This paper reflects the research and thoughts of a student at the time the paper was written for a course at Bryn Mawr College. Like other materials on Serendip, it is not intended to be "authoritative" but rather to help others further develop their own explorations. Web links were active as of the time the paper was posted but are not updated.
2001 Third Web Report
Each individual experiences reality in a different way. Show ten people the same picture, and each will present a different description of the scene. We all live in the same world and yet we all have different philosophies and ideas about reality and life. What do these differences tell us about objective reality? Do our senses detect the same reality, or does each person see a different picture in her head? To some extent, this difference seems to hold true. What holds significance for me does not seem important to other people. The reality I grasp is unique to me.
For a small portion of people their sensory reality differs radically from the accepted norms of society (1). They suffer from a rare condition, synesthesia, which remains constant throughout their lives. Synaesthetes, instead of having their senses in concrete, separate blocks, blend different senses. Many merge their perceptions of words and numbers with different colors. In stronger cases, people see colors and shapes floating in their visual field when they hear certain musical tones. In one extreme case, a man felt specific tactile sensations when he smelled different things. Even more radically, some scientists now say all humans may have undifferentiated senses in early stages of development.
This paper will explore the understanding of synesthesia in terms of sensory development. The first of four sections will establish contemporary criteria for synesthesia and will evaluate its usefulness. >From this foundation, the paper will explore the possible associative origins of their condition. It will then analyze the shared physical characteristics among synaesthetes, and the origins of synesthesia in infants. Finally, this paper will examine the neurological basis for synesthesia in adult synaesthetes.
Synaesthetes experience "cross-modal" sensory associations involuntarily, such that the experience of one sense stimulates the sensations of another (1). Cytowic defines five features of clinical synesthesia (1). First, people experience synesthetic phenomena involuntarily whenever presented with a certain stimulus. The experience is not a forced association, but one the subject has felt since birth. Also, an actively-induced synesthetic perception, rather than a passive experience, is not a genuine phenomenon. Synaesthetes project the sensation into peri-personal space; they sense an actual physical quality outside of the self, not an internal sensation or aura. In addition, the triggered synesthetic perceptions remain constant over time and are unelaborated, generic perceptions. Synaesthetes report the experiences to be memorable, and emotional. They express feeling that their extra sensations are "real and valid" (1).
All of these criteria demonstrate that synesthetic perceptions as part of the individual's reality parallel normal perceptions of sight and sound as the reality of the general population. People can be tested for consistency of their experiences. Experiments with a single synesthetic subject show perfect consistency for colour descriptions of words, compared to a 17% consistency rate of a control subject with similar intelligence and memory levels (3). Not only did the subject seem genuine, based on her high level of consistency, but she gave far more detailed and vivid descriptions of the associated colours than the control subject. In a larger group of nine subjects, the consistency rate for more than one year was 92.3%, compared to the 37.6% recall rate of the control group one week later (3). The synaesthetes could duplicate their original answers after long delays because their experiences were actual perceptions, rather than processes of forming and remembering word associations. All of the subjects displayed similar levels of memory and intelligence, but the synaesthetes managed to far outperform control groups.
One theory of how synaesthetes acquire these cross-modal associations is through associations with childhood memories. Day completed a statistical analysis of colored letters and sounds for 167 synaesthetes, trying to find any tendencies among the sensations (2). The study itself showed no levels of correspondence for any letter/color combination above 61%, for O being white. The rest of the associations all showed tendencies of less than 50% correspondence. Trends predominate among vowels, but exist for C, R, F, S, B, and Y, ranging from 44% to 19% similarity. The significance of the associations is not overwhelming, but trends still exist. Day suggests that synesthesia may result from association, citing evidence that "some remember distinctly the items from which the associations derived . . . colours that readily match household or classroom items from their childhood" (2). The results of the study may only apply to those with colored letters and sounds synesthesia which Day tested. It also does not include any non-synesthetic subjects to compare the results to.
