Department of Biobehavioral Sciences, Teachers College, Columbia University, New York 10027; and Department of Rehabilitation Medicine, College of Physicians and Surgeons, Columbia University, New York, New York 10032
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ABSTRACT |
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Salimi, Iran, Ian Hollender, Wendy Frazier, and Andrew M. Gordon. Specificity of Internal Representations Underlying Grasping. J. Neurophysiol. 84: 2390-2397, 2000. The present study examines anticipatory control of fingertip forces during grasping based on the center of mass (CM) of a manipulated object. Subjects lifted an object using a precision grip while the fingertip forces and the angle about the vertical axis (roll) were measured. The object's CM could be shifted to the left or right of the object's center parallel to the grip axis without changing it's visual appearance. Subjects performed 20 lifts with the CM in the center, left, and right side of the object, respectively. Subjects were instructed to lift the object while preventing it from tilting. Within three to five lifts, subjects were able to asymmetrically partition the load force development before lift-off such that it was higher in the digit opposing the CM. This anticipatory load force partitioning prevented the object from rolling sideways at lift-off. To determine whether the internal representation underlying the anticipatory control is specific to the effectors used to form it, subjects performed five lifts with the right hand with the CM on one side. Following these lifts, they rotated the object 180° around the vertical axis and performed one lift with the same hand or they translated the object to the left side of the body (with or without rotating it) and performed one lift with the left hand. Despite subjects' explicit knowledge of the new weight distribution, they were unable to appropriately scale the load forces at each digit, resulting in a subsequent large roll of the object. The findings suggest that within a few lifts subjects achieve a stable internal representation which accounts for the object's CM and is used to scale the fingertip forces in advance. They also suggest that this representation, which is used for anticipatory control of fingertip forces, is specific to the effectors used to form it. We propose that multiple internal representations may be used during the anticipatory control of grasping.
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INTRODUCTION |
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During precision grasping,
sensory information is used to trigger the release of motor commands
and modulate the motor output (feedback) (see Johansson 1996,
1998
for review). Sensory information signaling object weight
and texture is also used to scale the fingertip force output in advance
(planning) during subsequent manipulations. This anticipatory
control is based on internal representations related to the weight
(Johansson and Westling 1988
) and texture
(Johansson and Westling 1984
; Westling and
Johansson 1984
) of the manipulated objects gained during prior
manipulatory experience. Yet the nature of these representations is not
well understood. Are these representations specific to the effectors that were used to achieve them? What characteristics besides the weight
and texture are represented?
Recent evidence suggests that internal representations of the object
may also include other object features, such as the contact surface
shape (Jenmalm and Johansson 1997) and center of mass (CM) (Johansson et al. 1999
; Wing and Lederman
1998
). In the latter case, Wing and Lederman
(1998)
noted that subjects scale their grip forces in
anticipation of the resulting load torque. In their experiment, the
load was equally distributed between the two opposing digits; i.e., the
CM was located distal to the grip axis joining the fingertips.
Experiments requiring subjects to perform successive lifts with each
hand suggest that internal representations underlying grasping are
independent of the effectors employed. For example, Johansson
and Westling (1984) showed that when the fingertips of one hand
were anesthetized, subjects could use information about the texture of
the object gained during previous lifts with the contralateral hand to
appropriately scale the fingertip forces. Similarly, the ability to
transfer weight-related information between the hands has also been
documented (Gordon et al. 1994
). In contrast to these
studies, other evidence suggests that internal representations may be
specific to the effectors that were used to form them. During a
precision grip task when the surfaces (silk or sandpaper) in contact
with the thumb and index finger differed, Edin et al.
(1992)
found that the forces were independently adjusted to the
local frictional condition, resulting in a slight tilt of the object.
The initial force development was also scaled in anticipation of the
frictional condition based on prior manipulatory experience.
Interestingly, if subjects rotated the object (reversing the surface
condition at each digit), they usually were unable to appropriately
scale the force increase at each digit during the subsequent lift.
