LaMotte, Robert H. Softness discrimination with a tool. J. Neurophysiol. 83: 1777Neurophysiol. 83: -1786Neurophysiol. 83: , 2000. The abilities of humans to discriminate the softness of rubber objects of differing compliance with a handheld tool (a stylus) was measured under experimental conditions that differed as to how the tool was used and the kind of sensory information available. When the subject actively tapped or pressed the compliant objects, they discriminated softness as well by means of a stylus as they did by contacting the objects directly with the fingerpad. Discrimination with the stylus was unaffected by whether the stylus was controlled by one or two fingers. While tapping or pressing a stylus held in a precision grip, the grip force increased before, reached a maximum at the same time as, and decreased in parallel with the compressional force. This relationship was suggestive of anticipatory motor control based on an internal model of the motor system and the physical properties of the object. Discrimination was significantly better when tapping as opposed to pressing the objects with the stylus. This was hypothesized as due to the presence of tactile cues generated by the rapid increase in force rate as the stylus struck and indented the object during tapping. During tapping, the magnitude and rate of compressional force produced by the stylus against the object were greater, the harder the object. An additional cue, possibly kinesthetic, during pressing and tapping was the magnitude of indentation of the specimen by the stylus that was greater, the softer the object. Subjects could discriminate differences on softness by tactile cues alone in the absence of kinesthetic when compliant objects were tapped at approximately the same velocity by the experimenter against a stylus in contact with the subject's passive fingerpad. Discrimination deteriorated if the softer specimen of a pair was tapped with a slightly greater velocity than the harder and not possible if the specimens were pressed against the stylus without generating tactile cues of mechanical contact. In contrast, discrimination was possible during active pressing and unaffected by variations in velocity during active tapping. It is concluded that during active movements, kinesthetic information and knowledge of central efferent commands provide useful cues that are not present during passive touch. These cues allow the observer to discriminate differences in object compliance not confounded by differences in applied velocity. The softness of an object can be perceived through a tool by such means as tapping or pressing, as one can easily verify by means of a pencil or any other stiff object held in the hand. Perceived softness is the subjective assessment of an object's compliance, the amount an object deforms in response to an applied force. The effective use of a tool usually requires sensing those physical properties of the target object that are transmitted through the tool. When the tool is held in the hand, forces are applied via the tool to the object. Simultaneously, forces are applied by the hand to the tool itself to maintain a stability of grasp. As the tool contacts the object, tactual signals are generated that provide information about the object's compliance and perhaps other physical properties such as its shape and texture. Sensing such properties, whether by tactual or other sensory cues, may be useful for optimal exploration or manipulation of the object with the tool. The forces applied to the tool must be coordinated with the forces applied to the object. If the grasping force is insufficient for the force of impact on an object, the tool may slip from the hand. If the force applied by the tool is too great, the object or the tool may be damaged. Although sensory and motor processes in the dexterous manipulation of objects has received considerable attention (Johansson 1996) , relatively little is known about the role of tactual signals in relation to the different ways in which a tool is used to sense the geometric and material properties of objects. To what extent is the force of grasping a tool coordinated with the force applied to an object and how does the generation and use of sensory cues change with differences in tool use, for example when pressing as opposed to tapping the tool against the compliant object? Tactual cues are clearly important when sensing the physical properties of objects by means of a tool (e.g., LaMotte et al. 1994; Wellman and Howe 1995) . But information is lacking on the specific role of tactile and kinesthetic signals in the discrimination of softness with a tool. In the present experiments, the ability of humans to discriminate softness, using a stylus as a generic tool, was measured under experimental conditions that differed in the motor performance required and the type of sensory information made available. M E T H O D S Methods of construction and measurement were described in a previous study in which the same or similar objects were palpated with the fingerpad (Srinivasan and LaMotte 1995). The compliant specimens were made from transparent silicone rubber solution (RTV 615, General Electric) mixed with varying amounts of a diluent (RTV 910). As each specimen was indented with a flat-ended cylindrical probe (0.25 in. diam) at a constant indentation velocity of 0.5 mm/s, simultaneous measurements were made of force and displacement (Srinivasan and LaMotte 1995) . Compliance was defined as the average slope of the approximately linear function relating displacement to force. The compliances of the specimens used in the present study were (in m/mN): 0. 188, 0.278, 0.312, 0.407, 0.585, 0.692, 0.784, 0.946, 1.109, and 2.243. Softness discrimination was measured under two testing procedures. Under the first, subjects were allowed to rank the softness of all 10 specimens, under a variety of experimental conditions, by actively palpating each specimen, in a natural way, either directly with one The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
When a tool was used for exploration, additional kinaesthetic cues were found to be necessary for all types of objects REF .
Softness Discrimination With a Tool
Journal of neurophysiology
Journal of neurophysiology
Psychology Medicine