Tactile Illusion: An Explanation

Before anything else, check out this BuzzFeed video about how to trick your sense of touch:

Coolbeans, yeah? But the question is, how is it possible that we’re able to feel all these illusions even though we know we’re being tricked? Here are the scientific and psychological answers to those questions.

1.     1. Sound and Touch – the first illusion

In this first trick, the guy in the video is writing something on the chalkboard while hearing the scraping of the chalk on the board. Afterwards, he is asked to wear earplugs so he wouldn't hear anything. He reported to have felt like the texture was smoother than before. How is this possible?

This is actually possible due to the connections in our brain. We don’t feel the senses separately, we actually integrate the senses together in our brain. In our auditory cortex in the brain, which is right about here:

There is a belt surrounding the auditory cortex, wherein the integration of the senses of touch and hearing can occur. Hearing something and touching something at the same time is more likely to be associated with one another as well. The integration of senses happens in the brain. (Kayser, et. al, 2005)

However, it seems that the texture of the surface (whether rough or smooth) is an exception (Lederman, 1979). Other studies showed that haptic sense (or touching the object) actually overpowers the auditory senses in other characteristics but surface texture is more affected by auditory than haptic cues (Lederman, et. al, 2002)

2.     Imaginary String – Suggestion and Sense of Touch

Amazingly enough, this one can be explained with fatigue. The muscles on your fingers aren’t meant to stay that way, so once you start relaxing your finger muscles, they come back together. However, suggestion and sense of touch do come hand in hand.

In one research, it was found that a request made by the “toucher” to the “touchee” was more likely to be granted than if the “touchee” wasn’t being touched. This was in a restaurant setting, wherein the touchers were the customer service crew and the touches were the customers. The customers gave higher ratings to the service crew and gave bigger tips if they were touched than if they weren’t. The service crew would lightly touch the customer’s forearm. Unfortunately, it’s still unknown as to why this is the case today. All that’s known is that there’s a correlation between suggestion and touch in consumer behaviour research (Gueguen, Jacob, & Boulbry, 2007)

3.     Hot and Cold – Thermal Grill Illusion

For this trick, the woman puts her hands in hot and cold and then puts her hands in lukewarm water and describes the feeling as strange and confusing as to what the actual temperature of the warm water would be. Pushed further, it is something called the thermal grill illusion.

The thermal grill illusion is when you switch from hot to cold and cold to hot quickly and your brain gets confused about what sense to feel. Research shows that it’s actually more uncomfortable than feeling normal hot and cold (Bach, et al, 2011).

Why exactly do we feel this way? According to a recent research study which recorded the fMRI of people experiencing the Thermal Grill Illusion, the biggest influence in our brain (aka, the one most active during the experience) is actually the thalamus (Lindstedt, et al, 2011). The thalamus’ job is to regulate the senses and the body.

So why exactly is it important to study the reasoning behind the illusion? Well, there is this certain disorder called the cold allodynia, which makes a person feel like the normal cold is more painful than normal. Research has been conducted on the pain receptors before, but it’s much more data enriching and useful to know which parts of the brain are activated in healthy human beings in order to identify what could be going wrong in people with these disorders and what to prescribe or do with them.

4.     Dead man’s Finger – All in the Brain 

In this illusion, stroking the finger of another person managed to make your finger feel numb. Surprisingly, the answer for this question is more biological than psychological. According to research, it was found that the key to getting the “Numbness Illusion” to your brain is not the visual or the motor senses alone but rather is influenced by the tactile inputs in the primary somatosensory cortex (Dieguez, et al, 2009), thus making it a top down processing rather than a bottom up.

References:

Bach, P., Becker, S., Kleinböhl, D., & Hölzl, R. (2011). The thermal grill illusion and what is painful about it. Neuroscience letters, 505(1), 31-35.

Dieguez, S., Mercier, M. R., Newby, N., & Blanke, O. (2009). Feeling numbness for someone else's finger. Current Biology, 19(24), R1108-R1109.

Guéguen, N., Jacob, C., & Boulbry, G. (2007). The effect of touch on compliance with a restaurant's employee suggestion. International Journal of Hospitality Management, 26(4), 1019-1023.

Kayser, C., Petkov, C. I., Augath, M., & Logothetis, N. K. (2005). Integration of touch and sound in auditory cortex. Neuron, 48(2), 373-384.

Lederman, S. J. (1979). Auditory texture perception. Perception, 8(1), 93-103.

Lederman, S. J., Klatzky, R. L., Morgan, T., & Hamilton, C. (2002). Integrating multimodal information about surface texture via a probe: relative contributions of haptic and touch-produced sound sources. In Haptic Interfaces for Virtual Environment and Teleoperator Systems, 2002. HAPTICS 2002. Proceedings. 10th Symposium on (pp. 97-104). IEEE.

Lindstedt, F., Johansson, B., Martinsen, S., Kosek, E., Fransson, P., & Ingvar, M. (2011). Evidence for thalamic involvement in the thermal grill illusion: an FMRI study.