Vision and audition do not have the affective influence that somatosensation does.
If you take a quick survey of the number of publications on different sensory systems, you’ll quickly notice that there are about 100 times as many papers published on vision as there are on somatosensation. Vision does, after all, take up about 30% of cerebral cortex and the somatosensory cortex, the region that deals with the sense of touch, only takes up about 8%. This is one reason that many people like to test brain health visually with the idea that if any thing goes wrong with the brain, then there will be something wrong with vision. The problem with that is the extensive nature of the visual cortex and the difficulty in trying to pin down what is wrong mechanistically with the brain if something is awry with the visual system. One advantage of the somatosensory system is that you can test how adjacent groups of neurons are interacting with fairly good fidelity.
But there are other aspects of the sensory systems to consider. For example, what happens when someone is born without the ability to see or hear? Many born without sight or hearing go on to lead productive lives and are considered developmentally normal. However, what happens if someone does not receive input to the somatosensory system at an early stage in life? Unfortunately, we know the answer to this from observational studies conducted at over-crowded orphanages in Romania in the 1980s and 1990s. Children that received no contact suffered many developmental problems, but if therapy was introduced in the form of affective touch (hugging, etc.) for 30 minutes a day before the age of two, then there was a profound improvement in the psychological status of the child.
Most people are aware of C afferents as being the nerve fibers that deliver noxious or painful stimuli from the skin to the cortex (actually goes to area 3a in somatosensory cortex, and you can read more about that here). What many people are not aware of are a class of C fibers that respond to a different kind of input – inputs that are not noxious or painful. These are called Ct afferents, and they respond to slow and gentle moving stimuli. The are unmyelinated (so speed of stimulation is not an issue) but they do respond to mechanical stimulation.
Francis McGlone, one of our long time collaborators, has studied the Ct afferent and it’s role in affective touch for decades (you can read a bit more about that here.)
In short, he believes that the Ct afferent – which provides input from gentle stroking or gentle touch – plays a significant role in not only development, but in how we communicate and interact with one another. Social touch plays an important role in society, but he worries that societal issues are leading to less touch between people. He may have a point – social media seems to have reduced our apparent necessity for direct interactions with other people.
Ever imagine what your day would be like without the sense of touch? It’s probably the most taken for granted sense. We don’t think about what feels comfortable (we put on our clothes and quickly adapt to what they feel like) and we don’t constantly think about how things feel. Quite simply, we adapt to our tactile surroundings and are not distracted by them, unless there is some positive affective or negative pain component to that touch. Perhaps this is why the Brain Gauge is such a good tool for testing brain health – there are very few tactile distractions and the only thing that someone testing is aware of in that sensory modality are the two places that are being stimulated (the two finger tips). In essence, the lack of distractions provides a very well controlled laboratory template for testing. We do know that pain can negatively impact Brain Gauge results, but based on what we know about positive affective touch, we suspect that those could have a very positive effect on the results. In other words, you will test better when you feel better, and this would be one way to put a quantitative objective number on that elusive feeling.