Haptic Perception

The haptic sensory modality is based on cutaneous receptors lying beneath the skin surface and kinesthetic receptors found in muscles, tendons, and joints (Loomis and Lederman 1986). The haptic modality primarily provides information about objects and surfaces in contact with the perceiver, although heat and vibration from remote sources can be sensed (see also PAIN). Haptic perception provides a rich representation of the perceiver's proximal surroundings and is critical in guiding manipulation of objects.

Beneath the surface of the skin lie a variety of structures that mediate cutaneous (or tactile) perception (see, e.g., Bolanowski et al. 1988; Cholewiak and Collins 1991). These include four specialized end organs: Meissner corpuscles, Merkel disks, Pacinian corpuscles, and Ruffini endings. There is substantial evidence that these organs play the role of mechanoreceptors, which transduce forces applied to the skin into neural signals. The mechanoreceptors can be functionally categorized by the size of their receptive fields (large or small) and their temporal properties (fast adapting, FA, or slowly adapting, SA). The resulting 2 x 2 classification comprises (1) FAI receptors, which are rapidly adapting, have small receptive fields, and are believed to correspond to the Meissner corpuscles; (2) FAII receptors, which are rapidly adapting, have large receptive fields, and likely correspond to the Pacinian corpuscles (hence also called PCs); (3) SAI receptors, which are slowly adapting, have small receptive fields, and likely correspond to the Merkel disks; and (4) SAII receptors, which are slowly adapting, have large receptive fields, and likely correspond to the Ruffini endings. Among other cutaneous neural populations are thermal receptors that respond to cold or warmth.

By virtue of differences in their temporal and spatial responses, the various mechanoreceptors mediate different types of sensations. The Pacinian corpuscles have a maximum response for trains of impulses on the order of 250 Hz and hence serve to detect vibratory signals, like those that arise when very fine surfaces are stroked or when an object is initially contacted. The SAI receptors, by virtue of their sustained response and relatively fine spatial resolution, are implicated in the perception of patterns pressed into the skin, such as braille symbols (Phillips, Johansson, and Johnson 1990). The SAIs also appear to mediate the perception of roughness, when surfaces have raised elements separated by about 1 mm or more (Connor and Johnson 1992; see TEXTURE).

The responses of haptic receptors are affected by movements of the limbs, which produce concomitant changes in the nature of contact between the skin and touched surfaces. This dependence of perception on movement makes haptic perception active and purposive. Characteristic, stereotyped patterns of movement arise when information is sought about a particular object property. For example, when determining the roughness of a surface, people typically produce motion laterally between the skin and the surface, by stroking or rubbing. Such a specialized movement pattern is called an exploratory procedure (Lederman and Klatzky 1987).

An exploratory procedure is said to be associated with an object property if it is typically used when information about that property is called for. A number of exploratory procedures have been documented. In addition to the lateral motion procedure associated with surface texture, there is unsupported holding, used to sense weight; pressure, used to sense compliance; enclosure, used to sense global shape and volume; static contact, used to determine apparent temperature; and contour following, used to determine precise shape. The exploratory procedure associated with a property during free exploration also turns out to be optimal, in terms of speed and/or accuracy, or even necessary (in the case of contour following), for extracting information about that property; an exploratory procedure that is optimal for one property may also deliver relatively coarse information about others (Lederman and Klatzky 1987).

The exploratory procedures appear to optimize perception of an object property by facilitating a computational process that derives that property from sensory signals. For example, the exploratory procedure called static contact promotes perception of surface temperature, because it characteristically involves a large skin surface and therefore produces a summated signal from spatially distributed thermal receptors (Kenshalo 1984). Texture perception is enhanced by lateral motion of the skin across a surface, because the scanning motion increases the response of the SA units (Johnson and Lamb 1981). It has been proposed that weight can be judged by wielding an object (as occurs during unsupported holding), because the motion provides information about the object's resistance to rotation, which is related to its mass and volume (Amazeen and Turvey 1996).

With free exploration, familiar common objects can usually be identified haptically (i.e., without vision) with virtually no error, within a period of 1-2 s (Klatzky, Lederman, and Metzger 1985; see also OBJECT RECOGNITION). The sequence of exploratory procedures during identification appears to be driven both by the goal of maximizing bottom-up information and by top-down hypothesis testing. Object exploration tends to begin with general-purpose procedures, which provide coarse information about multiple object properties, and proceed to specialized procedures, which test for idiosyncratic features of the hypothesized object (Lederman and Klatzky 1990).

