Perceptions that occur in the absence of external stimuli:

Auditory:  Occur most frequently in the psychoses of schizophrenia, mania, and depression.

Visual:   Occur more frequently in psychotic episodes due to medical, neurologic, and toxin-induced disorders although are often associated with primary psychiatric disease.

Olfactory and Gustatory:  Often occur as prodromal symptoms of complex partial seizures.

Tactile:  Frequently occur in delirium i.e., formication is the sensation of bugs crawling on the skin.