overruled by perceptual mechanisms. Thus, these results suggest that the size information may be used in the anticipatory control of the loading phase along ...
Exp Brain Res (1991) 85:226-229
Experimental Brain Research Springer-Verlag 1991
Research Note Integration of sensory information during the programming of precision grip: comments on the contributions of size cues 1
2
A.M. Gordon’, H. Forssberg , R.S. Johansson , and G. Westling
2
Nobel Institute for Neurophysiology and Department of Pediatrics, Karolinska Institute, S-104 01 Stockholm, Sweden Department of Physiology, Umeb University, S-901 87 Umeå, Sweden Received October 22, 1990 / Accepted February 14, 1991
Summary. Evidence has recently been given by Gordon et al. (1991a, b) for the use of visually and haptically acquired information in the programming of lifts with the precision grip. The size-related information influences the development of manipulative forces prior to the lift-off, and the force output for larger objects is adjusted for a heavier weight even if the weight of the objects is kept the same. However, the size influences on the force output were small compared to the relative effects of the expected weight in previous trials (Johansson and Westling 1988). In the present study, both the size and weight of objects were changed between consecutive lifts to more fully determine the strength of visual size cues. During most trials, the size and weight covaried (i.e. the weight was proportional to the volume). However, in some trials, only the size was switched while the weight was kept the same to create a mismatch between the size and weight. The forces were still appropriately scaled towards an expected weight proportional to the volume of the object. It was concluded that visual size cues are highly purposeful. The effects were much larger than previously reported and were similar in magnitude to the effects based upon the expected weight. Thus, the small effects reported in the previous experiments may have been a result of conflicting “size-weight” information. Key words: Precision grip - Motor control - Motor
programming - Vision - Size-weight - Illusion - Human
Introduction
Anticipatory control is necessary to appropriately adjust the isometric force development to the weight of a lifted Offprint requests to: H. Forssberg, Department of Pediatrics, Karolinska Hospital, S-104 01 Stockholm, Sweden
object. Thus, the force output is characterized by a decreased force rate prior to lift-off and iscritically adjusted to provide for similar movement trajectories. Coupling grip force and load force generating circuits provides a useful synergy allowing these forces to be scaled in parallel toward the anticipated weight of the object (Johansson and Westling 1988). Recently, Gordon et al. (1991a, b) demonstrated that visually and haptically acquired information regarding the size of the object influences the anticipatory control of the force output during the loading phase. When various sized objects were attached to an instrumented grip handle, subjects scaled the grip and load force rates higher for larger objects even though the weights of the objects were identical. The higher force rates for larger objects occurred in spite of the fact that subjects perceived the small box to be heavier. The latter, which is consistent with Charpentier’s (1891) size-weight illusion, means that the influences of size information cannot be overruled by perceptual mechanisms. Thus, these results suggest that the size information may be used in the anticipatory control of the loading phase along with information regarding the weight and friction of the object acquired during previous lifts (cf. Johansson 1990). It was noted, however, that the influences of the object’s size on the isometric force output were small compared to the relative effects of the expected weight of the object (Johansson and Westling 1988). This may not be consistent with the general importance of vision during the programming of reaching and grasping (cf. Jeannerod 1986; von Hofsten and Rönnqvist 1988). In contrast to our previous studies in which the weight of the objects was kept constant, both the size and weight of objects were changed in the present paradigm to more fully determine the relative contribution of visual size cues in the programming of manipulative forces during precision grip.
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Material and methods
Size-weight covariation (part ZZZ). Following part II, the large box was again unexpectedly switched back to 1200 g, and the subject lifted each box 6 times in a pseudorandom order. After this series, the small box was unexpectedly switched to 1200 g and lifted once.
Subjects Twelve healthy right-handed subjects (all female; aged 1844) participated in the study. All subjects were naive to the purpose of the study. Two subjects initially recruited were replaced due to excessive grip force and grossly awkward lifts.
Data acquisition and analysis Signals from the test object were sampled at 400 Hz, digitized with 12 bit resolution and stored in a microcomputer. The first time derivative of grip force and load force, and the second time derivative of vertical position (acceleration) were calculated using a +/-5 point numerical differentiation. These parameters were measured at their positive peaks. Student’s t-tests were used to assess statistically significant differences between mean values.
Apparatus Four boxes (with similar surfaces made of balsa) were used as test objects; two boxes measured 8 x 8 x 8 cm and two measured 12.7 x 12.7 x 12.7 cm resulting in a volume ratio of 1: 4. One box of each size weighed 140 g while the other weighed 1040 g. A small instrumented grip handle, weighing 160 g, slid into a track on top of each box. Thus the weight of the boxes and the instrument was either 300 g or 1200 g, i.e. a weight ratio of 1: 4. The instrumented grip handle was a modified version of an earlier described instrument with grip surfaces (35 mm x 35 mm) covered with sandpaper (no. 200) at the top of the instrument (see Gordon et al. 1991a). Grip force and load force were measured by strain gauge transducers (d.c. - 160 Hz). The vertical position was measured (d.c. - 1.5 kHz) by infra-red light emitting diodes attached to the instrumented grip handle and a light-sensitive photo-resistor (United Detector Technology, SC/28) located in a camera housing unit.
