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Auditory-tactile Synesthesia
Synesthesia has been described by Professor Richard Cytowic as a ‘union of the senses’. It is a perceptual condition whereby a stimulus that produces a normal sensory response simultaneously produces a response from an additional sensory pathway. People who experience this are referred to as synesthetes. Synesthesia can be present from early childhood or acquired because of a brain injury, for example. Synesthetic experience is involuntary, and the cross-sensory associations experienced by the synesthete remain consistent over time and contexts. There are many types of synesthesia, within the names of the different types the first word describes the stimulus that induces a sensory response, and the second word, the additional sensory response that the person experiences (for example, grapheme-colour and lexical-gustatory). At present there is no one standardised diagnostic criteria for synesthesia which makes it difficult to verify genuine cases of synesthesia

What is Auditory-tactile Synesthesia?
Auditory-tactile synesthesia (also known as hearing-touch a or sound-touch synesthesia) is a rare type of synesthesia in which specific auditory input induces a simultaneous physical sensation in the body. For example, in response to a specific sound, an auditory-tactile synesthete might feel tactile sensations, tingling or localised pressure in the absence of being physically touched.

The relationship between sound and touch brain regions
Studies have found that the regions of the brain associated to sound (somatosensory cortex) and touch (auditory cortex) are closely related. Researchers have used the examples of the high-pitched noise of fingernails screeching on a chalkboard and the sharp sound of smashing glass as examples of auditory stimuli that elicit strong physical responses. It has been suggested that these responses may be because the areas of the brain associated with sound and touch have similar mechanisms of encoding information, and they exist in adjacent regions of the brain, because of this there might be some cross-activation between the two areas. Consistent with this theory, neuroimaging studies have found that is extensive white matter connectivity between auditory and somatosensory regions of the brain White matter is an important structure in the brain that coordinates communications between the different brain regions. Researchers therefore suggested that people who experience auditory-tactile synesthesia might have stronger cross-sensory sound-touch connections in the brain

The neural basis of Auditory-tactile Synesthesia
A case of Auditory-tactile synesthesia reported by Professor Tony Ro and his colleagues was the first to shed light on the underpinning neural mechanisms of the condition. A second acquired case of a man who acquired multiple types of synesthesia, including an auditory-tactile type has also been reported in the scientific literature. Both patients suffered damage to the thalamus associated with a stroke. Over time, the brains of both patients showed heightened neural activity between auditory and somatosensory cortex in comparison with healthy controls. Researchers believe this to be the neural basis of their experiences of synesthesia.

Acquired cases
Patient ‘SR’ developed an unpleasant form of auditory-tactile synesthesia following an acquired brain injury. SR suffered a stroke, and Magnetic Resonance Imaging scans (MRI) conducted fifteen months after her stroke found a lesion to her right ventrolateral nucleus of the thalamus (VL) but the scan found no disparity in white matter between the damaged and intact hemispheres of her brain. SR reported some problems with attention and sensory abilities but did not report any experience of synesthesia

However, eighteen months after her stroke, SR first reported experiences of auditory-tactile synesthesia, she reported intense feelings of tingling or pressure on her left arm and hand that were induced when she heard a specific announcer on the radio. Another MRI scan two months later found atypical white matter in the damaged lesion of SR’s brain

MRI studies revealed that the SR’s auditory-tactile synesthesia may have occurred due to compensatory cross modal plasticity in the brain. Over time the somatosensory region of SR’s brain had become more receptive to sound because it had become deprived of somatosensory input. Consistent with this, another study used a MRI technique called diffusion tensor imaging techniques and found increased white matter connectivity between auditory and somatosensory regions in the damaged region of SR’s brain in comparison to non-synesthete controls.

This second reported case of acquired auditory-tactile synesthesia was reported by a male patient who suffered a stroke involving a lesion to the left lateral posterior nucleus of the thalamus. This resulted in multiple synesthetic experiences including an auditory-tactile type. Six months after his stroke he started to experience sensitivity to sounds, for example an ambulance siren would induce warm feeling in his body. Nine months after his stroke he felt an unusual experience of motion when he listened to certain sounds for example, Chinese music. . MRI studies conducted eighteen months after his stroke found enhanced neural responses to sound in different brain regions, including the auditory and somatosensory cortex

Researchers have suggested that irregular changes to the circuitry of the thalamus might result in the abnormal cortical connectivity found in the brains of synesthetes. This has been found in different types of congenital synesthesia such as grapheme-colour synesthesia The two acquired cases of auditory-tactile synesthesia support this idea. Both patients acquired their synesthesia following a stroke affecting their thalamus.

Autonomous Sensory Meridian Response (ASMR)
Auditory-tactile synesthesia has also been compared to the experience of Autonomous sensory meridian response (ASMR). People who experience ASMR experience an atypical tactile and emotional response to auditory stimulus in comparison with the general population. An ASMR response occurs when certain visual and auditory stimuli induce a ‘static-like’ or tingling feeling on the scalp, neck, and spine ASMR is distinct from the similar sensation of musically induced ‘frisson’ because the stimuli that induces the experience are generally more reliable, in that they consistently elicit the same response from the individual. To date, only one study has explored the brain mechanisms associated with the experience of ASMR The fMRI study assessed a resting state network of the brain called the default mode network (DMN) to determine whether the brains of people who experience ASMR are different to control participants. Results showed differences in functional connectivity in the brains of ASMR participants in comparison to the control group. The researchers thought these differences could reflect the ASMR participants inability suppress sensory-emotional experiences in the way that most individuals can. The researchers also found reduced connectivity in the medial dorsal thalamus in the ASMR participants and suggested that this also might play a role in the auditory-tactile and emotional responses elicited in people who experience ASMR.