[M.D. Holloway]

Synesthesia - a mixing or combination of senses - is a concept relatively familiar in the Western world. History is littered with descriptions of people tasting words and seeing sounds; the folklore of creative and expressive arts, in particular, filled with stories of famous "synesthetes." Russian author Vladimir Nabokov, German painter Wassily Kandinsky, American artist David Hockney, and Russian composer Alexander Scriabin have all been labeled as such, and books on the topic were published as early as 1890 .

Though at that time interest in synesthesia was mainly from those involved in humanities disciplines, it was clear that synesthesia was an unusual form of perception, which opened the door for the phenomenon to be studied by neuroscientists. As these scientists gained the intellectual apparatus (more complex theories of the human nervous system, for example) to approach these issues of perception, synesthesia again became a topic of interest in the 1980's and '90's. Historical reasons for this are perhaps due to the increased availability and interest in technology during the Cold War and Americans' resulting faith in expertise of science. Also, the discovery of LSD and trend of recreational hallucinogen use in the 1960's and '70's could have influenced this interest, as hallucinogens (particularly LSD) are known to induce synesthesia.

Though this historical background may not seem important to a neurobiological paper, I wish to in part use this paper to show links between culture and scientific "objectivity." The controversy surrounding current theories of synesthesia is most certainly culturally informed, and scientists working in this area are as likely to cite philosophers as they are scientific research . This intellectual debate surrounding synesthesia, which is both philosophical and scientific, can also inform our class discussion of the notion that "brain equals behavior."

Cytowic, who is one of the foremost writers on the topic of synesthesia posits a complex mŽlange of scientific and epistemological points to analyze the phenomenon. An explanation of his work is crucial to getting inside the synesthesia debate. Instead of viewing synesthesia as a neurobiological concern, Cytowic explains how multidisciplinary work will provide insight into "consciousness, the nature of reality, and the relationship between reason and emotion" (1). He aims to look not only at experimental data but at a broad overview of the scientific concept of the brain in order to answer the synesthesia question. His work is circumscribed by historical and philosophical frameworks as are mentioned above.

Important to Cytowic's work is his definition of synesthesia. His clinical diagnosis is comprised of several pieces - that synesthesia is :


* not voluntary or controllable by the subject, but it is also not constant - it is usually triggered by some stimulus
* "projected" - perceived to take place in the area immediately surrounding the subject
* "durable and generic" - associations between the senses will be constant over time and will also be relatively abstract (e.g., associating a certain color or shape - not a complex image - with a specific sound).


Though synesthesia is described as any mixing of senses, certain senses - sight and sound - are far more often involved than others. The relationship of the mix is also unidirectional, meaning that someone may see a certain color when they hear a certain sound but will nor hear the sound when they see the color. According to Cytowic, smell and taste are uncommon in synesthetic situations (1). I find this particularly interesting in light of the fact that humans use taste and smell together and also that smell is an underdevloped sense in humans (for example, humans do not receive olfactory inputs in their sleep).

There are also patterns in the population of synesthetes. Because synesthesia has been seen in many generations of families, yet never passed on from father to son (1), it has come be accepted that synesthesia is a X-chromosome-linked trait. Though Cytowic's estimate of a frequency of 1/25,000 is often cited, it is important to note that few other researchers have offered estimates and also that most research on synesthesia has been done only in the U.S. and in the U.K. (1). It is also agreed that synesthesia is more common in women (this also supports the hypothesis that synesthesia is linked to the X-chromosome), but the data on this provide ratios as disparate as 3:1 (Cytowic, in the U.S.) and 8:1 (Baron-Cohen, in the U.K.) (4). The population of synesthetes performs normally on neurological exams.

Several traits found in synesthetes indicate that they may have one cerebral hemisphere which dominates. Synesthetes are more often non-right-handed than non-synesthetes. They also exhibit tendencies toward being very neat and organized and also have particularly good memories - many attribute this skill to the way they perceive senses together. Deficiencies are often seen in mathematical skills and in sense of direction. Another interesting commonality between synesthetes which Cytowic cites is a high frequency of "unusual experiences" like dŽjˆ vu, clairvoyance, and premonitions (1).

Observational data also show neurochemical bases for synesthesia or synesthetic states. Cytowic explains how synesthesia can be induced in non-synesthetes during seizure discharges in the hippocampus, which is part of the limbic system. If these seizures take place in not only the hippocampus but also the cortex of the temporal lobe, the sensation is more detailed. He sees an importance in the fact that synesthesia is memorable and emotional (subjects feel sure that they have had the experience). He points to the fact that synesthetes disagree on the associations between senses as evidence that synesthesia is perhaps not a condition, but rather a mode of perception in which humans take part to varying degrees (1).

Cytowic asserts that synesthesia is not a cortical function, and cites several pieces evidence. He has shown that synesthesia takes place in the presence of increased limbic and decreased cortical function. This is interesting because increased cortical metabolism usually accompanies any activity, yet the test subject with increased limbic function performed normally on cognitive and neurological texts. Blood flow in the subject's left cerebral hemisphere, usually at a lower level than circulation in the right hemisphere, dropped another 18% in relation to the right hemisphere. Cytowic uses this for his somewhat radical notion of the primacy of the limbic system (sometimes seen as an evolutionary dinosaur) in the synesthetic response (1).