Satoru Kawai

Contributions and co-ordination of individual fingers in multiple finger prehension

The contributions and co-ordination of external finger grip forces were examined during a lifting task with a precision grip using multiple fingers. The subjects (n = 10) lifted a force transducer-equipped grip apparatus. Grip force from each of the five fingers was continuously measured under different object weight (200 g, 400 g and 800 g) and surface structure (plastic and sandpaper) conditions. The effect of five-, four-, and three-finger grip modes was also examined. It was found that variation of object weight or surface friction resulted in change of the total grip force magnitude; the largest change in finger force, was that for the index finger, followed by the middle, ring, and little fingers. Percentage contribution of static grip force to the total grip force for the index, middle, ring, and little fingers was 42.0%, 27.4%, 17.6% and 12.9%, respectively. These values were fairly constant for all object weight conditions, as well as for all surface friction conditions, suggesting that all individual finger force adjustments for light loads less than 800 g are controlled comprehensively simply by using a single common scaling value. A higher surface friction provided faster lifting initiation and required lesser grip force exertion, indicating advantageous effect of a non-slippery surface over a slippery surface. The results indicate that nearly 40% force reduction can be obtained when a non-slippery surface is used. Variation in grip mode changed the total grip force, i.e., the fewer the number of fingers, the greater the total grip force. The percent value of static grip force for the index, middle, and ring fingers in the four-finger grip mode was 42.7%, 32.5%, and 24.7%, respectively, and that for the index and middle fingers in the three-finger grip mode was 43.0% and 56.9%, respectively. Therefore, the grip mode was found to influence the force contributions of the middle and ring fingers, but not of the index finger.

The mechanical properties of the heel pad in elderly adults

The shock absorbing characteristics of the heel pad in vivo were examined in two groups of active elderly individuals whose ages ranged between 60 and 67 years (n = 10) and between 71 and 86 years (n = 10). For comparative purposes, young adults (n = 10) aged between 17 and 30 years were also examined. A free-fall impact testing device which consisted of an instrumented shaft (mass 5 kg), accelerometer and position detection transducer was used to obtain deceleration and deformation of the heel during impact. The data were obtained from impact velocities of 0.57 m · s−1 (slow) and 0.94 m · s−1 (fast). Peak values of the deceleration and deformation, as well as the time to these peaks from onset of impact, and energy absorption were evaluated. At the slow impact velocity, no age effect was found for the parameters examined except for the energy absorption. At the fast impact velocity, there was higher peak deceleration and smaller deformation for the elderly than for the younger adults. The energy absorbed was less for the elderly than for the younger adults. It was concluded that the capacity for shock absorbency of the heel pad declines with age.

A reexamination of the size-weight illusion induced by visual size cues.

The size–weight illusion induced by visually perceived sizes was reexamined to investigate whether this illusion is a sensory based or cognitive-based phenomenon. A computer-augmented environment was utilized to manipulate visual size information of target objects independently of their haptic information. Two physical cubes of equal mass (30.0xa0g) and size (3.0xa0×xa03.0xa0×xa03.0xa0cm) were suspended in parallel by wires attached to small graspable rings, in order to keep haptically obtained information constant between lifts. Instead of directly seeing each physical cube, subjects viewed 3D graphics of a cube with a wire and a ring that were precisely superimposed onto each physical cube. Seventeen subjects vertically lifted these augmented cubes, one after the other, by grasping the attached rings, and then reported their perception of cube heaviness. The graphical size of a comparison cube pseudo randomly varied for every comparison from 1.0xa0×xa01.0xa0×xa01.0 to 9.0xa0×xa09.0xa0×xa09.0xa0cm, while that of a standard cube remained constant (5.0xa0×xa05.0xa0×xa05.0xa0cm). Results indicated that the size–weight illusion frequently and systematically occurred for all the subjects such that when the comparison cube was relatively smaller than the standard cube, it was perceived to be heavier, and vice versa. As the size difference increased between the standard cube and the comparison cube, more subjects experienced the illusion, and vice versa. Follow-up tests showed occurrence of the size–weight illusion was significantly correlated with subject’s sensitivity to discriminate weight, but not with sensitivity to discriminate visual size. Results suggest that the size–weight illusion induced by only visual size cues in an augmented environment is sensory based, and depends on an individual’s integrated perception based on multimodal sensory information.

Heaviness perception. II. Contributions of object weight, haptic size, and density to the accurate perception of heaviness or lightness.

