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Although scientists do not fully understand the workings of the brain and perception, the basic concept seems fairly simple on an intuitive level. The brain interprets one set of stimuli in a specific way. Certain people with synesthesia, however, can experience a single stimulus in different ways. Are they naturally predisposed to hear red? Do these people have extra neural connections allowing them to taste green? Some scientists claim that all humans begin their lives as synaesthetes until they learn to differentiate their senses. It brings into question the nature of perception, and how the brain perceives reality.
Synaesthetes experience "cross-modal" associations involuntarily, so that the feeling of one sense stimulates the sensations of another (1). Five features for clinical synesthesia exist (1): People with the rare condition of synesthesia experience it passively, but it requires a certain stimulus to elicit a response. The individual projects the sensations into his peri-personal space. The sensations do not change over time, and are generic, unelaborated sensory perceptions. In addition, the experiences are very memorable; one subject said that "he was merely a passive observer as the reminiscence unfolded itself" (1).
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). Synesthesia predominantly affects females and shows sex-linked transmission (5).
Cytowic suggests that the "link between a stimulating sensation and the synesthetically-perceived one" exists on a lower level of the neuraxis (1). Synesthesia involves metabolic shifts, occurring only in the left-hemisphere of the brain, away from the neocortex toward limbic structure, reducing rational processes in favor of emotional ones. The hippocampus also contributes greatly to the synesthetic experience, corroborated by altered states of consciousness from seizures and drugs which effect the hippocampus and also produce synesthesia (1).
Further experimentation using PET scans of the brain shows that blindfolded synaesthetes given certain auditory stimuli showed activity in the visual centers of the brain than their control-group counterparts (3). Thus, conscious visual perceptions occur without activation in the primary visual area for those with synesthesia, lending physical evidence to subjects' experiences. The condition has a physiological basis for the neural connections, yet it only appears to effect very few individuals.
Because the way the brain receives all sensory information is the same, all feelings may have a similar neurological origin. Thus, we may all start life in a state of synesthesia until our brains learn to differentiate our sense into different modalities. Maurice Merleau-Ponty believed that "synesthetic perception is the rule, and we are unaware of it only because scientific knowledge shifts the center of gravity of experience, so that we unlearn how to see, hear, and generally speaking, feel" (2). Perhaps then the brain employs a "hybrid sensory system" (4).
Experimental observation of human newborns suggests that a spatial link exists between vision and auditory senses in the prenatal brain (4). Additionally, studies in neonatal animals show clear, though transient, connections among sensory areas of the brain (5). 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). Most humans outgrow the sensation synesthesia and learn to differentiate different sensations, perhaps to prevent sensory overload.
Thus, looking at synesthesia provides clues to sensory perception overall. The purpose of sensory perception is not only to experience reality, but also to grasp which sensations are important and significant, making certain perceptions stand out more than others. Synesthesia originates in the limbic area of the brain, the irrational, "emotional" brain responsible for "qualitative alteration of consciousness" (1). The objective reality expected, and not experienced by synaesthetes, although they believe their perceptions to be real, may not exist for non-synaesthetes either. The limbic brain also gives all experiences significance over others ones. Moreover, synesthesia may represent premature perceptions, before the brain differentiates them into specific sensations. Thus, perception may be "holistic, constrained by sensation as it unfolds from within" (1).
The holistic nature of perception makes sense given the uniform nature of neural transmissions, which differentiate only according to areas of the brain connected. Synesthesia demonstrates this differentiation by comparison with a lack of differentiation, so that sensations overlap. A trait all humans start with, until the brain learns to separate sensations. The source of the connections, the limbic brain, still serves as an emotional guide to the rational objective world, giving reality different perspective for each individual.
2)http://www.psychiatry.cam.ac.uk/isa/whatis.html, Overview on the ISA homepage
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.cnn.com/HEALTH/9511/synesthesia/, article on CNN
7)http://www.ad-i.com/viral/what/synes2.html, paper by Kamel
8)http://www.healthlink.usa/synesthesia.html,health information about synesthesia from Healthlink
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