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).
Cytowic urges that the role of the limbic system in synesthesia, and indeed in the nervous system, be taken seriously. He argues that because of the Western notion of a dichotomous split between reason and emotion, hierarchical, corticocentric models of the brain have been established. He pushes for a "multiplex" concept of the brain, which is less centralized and allows for more environmental influence than our idea of "brain equals behavior." He subscribes the ideas of Ommaya, a critic of current models of brain organization and asserts that, "we are irrational creatures by design and that emotion, not reason, may play the decisive role both in how we think and act." The fact that humans have both well-developed neocortical and limbic features is used to back up his idea that "the relationship between cortex and subcortical brain is not one of dominance and hierarchy . . . but of multiplex reciprocity and interdependence." Again, he establishes the primacy of emotion with studies of people emerging from comas, whose "intellect" recovers after their more primal limbic responses return. Cytowic challenges "computational" models of the brain and instead proposes a holistic vision of the perception process (1).
Ctyowic accredits the post-1930 drop in interest to the inability of psychology and biology of the time to grapple with necessary issues. However, he states, when these sciences caught up to the phenomenon they had earlier attempted to study, the rejection of personal experience as a basis for academic work has kept synesthesia out of the scientific limelight. From 1890-1930, until now the hey day of the interest, synesthesia was primarily discussed by artists as an experiential phenomenon in which they wished to partake free of scientific explanation (1). Kandinsky is quoted as saying: Lend your ears to music, open your eyes to painting, and . . . stop thinking! Just ask yourself whether the work has enabled you to "walk about" into a hitherto unknown world. If the answer is yes, what more do you want? (1)
Despite being a scientific researcher, Cytowic agrees that "our passion for a detached and 'objective' point of view has diminished other kinds of knowing" (1). Though Cytowic is seen as an expert on synesthesia and his work is cited frequently, it is not surprising that there is challenge to his ideas from other members of the scientific community. Burt and Smith-Laittan, of Cambridge University, cling to the notion of objectivity and researcher expertise, as evidenced by a comment in their article that subjects who report themselves as identical twins are often wrong and therefore research involving them may be disregarded (in this case, it is convenient for Burt and Smith-Laittan to do so). Though they are faithful to this idea of Scientific Objectivity, Burt and Smith-Laittan have written the article in which it is most clear how scientific conclusions are informed by cultural values. To back up their statement that limbic and cortical regions work together, they cite Descartes. They continue to state that: Current perspectives appear to owe as much to the notion of hierarchy of science and so do not as much compete as models of brain function as provide explanations couched in vocabulary appropriate to the discipline.
They suggest an explanation of synesthesia based on a modular model of the brain, i.e., that different types of perception might rely on different neurological pathways (2). Another insight into synesthesia which has yet to be significantly explored is the neo-natal sensory system. Burt and Smith-Laittan state that the genetic basis of synesthesia may cause abnormal differentiation between visual and auditory pathways (2) and other researchers discuss infants' perception. There is a sense that certain parts of the brain, like the frontal lobes (which are in the neocortex), do exhibit significant sensory integration. There is evidence that infants, perhaps immediately from birth, experience visual and auditory stimuli together (3). In 1993, Baron-Cohen has posited the Neonatal Synaesthesia Hypothesis: . . .that early in infancy, probably up to four months of age, all babies experience sensory input in an undifferentiated way. Sound triggers both [sic] auditory, visual and tactile experiences. A truly psychedelic state (4). This transitions nicely into an idea on synesthesia which includes aspects of many theories on synesthesia. Andrews explains that: . . . in creatively encountering one's environment, one's experiences are not merely visual, nor are they an oscillation between various diverse sensory stimuli. One's perceptions are polysensory with the various sensory stimuli homogeneously combined so that when one is perceiving a situation, their encounter may include hearing, seeing, and feeling. Another time it may include seeing, tasting, and smelling . . . . explaining synesthesia as a mode of perception not necessarily so far removed from standard perception (4). The common thread between all these researchers is that they see some neurochemical basis for their understanding of synesthesia, and that they must work this understanding in with their vision of the way different parts of the brain relate to each other. All agree that synesthesia is a perceptual mode and that there is some genetic basis for its strong expression. For scientists interested in mapping out life and giving out maps for humans to navigate it, this genetic relationship gives them a chance to add something to the list for the Human Genome Project. Yet for those more interested in looking at trends and patterns in the population, there is a sense that synesthesia may not be an experience very far removed from "normal perception." The fact that this community of scientists, often using the same data, has such divergent ideas about synesthesia shows the mythical quality of this scientific objectivity. While the phenomenon we have designated synesthesia has not changed over time, our ideas of it have, and the contemporary scientific community debates it. As Cytowic strives for a holistic vision of the parts of the brain, we must strive for a holistic idea of the scientific process so that we can identify our biases and use debate to constructively analyze problems instead of to attack each others ideas.
1. Synesthesia: Phenomenology And Neuropsychology - A Review of Current Knowledge by Richard E. Cytowic, from PSYCHE: an interdisciplinary journal of research on consciousness, 2(10), July 1995
2. Current Work And Theory Of Synaesthesia: How many people hear in colour? by Alison Motluk, from New Scientist 27 May, 1995, page 18:
3. Synesthesia - the mixing of the senses
4. Synesthesis: To Hear Colors, Taste Shapes, and Experience Other Startling Sensory Blending
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