However, it should be noted that subjects were not instructed to
prevent the object from tilting.
The present study further investigates the underlying mechanisms of
anticipatory control. Unlike earlier studies which manipulated the CM
anterior to the grip axis, the weight distribution of the object used
in our study was changed parallel to the grip axis (i.e., the CM is
located lateral to the object's center). This requires an asymmetric
partitioning of the load forces at the thumb and the index finger
creating a torque before lift-off to prevent subsequent tilting. Thus,
we will first examine whether the internal representation related to
the CM can be used to scale the load forces at each digit independently
when the load force is not equally distributed between the two digits.
If so, we will determine how many trials are needed to achieve this
internal representation. We will also determine whether this
representation is specific to the effectors which were used during
previous manipulations (as suggested by the findings of Edin et
al. 1992), or is generalizable across effectors (as suggested
by the findings of Johansson and Westling 1984
and
Gordon et al. 1994
).
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Methods |
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Subjects
Eight healthy right-handed subjects (aged 20-51, three males and five females) participated in both experiments I and II. Sixteen different right-handed subjects (aged 19-53, nine males and seven females) participated in experiments III and IV (eight in each experiment). All subjects gave their informed consent according to the Declaration of Helsinki and were naïve to the purpose of the study.
Apparatus
The apparatus (Fig. 1) used in all experiments consisted of two parallel grip surfaces (covered with 200-grit sandpaper, 19 mm diameter, 4.25 cm apart, Fig. 1A) attached 2.25 cm above the center of the upper surface of an aluminum box (12.5 × 7.6 × 10 cm, Fig. 1B). The grip surfaces covered force-torque sensors (Nano F/T transducer, ATI Industrial Automation, Garner, NC) which measured the orthogonal force components (Fy and Fz, 0.025 and 0.05 N resolution, respectively). An electromagnetic position-angle sensor (Polhemus Fastrack, 0.05° resolution, Fig. 1C) mounted on the apparatus measured the position and the angle about the vertical axis (roll). The box contained three compartments, one central and two lateral, in which a 300-g weight could be inserted. The whole apparatus (including the 300-g weight) weighed 680 g. When the weight was placed in the left or right compartment, the CM (Fig. 1B, asterisk) was shifted 2 cm laterally (i.e., parallel to the grip axis) from the object's center in either direction (but still remained between the two contact surfaces). The CM was always located 7.25 cm below the center of the grip surfaces but was centered in the transverse and sagittal planes of the box. The aluminum casing prevented the subjects from seeing the location of the CM at all times.
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Procedures
Subjects washed their hands prior to the experimental session. Subjects sat comfortably in an adjustable chair in front of a table such that when the object was grasped the forearm was parallel to the floor. The apparatus was located such that the axis joining the center of the grip surfaces was perpendicular to the sagittal plane through the subject's shoulder. Before beginning the experiments, the object was placed (sideways) on the subjects' palm so they would know the weight of the object but not the location of the CM. Subjects were instructed to use the thumb and index finger (precision grip) to grasp and lift the object such that the bottom of the box was aligned to a marker 10 cm above the table surface and hold it for 6 s. Timed auditory cues instructed the commencement of each self-paced lifting trial and replacement of the object to the table. Subjects were asked to grasp the object with their fingerpads on the center of the grip surfaces. The experimenter visually monitored the appropriate location of the digits. Subjects were informed that the object's weight distribution may vary and that they should lift and hold the object such that its lower surface was parallel to the table surface. Effort was made to keep the time as consistent as possible between trials. In all experiments, this time was approximately 4-6 s (always <10 s).
experiment i (acquisition). The aim of the first experiment was to examine the acquisition of anticipatory fingertip force scaling to the CM of the manipulated object. Subjects performed 20 consecutive lifts with the right hand with the CM positioned either in the center, left, or right side of the object. The order was randomized across subjects, and five lifts were performed (based on the results of pilot data) with the CM centered in between conditions to "neutralize" the influence of the lateral CM in previous lifts.
