Synaesthesia is a neurological condition in which stimuli of one sensory modality evoke experiences in another modality. This is thought to occur as a result of insufficient "pruning" during development, so that most of the pathways connecting parts of the brain mediating the different senses remain in place instead of being eliminated. Consequently, there is too much cross-talk between sensory systems, such that activation of one sensory pathway leads simultaneously to activity in another.

Once believed to be extremely rare, synaesthesia is now thought to be relatively common. The cross-modal connections implicated in the condition are present in all of us, to a greater or lesser extent. Thus, some researchers argue that we all experience synaesthesia-like sensations to some degree, but that these sensations are particularly intense in only some individuals.

Earlier this year, researchers from the California Institute of Technology described a new form of the condition, called hearing-touch synaesthesia, in which moving visual stimuli evoke sounds. Now Vilayanur Ramachandran and David Brang of the Center for Brain and Cognition at the University of California, San Diego report another unusual form of the condition. In the journal Neurocase, they describe the first two known cases of individuals with what they have called tactile-emotion synaesthesia, who experience a specific emotion whenever they touch a particular texture.

Ramachandran and Brang investigated the stability of these experiences over time, by testing the two synaesthetes' responses to a battery of textures and then re-testing them with the same textures 8 months later. They also recorded the subjects' facial expressions during the tests, and used the skin conductance response as an objective measure of the emotional impact of each texture. (This response cannot be used to distinguish between emotions, but is linked to emotional arousal, as strong emotions often cause sweating.) These tests confirmed that the emotions experienced in response to textures were remarkably consistent over time. In both subjects, the same textures evoked the same emotions during both tests, and with the same intensity. Both of them experienced a wide range of emotions in response to different textures, and in both there were textures which elicited no emotions whatsoever.

In patient AW, a 22-year-old female, the most vivid emotions are evoked by denim, which causes in her strong feelings of depression and disgust, and silk, which produces feelings of happiness and contentment. Other textures evoked a wide variety of emotions and feelings: when she touched corduroy, AW felt confused; leather aroused feelings of receiving criticism; multicoloured toothpaste made her feel anxious; wax made her feel embarrassed; tylenol gel caps made her feel jealous; and different grades of sand paper made her feel either guilt, relief, or as if she was telling a white lie. In patient HS, a 20-year-old female, the same textures often evoked different feelings. She felt no real emotion when touching denim but was disgusted instead by the texture of fleece and wax; corduroy made her feel disappointed; bok choy made her feel irritated, but smooth metal made her feel sedated and calm. In this subject, the strongest emotion was evoked when she touched soft leather, which made her feel extremely scared - she described the sensation as "making my spine crawl."

The tests also showed that AW's tactile-emotion synaesthetic experiences are best evoked by the forefinger and little finger on both hands. Interestingly, they found that tactile inputs from her feet also evoked emotions, albeit not as intense as those evoked by touching textures with the hands. In some cases, the evoked emotion varies depending on the limb used to touch it: a ceramic tile elicied feelings of comfort when touched by the hand, and feelings of power when touched by the feet. By contrast, subject HS only experiences synaesthetic sensations when touching textures with her hands, and for her the intensity of the emotion elicited by a particular texture depends upon the size of an object. For example, she finds small stones mildly comforting, but larger stones provoke more intense feelings of comfort.



The authors suggest that these experiences arise as a result of cross-activation between the somatosensory cortex, which processes sensory information from the body, and the insula, a component of the limbic system, which is located in the temporal lobe and which is involved in emotion. Another possible (but not mutually exclusive) mechanism is reduced inhibition between activity in the different sensory pathways, as a result of a neurotransmitter imbalance. Evidence for this comes from an earlier study, in which the researchers showed that synaesthetic experiences could be temporarily blocked by selective serotonin reuptake inhibitors (SSRIs) such as Prozac, which are commonly prescribed for depression. Based on these observations, the authors also suggest that synaesthesia is caused by mutation of a gene encoding a serotonin transmitter subtype.



Ramachandran and Brang also speculate about the evolutionary origins of the new form of synaesthesia they have characterised. They propose that our primate ancestors may have evolved unconscious mechanisms for predicting the potential of an object to cause harm. Thus, tactile sensations which may be beneficial to survival (such as soft furs, for example, which provide warmth) may activate the parts of the limbic system mediating pleasure, whereas others which may be harmful (such as jagged stones) may be connected to those areas mediating aversion. With time, these pathways would have incorporated stimuli of different intensities and across sensory modalities, to give rise to our preferences of some textures over others. In tactile-emotion synaesthetes, however, the pathways and associations encoded by them become randomly enhanced, so that cross-talk between the pathways generates "junk" patterns of neural activity which lead to indiosyncratic experiences.

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(Found at BPS Research Digest)

Ramachandran, V. S. & David Brang, D. (2008). Tactile-emotion synesthesia. Neurocase, 14: 390-399. DOI: 10.1080/13554790802363746