footing as reality.
The corpus callosum connecting the two halves of the cerebrum can be severed, leaving each side with an independent partially-disconnected consciousness.