Abstract. The present study investigated the contributions of object weight, haptic size, and density to the accurate perception of heaviness or lightness in the process of discriminating differences in weight between pairs of cubes with cue conflicts such as that resulting from the size-weight illusion. Fifteen subjects, with visual input blocked and relying on the input gained by grasping the cubes with only their fingertips, attempted to accurately discriminate possible differences in weight factor between the two respective cubes in each step of the trials. Three sets – one set each of copper (CP), aluminum (AL), and plastic (PL) – of seven cubes of various weight (0.10–0.74xa0N) were used. All of the cubes were covered with smooth, thin vinyl to eliminate possible input concerning density or material per se. Screens were strategically placed to eliminate any visual cues. One hundred and ninety-six trials with 37 combinations were pseudorandomly presented to subjects in the following conditions: PL versus AL, AL versus CP, and CP versus PL. Trials included 2×3 combinations on the basis of density (98 trials for higher and 98 for lower conditions) and weight (84 ascending trials for heavier, 28 for identical, and 84 descending for lighter conditions). The response for each trial given by each subject was regarded as correct when it accurately identified the weight relationship between the first and second cube. It was found that the subjects fairly accurately identified the weight relationship when density and weight both increased for the second cube (95.6% for given trials), and when density and weight both decreased (94.6%). The current results were markedly greater than those in the constant-density conditions obtained previously, suggesting that changes in density may be as much of an aid in the perception of heaviness and lightness as is weight. Whenever two cues conflicted directionally with each other, however, accuracy fell dramatically to 33.6% for lower density/ascending weight, and to 22.7% for higher density/descending weight. These results indicate the possibility of two different cues contributing to the perception of heaviness and lightness. Cue conflict such as the size-weight illusion naturally occurs when discriminating weight between objects. The present results, however, suggest that a person may perceive heaviness on the basis of the well-regulated relations between changes of density, size, and weight. The way in which these two cues are related through the haptic size is discussed.

Heaviness perception. IV. Weight x aperture -1 as a heaviness model in finger-grasp perception

The present study verified that a simple division of Weight/Aperture (W/A) from information obtained from the individual cubes could describe perceived heaviness when pairs of cubic objects were haptically held one-by-one for comparison utilizing a thumb/index finger grasp. To test the effect of W/A, 15 subjects judged the heaviness between a pair of cubes in three experimental conditions with all visual and material input blocked: (1) cubes with similar ratios, but different weights; (2) cubes with different ratios, but of the same weight; and (3) cubes with dissimilar ratios and weights. The largest percentage of errors (67.4%) was made when objects with similar ratios but different weights were presented, because subjects tended to perceive these objects as being of equal weight. In the condition “equal weight and different ratio”, the percentage of correct responses (17.7%) was relatively small, as the subjects tended to perceive the objects as being of different weights. These results strongly suggest that the W/A ratio is an authoritative model to explain human performance in the process of discriminating heaviness. The division model is proposed as Weight × Aperture−1 on the basis of a recently expounded concept of multiplicative neural circuits.

Heaviness perception. I. Constant involvement of haptically perceived size in weight discrimination

Abstract. With visual input blocked, subjects in this study utilized fingertips only to investigate the involvement of haptically perceived size in heaviness perception among humans. The objects used for testing consisted of three sets – copper (CP), aluminum (AL), and plastic (PL) – of ten cubes of various weights (0.05–0.98xa0N). All of the cubes were covered with a smooth vinyl material to eliminate any extraneous input concerning the actual composition. Screens enclosed the working space to eliminate any possible visual cues. Each comparison was between a pair of cubes of the same material to eliminate the effect of density. Fifteen subjects (M=19.2, SD=0.68xa0years) attempted to judge differences in heaviness between the first and second cube in each trial that had been handed to them by the experimenter and were grasped between the thumb and the index finger. A total of 340 trials with 70 combinations of weight composed of 160 ascending trials (heavier), 160 descending trials (lighter), and 20 identical weight trials were pseudo-randomly presented to each subject for each material. Combinations of difference in weight and the number of trials were identical for all materials so that haptic size was regarded as the single independent factor. Accuracy of the subjects responses for identical weight differences that resulted from placing a pair of cubes of the same combination was compared among the three materials. It was observed that a material like CP that had a lesser size effect facilitated significantly more accurate discrimination of the identical weight differences than PL with its greater size effect. This suggests that small changes in haptic size by the fingertips have a direct influence on heaviness perception when comparing objects of equal density. This finding, therefore, can be considered analogous to the size-weight illusion when comparing objects of unequal density. The findings of this study also suggest the constant involvement of haptic size in heaviness perception by humans along with the existence of a processing mechanism that integrates the factors of weight and haptic size in which heaviness increases either as weight increases or as size decreases, and vice versa.

