Synesthesia is a phenomenon in which a sensory experience in one modality triggers a sensory experience in another modality. This experience may take many forms with the pairings of various senses. One synesthete, for instance, may hear particular tones which correspond to particular colors, another may experience a particular taste when viewing a particular shape. In its most common form, each number or each letter of the alphabet will automatically elicit a specific color. In each case, there is an initial stimulus, the 'inducer', and an automatically elicited 'concurrent'. These paired associations are stable and consistent across the synesthete's lifespan.

Synesthesia runs in families and more than 75 percent are women (Cytowic, 2002). Modern scientists have known about synesthesia since Francis Galton published a paper on the subject in 1880 (Ramachandran & Hubbard, 2003) but, until recently, synesthesia was often regarded as a hoax or a product of the synesthete's active imagination. Synesthesia can result from epilepsy or the ingestion of hallucinogens but synesthesia most often exists without external influence or brain abnormality (Cytowic, 2002) In recent years, serious scientific interest in the phenomenal experience and the possible neural mechanisms of synesthesia has emerged. From the results of various studies, scientists now agree on the credibility of synesthetic experience and several remarkable theories on the subject have been proposed. One of the most notable theories, the Neonatal Synesthesia (NS) hypothesis, proposes that, although synesthesia is somewhat rare in the adult population (sometimes suggested as affecting approximately one in every two hundred people) (Ramachandran & Hubbard, 2003), all infants up to about four months of age are synesthetes (Baron-Cohen, 1996). The NS hypothesis argues that "babies experience sensory input in an undifferentiated way.

Sounds trigger both auditory and visual and tactile experiences" (Baron-Cohen, 1996). A very similar but perhaps less controversial hypothesis is the Cross-Modal Transfer (CMT) hypothesis. This hypothesis argues that "objects can be recognized in more than one modality" and this implies that, for example, "babies can recognize one object versus another from their appearance, even if they have previously only touched them without seeing them" (Baron-Cohen, 1996). Research evidence for the CMT hypothesis has been growing.

"Rose, Gottfried, and Bridger (1978) found that 12 month olds look longer at an object they had just explored orally" and similar results have been found for 1 month olds (Baron-Cohen, 1996). Lewkowicz and Turkewitz (1980) found evidence for the ability of 1 month olds to recognize 'matched intensity' between two sensory modalities. The experimenters used the infants' change of heart rate as an indication of their recognition of a novel (or 'unmatched') stimulus. An increase in heart rate was assumed to indicate that a stimulus was novel or 'unmatched' in intensity with reference to another stimulus.

The experimenters found that the least heart rate change occurred when an image of white light was followed by white noise which adults rated as "matched in intensity, but they showed significant heart rate change when the light was followed by a sound of very high or low intensity" (Baron-Cohen, 1996). Although the CMT hypothesis is now widely accepted, the NS hypothesis is relatively new and fairly controversial. The NS hypothesis builds on evidence which has supported the CMT hypothesis but goes further to propose that, not only can infants correlate sensory modalities in relation to a stimulus, they experience an undifferentiated sensory response to a stimulus. The NS hypothesis proposes that the modularization of sensory modalities (so that one modal experience will no longer elicit another) is a normal developmental progression and that, with synesthetic adults, this progression has not taken place (Baron-Cohen, 1996).

Maurer points to a great degree of neural connectivity between particular sensory modules within the brain in support of his hypothesis. Hoffman (1978) found a greater degree of widespread activity in the brain in infants as compared to adults when evoked responses to visual stimuli were recorded (Baron-Cohen, 1996). It is also assumed that the progressive modularization of the sensory centers of the brain (that is, less 'cross-talk') serves an adaptive function as the child develops, allowing information processing to become more efficient. Although most synesthetes do not report that their abilities have disrupted their lives in any way which could be seen as maladaptive, some synesthetes with a more reciprocal brand of synesthesia report feelings of stress and "information overload" (Baron-Cohen, 1996). An example of this type of synesthesia would be seen in someone who not only saw colors in response to sounds, but also heard sounds in response to colors. In this case, the evolutionary argument for the maladaptive ness of dysmodularity could be proposed.