Other studies contradict an associative explanation of synesthesia. One set of experiments also tested 2 subjects with synesthetically colored letters or numbers along with 10 to 20 non-synesthetic control groups (6). The experiment tested the appearance of synesthetically colored characters as they moved away from the center of the visual field. Both subjects demonstrated a limit beyond which synesthetic colour projection did not occur, despite the fact that characters and colours were within visual range. Ramachandran and Hubbard believe this evidence denies synesthetic ties to memory because the subjects still see the characters but fail to evoke a synesthetic response at all positions. Instead, they infer that "synesthesia arises from cross-wiring between adjacent brain maps," especially between central V4 vision areas and colour areas in the fusiform gyrus of the brain (6). These claims, despite their indirect justification, require more tangible proof from the brain itself.
This finding, however, does not really contradict previous experimental results. Simply because synaesthetes do not rely entirely upon childhood associations for colours does not deny the possibility that colour associations originated them. Just as a child must learn letters and numbers at an early age, their synesthetically-inclined child brain ėlearns' colour associations at the same time. Interestingly, both research subjects only associated colours with Hindu-arabic numerals, not Roman numerals (6). The association may imply a learned association between the characters and colours necessarily takes place at the time when a child first learns language. Children typically learn numbers and letters at the same time, but learn Roman numerals several years later, which possibly accounts their developing no synesthetic associations with them.
Beyond first-hand accounts, there exist basic physical tendencies that help substantiate synesthesia. Synesthesia predominantly affects females and non-righthanders (5). Synesthesia does not discriminate, however, across cultural barriers either. Because synesthesia is associated with physical rather than cultural traits, it suggests a genetic link, be it sex-linked or autosomal. This genetic transfer necessarily must exist if synesthesia relates to the structure of the brain, as genes determine its formation, as they do for all parts of the body.
In addition to the genetic aspect, synesthesia appears to involve some link to development of the brain. Because the brain receives all sensory information in the same fashion or manner, all sensations may have a similar neurological origin. All may start life in a state of synesthesia until our brains learn to differentiate our senses into different modalities. Though most humans outgrow synesthesia and learn to differentiate different sensations, perhaps to prevent sensory confusion.
Experimental observations of human newborns confirm that a spatial link exists between vision and auditory senses in the prenatal brain. Both audio and visual stimuli elicit spatially coordinated eye movements in infants (4). Additionally, studies in neonatal animals show clear, though transient "multisensory convergence in structures known to be unimodal at maturity" (4). Human newborns (younger than four months) also exhibit increased blood flow, indicating neural activity, in both auditory and visual areas of the brain, when only auditory stimuli are present (5). Infants appear to be experiencing synesthesia before the brain completes its development. This evidence provides significant indication that the development of synesthesia arises from natural tendencies present in the brain.
Though the condition has a physiological basis for the neural connections, it only appears to affect very few individuals. Baron-Cohen claim that "several studies have confirmed that ėselective cell death' is part of infant brain development (8). Different senses, then, may lose their connections to each other. The synaptic bridges that used to unite different sensory areas become cut off. When this happens, the senses fully differentiate, ending the synesthetic period in development. The exact impetus for the lack of differentiation among synaesthetes still remains unclear. Exploring the physiology of the adult synesthetic brain could help understand their critical difference.
What evidence, then, exists to confirm synesthesia in adults? During synesthesia Cytowic observed metabolic shifts, occurring only in the left-hemisphere of the brain, that moved away from the neocortex toward the limbic structure (1). He observed reduced blood flow in the cortex during synesthetic perception, reducing rational processes in favor of emotional ones in the limbic structures. According to Cytowic this evidence suggests that the "link between a stimulating sensation and the synesthetically-perceived one" exists on a lower level of the neuraxis (1). The higher cortical levels of the brain associated with rationality did not seem to be activated during synesthetic perception. This research, however, has a limited scope because Cytowic only tested one synaesthete and showed no comparisons to a control group.