Results Size-weight covariation (part I) All subjects used visual size information to adequately scale the forces according to the weight of the object (Table 1). This resulted in significantly greater maximum grip and load force rates, and larger grip force peaks for the large box (p < 0.01 in all instances). After a decrease of load force rate prior to lift-off, the load force following this moment accounted for an acceleration profile providing a critically damped lifting movement resulting in similar accelerations. When the size was switched to the large box while the weight remained at 300 g, the forces were scaled for the heavier, 1200 g object (Fig. lA, Table 1). The strong load force drive at lift-off resulted in an increased acceleration and a pronounced position overshoot. The peak acceleration was approximately twice that observed during the previous lifts. However, large differences were not seen in the maximum load force rate since the object was lifted prior to the programmed maximum, while the grip force rate continued to increase until terminated by sensory feedback (cf. Johansson and Westling 1988).
Procedure The task required subjects to lift the object approximately 10 cm by the grip handle using the precision grip (between the thumb and index finger), maintain the lift 3-4 s, replace and release the instrument. To prevent the subject from knowing the weight of the box prior to lifting, the boxes were changed behind a screen prior to each lift by sliding the grip handle out of the track on one box into the track of another. All boxes not being used during a given trial were kept behind the screen. The time between consecutive lifts was approximately 10 s. The experiment was divided into three parts, although there was no pause in between each part. These included: Size-weight covariation (part Z). After several practice trials with the small box at 300 g and the large box at 1200 g, these two objects were pseudorandomly presented such that the subject lifted each 6 times. Immediately following this series, the large box was unexpectedly switched to 300 g and lifted once.
Size variation (part II) Differences were again found between the small and large box for the maximum grip and load force rates and for the grip force peaks (Table 1). These were statistically significant (p < 0.01) except for the maximum load force
Size variation (part ZZ). After the switch in weight for the large box,
the small and large box were pseudorandomly presented 6 times each while the weight of each box remained at 300 g. This paradigm was similar to that used by Gordon et al. (1991a, b).
Table 1. Descriptive statistics of dependent measures as a function of box and condition
Grip force Grip force rate Load force rate Acceleration
Part III
Part II
Part I (300 g)
LG 1200 g)
1st LG (300 g)
SM (300 g)
LG (300 g)
SM (300 g)
LG (1200 g)
1st SM (1200 g)
12.1 103.9 75.7 3.6
21.7 142.3 109.2 3.5
17.7 150.4 82.3 6.5
7.5 67.8 68.7 2.8
10.5 97.1 72.7 4.5
9.2 95.8 79.4 3.3
20.1 147.0 119.5 3.4
19.5 90.4 89.4 1.8
(4.4) (46.9) (24.5) (0.2)
(4.5) (51.7) (27.1) (0.8)
(5.6) (64.0) (31.9) (1.2)
(3.2) (36.1) (25.9) (0.6)
(4.7) (56.3) (24.5) (1.2)
(3.3) (59.6) (36.8) (0.9)
(5.2) (94.9) (35.0) (1.2)
Numbers represent means and standard deviations (parentheses) of the individual means (n = 6 trials for each box except 300 g and the 1st SM 1200 g) for all subjects 12). All values were measured at their corresponding peaks
(5.8) (65.2) (55.1) (0.3)
= 1 for the 1st
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of the object. Size cues seem to be used to indirectly provide weight information for the programming of the manipulative forces. Thus, an assumption of constant density for similar looking materials may be the link between size and weight. However, the use of size information also relies on the relationship between various objects since no differences are found in the programming of manipulative forces when boxes differing in size but equal in weight are lifted consecutively (Gordon et al. 1991b). Size cues are thus used to relate the physical properties of objects and appear to be an important source of feedforward information during the programming of manipulative forces. Acknowledgements. The first author was supported by a grant from the Fulbright Commission and Stiftelsen Wenner-Gren Center. This study was supported by the Swedish Medical Research Council (projects 4X-5925, 4P-8885 and 14X-08667), Stiftelsen Sven Jerrings Fond, the First of Mayflower Annual Campaign for Children’s Health, Stiftelsen Solstickan, Sunnerdahls Handikappfond, and the University of Umeå. The authors also wish to thank AnnChristin Eliasson for assistance with figure preparations, Ingmarie Ericsson for assistance with data collection, and Tommy Nord for technical assistance.
References Charpentier A (1891) Analyse experimentale de quelgues elements de la sensation de poids. Arch Physiol Norm Patho1 3 : 122-135 Gordon AM, Forssberg H, Johansson RS, Westling G (1991a) Visual size cues in the programming of manipulative forces during precision grip. Exp Brain Res 83 : 477482 Gordon AM, Forssberg H, Johansson RS, Westling G (1991b) The’ integration of haptically acquired size information in the programming of the precision grip. Exp Brain Res 83:483-488 Hofsten C von, Rönnqvist L (1988) Preparation for grasping an object: a developmental study. J Exp Psycho1o1 14:610-621 Jeannerod M (1986) The formation of finger grip during prehension: a cortically mediated visuomotor pattern. Behav Brain Res 19:305-319 Johansson RS (1990) How is grasping modified by somatosensory input? In: Humphrey DR, Freund HJ (eds) Motor control: concepts and issues. Dahlem Konferenzen. John Wiley & Sons Ltd., Chichester, pp 331-355 Johansson RS, Westling G (1988) Coordinated isometric muscle commands adequately and erroneously programmed for the weight during lifting task with precision grip. Exp Brain Res 71:59-71 Poulton EC (1980) Range effects and asymmetric transfer in studies of motor skills. In: Nadeau et al (eds) Psychology of motor behavior and sport. Human Kinetic Publ, Champaign, pp 3399359
