[Paleopsych] SW: On Synesthesia

Tue Jul 26 19:45:10 UTC 2005

Neuroscience: On Synesthesia
http://scienceweek.com/2005/sw050722-2.htm

    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
    --------------------------------
    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".
    --------------------------------
    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