Sensory tremens
Sensory tremens was a cognitive disorder which could cause realistic auditory and visual hallucinations, such as those suffered by Chakotay while in proximity to chaotic space. 

This disorder was characterized by a degradation in neuron protein insulation in the optic and auditory nerves. Treatment consisted of controlling the hallucinations with neural suppressants. The only way to cure this condition was to deactivate the offending gene. ()
Medical conditions