Magnitude estimation of softness

Abstract

The human capacity to estimate the magnitude of softness of silicone rubber disks of differing compliance was studied under experimental conditions that altered the mode of contact. Subjects were able to scale softness regardless of whether they (1) actively indented each specimen by tapping or pressing it with the finger pad, (2) received passive indentation of the finger pad by each specimen via a force controlled tactile stimulator, thus eliminating kinesthetic cues, or (3) actively indented each specimen with a stylus that was manipulated either by tapping with one finger, or held by two fingers in a precision grip, thereby removing tactile cues provided by direct mechanical contact between the finger pad and specimen. Ratings of softness were independent of moderate variations in peak compressional force and force-rate. Additionally, functions for scaling softness were affected by the mode of contact; the slopes of the functions were greater in the tasks with a complete complement of compliance cues. When subjects were asked to classify objects as either hard or soft, specimens were classified as soft if the compliance were greater than that of the human finger. This suggests that the classification of softness depends on whether the object conforms to the body, and that tactile information about the spatial profile of object deformation is sufficient for the magnitude scaling of softness. But typically, kinesthetic information about the magnitude of object displacement, along with contact vibratory cues is also used while judging softness especially in the absence of direct skin contact with the object when using a tool.

Fig. 1

Measurements of the compliance of the rubber specimens and the finger pad. Each specimen and the finger pad of a human subject were indented with a flat-ended cylindrical probe of 10 mm diameter at a constant velocity of 0.5 mm/s as both force and displacement were recorded. Compliance (in mm/N), defined as the slope of the function relating displacement to force, is provided to the right of the curve for each specimen (solid lines) or the middle finger pad (dashed line). Inset: Each specimen is separated from the adjacent one by 1.3 log units and ranked from the hardest (1) to the softest (8)

Fig. 2

Subjects scaled softness in five tasks. A Active tapping or pressing with the finger pad. Each specimen (a) was mounted to a load cell (b) that was used to measure compressional force. B Active tapping with a two-finger tool. The unconstrained, two-finger stylus (c) was tapped once against each specimen. The stylus had a shaft diameter of 12 mm and a sphere, 8 mm in diameter, at the end that contacted the specimen. C Active tapping with an one-finger tool: The one-finger stylus (d) had tip diameters of 8 mm at each end. The stylus was attached to a lever (e) that pivoted on the shaft of a torque motor. The motor was used to exert a constant downward force of 10 mN on the finger pad. This served to maintain contact between the tip of the stylus and the finger pad. D Passive pressing of the finger. A torque motor (f) pressed a specimen against a subject's finger pad via a lever (g) that was attached to the shaft of the motor. The dorsum of the middle finger was held against a holder (h) with double-sided tape. E–I Compressional forces measured for the five tasks during contact with specimen that had a compliance of 2.58 mm/N. E Active pressing with the finger (A). F Active tapping with the finger (A). G Active tapping with the two-finger stylus (B). H Active tapping with the one-finger-stylus (C). I Passive pressing of the finger pad (D). The motor held the specimen against the finger pad with a downward force of 1 g (9.8 mN) prior to pressing it against the skin with one of 6 trapezoidal waveforms of compressional force

Fig. 3

Perceived softness as a function of the compliance of each specimen actively tapped or pressed with the finger pad. a Magnitude estimates of softness obtained from individual subjects while pressing the specimens. b Magnitude estimates of softness obtained from individual subjects while tapping the specimens. c Mean magnitude estimates of softness obtained from tapping and pressing the specimens. d–i The peak compressional forces plotted for each subject as a function of compliance and obtained during pressing (d) and tapping (e) and averaged for all subjects (f). Similarly presented are the individual force rates for pressing (g) and tapping (h) and the means for all subjects (i)

Fig. 4

Effects of compressional force on the perceived softness of specimens actively pressed with the finger pad. The magnitude estimation functions from subjects were divided into two natural groups, based on whether a subject tended to indent specimens with forces either greater or lower than 2.5 N. (Fig. 3d)

Fig. 5

Perceived softness as a function of the compliance of each specimen pressed onto the passive finger pad. a Mean magnitude estimates of softness for indentations of the specimens for each of the six compressional forces. b Relationship between magnitude estimates of softness and peak compressional force (or force-rate) for each specimen

Fig. 6

Perceived softness as a function of the compliance of each specimen actively indented with a stylus. a Magnitude estimates of softness of individual subjects who were required to use one finger to tap each specimen with a stylus b Magnitude estimates of softness of individual subjects who were required to tap each specimen with a stylus held in a precision grip. c Mean magnitude estimates of softness for the two modes of tapping. d–f Peak compressional forces as a function of compliance obtained from each subject during tasks that required tapping either with the one-finger stylus (d) or the two-finger stylus (e) and averaged for all subjects for each of these tasks (f)

Fig. 7

Mean magnitude estimates of softness for the five different modes of contact. Individual estimates from each subject were averaged for each specimen during tasks that differed according to whether the specimen was pressed against the restrained finger pad (“Passive press”) or actively tapped or pressed with the finger pad (“Tap with finger”, “Press with finger”) or actively tapped with a stylus controlled by one- or two fingers (“1 finger”, “2 finger”)

Fig. 8

The incidence with which objects were categorized as “hard” or “soft”. The mean percentage of categorizations by subjects of the silicone specimens (a) as a function of compliance, and of common household objects (b) ranked in order of the percentage called soft. The household objects included a small cotton ball (“cotton ball”), a full roll of toilet paper (“toilet paper”), a synthetic sponge (“sponge”), a partially filled latex balloon (“balloon”), a gummy frog (“gelatin candy”), and synthetic clay (“Play-Doh™”). The particle objects (“sugar”, “talc”, “flour”) were poured into empty Petri dishes and loosely covered with plastic wrap. Seven subjects were instructed to press each object once with the middle finger pad. The specimens and household objects were intermixed and pseudorandomly presented. Specimens with compliances of 0.72 mm/N or less were categorized as hard; specimens with compliances of 2.58 mm/N or more were categorized as soft. The vertical ellipse marks the compliance of the finger pad