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ScienceWeek
NEUROSCIENCE: ON SYNESTHESIA
The following points are made by C. Mulvenna and V. Walsh (Current Biology 2005 15:R399):
1) The term "synesthesia" refers to a phenomenon in which an individual experiences a sense other than the one being stimulated. This unusual pairing is automatic, present since childhood and consistent across time. A specific experience will be activated by the same stimulus with a seemingly arbitrary connection. For example, the sight of the letter "q" may always activate the experience of a deep red color; or a middle C played on a violin may always activate the experience of the taste of tuna. The pairings can be more complex for some synesthetes; for example, a sequence of pitches may activate the sensation of gold, yellow and white moving rapidly upwards and at an angle to the right, like a rippling stream . The condition is also referred to as "sensory cross-activation".
2) Synesthesia does not apply to forced or acquired associations, such as the word "Christmas" having connotations with the color red, the smell of mince pies, or the general sound of Christmas carols. It also does not include sensations triggering memories, such as a song eliciting the memory of a person or place.
3) The first known reference to synesthesia in scientific writing is John Locke's account of a blind man who described the color scarlet as the sound of a trumpet in 1690. Similar isolated case-studies continued for some time, and it was described in detail by Francis Galton in 1883. Since then synesthesia has suffered repeated waves of dismissal as a phantom condition, despite continual reports of its existence. It is only relatively recently, with the application of brain imaging techniques, that it has gained creditability in the scientific world as a genuine neurological condition, and this acceptance has led to the current surge in synesthesia research.
4) Sensory cross-activation in the brains of synesthetes has now been observed by positron-emission tomography (PET) and functional magnetic resonance imaging (fMRI). Activation of brain regions associated with visual perception was observed in blindfolded synesthetes listening to words that evoked visual experiences. These activations were shown to be clearly different from those evoked in either non-synesthetes or the same synesthetes listening to tones that did not evoke visual experiences. Activation of areas strongly associated with the perception of color was observed in a group of word-color synesthetes. This was not observed in non-synesthetes, even after they were trained to associate pairings of words with colors. Current investigations are examining if this neurological trend is observable across subtypes involving other senses.
5) One theory suggests that, rather than synesthesia being caused by extra connections "growing" between sensory areas, the apparent cross-activation could be a result of reduced apoptosis which aids differentiation of the sensory areas of the brain in the first months after birth. Because of this increased sensory connectivity, some experiences between certain senses in infancy may stay fixed in the brain. If this is the case, we were all synesthetes at one stage, but sensory modularity developed more explicitly in non-synesthetes.[1-5]
References (abridged):
1. Baron-Cohen, S. (1996). Is there a normal phase of synesthesia in development?. Psyche. An Interdisciplinary Journal of Research on Consciousness. Volume 2, number 27.
2. Baron-Cohen, S., Burt, L., Smith-Laittan, F., Harrison, J., and Bolton, P. (1996). Synesthesia: Prevalence and similarity. Perception 25, 1073-1080
3. Baron-Cohen, S. and Harrison, J.E. (1997). In: Synesthesia: Classic and contemporary readings.. (1997). Cambridge, Massachusetts: Blackwell Publishers
4. Cohen-Kadosh, R., Sagiv, N., and Linden, D.E.J. (2005). When blue is larger than red: colors influence numerical cognition in synesthesia. J. Cogn. Neurosci., in press
5. Galton, F. (1883). Inquiries into human faculty and its development. London Press
Current Biology http://www.current-biology.com
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Related Material:
THERMAL STIMULATION OF TASTE SENSATION
The term "chemoreceptors" refers to biological cells specialized to respond to chemical stimuli, and the function of such a cell is to signal to the nervous system a change in the chemical environment. In humans, for example, major use of chemoreceptors occurs in those parts of the body specialized for taste (gustatory sense) and smell (olfaction). Taste receptors are found in the epithelium of the tongue, and these receptors are responsible for sour, sweet, salty, and bitter sensations from food applied to the tongue. Taste receptors are also found in the pharynx and the upper part of the esophagus.