experiment ii (object rotation). The aim of the second experiment was to determine whether anticipatory control based on the object's CM could be used immediately following object rotation. In this experiment, the CM was constantly located on one side of the object. Subjects performed five lifts (based on the results of pilot data and experiment I) with their right hand to achieve a stable representation of the weight distribution. They then rotated the object 180° (horizontally) without lifting it and performed one lift with the same hand with the object's CM located on the opposite side (thus, a total of six lifts consisting of five practice trials and one test trial with the CM in a given location). They then performed five more lifts with the CM in this new location before rotating the object again and performing one lift. This entire procedure was repeated five times resulting in a total of 60 trials (five practice and one test trial performed five times in each of the two CM locations).
experiment iii (object translation). The third experiment examined anticipatory control based on the object's CM following translation of the object to the contralateral hand. Subjects performed five lifts with the right hand (same procedure as experiment II). Then, the subjects translated the object approximately 40 cm to the left side of the body (such that the sagittal plane through the left shoulder now coincided with the mid-point of the grip axis) with or without rotating (in a unpredictable order) the object 180° and performed one lift with the left hand (the left hand was only used for the first trials after object translation). When the object was translated but not rotated, nonhomologous digits of the contralateral hands contacted the heavier side (but the CM was identical), whereas when the object was both translated and rotated, homologous digits contacted the heavier side (but the CM was reversed). The procedure (five practice trials and one test trial) was repeated five times for each of the two CM locations (left or right) in each condition (translated or rotated and translated), for a total of 120 trials.
experiment iv (object and subject translation). To exclude the possibility that any change in object location disrupts subsequent anticipatory control, a fourth experiment was performed in two parts. In the first part, subjects only used their right hand. While the CM was located on the left side of the object, subjects lifted the object five times. Subjects then translated the object toward the left side 40 cm without lifting it, moved themselves the same distance so that the object was again aligned with the right shoulder (to insure that the object location was identical relative to the body), and then lifted the object once with the same (right) hand. This procedure was then repeated with the CM located on the right side of the object. This process (five practice trials and one test trial) was repeated five times for each of the two (left and right) CM locations for a total of 60 trials. Thus, the protocol was identical to experiment II except that in this experiment the object was translated rather than rotated.
In the second part of this experiment, both hands were employed. Subjects lifted the object with their right hand five times while the CM was on the one side of the box. They then translated their chair so that the object was aligned in the same manner with their left shoulder without moving the object and lifted it once with the left hand. This procedure was repeated five times for each CM location for a total of 60 trials. Thus, the protocol was similar to experiment III except that after the fifth trials with the right hand the subjects translated themselves to the right rather than translating the object to the left.Data analysis
The grip and load forces at each digit and the position (vertical position and roll) were sampled at 400 and 120 Hz, respectively. The signals were digitized with 12-bit resolution and stored in a laboratory computer system (SC/ZOOM, Umeå University). When the object's CM was on the left or right side of the object, subjects were required to create torques about the Y-axis by asymmetrically partitioning the load forces between the thumb and index finger to prevent subsequent roll (although both forces were still positive to counteract gravity since the CM was still located within the grip aperture). Since unequivocal information about the weight (and weight distribution) is not available until lift-off, the rate of load force development (dLF/dt, calculated using a ±12.5-ms moving average) prior to lift-off must be scaled in advance at each digit independently to achieve the appropriate load forces (i.e., higher in the digit opposing the CM) and to prevent object roll. Therefore, the maximum load force rate before lift-off and roll after lift-off were measured. The average grip force, load force, and the degree of roll were also calculated during the static phase (defined as the last 3 s prior to initiation of object replacement) to determine the effect of a lateral CM on the force distribution and the extent to which subjects were adhering to the instructions to maintain the object upright. The time from roll initiation to the maximum roll was measured to determine how long it took to initiate a correction. Repeated measures analysis of variance (ANOVA) were used in all experiments followed by Newman-Keuls posthoc tests (at the P < 0.05 level).
