Certain sounds can cause ordinary people to experience sensations in their body. Think of the sound of chalk on a blackboard and the cringing sensation that accompanies it. This is similar to one of the rarest forms of synesthesia called auditory-tactile synesthesia. Synesthesia is the cross-integration of senses due to cross-connectivity within the brain. This can include the association of two or more senses at the same time. Some synesthetes with this form of synesthesia experience the sound of instruments while at the same time feeling them someplace on their bodies, feelings like brushing or tickling. Typically, individuals affected by auditory-tactile synesthesia associate sounds with particular tactile (touch) feelings.
Types of Auditory-Tactile Synesthesia
Auditory-tactile synesthesia seems to have two forms: individuals who have had it since birth and individuals who acquire it later in life. For those individuals who have had this form of synesthesia since birth, it can be thought of as merely a different, more enhanced way of experiencing the environment. Usually these experiences are automatic, involuntary, and consistently unique to the individual. Acquired auditory-tactile synesthesia usually occurs as a result of some sort of neurological damage to the brain, especially the thalamus, although sensory deprivation and hallucinogenic or psychedelic drug usage can also induce it. This form of auditory-tactile synesthesia typically has resultant unpleasant and intense tingling sensations.
Connection of Hearing and Touch
It is hypothesized that abnormal or cross-modal connectivity results in inappropriate structural links between the sensory modalities of touch and hearing. Stroke-induced masking of somatosensory thalamic input might allow short-term, already existing connections between the adjacent regions of the brain. Both feeling and hearing depend on the transduction of physical events into frequency-based neural codes, which suggests that the two systems may be linked in every individual. Synesthesia might be an enhanced form of these cross-cortical links that the brain utilizes every time it integrates audition and vision. Physiologically, auditory, and somatosensory processing begins with receptor organs that are similar in structure and function. Both touch and audition rely upon the displacement of mechanoreceptors (neuroreceptors that respond to physical deformation) to translate physical events into nerve signals. For touch, the cell membrane of mechanoreceptors lying within the skin detect any sort of physical deformation and can determine the amount of pressure and length of touch. Within the ear, hair cells are responsible for converting vibrations that enter the inner ear into neural messages that can then be passed on to the brain in order to create sound. Both are also frequency-dependent and tuned to different frequencies. One way that auditory-tactile synesthesia may come about is damage to the brain. Damage to the regions directly involved in the integration of tactile stimuli or auditory stimuli tends to encourage compensation by the brain near the lesion. This can result in crossover of the modalities.
Applications of Auditory-Tactile Synesthesia
It is speculated that the sense of hearing may have evolved from the sense of feeling. Research with twins demonstrates that touch sensitivity is correlated with hearing acuity and can be heritable. Additionally, recent research shows evidence that the middle ear bones (the stapes, incus, and malleus) may have evolved from certain jaw bones. A practical and exciting application of induced visual tactile synesthesia is work in patients with phantom limbs. The phantom limb is resurrected visually in a virtual reality box containing a mirror. The intact hand’s reflection is superimposed on the phantom limb, thus making it appear as if the phantom hand were moving when the normal hand moves. More than half of the patients studied had the kinesthetic sensation of the involved hand, and most of these patients obtained relief from the painful spasms of the phantom limb.
Carolyn Johnson Atwater
See also: Auditory System; Mirror-Touch Synesthesia; Synesthesia; Touch
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