Saturday, June 19, 2010

"Can't Touch This" - The neurophysiology of pleasant touch

Touch is crucially important for social communication, mediating how we perceive and interpret the actions of others. For example, waitresses who touch customers get better tips (Crusco & Wetzel, 1984). However, touch alone does not always lead to positive outcomes. For many years researchers have known that touch is moderated by factors such as type of touch, location of touch, situational context, and many others. Until recently however, they were uncertain as to how the social effect of touch was achieved. In this review by Olausson et al., the authors shed light on how pleasant and socially relevant aspects of touch are possibly mediated by a new distinct type of receptors called C-Tactile afferents.

To give a little background, human touch is generally considered to be mediated by 3 different type of receptors:
  1. Thermal Receptors allow us to feel warmth and heat.
  2. Mechanoreceptors allow us to feel pressure.
  3. Nociceptors allow us to feel pain from chemical or heat damage.
For the most part, the mechanoreceptors are myelinated afferents. "Myelinated" means that the part of the neuron that transmits the signals are covered with an electrical insulator that allows it to conduct impulses at high speed (50m/s). Myelination is essential for quick reaction. "Afferent" neurons are neurons which carry information towards the central nervous system.

In the early part of the 20th century, mechanoreceptors that were unmyelinated were identified in various mammals like cats. They were termed C tactile (CT) afferents. For a time it was thought that humans did not share this primitive and slower tactile system, which conducted impulses at a speed of 1m/s. However in the 1990s they were soon found in abundance in hairy skin around the arms and legs, but not on the hairless surface of the palm.

The only appropriate time to use a lolcat.

Immediate testing of the sensitivity of these CT afferents found that they were for lack of a better word, wierd. They could not discriminate between pin pricks and smooth probe pressure, but however responded best to a slow stroking with a soft brush.

Direct evidence for the specific role of CT afferents was however, hard to achieve, because it was not possible to stimulate CT afferents without also activating myelinated afferents in normal subjects. The authors studied subjects who suffered from sensory neuronopathy, a condition where subjects lacked myelinated afferents but have intact C fibres. These subjects were long thought to have lost all tactile sensations but they detected soft brush stroking. Much of the CT effect was however, below conscious level. They could not effectively locate where the touch was delivered, and were not able to describe it, only saying that it was slightly to moderately pleasant.

FMRI imaging of the neuronopathy subjects also found that the posterior insula cortex was activated, but not traditional somatosensory systems identified with other tactile stimulation. This region is considered important for integrating converging information and passing it on to emotional systems. For one thing, it has been shown to be involved in the perception of pain.

Based on the evidence, the authors believe that CT system is involved in social touch. For one, the poor conduction velocity and poor discriminative properties of CT afferents make them ill-suited for processing the complex features of touch, features which are easily handled by other mechanoreceptors that are myelinated. Why then would they be present in humans? For another, the CT system appears to be tuned to slow, light touch, which occurs most often during close interactions with loved ones. The evidence from the sensory neuronopathy patients appears to confirm this fact.

So, what does this tell us? For the scientist, this research opens up new areas of research into the biological foundations of the social and emotional aspects of touch, which has longed focused purely on physical sensations. For the layman, not much. Intuitively we already reach out to stroke someone who is upset when we have no idea how to comfort that person, even if we do not know whether it would help. This review, and the research that has been done, confirms that physiologically, there is an effect. So, keep touching (as long as its appropriate, of course) :)

Ed's note: For another noteworthy study which was not mentioned in this review, but which I'm guessing was done by a postgraduate student of the author, you can refer to
Olausson, H., Wessberg, J., Morrison, I., McGlone, F., & Vallbo, �. (2010). The neurophysiology of unmyelinated tactile afferents Neuroscience & Biobehavioral Reviews, 34 (2), 185-191 DOI: 10.1016/j.neubiorev.2008.09.011

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