In the phenomenon of synaesthesia, adequate stimuli in one sensory modality are accompanied by perceptions in one to four others. Even if descriptions of synaesthesia are well known in the scientific literature, only scarce empirical research results are available yet. Certain characteristics distinguish synesthesias from eidetic imagery. Idiopathic synaesthesias are more common in females, familial clustering occurs. Electrophysiology usually yields no abnormal results, functional imaging and neuropsychology hint to a reduced inhibition of secondary sensory associative areas by the temporal cortex of the speech dominant hemisphere as a neurobiological basis. Secondary synaesthesias are sometimes symptoms of diseases like epilepsy or migraine.
Autoscopy is a rare phenomenon, which consists of the pseudohallucinatory visual perception of the own body. The “Doppelgänger” exhibits characteristic features. Psychiatric comorbidity is common, occasionally the appearance of the double is associated with suicidality. The most common cause of autoscopy is temporal lobe epilepsy,a distinct lateralization, however, has not been demonstrated so far. Autoscopy can also be found in migraine attacks and has to be differentiated from other disturbances of body image.