It must be noted that hippocampal seizures can induce sensations similar to synesthesia. The Preserved Neural Connectivity theory, mentioned by Simon Baron-Cohen in his book Blue Cats and Chartreuse Kittens: How Synaesthetes Color Their World, states that
4.2 Synesthesia is involuntary but elicited. It is a passive experience that happens to someone. It is unsupressable, but elicited by a stimulus that is usually identified without difficulty. It cannot be conjured up or dismissed at will, although circumstances of attention and distraction may make the experience seem more or less vivid.
4.3 Synesthesia is projected. It is perceived externally in peri-personal space, the limb-axis space immediately surrounding the body, never at a distance as in the spatial teloreception of vision or audition. My subject DS, for example, is a college teacher who, on hearing music, also see objects - falling gold balls, shooting lines, metallic waves like oscilloscope tracings - that float on a "screen" six inches from her nose. Her favorite music, she explains, "makes the lines move upward."
4.4 Distinguishing the experience of perception as "near" (e.g., chemosensation, touch, proprioception, body schema, the orientation of one's body within Euclidean space) or "distant" (e.g., seeing, hearing) is concordant with concepts of classical neurology and neuroanatomy. This idea was most clearly articulated by Paul Yakovlev (1894-1983) who mapped "three spheres of motility" onto three anatomical divisions of the neuraxis (Yakovlev, 1948, 1970).
4.5 Synesthetic perceptions are durable and generic, never pictorial or elaborated. "Durable" means that the cross-sensory associations do not change over time. This has been shown many times by test-retest sessions given decades apart without warning. "Generic" means that while you or I might imagine a pastoral landscape while listening to Beethoven, what synesthetes experience is unelaborated: they see blobs, lines, spirals, and lattice shapes; feel smooth or rough textures; taste agreeable or disagreeable tastes such as salty, sweet, or metallic.
4.6 Though synesthetes are often carelessly dismissed as being just poetic, it is we who must be cautious against unjustifiably interpreting their comments. For example, my index case MW described the shape of mint as "cool glass columns." On analysis, this turned out to be his shorthand way of trying to convey the quality of the tactile experience - "what is it like." When pressed to elaborate the sensations he felt, he said:I can reach my hand out and rub it along the back side of a curve. I can't feel where the top and bottom end: so it's like a column. It's cool to the touch, as if it were made of stone or glass. What is so wonderful about it, though, is its absolute smoothness. Perfectly smooth. I can't feel any pits or indentations in the surface, so it must not be made of granite or stone. Therefore, it must be made of glass.
beyond the normal human fetal stage, the brain does not have direct neural connections between the auditory and visual areas. This theory goes on to suggest that, probably for genetic reasons, in individuals with synesthesia the pathways between the auditory and visual areas of the brain exist beyond the early embryonic stage, when normally such connections would die off. Certainly there is evidence that these connective pathways between the auditory and visual areas of the brain exist during fetal development in other species, such as the macaque monkey and the domestic cat. These pathways, or projections, are transient; typically, they disappear approximately three months after birth. There is some evidence that they may exist in human newborns and, as in cats and macaques, get pruned as the brain biologically matures.
We in cognitive science do not know the complete mechanisms by which synesthesia works, but certainly, the Preserved Neural Connectivity theory posits an idea about the developmental neurological aspects of this - synesthesia almost always manifests before age 4, and persists into one's adulthood.
Richard E. Cytowic, a cognitive scientist at Monash University in Australia, asserts that the limbic system is the critical brain center for synesthesia. PET and fMRI could serve as useful in evaluating this. Eraldo Paulesu at London University tested this by examining color-noise synesthesia in a PET scanner, giving subjects words or tones. Two brain areas of particular interest emerged: the posterior inferior temportal cortex and the parietal-occipital junction.
The posterior inferior temporal cortex participates in visual processing, particularly face recognition, sentence comprehension, writing, and spelling.
The parietal-occipital junction participates in understanding hearing.
Both of these areas activated in the brain while a synesthete listened to words. In a non-synesthetic person, neither area activated. The limbic system didn't activate either so Cytowic's theory was debunked, but new areas emerged as the focus for synesthesia.
Sensory research isn't my field, but the effects of this on cognitive processes contain some mental gymnastics, according to Simon Baron-Cohen, for those who have it.
Both as a neuroscience student and a synesthete, I can say that this has some implications for the future of developmental neuroscientific research, especially where comparisons are drawn between brains and a computer.