THE CUTANEOUS RABBIT
by Yasushi Kusume
‘You [can] create what you feel and where in a space you feel it. An amputee creates the feel of the phantom hand and places it at a distinct location in space.’
Visual Intelligence, Donald D. Hoffman.
‘The Cutaneous Rabbit’ was discovered by Frank Geldard and Carl Sherrick of Princeton University, while they were carrying out tests that called for three small vibrators to be connected to the forearm of a volunteer. Due to a technical error, the vibrators weren't synchronised; instead they delivered a sequence of taps that started first near the wrist, then moved to the middle of the forearm, and ended by the elbow. This series of sequential taps, the volunteer reported, felt as though a small rabbit was hopping up and down his arm from the wrist to the elbow. The sensation the volunteer felt was similar to that often experienced by amputees, who say they can still feel a missing limb.
The Cutaneous Rabbit is just one example of how somatic senses can work, and illustrates how our brain can construct the illusion of an object that doesn't actually exist. This has implications for design, most especially, for multi-sensorial design.
Somatic design
If you're going to embrace multi-sensorial design, you'll need to do it fully. That doesn't just mean designing for what you may think are the only senses we use, the traditional five means of human perception: sound, sight, touch, smell and taste. It also means using the somatic senses.
Somatic senses receive signals through the receptor cells in our skin and inner tissues. As with the input from our other five senses, these receptor signals are then sent to our brain, which combines them to form our perception of the world around us. Let me describe a few of them.
Touch
Touch receptors sense pressure and send signals to our brain. This happens not just when someone or something else touches our body, but when even the slightest pressure is applied; as, for example, when a breeze ruffles our hair. Touch is the most prominent of our somatic senses.
Thermoception
Themoreceptors in our skin - one for warm and one for cold - detect rises and falls in temperature. Thermoreceptors located in our organs and deeper tissues help our brains to track our core body temperature.
Nociception
Nocireceptors detect pain. There are many different types of pain, but all are detected and carried to our brains by two types of nerve. One carries fast and sharp pain; the other carries deep, burning pain. In her book Mapping the Mind, Rita Carter notes that the stimulation of one nocireceptor can block signals from the other by closing a neural gate in the spine. Which explains why ‘rubbing it better’ can sometimes be so effective.
Proprioception
Proprioception enables us determine the position of our limbs, so we can maintain our equilibrium. (It's the reason why we can touch our index fingers together with our eyes closed.) We possess two types of proprioceptor. One in our muscles and tendons measures how much they stretch. The other, in our cartilage, measures the load placed upon, and the rate of slippage, in each of our joints.
Understand your product
So if you're going to design for a multi-sensorial experience, and you're going to include the somatic senses, then you're going to need to understand the senses your product will affect. For example, if you're designing an air conditioning unit, a boiler or a thermostat, you'll need to investigate the multi-sensorial properties of air. Because it won't just be the look of the product that matters, it will also be the way it affects the user's somatic sense.