More current neuro-imaging experimentation using PET scans of the brain shows that blindfolded synaesthetes given certain auditory stimuli showed activity in the visual centers of the brain (3). Their control group counterparts, however, showed no activity in the V4 visual center of the brain. Thus, the experiment revealed that conscious visual perceptions occur without activation in the primary visual area for those with synesthesia, lending physical evidence to subjects' experiences. It also showed that synesthesia leads to activation in cortical areas of the brain, not deactivation.
This data compounds the speculations made by Ramachandran and Hubbard about the cross-wiring of the V4 visual area of the brain with the fusiform gyrus, also located in higher regions of the neuraxis, because of the increased activity observed in those areas. But, these experiments directly contradict Cytowic's findings. Instead of the cortex being deactivated during synesthetic experiences, the cortex turns on. PET scans showed increased cortical activity during synesthesia. His findings may be particular to the case he studied, but do not hold for the majority of synaesthetes. Cytowic's conclusions about the emotional processes stimulating synesthesia, then, collapse, because he bases these claims on the fact that the limbic, emotional structures receive more activity than the rational cortex.
Further studies also show increased brain activity among synaesthetes. A set of experiments recorded "event-related potentials (ERP's) . . . from multiple scalp sites" in control groups and synaesthetes who see color with letters (7). In comparison to the control group the synaesthetes showed increased activity in frontal and prefrontal scalp areas (such as the superior temporal gyrus). The synaesthetes experienced decreased activity to the anterior scalp regions. Both groups had increased activity in the primary visual areas. It appears that the frontal regions of the brain in synaesthetes contain multisensory neurons (7). Non-synaesthetes may also experience inhibition in the anterior region of the brain, preventing a synesthetic experience (7). Development of synesthesia may occur because people simply do not develop the inhibitory connections that occur in normal developments, which take over infant synesthetic tendencies.
Grossenbacher claims that sensory input in the brain "goes from single-sense cortical modules into multisensory areas" allowing people to relate the information of different senses (8). Thus, people have some synesthetic capabilities, but they remain largely inhibited. Also, people without synesthesia can experience the phenomenon through the use of certain drugs such as LSD and mescaline. These experiences show that ėnormal' brains contain the capacity and connections for synesthesia already, but they normally remain inhibited (8). Inhibition may occur over the infantile synesthetic connections rather than those connections simply dying because these connections can still be exhibited. If cell death in the cortex cut off the senses, then these interconnections could not be revived. But, people relate senses, even without the use of drugs, suggesting that the neuronal links among senses still exist in the adult brain.
The holistic nature of perception makes sense given the uniform nature of neural transmissions, which separate only according to areas of the brain connected. Synesthesia demonstrates the differentiation of normal sensory modalities by the very lack of differentiation exhibited in the brains of synaesthetes, which leads to sensory overlap. Synesthesia is a trait all humans start with, until the brain learns to separate sensations and inhibit synesthetic neuronal connections. All humans may have the capability of mixing their senses, but our brains choose not to. May-be the reality in our heads is all-alike, but our brains do not let us know.
1) Http://psy che.cs.monash.edu.au/v2/psyche-2-10-cytowic.html, in Psyche
2) Http://www.ncu.edu.y w/~daysa/Colored-Letters.htm, Day's Trends in Synesthesia
3) http://www.psychiatry .cam.ac.uk/isa/expinv.html, Experiments on the ISA website
4) http://www.ozemail.com.au/~ddiamond/synth.html,paper on Synesthesia
5) http:// psyche.cs.monash.edu.au/v2/psyche-2-27-baron_cohen.html, in Psyche
6) Http://www-psy.ucsd.edu/~edhubbar d/
8)Http://www.discover.com/d ec_99/featsyn.html, Discover article
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