Grasping an augmented object to analyse manipulative force control

Augmented reality allows changes to be made to the visual perception of object size even while the tangible components remain completely unaltered. It was, therefore, utilized in a study whose results are being reported here to provide the proper environment required to thoroughly observe the exact effect that visual change to object size had on programming fingertip forces when objects were lifted with a precision grip. Twenty-one participants performed repeated lifts of an identical grip apparatus to a height of 20 mm, maintained each lift for 8 seconds, and then replaced the grip apparatus on the table. While all other factors of the grip apparatus remained unchanged, visual appearance was altered graphically in a 3-D augmented environment. The grip apparatus measured grip and load forces independently. Grip and load forces demonstrated significant rates of increase as well as peak forces as the size of graphical images increased; an aspect that occurred in spite of the fact that extraneous haptic information remained constant throughout the trials. By indicating a human tendency to rely - even unconsciously - on visual input to program the forces in the initial lifting phase, this finding provides further confirmation of previous research findings obtained in the physical environment; including the possibility of extraneous haptic effects (Gordon et al. 1991a, Mon-Williams and Murray 2000, Kawai et al. 2000). The present results also suggest that existing knowledge concerning human manipulation tasks in the physical world may be applied to an augmented environment where the physical objects are enhanced by computer generated visual components.

WORST-CASE PREDICTION STRATEGY IN FORCE PROGRAMMING WHEN VISUAL INFORMATION IS OBSTRUCTED

A persons strategy for applying force while lifting an object is dependent upon visual cues. This study investigated the alteration of strategy in force programming when visual information about an objects size was obstructed at the moment of lifting. Seven subjects were instructed to use a precision grip for repeated lifts of a cube-like grip apparatus attached to a box. The grip apparatus was a special device designed to measure grip and load forces. Three different-sized plastic boxes of equal weight were pseudorandomly presented by attaching them beneath the grip apparatus to the subjects in two visual conditions. In the Full-vision condition, subjects could view the boxs size prior to lifting. In the Obstructed-vision condition, a screen prevented subjects from seeing the box size prior to lifting. In the Full-vision condition, the grip force and load force used by subjects on the grip apparatus increased with box size. In contrast, the subjects in the Obstructed-vision condition used forces appropriate for the largest box regardless of box size. The present results suggest that absence of size information may cause an alteration of strategy used to determine force output in that subjects may apply a maximum force adequate for the largest box, which could be called a “worst-case” prediction strategy, i.e., when there is doubt, the most secure lift may be selected for all possible cases.

Contribution of visually perceived size to the scaling of fingertip forces when lifting a 'small' object.

The effects of visually perceived size of an object on the scaling of fingertip forces during lifting tasks were investigated using a small and lightweight object. A grip apparatus was attached to the top surface of three different size boxes of equal weight. 15 healthy adults were asked to grasp the grip apparatus with the thumb and index finger, lift it to a height of 5 cm, hold it for 8 sec, and then put it down. Force transducers embedded in the grip apparatus measured grip and load forces. When subjects lifted the same size object repeatedly, there were no size effects on the grip and load forces used by each subject. When the size was pseudorandomly varied, however, the grip and load forces exerted by 7 subjects significantly increased with increased size, while there were no size effects for the remaining subjects (n = 8). These results suggest a smaller contribution of information on size to the force programming when lifting a small object than when lifting a larger and heavier object as used in previous studies.

Heaviness perception. III. Weight/aperture in the discernment of heaviness in cubes haptically perceived by thumb-index finger grasp.

Weight/aperture (W/A) and weight/square aperture (W/A2) rather than weight (W) previously emerged as possible heaviness models for when a cube is haptically held by thumb–index finger grasp. This is based on the convincing evidence that discernment of a cube’s heaviness depends on the integration of sensory information about W and size of finger aperture (A). The present study, therefore, determined which model would be the best predictor for subject-perceived heaviness. Fifteen subjects were asked to judge whether the second of a pair of cubes was heavier, lighter, or identical to the first in 3×3 (density × weight) conditions. The subject responses were compared with the expected responses for each of the possible heaviness models as well as W. The percentage of consistent trials, i.e., subject response matched the expected response, was then compared among the three models and conditions within each model. The results indicated that subject responses were significantly more consistent with the expected responses determined from W/A compared with those determined from W or W/A2, suggesting W/A as a best predictor of perceived heaviness by finger-grasp perception. W/A2, however, reflected subject responses almost as accurately as W/A could and was, therefore, discussed as one of possible heaviness models when subjects perceive heaviness with different sensory modality.