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RESULTS |
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Experiment I: acquisition
This experiment examined the time course for the development of anticipatory control based on the object's center of mass. Figure 2 shows force and position recordings for a representative subject when the CM was located on the left and right side of the object, respectively. During the first encounter (dotted traces) with the object when the CM was located on the left side, the load force rates were similar for the thumb and the index finger (i.e., they were scaled as if the CM were in the object's center). As a result, there was a sudden counter-clockwise roll of the object to the left (heavier) side, which was quickly corrected toward the horizontal. By the fifth lift (solid traces), there was a significant increase in the load force rate in the thumb, and conversely a significant reduction in the load force rate in the index finger compared with the first lift, although there was still a slight roll as the object was lifted. During the static phase when the object was held in the air, the load forces were not evenly distributed between the two fingers. Rather, most of the load force was distributed to the digit opposing the CM (thumb) to maintain an upright position. Nevertheless, the object was still held at a slight angle. A similar finding (but opposite relationship between the two digits) was observed when the CM was located on the right side of the object.
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Figure 3 shows that these findings were generally representative of the subjects we tested. The mean (±SE) peak load force rate at each digit and the maximum roll of the object for each of the 20 consecutive lifts (across all eight subjects) when the CM was located in the center, left, or right side of the object are plotted in Fig. 3A. As expected, when the CM was located in the object's center, the rates of load force increase of the thumb and index finger were not significantly different from each other and did not change with practice (Fig. 3A, top, P > 0.05). Consequently, the maximum object roll after lift-off was insignificant (always <0.2°, Fig. 3A, bottom). When the CM was located on the left or right side, the load force rates in the thumb and index finger were not appropriately scaled during the first few lifts; rather, equal force rates at each digit were employed. As a result, there was a large (>10°) roll in the object toward the CM which was subsequently corrected (on average 176 ms after it began). On the second lift, the force rates began to be asymmetrically partitioned in a manner which was higher in the digit opposing the CM, reducing the amount of roll. By the fourth or fifth lift, the load force rates at each digit appropriately reflected the location of the CM and these were generally consistent thereafter (P > 0.05 for lifts 6-20). This force scaling prevented the object from rolling appreciably. Interestingly, the differences in the two force rates were somewhat smaller when the CM was located on the right side, which may be explained by the reduced asymmetric partitioning of the final load forces achieved during the static phase (Fig. 3B, middle).
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Figure 3B shows the average grip force, load force, and roll for all trials during the static phase (last 3 s before replacement). The grip forces of the thumb and index finger (Fig. 3B, top) were similar to each other regardless of the CM location (P > 0.05). However, they were both slightly higher when the CM was not located in the object's center (P < 0.05) (the load force was higher at the digit opposing the CM, requiring a higher overall grip force to prevent slips and object tilt). The load forces of the two digits (Fig. 3B, middle) were significantly different from each other when the CM was either on the left or right side of the object (P < 0.05 in both cases), although the asymmetric partitioning between the two digits was slightly reduced when the CM was on the right side as described above. The average roll during the static phase (Fig. 3B, bottom) was always minimal (<2°), although it generally was in the opposite direction of the CM location (i.e., subjects slightly overcompensated).
Experiment II: object rotation
This experiment examined whether subjects would use appropriate anticipatory control immediately following rotation of the object. Subjects lifted the object five times with the right hand, rotated the object 180°, and lifted it again with the same hand. Figure 4 shows recordings from a representative subject for the last (fifth) lift when the CM was located on the left and right side of the object, respectively, and the first lift following rotation of the object. On the fifth lift (solid traces) of each condition, the load force rates were appropriately scaled in the two digits as described in experiment I (i.e., they were higher in the digit on the side of the CM), resulting in minimal roll. The appropriate partitioning of the load force rates prior to lift-off that occurred during the fifth lift was not seen following rotation (dotted traces). Rather, the force rates were similar in both digits regardless of which side of the object the CM was located. This resulted in a large object roll toward the CM compared with the fifth lifts.
