Synaesthesia is a complex experience and, being rare, it is difficult to study. Nevertheless, several interesting theories have
been proposed and tested. The idea that synaesthesia is a mental illness
is long gone, but that doesn't mean it's not "all in the head"! Positron Emission Tomography (PET)
scans have shown that different areas of the brain are active for
synaesthetes experiencing a cross-modal association than for non-synesthetes engaged in
the same task. The synaesthetic experience depends exclusively on the left brain
and is associated with a decreased blood supply to the neocortex. This results in enhanced limbic expression. Therefore, we can assume that the system responsible for synaesthesia is located or influenced by the limbic system more than the neocortex, which is not what most people would predict without the evidence from the PET scans. Additional support for this is that there is an emotional aspect associated with a synesthetic experience. In fact, in order to fulfill the diagnostic criteria for synaesthesia an emotional response must be present.
Cases of synaesthesia may be idiopathic (developmental), with the person having experienced synaesthesia as long as they can remember, or non-idiopathic, resulting from a
known etiology or mechanism which is acquired and produced synaesthesia.
One type of non-idiopathic synaesthesia is epileptic synaesthesia.
This type of synaesthesia is experienced during electrical discharge (seizure) in the
limbic region of the brain and is found in less than 7 percent
of all temporal lobe - limbic seizures. Seizures which remain
confined to the hippocampus in the limbic area of the brain produce
elementary sensations, such as a simple taste like bitter. Those which
spread to the cortex of the temporal lobe produce more specific and
elaborated perceptions, such as the taste of a chocolate chip cookie.
Indeed, it has been found that electrical
stimulation of the temporal cortex alone evokes synaesthesia.
Synaesthesia may also be induced by drugs, especially hallucinogenic compounds
such as mescalin, peyote or LSD. This, however, is relatively infrequent.
Another cause may be deafferation, the loss of
certain neurons which make up nerves, or sensory deprivation.
An example is that a sound evoking simple visions, such as a flash of light, may be seen in the portion of the visual field in which a person is blind.
Brain Damage Induced
Acquired synaesthesia tends to arise from damage to anterior
portions of the brain, often the optic nerve. Frazinno, Yap, and Gross (1994),
in accordance with this observation, found that 27-31% of neurons in
the ventral portion of the premotor cortex are bimodal. This means that
some neurons in all people respond to both vision and touch stimuli. These sensory systems are often found in parallel in the brain. There
have been a few cases in which synaesthesia was brought on by gross
brain stem lesions. The removal of one patient's brain stem tumor
put an end to his experience of synaesthesia.
Spinal Cord Damage Induced
A unique experience
that is technically synaesthesia may occur in about 12% of people
with spinal cord injuries. In some people, a touch above a lesion
causes a local sensation as well as a sensation in other parts of the body not normally related to the stimulated area. These phantom
sensations appear to come from an artificial synapse located near the
In two percent of concussions, photo- or audio-algesic synaesthesia
occurs. This is a temporary condition where sudden noises or bright light cause momentary
pain extending into the trunk of the body or an extremity.
Most cases of synaesthesia, however, are idiopathic; it is their normal mode of perception. There have been a number of theories put forward to explain this phenomenon. Those which follow are not necessarily mutually exclusive, for no single theory is likely to account successfully for all the various forms of developmental synaesthesia.
Uninhibited Natural State
One view, held by Richard Cytowic, is that everyone is synaesthetic in reality; most of us are just not consciously aware of it. Under this theory, it is posited that a stimulus causes a rebalancing of regional metabolism. This is the way a migraine is produced. Only, with synaesthesia, the stimulus causes parts of the brain to become electrically disconnected from one another so that the normal processes of the limbic system of the brain are released and can be sensed in our conscious state. As yet, there is little evidence for this point of view.
Neonatal Synaesthesia Hypothesis
Another school of thought is that there is a normal phase of synaesthesia in development, that all babies experience sensory input in an undifferentiated way in early infancy, probably up to about 4 months in age. This intermodal unity has been the focus of many studies. It has been found, for example, that neonates are able to detect tactile (touch)-visual correspondences. At a critical period, the senses may become increasingly modular, presumably because modularity leads to more rapid and efficient information processing and is therefore highly adaptive. In adults with synaesthesia the process of modularization of the senses may not have been completed. Connections between different sensory modalities in synaesthetes may have either grown or not died off. Although no conclusive findings have been produced for humans, it is known that such connections exist between auditory and visual areas in the early developing brain of many species. In the macaque monkey and domestic cat, transient connections exist up to three months after birth. There exists the possibility that the normal human brain also contains these connections but that there is an inhibitory mechanism, like a kind of gate, which prevents their activation. This inhibitory mechanism may be dysfunctional in synaesthetes.
Certain studies have led researchers to believe that, in late infancy, there is a change in the aspects of stimulation attended to. Infants respond only to differences in amount of stimulation whereas adults, while retaining this ability, respond to differences between modalities of stimulation instead. In this sense, we all have synaesthetic capabilities.
Influence of Genetics
There is evidence that some people may be more prone to developing synaesthesia due to their genetic makeup. This would involve the expression of genes that regulate the migration and maturation of neurons in the developing brain, or 'neuronal pruning' of the sort mentioned earlier. It is speculated that women are at least six times as likely to exhibit this condition than men. This shows that the condition is sex-linked and dominant. It has been found by others, that synaesthesia can be transmitted by any sex combination, leading to the belief that the trait is not sex-linked but, rather, autosomal.
For colored hearing synaesthesia in particular, three psychological theories have been put forward. The doctrine of the unity of the senses or linkage theory, proposes that the perpetuation of a primitive perceptual experience in the limbic system is the root cause of color synaesthesia. As this system evolved, the perception was differentiated into two separate senses, hearing and vision.
A similar theory, the crosstalk theory, holds that auditory and visual information
pathways may cross in synaesthetes. These cross-modal neural
connections may be numerically greater than usual or simply used
in different ways.
Some believe higher cognitive/cortical level processing (the limbic system is thought to be lower level) to be involved. According to this view, colored hearing synaesthesia is the result of a chain of mental associations, some of the intermediate links having dropped out of awareness. For instance, a person may see red every time they hear a trumpet because of the red uniforms of a brass band.
Feedback connections aid us in imagery, memory, sensory attention
and other cognitive functions, but could they also result in
synaesthesia? Auditory and visual information must meet somewhere
in the brain or we could not process them in conjunction as they
occur. These systems may contain feedback pathways normally but, in synaesthetes, they may be altered to include information from the other
As you can see, synaesthesia is an extremely complex
As methods for exploring the way the brain works improve,
the mechanisms of synaesthesia may be revealed.