Haptics, Senses of Touch – Part 2: Thermoception

 

Temperature

Part two of my intro to haptics and the senses of temperature. Check out the video, or read below to learn more.

Let’s take a look at the sense of temperature. Temperature is of particular interest as it can be used in order to intensify perception of other variables of touch.  For example, cooling may make a heavy object be perceived as heavier.

Temperature receptors are distributed throughout the skin, different areas having different densities of receptors.  The hand, face (particularly the tongue and lips), and fingertips have the highest densities of these temperature receptors, while for example the sole of the foot has the least.

The information can be organized visually as sensory spots.  The main idea is that the smaller the spots (higher density of receptors) the higher temperature sensing resolution exists on the skin.  It is important to note that warmth sensory spots (areas that detect warmth) are relatively rare; our skin primarily consists of cold sensory spots.  For this reason, a person is able to detect changes in cold temperature far more accurately than hot.

Thresholds for warm receptors are twice those of cold.  Cold receptors have a resolution of roughly 0.02 to 0.07 degrees Celsius, more accurate then heat receptors, which tend to have a resolution of 0.03 to 0.09 degrees Celsius.  Resolution in this case is the accuracy at which temperature change can be detected; the skin is quite sensitive to very small changes in temperature.

However, this does not take into account the speed of the temperature change.  Fast or extreme changes (dangerous) are detected far sooner than slow changes.  As such, a change of 5 degrees Celsius may go unnoticed as long as it is within the neutral area (an area of temperature that is not detected as too cold or too hot).  This is because of adaption, which can happen anywhere between 17 and 40 degrees Celsius.

In order to determine what is perceived as hot or cold, a neutral or null point needs to be defined.  Again, the problem is that the physiological zero is never always the same.  This is because a subject’s perception will eventually adapt to their surrounding environment.

A simple experiment is to put one hand in cold water, and another in warm water.  After five minutes, dipping one’s hand in water that was previously at the physiological zero will feel hot to the hand that was laying in cold water, and cold to the hand that had been in warm water.

Another aspect to consider is spatial summation; the larger the area of skin that is stimulated, the greater magnitude of response will exist, which in turn reduces the reaction time to a stimulus.

Temperature receptors are poor at spatial recognition.  The only exception is as temperature nears the pain threshold, higher intensities are spatially easier to detect.  It is interesting to note that thresholds diminish if applied simultaneously at symmetric locations of the body.  This is not the case asymmetrically.  Consequently, temperature spatial recognition is mediated centrally.

Next up: Sense of Pressure – Mechanoreception

Music: Searching For A Warmupper #007 (Mixed By ELF) by ELF_ is licensed under a Creative Commons License.

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