The age-old philosophical question of perception, “Is my green the same as your green?” remains a problem that by its very nature science will never be able to answer. This question addresses whether one solitary sensory system perceives input precisely as another’s. But for some people, this question is a resounding no. For these individuals one sensation is not limited to one sensory field. Instead, a mixed wiring occurs between senses. Sounds are given colors; shapes are given smells. Synesthesia ensnares the senses in ways unfathomable to the average person.
A common method to determine the cause of deviant attributes is to look for discrepancies between the abnormal structure and the typical structure. In synesthetes sensory areas of the brain are wired to fire together. In other words, one input fires two senses which give rise to artificial attributes (e.g. green music). For most synesthetes this misfiring has been present since birth. The cause of this sensory blending is hereditary.
Not all synesthesia is the result of genetic predisposition though. In some cases synesthesia can be induced, known as adventitious synesthesia. The most popular form of this is often associated with lysergic acid diethylamide or LSD. Patients who use this compound often report a temporary, psychedelic blending of various senses. Other methods have been reported to elicit similar effects, both temporary and permanent. Another temporary occurrence of synesthesia can be induced by seizures within the temporal lobe. This is unsurprising as many studies of synesthetes show abnormal activity in this area.
Permanent synesthesia can be induced in people by a variety of sources. In the case of neural damage inflicted by stroke, the effects can be seen as the result of plastic change attempting to offset the impacted areas. Another example involving neuroplasticity is that of a sudden loss of a sensory system, such as blindness or deafness. These sensory areas of the brain devoted to these processes are suddenly left without input and as a result begin to lose connections. As the pathways dissolve, other systems begin to utilize this suddenly free space. As a result, some processes wire together and fire erroneously when subjected to a sensory signal.
Much can be gleaned from adventitious synesthesia. If looking for deviant processes, the best comparison would be to have a before and after image of the same individual. While LSD is illegal in many countries and inducing seizures in patients is generally frowned upon some researchers have turned to a novel method to induce synesthesia, hypnosis. This technique is a source of great controversy within the scientific community. While some consider it to be an apt method of bridging the gap between the conscious and subconscious mind others consider it no more than well-played showmanship or suggestive influence from the researcher.
A group of researchers, Kosslyn et al., sought to find more concrete data that shows a controlled correlation between hypnotic techniques and actual changes sensory perception. To do this they utilized an arrangement of colored squares in two conditions, normal and greyscaled. Then they used PET imaging while subjects were under hypnosis and exposed them to these photos under four conditions: 1) perceive the greyscaled photo with color, 2) remove color from the normal photo, 3) view the grayscale as it appears, and 4) view the normal photo as it appears. Individuals who were not sensitive to hypnotic techniques were also included as a control method.
What they found was a shift in blood flow to key areas generally associated with synesthesia, such as the fusiform gyrus. While there was an increase in blood flow for nearly every parameter only the condition in which participants perceived grayscale with color proved significant. A peculiar difference was noticed between left and right hemisphere activity. Whereas the left hemisphere only changed in activity within the hypnotized group, the right hemisphere altered activity across both groups but in response to what the subjects were instructed to perceive.
This study shows that, at the very least, the induced effect of hypnosis to perceive color on a grey template has a physical basis within the brain. While the other attributes may still certainly be open to bias from confounding variables, this small portion of synesthesia-like behavior has an explanation other than subjective perception or role-playing. Although this shows that indeed something is occurring in this area of the brain it fails to provide any causal evidence.
Two hypotheses currently exist to describe the mechanism behind these abnormal interactions between senses. The first suggests an over abundance of neuronal connections between areas and that this phenomena is due to said hyperconnectivity. The other suggests a normal connectivity between areas. Whereas normally signals are inhibited across these regions, in synesthetes these signals are dis-inhibited. A study performed by Kadosh et al. examined the likelihood of the first hypothesis within a short time frame of induced synesthesia-like activity.
A common test given to grapheme-color synesthetes (those who see numbers with a color) is to place the number on a background with either the same color as their perceived number or an opposing color and test for recognition. This same test was given to a group of highly hypnotizable individuals who acted as their own controls. Participants received the test under two randomized conditions, no suggestion and post-hypnotic suggestion, and then scored for speed and accuracy. The suggestion received by subjects set the numbers 1 through 6 to uniform colors which corresponded with various backgrounds. What a non-synesthete would see on a red background would be the number 1; however, when an induced-synesthete sees this image the 1 would blend into the background. They tested the participants in four trials, ABBA and BAAB, with “A” being without hypnotic suggestion and “B” with the suggestions. The trials were separated by a two week interval between the third and fourth testing.
The results of the study yielded a significant difference in test performance between suggested trials and those without. The effects of the suggestion even carried over the two week gap in trial time. Due to the extremely short length between trials of this study it is highly unlikely that the first hypothesis can be deemed appropriate for these results. The initial hypothesis required a hyperconnectivity between key areas, but as the effects were quickly picked up and immediately dropped (BAAB condition) macroscopic growth and decay of connections is not plausible. Plasticity of that magnitude takes place on a much larger time scale. Instead focus should be placed upon the more micro-level changes associated with the latter hypothesis, dis-inhibition.
While this study neither disproves hyperconnectivity in congenital synesthetes nor proves disinhibition amongst induced-synesthetes, it does show that the brain is remarkably complex in its role of regulating our perception. Subconscious processes alone can induce a phenomenon long thought to be solely the result of brain injury or congenital anomalies. This event can help give insights into the microscopic realm of neuroplasticity by how small inconceivable changes in neuronal firing can result in the blending of sensory systems.
Hypnosis is a technique that has long been scrutinized if not simply seen as fraudulent, and not without good reason. Yet, as the field comes under greater scrutiny and is applied to the proper field, such as synesthesia, we find that there is much to be learned. It is in response to the latter study of induction that we can begin to wonder if instead of turning on this bridging of senses, might these techniques enable synesthetes to disconnect their senses from one another. This experiment, at this time, appears not to have been attempted. As the field of neuroscience continues its growth into new areas of interest a greater, more diverse array of techniques are becoming commonplace.
Cohen Kadosh, R., Henik, A., Catena, A., Walsh, V., Fuentes, L. J., 2009. Induced Cross-Modal Synaesthetic Experience Without Abnormal Neuronal Connections. Psychological Science. 20, 258-265.
Kosslyn, S. M., Thompson, W. L., Costantini-Ferrando, M. F., Alpert, N. M., Spiegel, D., 2000. Hypnotic Visual Illusion Alters Color Processing in the Brain. Am J Psychiatry. 157, 1279-1284.