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Figure 5 shows that these results were representative of the subjects we tested. It compares the mean (±SE) load force rate for the thumb and index finger (Fig. 5A) and roll (Fig. 5B) for the first and fifth lifts of the object for each CM location (before and after object rotation). Again, on each of the fifth lifts the load force rates were appropriately scaled in the two opposing digits (i.e., they were higher in the digit on the side of the CM, P < 0.05). In contrast, following rotation, the load force rates before lift-off were neither scaled appropriately according to the new CM location nor scaled according to the CM location in the previous trial. Rather, they were nearly equal in each digit (P > 0.05) regardless of which side of the object the CM was located (Fig. 5A). This resulted in a significant (P < 0.05) maximum object roll in the direction of the CM compared with each of the fifth lifts (Fig. 5B). This suggests that subjects were unable to anticipate the object's weight distribution following rotation of the object. Of the eight subjects tested, only one seemed to attempt to scale the load force to the CM (when it was located in the right side), but there was still a significant roll as the object was lifted.
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Experiment III: object translation
This experiment examined whether subjects would use appropriate anticipatory control immediately following either object translation or both object rotation and translation between the two hands. Subjects lifted the object five times with their right hand before either translating the object or both rotating and translating it to their left side. Thus, we were interested in each of the first lifts with the left hand. Figure 6A compares the mean (±SE) load force rate for the thumb and index finger for the initial lifts with the left hand for each condition. After translating or both rotating and translating the object and lifting it with the left hand, there were no differences in the load force rates between the index finger and thumb when the CM was on the left or right side (P > 0.05). The only exception was that the load force rates were slightly higher in the index finger when the object was translated to the left hand with the CM on the left side (P < 0.05). Nevertheless, there was still a significant roll of the object (P < 0.05) compared with the fifth lift of the right hand (not shown) regardless of the CM location and condition, suggesting a poor internal representation of the object.
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Experiment IV: object and subject translation
The results of the above experiments suggest that subjects were unable to appropriately scale the fingertip forces following object rotation or translation between the two hands. To determine whether any alteration of object location would alter the anticipatory control, experiment II was repeated, but instead of rotating the object after the fifth lift, it was simply translated prior to lifting. Figure 7A (top) compares the mean (±SE) load force rate for the thumb and index finger for the fifth lift and the first lift with the same (right) hand after object and subject translation. The appropriate scaling observed in the fifth lift was preserved on each of the first lifts after the object was translated (without rotation); i.e., the force rates were higher in the digit on the side of the CM (P < 0.05 in both cases) and there was minimal object roll (Fig. 7A, bottom). Thus, simply translating the object did not diminish the anticipatory control during the subsequent lift.
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Furthermore, we repeated experiment III, but now the subjects shifted themselves instead of translating the object to the other hand. Figure 7B (top) compares the mean (±SE) load force rate for the thumb and index finger of the fifth lift with the first lift with the contralateral hand after the subject was repositioned (while the object remained stationary). As observed in experiment III, the appropriate scaling was not preserved in each of the first lifts after the subjects shifted themselves and the object was lifted with the contralateral (left) hand (P > 0.05 in both CM locations). Consequently, there was significant object roll compared with the previous lift (P < 0.05). The results suggest that it was not simply an alteration of the object location which prevented anticipatory control.