Although haptic object identification usually has a timecourse of seconds, considerable information about objects can be acquired from briefer contact. Intensive properties of objects -- those that can be coded unidimensionally (i.e., not with respect to layout in 2-D or 3-D space) -- can be extracted with minimal movement of the fingers and in parallel across multiple fingers (Lederman and Klatzky 1997). When an array of surface elements is simultaneously presented across multiple fingers, the time to determine whether an intensively coded target feature (e.g., a rough surface) is present can average on the order of 400 ms, including response selection and motor output. Properties extracted during such early touch can form the basis for object identification: a 200-ms period of contact, without finger movement, is sufficient for identification at levels above chance (Klatzky and Lederman 1995).

A critical role for haptic perception is to support manipulatory actions on objects (see also MOTOR CONTROL). When an object is lifted, signals from cutaneous afferents allow a grip force to be set to just above the threshold needed to prevent slip (Westling and Johannson 1987). During lifting, incipient slip is sensed by the FA receptors, leading to corrective adjustments in grip force (Johannson and Westling 1987). Adjustments also occur during initial contact in response to perceived object properties such as coefficient of friction (Johannson and Westling 1987). Age-related elevations in cutaneous sensory thresholds lead older adults to use grip force that is substantially greater than the level needed to prevent slip (Cole 1991).

Additional links

The Haptics Community Web Page

-- Roberta Klatzky

References

Amazeen, E. L., and M. T. Turvey. (1996). Weight perception and the haptic size-weight illusion are functions of the inertia tensor. Journal of Experimental Psychology: Human Perception and Performance 22:213-232.

Bolanowski, S. J., Jr., G. A. Gescheider, R. T. Verrillo, and C. M. Checkosky. (1988). Four channels mediate the mechanical aspects of touch. Journal of the Acoustical Society of America 84(5):1680-1694.

Cholewiak, R., and A. Collins. (1991). Sensory and physiological bases of touch. In M. A. Heller and W. Schiff, Eds., The Psychology of Touch. Mahwah, NJ: Erlbaum, pp. 23-60.

Cole, K. J. (1991). Grasp force control in older adults. Journal of Motor Behavior 23:251-258.

Connor, C. E., and K. O. Johnson. (1992). Neural coding of tactile texture: Comparison of spatial and temporal mechanisms for roughness perception. The Journal of Neuroscience 12:3414-3426.

Johannson, R. S., and G. Westling. (1987). Signals in tactile afferents from the fingers eliciting adaptive motor responses during precision grip. Experimental Brain Research 66:141-154.

Johnson, K. O., and G. D. Lamb. (1981). Neural mechanisms of spatial tactile discrimination: Neural patterns evoked by Braille-like dot patterns in the monkey. Journal of Physiology 310:117-144.

Kenshalo, D. R. (1984). Cutaneous temperature sensitivity. In W. W. Dawson and J. M. Enoch, Eds., Foundations of Sensory Science. Berlin: Springer, pp. 419-464.

Klatzky, R., S. Lederman, and V. Metzger. (1985). Identifying objects by touch: an "expert system." Perception and Psychophysics 37:299-302.

Klatzky, R. L., and S. J. Lederman. (1995). Identifying objects from a haptic glance. Perception and Psychophysics 57(8):1111-1123.

Lederman, S. J., and R. L. Klatzky. (1987). Hand movements: a window into haptic object recognition. Cognitive Psychology 19:342-368.

Lederman, S. J., and R. L. Klatzky. (1990). Haptic object classification: knowledge driven exploration. Cognitive Psychology 22:421-459.

Lederman, S. J., and R. L. Klatzky. (1997). Relative availability of surface and object properties during early haptic processing. Journal of Experimental Psychology: Human Perception and Performance 23:1680-1707.

Loomis, J., and S. Lederman. (1986). Tactual perception. In K. Boff, L. Kaufman, and J. Thomas, Eds., Handbook of Human Perception and Performance. New York: Wiley, pp. 1-41.

Phillips, J. R., R. S. Johansson, and K. D. Johnson. (1990). Representation of braille characters in human nerve fibres. Experimental Brain Research 81:589-592.

Westling, G., and R. S. Johannson. (1987). Responses in glabrous skin mechanoreceptors during precision grip in humans. Experimental Brain Research 66:128-140.

Haptic Perception

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References

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