Also known as synesthesia, synaesthetic hallucination, synaesthetic experience, reflex false perception, secondary sensation, and secondary sense perceptions (German: sekundären Sinnesempfindungen). The term synaesthesia comes from the Greek words sun (together, unified) and aisthanesthai (to notice, to perceive). It translates loosely as 'joined sensation'. The coiner of the term synaesthesia is unknown. At least from the late 19th century onwards, it has been used to denote either a hallucination that does not tally with the sensory modality in which it is experienced (i.e. what is more properly referred to as an * extracampine hallucination) or a hallucinated secondary percept which is triggered by a sense perception in a different sensory modality (for example, a hallucinated patch of blue which is triggered by the sound of a clarinet). The person usually credited with the first reference to synaesthesia is the British empiricist philosopher John Locke (1632-1704). In his 1693 Essay Concerning Human Understanding, Locke recounts the story of a blind man who associated the colour scarlet with the sound of a trumpet. But the man who actually opened up the scientific study of synaesthesias is the Austrian philologist F.A. Nussbaumer, who in 1873 described a systematic association between colours and musical tones (i.e. * coloured music) which he and his brother had experienced since childhood. Phe-nomenologically, the group of synaesthesias is extremely diverse. On the basis of the work of the Hungarian-Dutch experimental psychologist Géza Révész (1878-1955), three broad classes can be distinguished: * perceptual synaesthesias, in which the secondary percepts (i.e. hallucinations) are actually perceived, * conceptual synaes-thesias (in which the secondary percepts are envisaged in the form of an ideated sensation), and * mental synaesthesias (in which the secondary percept comes to mind when its name is suggested). The expression synaesthesia has traditionally been applied to hallucinations occurring simultaneously with - or in close succession to - regular sense perceptions, but in a different sensory modality. The following definition, borrowed from the German psychiatrist Johannes Stein (1871-1951), captures the phenomenon in its classic sense: "We speak of a synaesthesia when stimulation of one sense organ evokes a response not only from this one, but also from a sense organ which was not directly stimulated." Examples of this phenomenon are tastes accompanied by visual hallucinations (*taste-photisms, or taste-chromatisms) and colours followed by music (* light-phonisms). The most conspicuous phenomenon within this class is * colour hearing, i.e. a visual sensation triggered by an auditory stimulus. A second sense in which the term synaesthesia is used is when voices are heard within the knee or the stomach, for example, as if they were functioning as an acoustic organ. Examples of this type of synaesthesia can be found in many classic textbooks of psychiatry. The German neurologist Carl Wernicke (18481904), for example, reports on *phonemes experienced inside the trunk, a leg, and a boot. The German psychiatrist and neurologist Georg Theodor Ziehen (1862-1950) describes various cases of voices heard within a neuralgic part of the thorax, and one patient who experienced visual hallucinations inside the thorax. The Swiss psychiatrist Eugen Bleuler (1857-1939) drew up a whole list of such phenomena, including voices in the abdomen, the womb, the penis, the urine in a person's bladder, and in a polyp on a person's nose. Bleuler was also the one who introduced the apter term extracampine hallucination to designate these phenomena. The reason why phenomena such as a voice 'heard' by the knee or the thigh have often been included in the group of synaesthesias is that their genesis is considered more or less similar. The Heidelberg psychiatrist Hans Walther Gruhle (1880-1958), for instance, explains a voice 'heard by' the thigh as follows: "The explanation must be that we are dealing here with synaesthesias in the - not entirely correct - sense that the patient has a sensation in the thigh, for example, at the very moment of hearing a voice, and now both sensations fuse in such a way that he identifies them topically." Current diagnostic criteria of synaesthesias, as issued by the American neurologist and synaesthesia expert Richard Cytowic, state