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.
- The sound of dots moving: A new form of synaesthesia
- The neuropsychology of synaesthesia
- Stroke causes woman to feel sounds
(Found at BPS Research Digest)
Ramachandran, V. S. & David Brang, D. (2008). Tactile-emotion synesthesia. Neurocase, 14: 390-399. DOI: 10.1080/13554790802363746
Whenever I read articles like this, I find myself thinking thoughts that range between "That's not so weird" and "Well, yeah, doesn't everyone?"
I recognise that we are at risk of self-diagnosing every condition we learn about - it's a common occurrence in psychology students, I understand.
Nevertheless, from the time I first saw Ramachandran's address to TED, I have felt this enormous sense of relief. I completely identified with his descriptions of synaesthesia, and discovered in the process that I wasn't a (complete) freak. Subsequently, I have discovered boatloads of people who similarly identify with aspects of the condition. Perhaps it's not such a rare phenomenon.
The first time I realised that other people don't 'taste and smell colours' was when I told someone I didn't like beansprouts because they tasted like green. I didn't help my case when the person scoffed and I asked what colour they thought beansprouts tasted like.
On a recent trip to visit my family, I was explaining to my Mom about the condition and how I thought I might have an element of it. She thought for a while before saying, "Well that explains a lot of things!" and related several instances from my childhood when I had made remarks that sounded at the time like the sort of nonsense a sleeptalker spouts.
But unequivocally I can tell you that onions taste silver and soap tastes turquoise. Most tastes have colours I can't name because there isn't a word for them.
I reckon many perfumiers must be synaesthetic, too, because I will often smell a fragrance, associate it with a colour and later discover that the packaging or the liquid itself is a very close match to the 'colour' of the fragrance.
Oh and just to bring the whole tone of the conversation right down: most farts smell like the colour of a bruise!
I've always appreciated the fact that 4 is green, 6 is cyan, 7 is navy blue and 9 is obsidian. It made arithmetic homework in elementary school significantly more enjoyable. With every new form of synaesthesia which the researchers document, I have something new to be jealous about.
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.)
Aha! It's a Voight-Kampff test!
It occurs to me that one further way to test the specificity and regularity of the elicited emotions is to appeal to the theory of hemispheric asymmetry. As Richard Davidson and others have demonstrated, the LH encodes more positive emotions and mediates approach behaviors while the RH encodes more negative emotions and mediates withdrawal behaviors.
One could get baseline hemispheric reactions (say via ERP) to negative and positive pictures in these patients, and then compare this to the elicited positive and negative emotions from textured objects. If they are coincident, I think this would buttress the consistency argument further.
I recall being in school and feeling the same way chalkboards raked by nails sound whenever I felt the texture of paper with dry hands.
My parents still think I was just being a disobedient kid.
The whole idea of tactile-emotion synesthesia is a goldmine for dirty jokes. Or indeed pick-up lines. I'm just in a lame mood today and can't think of any.
Surely sandpaper is angry?
Hmm. TomK's reminiscence (#5) reminds me of my least favorite sensation - the failed, dried-out metal tip of a ballpoint pen scraping across paper. It actually makes me shudder with revulsion even to think about that sensation, but I never thought of it as a synaesthetic experience before...
Just yesterday I was browsing around YouTube and ran across a video about schizophrenia. As people described what it was like to be schizophrenic, I found myself suddenly thinking that their experiences actually sounded a lot like a form of synaesthesia. I'm not sure if I'd characterize it as a thought-emotion, an emotion-sensation, or maybe thought-sensation synaesthesia, but the descriptions really sounded like there was some kind of cross-sense contamination going on, and those who were experiencing it found it really disorienting and upsetting.
I'm a 41 yr old woman, american, who has had varieties of synaesthetic experiences for as long as I can recall. I've been described as 'visual to a fault' and certainly this tendency to have extra layers of info to perception seems to have been an obstacle in certain ways in my life. As a child I had huge problems with math because I was very distracted by not only colors of each number and combination, but genders and personalities for each digit. It got complicated fast. The many small and large overlaps of sensory info can give you an extra edge but then can also be too much static.
I also have to note that, for me, synaesthesia was something that fluctuated throughout my life but never disappeared, and something I realized early I needed to try to suppress to get by. I'm sure it was the same as any neurology that is atypical in that respect, socially a liability. It's validating to see research that shows it to be a concrete condition.
In regards to Michael Anes comment about the theory of hemispheric asymmetry, if the LH encodes more positive emotions and mediates approach behaviors while the RH encodes more negative emotions and mediates withdrawal behaviors does that correlate with great levels of depression and social withdrawal in predominately left handed people?
When I touch suade, I feel like I can't breathe. As if it almost takes my breath away. Is this a form of Tactile-emotion synaesthesia? I've always wondered, because it's not something I can explain and most people think I'm just being silly for feeling this way. It's not something I can fake and it happens EVERY time I touch suade. Tell me what you think...
@Red: I'm not sure what to think. Perhaps you should approach someone doing synaesthesia research.
i always thought what i had was called haptodysphoria i can't touch this velvety substance, but after reading this and other articles on tactile-emotion synesthesia i completely identify with the description. i often touch surfaces because i know i will get pleasurable sensations from touching them especially rough things like wood. yet if it is something both dry and slimy i feel distressed. only very few things i dont like the feeling of though, the velvety microfibre is the most intense - i shudder feel incredibly intensely uncomfortable, if i have to continue being in contact with it i cry, my hands feel really dry, i feel like i have to put them in water and my face tingles. its incredibly stressful to touch that fabric. mostly it is good pleasurable emotions from touching different fabrics i love rubbing my hands a long surfaces to touch them and the feelings of pleasure differ slightly with different surfaces, yet many surfaces i dont respond to at all in this way. i feel angry when i touch certain surfaces and soothed when i touch others.
Are there any genetic components to this form of synesthesia? On mother's day we were talking about our Grandmother who died in the 1980's. My parents and cousins and siblings all recall that my grandmother used to touch things. When we were little if we sat next to her she would always rub our forearm for no apparent reason. It wasn't really a rub more of a feathery touch. She did this to adults as well. There was nothing creepy or sexual about it she also would touch counter tops and fabrics. My Mom said that sometimes Grandma would touch her neck below the ear. This drove my mother crazy and she would usually walk away. I asked grandma once why she did that and she told me she liked how I felt. When I asked her how I felt to her, she would just smile and say that I felt sweet. I usually had the patience to let her do it for a while. I've noticed recently that my aunt and my dad, in their old age, also touch my arm in the same way. This might be a learned behavior from their mother, but my daughter has refused to wear certain fabrics from the time she was 2 years old. In particular she can't stand that velvety fake suade material. Recently I've seen bedspreads and baby blankets made out of this material. Once at JC Penney's I asked my daughter what she thought of a comforter (it was made of that material). I had her sit on the made up bed in the store and she actually vomited. She says the feel of it makes her sick to her stomach. Another wierd thing about my Dad's family is that more than half of them have perfect pitch. My kids were taking music lessons at a nearby university and when they both passed the pitch test the University asked to test others in my family. Both my Dad my Aunt, and several of my siblings have perfect pitch. This is why I wonder if this form of synesthesia is passed from generation to generation in families.
@Jen S.: This is the latest form of the condition to be identified, so very little is known about it. Here's another post about the genetics of synaesthesia.
Some comments here seem like tactile defensiveness which is sometimes noticed in children and I think carries through to adulthood. Is this synesthesia? My some has the music/visual crossover. He's made an animation about what he sees in music.