A number of theorists have attempted to explain the apparent dysmodularity of the sensory systems in synesthetes. A number of theories of synesthesia may exist against the background of a more general theory of neural development and organization: Gerald Edelman's theory of Neuronal Group Selection (Edelman & Tononi, 2000). Edelman and Tononi stress the remarkable individuality and variability of each brain and that, within each brain, there are numerous pathways and systems of neural interaction, which are not structurally identical, by which a particular outcome may occur. This is similar, conceptually, to the fact that many different DNA sequences may provide the specifications for the production of the same protein. This variability, however, confounds attempts at a general theory of global brain functioning. Edelman proposes that, while in early development, initial brain anatomy is constrained by genes, beginning in the early embryonic stages, synapses are established by selection.

Neural connections initially branch far and wide in the brain but are strengthened or weakened based on individual patterns of neural firing and, throughout an individuals life, are selected for based on behavioral experience. The neural architecture of an individual's brain also shows evidence of a quality Edelman calls 'reentry' (Edelman & Tononi, 2000). Reentry describes a massive network of reciprocal parallel paths between brain areas, each influenced by every other, yet each initializing its own signals. In keeping with the idea of a reentrant neural system, Catherine Kavas salis states that "a quick scan of the literature soon generates an overwhelming number of studies reporting evidence for cross modal brain connections and multimodal, multifunctional neurons in every imaginable animal" (Baron-Cohen, 1996).

Theories of synesthesia often focus on either the varying degrees of these cross modal connections or the degree of individual awareness of the associations generated by them. Theories of synesthesia have begun to make use of recent neurological studies which illuminate some of the physiological correlates of synesthetic experience. Using functional MRI scanning, Julie Nunn and her colleagues found that synesthetes who see color in response to spoken words showed activation in the V 4 (color responsive) area of the left hemisphere of the brain but not in the V 1 (primary visual cortex) area. Although control and experimental (synesthetic) subjects showed activation in auditory and language areas, control subjects showed no such V 4 activation (Cytowic, 2002).

Unexpectedly, an additional finding of the study was that synesthetes showed no such activation of V 4 in the left hemisphere when viewing colored surfaces. The right V 4 area, however, was similarly active in control and experimental subjects when viewing the colored surfaces. The control group, however, showed activity in the left hemisphere as well as right hemisphere V 4 areas (Cytowic, 2002). Additionally, it has been found that nonsynesthetes who merely imagine colors do not show activation in the V 4 area. Therefore, "the brain basis of synesthetic color experience is consistent with real color perception rather than color imagery" (Cytowic, 2002).

Richard Cytowic, a leading researcher on synesthesia, has amassed significant evidence on the perceptual reality of synesthetic experience and has shown how important a role an understanding of the phenomenon may play in understanding consciousness itself. Synesthesia studies have lead Cytowic to favor a model of brain organization called the distributed system. The central features of this model are the distribution and simultaneity of function across brain structures (rather than the more familiar and traditional hierarchical and sequential models) (Cytowic, 2002). As scientists continue to pursue an understanding of the phenomenal experience and neurological underpinnings of synesthesia, they will continue to make strides toward understanding the nature of perception and consciousness.

The idea that we are all naturally synesthetic as infants is an intriguing possibility which prompts us to investigate, with fascination, the changes in neural architecture and function which occur throughout our development.

Bibliography

Baron-Cohen, S. (1996).
Is there a normal phase of synesthesia in development? Psyche, 2 (27). Cytowic, R.E., (2002).
Touching tastes, seeing smells - and shaking up brain science. Cerebrum, 4.3, Summer 7-26. Edelman, G. & Tononi, T. (2000).
A universe of consciousness: How matter becomes imagination. New York: Basic Books. Ramachandran, V.S. & Hubbard, E.M. (2003).