that cases of synaesthesia are always involuntary, that they involve the 'outward projection' of the secondary percept (rather than their being imagined), that the associations are consistent and discrete, that they are easily remembered, and that they are accompanied by an unshakable conviction and sense of validity. The terms * idiopathic and * developmental synaesthesia are used to denote synaesthesias occurring in the absence of any demonstrable pathology. This type of synaesthesia tends to occur in healthy individuals from birth or early youth onwards. Synaesthesias occurring in the context of overt pathology are referred to as * non-idiopathic synaesthesias. This latter group includes * drug-induced synaesthesias, * epileptic synaesthesias, * brain-damage-induced synaesthe-sias, and * spinal cord-damage-induced synaes-thesias. As in all types of hallucination, synaesthesias can sometimes be of use to the individual experiencing them. Reportedly, the German Egyptologist Karl Lepsius (1810-1884) employed his * chromatisms as a guide in his philological inquiries. The British scientist Sir Francis Gal-ton (1822-1911) describes the case of a woman who made use of the colours associated with the letters of the alphabet when she had difficulty with the spelling. And Wassily Kandinsky (1866-1944), the Russian expressionist painter, employed his synaesthesias in designing performances which combined colour and music, in collaboration with the composer Alexander Scri-abin (1872-1915). Reportedly, Scriabin and his compatriot Rimsky-Korsakov (1844-1906) were also familiar with synaesthesias. As to the prevalence of synaesthesias in the general population, estimates vary significantly: Cytowic came up with a point prevalence of 1 in 20,000, the British neuroscientists Simon Baron-Cohen et al. made it 1 in 2,000, and Galton's estimate was 1 in 20. Starting with Galton, the propensity to experience synaesthesias has been associated with heretability. Linkage analyses and population studies have confirmed that this is the case for the idiopathic types of synaesthesia. As to the neurophysiological correlates of synaesthe-sias, four major hypotheses exist. The best known of these is perhaps the cross-activation hypothesis, which suggests that perceptual information from one sensory modality may cross over into another. Competing models attribute the mediation of synaesthesias to disinhibition between adjacent brain areas, to an increase in feedback connections between successive stages of the sensory hierarchy, and to excess activity between these successive stages, for example as a result ofa disinhibition of feedback connections. Conceptually, the notion ofsynaesthesia shows certain similarities to other cross-activation symptoms, such as the *functional hallucination and the *reflex hallucination (both terms referring to hallucinations evoked by a regular sense perception), the * Tullio phenomenon (involving the occurrence of vestibular phenomena in response to auditory stimuli), and the Proust phenomenon (involving odours evoking autobiographical memories, named after the French author Marcel Proust (1871-1922)). The term *pseudosynaesthesia is used to denote metaphorical references reminiscent of synaesthesias. Today synaesthesias enjoy something of a cult status among people calling themselves synaesthetes, who exchange experiences at conferences and via the internet.
   Cytowic, R.E. (2002). Synesthesia. A union of the senses. Second edition. Cambridge, MA: MIT Press.
   Galton, F. (1883). Inquiries into human faculty and its development. London: J.M. Dent & Sons.
   Harrison, J. (2001). Synaesthesia. The strangest thing. Oxford: Oxford University Press.
   Nussbaumer, F.A. (1873). Ueber subjektive Farbenempfindungen, die durch objektive Gehörempfindungen erzeugt werden. Eine Mittheilung nach Beobachtungen an sich selbst. Wiener Medizinische Wochenschrift,3, 52-54.
   Ramachandran, V.S., Hubbard, E.M. (2001). Synaesthesia - A window into perception, thought and language. Journal of Consciousness Studies, 8, 3-34.
   Révész, G. (2001). Introduction to the psychology ofmusic. Translated by de Courcy, G.I.C. Mineola, NY: Dover Publications.
   Stein, J. (1928). Die Synästhesien.In: Handbuch der Geisteskrankheiten. Band I. Allgemeiner Teil I. Edited by Bumke, O. Berlin: Verlag von Julius Springer.

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