In contrast to olfactory receptors, taste receptors do not have their own output extensions (axons) to send signals to the central nervous system, but instead taste receptors stimulate the endings of nerve fibers that send input to the central nervous system ("afferent fibers"). Taste receptor cells are gathered into groups as "taste buds", and the sensing of taste stimuli occurs in finger-like projections (microvilli) at the surface of these taste buds, with various chemical mechanisms proposed to account for transduction of taste stimuli. In general, sourness depends primarily on the acidity of a chemical stimulus, and salty sensations are evoked by solutions with a high sodium concentration. Sweetness and bitterness, on the other hand, are apparently transduced by specific *receptor cell membrane receptors for sugars, amino acids, and other chemicals
Threshold concentrations for taste sensations produced by most ingested substances are relatively high. For example, the threshold concentration for sodium chloride is approximately 10 millimolar, for sucrose, 20 millimolar, for citric acid 2 millimolar. The threshold is much lower for certain bitter-tasting potentially dangerous plant compounds: the threshold concentration for quinine is 0.008 millimolar, and for strychnine 0.0001 millimolar.
In humans, approximately 4000 taste buds are distributed throughout the oral cavity and upper alimentary canal. Taste buds are approximately 50 microns wide at their base and approximately 80 microns long, each bud containing 30 to 100 taste receptor cells. Approximately 75 percent of all taste buds are found on the upper (dorsal) surface of the tongue.
The following points are made by A. Cruz and B.G. Green (Nature 2000 403:889):
1) The authors point out that the first electrophysiological recordings from animal and human taste nerves (in 1935 and 1985 respectively) provided clear evidence of thermal sensitivity, and studies have indicated that as many as half the neurons in the mammalian taste pathways respond to temperature. Since temperature has never been shown to induce sensations of taste, it has been assumed that thermal stimulation in the taste system is somehow nullified.
2) The authors report, however, that heating or cooling small areas of the tongue can in fact cause sensations of taste: warming the front (anterior) edge of the tongue (which is innervated by the chorda tympani nerve) from an initially cold temperature can evoke sweetness, whereas cooling can evoke sourness and/or saltiness. Thermal taste also occurs on the rear of the tongue (which is innervated by the glossopharyngeal nerve), but the relationship between temperature and taste is different in that location from that found in the front of the tongue.
3) The authors suggest these observations indicate the human taste system contains several different types of thermally sensitive neurons that normally contribute to the sensory code for taste, and that although there is evidence for neurons whose chemosensitive mechanisms are temperature sensitive, thermal sensitivity in some taste neurons may arise from cellular processes unrelated to chemosensory transduction.
Nature http://www.nature.com/nature
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Notes by ScienceWeek:
receptor cell membrane receptors: This phrase is a good illustration of the two uses of the term "receptor" in cell biology. Biological cells specialized to respond to specific physical or chemical stimuli are called "receptors cells", or merely "receptors". However, specific proteins or groups of proteins, embedded in the surface of a single cell, and which respond to interactions with specific ions, chemical groups, or molecules, and send molecular-level signals to the interior of the cell, are also called "receptors".
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Related Material:
CROSS-MODALITY SENSATION
The following points are made by Alison Motluk (The Scientist 2001 11 Aug):
1) The classical and prevailing view of the brain holds that there are 5 separate senses feeding into 5 distinct brain regions genetically wired to handle one and only one sense each. Sensory information is thus parcelled up and analyzed in isolation. Recent research, however, demonstrates that people who are born blind use the visual cortex when they read Braille, and this has led to the idea that everyone has the capacity to use non-classical regions for the analysis of sensory information under certain circumstances, and that the brain is much more versatile than many researchers have believed.
2) The brain is apparently able to quickly recruit new areas for sensory analysis and also able to quickly reverse the recruitment, with a time-scale apparently too short to involve new connections. Tactile and auditory input into the visual cortex is apparently present in all people and can be utilized for analysis if behaviorally desirable.
3) Some researchers now believe that the brain is not organized into specific sensory modalities, but instead it is split into units with specific tasks or particular problems to solve, and these task-oriented or problem-solving units simply use the most relevant information available. The units may prefer certain senses at certain times and under certain conditions, and prefer other senses at other times. Vision, for example, may be the preferred way to judge distances, but in the absence of vision, hearing or touch sensation may be used to complete the same analysis.
The Scientist http://www.thescientist.com
ScienceWeek http://scienceweek.com
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