Academic journal article Perception and Psychophysics

Tactile Perception of Thermal Diffusivity

Academic journal article Perception and Psychophysics

Tactile Perception of Thermal Diffusivity

Article excerpt

The thermal diffusivity of an object is a parameter that controls the rate at which heat is extracted from the hand when it touches that object. It is an important feature for distinguishing materials by means of touch. In order to quantitatively describe the ability of human observers to discriminate between materials on the basis of heat extraction rate, we conducted an experiment in which this heat extraction was performed in a controlled way. In different conditions, subjects were repeatedly asked to select from two stimuli the one that cooled faster. The discrimination threshold was around 43% of the extraction rate. A rate that was twice as slow also yielded twice the absolute threshold. When we halved the temperature difference between the beginning and end of the stimulus, the threshold did not change as much. In separate experiments, we investigated the different cues that were available in the stimulus: initial cooling rate and end temperature. Both cues were used for discrimination, but cooling rate seemed to be the most important.

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The thermal diffusivity of a material describes the way heat spreads through it. It is an aspect of materials that is very important for discriminating between them by touch. A wooden and a metal surface with exactly the same surface texture are easily distinguished by touch just on the basis of their different thermal behavior. A material with a high thermal diffusivity conducts heat away quickly upon touch and therefore feels cold. Conversely, a material with a low thermal diffusivity will feel warm to the touch. What is often perceived as "warm" or "cold" has little to do with temperature but is, in fact, caused by different rates of heat extraction. Walking around in air of 10?C is a very different thermal experience from swimming in water of 10?C. This effect is very well known in daily life, and it is therefore surprising that so little is known about the tactile perception of heat extraction rate.

At the basis of heat extraction perception lies temperature sensation. There is evidence that temperature sensation is mediated by separate channels for warm and cold. Both warm and cold temperature receptor populations have been found to include Ad and C type fibers (Spray, 1986). Differences in responses to cold and warm suggest that these stimuli are processed separately (Greenspan, Roy, Caldwell, & Farooq, 2003). Phenomena like synthetic heat and paradoxical cold suggest that warm, cold, and pain are signaled by both separate fibers and nonspecific multimodal fibers (Green, 2004).

Although equipped with these warm- and coldsensitive systems, humans are not very good at judgments of absolute temperature, as compared with judgments of temperature differences. The reason for this is thought to be that there is no fixed reference point for temperature perception, which is due to adaptation. Adaptation occurs when the hand is immersed in, for example, water or air of a specific temperature. Often, the perceived temperature of an object depends on the temperature difference between adaptation temperature and object temperature, rather than on the absolute temperature (Tritsch, 1988). Indeed, people perform better at the perception of temperature differences; depending on the temperature at which a measurement takes place, the threshold for detecting a difference between the two hands varies from 0.5?C to 5.2?C (Abbott, 1914). People are even more sensitive to changes in temperature at a single location on the body. With a rate of change of 0.1?C/sec or faster, the cool threshold is between 0.1?C and 0.3?C (Kenshalo, Holmes, & Wood, 1968). With slower rates of change, the threshold goes up. Still better is the discrimination between two subsequent temperature changes or cooling pulses: The threshold ranges from 0.03?C to 0.06?C for different intensities of the cooling pulses on the thenar eminence (Johnson, Darian- Smith, & LaMotte, 1973). …

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