Academic journal article Perception and Psychophysics

Contribution of Thermal Cues to Material Discrimination and Localization

Academic journal article Perception and Psychophysics

Contribution of Thermal Cues to Material Discrimination and Localization

Article excerpt

The objective of these two experiments was to determine the role of thermal cues in material discrimination and localization, using materials that spanned a range of thermal properties. In the first experiment, the subjects were required to select the cooler of two materials presented to the index fingers. In the second, the finger that was in contact with a material that was different from that presented to the other two fingers on the same hand had to be identified. The results indicated that the subjects were able to discriminate between materials, using thermal cues, when the differences in their thermal properties were large. The changes in skin temperature when the fingers were touching the materials were, however, smaller than those predicted by the theoretical model. The ability to localize the thermal changes when three fingers on the same hand were stimulated was poor and depended on both the thermal properties of the target and the distractor materials.

The ability to identify and discriminate between objects by touch is based on the perception of a number of properties, including shape, surface texture, compliance, and thermal characteristics. These cues become particularly important when objects must be identified in the absence of vision. The human hand is capable of resolving remarkably fine variations in texture, as is shown by its capacity to detect periodically ordered elements that are only 0.06 µm high when there is a relative motion between the texture and the finger pad (LaMotte & Srinivasan, 1991). The ability to discriminate between the compliance of objects depends on whether the objects have deformable or rigid surfaces. With deformable surfaces, cutaneous cues from skin deformation are sufficient to discriminate compliance, whereas for rigid objects both cutaneous and proprioceptive cues are necessary for discrimination (Srinivasan & LaMotte, 1995). The thermal cues that are used to assist in identifying an object arise from changes in skin temperature that occur when the object is held in the hand. Warm and cold thermoreceptors in the skin discharge in response to these local thermal transients. The resting temperature of the skin is generally higher than the temperature of the object in contact with the skin, and so it is the cold thermoreceptors that signal the decrease in skin temperature upon contact.

The ability to perceive thermal changes depends on many factors, including the amplitude and rate of temperature change, the baseline temperature of the skin, and the site stimulated (for reviews, see Darian-Smith, 1984; Stevens, 1991). The threshold for discriminating the difference in the amplitudes of two temperature pulses delivered to the thenar eminence of the hand is 0.02°-0.07°C for cooling pulses and 0.03°-0.09°C for warming pulses (Johnson, Darian-Smith, & LaMotte, 1973; Johnson, Darian-Smith, LaMotte, Johnson, & Oldfield, 1979). This is considerably lower than the threshold for discriminating a change in skin temperature. When the skin temperature of the thenar eminence is maintained at 33°C, the differential threshold is 0.20°C for warming and 0.11°C for cooling (Stevens & Choo, 1998). If skin temperature changes very slowly-for example, at a rate of less than 0.5°C/min-an observer can be unaware of a change of up to 4°-5°C, provided that the temperature remains within the neutral thermal zone of 30°-36°C (Kenshalo, 1976).

Several studies have suggested that subjects can identify objects varying in thermal properties, using only thermal cues. Caldwell and Gosney (1993) recorded the thermal changes on a teleoperated robotic hand as it made contact with a variety of objects, including an ice cube, a heated soldering iron, an aluminum block, and a piece of insulation foam, and then presented these thermal transients to subjects. A signal from a temperature sensor (thermocouple) on the robotic hand maintained at 40°C indicated the type and magnitude of the thermal transient associated with contact with the various materials. …

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