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DISCUSSION |
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Acquisition of anticipatory control
The findings suggest that subjects employ an anticipatory control
strategy which accounts for the weight distribution of the object by
appropriately partitioning the load force development between the two
digits according to the CM location. This is in agreement with other
studies which show that grip forces are scaled based on the predicted
torsional load when the center of mass is located distal to the grip
axis joining the fingertips (Johansson et al. 1999;
Wing and Lederman 1998
). An important difference is that
the CM was moved laterally along the grip axis in our study resulting
in asymmetric partitioning of the load force development. Furthermore,
the findings show that anticipatory control develops within just a few
lifts. This is in agreement with earlier work suggesting that accurate
representations related to the object's weight (when the CM is
centered) are formed within one or two lifts except for objects of
unusual density (Gordon et al. 1993
).
Specificity of internal representations
Following self-rotation of the object or translation between the
two hands, subjects employed load forces which were nearly equal in
each digit. Several studies suggest that there is an effector (digit)
specific adjustment of the grip-load force ratio (Birznieks et
al. 1998; Burstedt et al. 1997a
,b
; Edin
et al. 1992
). In an analogous study, Edin et al.
(1992)
found that the initial force development reflected the
friction at each digit-object interface during prior manipulation.
Similar to our study, if the subjects rotated the object, they
subsequently were unable to appropriately scale their fingertip forces.
Although subjects in our study rotated the object themselves and were
consequently aware of the location of the CM (they had explicit information mediating the conscious interaction
between the subject and environment), this awareness alone was not
sufficient for the subsequent fine force adjustment. Our findings
suggest that the internal representation related to the object's CM is based on an implicit process (related to the dynamics of the
force development) unavailable to consciousness (see Gentile
1998, 2000
) and is used subsequently only if the
effector-object interface remains unchanged. Indeed, when asked
informally, subjects reported subjectively that they knew where the CM
was immediately following rotation but that they could not use this
information to control the forces accordingly.
In contrast to the present findings, the object's overall weight and
texture can be generalized for use between the two hands (Gordon
et al. 1994; Johansson and Westling
1984
). So what differentiates these two behaviors? It
is conceivable that separate internal representations (or different
levels of the same representation) control these behaviors. Several
studies suggest that more than one representation may underlie
arm-movement planning (e.g., Blakemore et al. 1998
;
Flanagan et al. 1999
; Kawato and Wolpert
1998
). In the present context, the internal representation
related to the object's overall weight and texture may be a "higher
level" representation, in which the relevant muscles are
appropriately parameterized together. Such global parameterization
would likely include the scaling of postural muscles to counteract the
object's weight during lifting (e.g., Bouisset and Zattara
1987
; Wing et al. 1997
; Winstein et al.
2000
). In contrast, the internal representation relevant to the
object's weight distribution (or texture at each digit) may reside at
a "lower level" in which the appropriate force partitioning is
controlled by independent neural mechanisms operating in an
effector-specific manner (Burstedt et al. 1997b
; Edin et al. 1992
).
It is conceivable that the lack of initial force differentiation
following translation between the hands is the result of a reduced
ability to coordinate the fingertip forces in the nondominant (left)
hand. However, as described above, weight information for symmetrically
distributed objects has been shown to be transferable from the dominant
to the nondominant hand (Gordon et al. 1994). Also, we
have found little difference in the coordination of fingertip forces
based on handedness (Gordon et al. 1994
, 1999
).
Furthermore, a lack of initial force differentiation is also observed
in the dominant hand following object rotation. Therefore, handedness does not likely account for these findings.
It is also possible that simply altering the object's position "erases" the internal representation, preventing anticipatory control during subsequent lifts. However, our results do not support this notion. When the object was translated and the subject was asked to lift it with the same hand, anticipatory control was preserved. In contrast, when the object position was unaltered between lifts but the subject moved and lifted it with the contralateral hand, anticipatory control disappeared. These findings suggest that it was the disruption in the mapping between the effector and object characteristics which accounted for the loss of anticipatory force scaling rather than simply movement of the object.
Object versus effector representation
Do subjects have an internal representation of the object's
physical properties or a "sensorimotor memory" of the forces
employed during previous manipulatory experiences? Following rotation
of the object or translation between the hands, the fingertip forces neither reflected the new location of the CM nor the previous CM
location. This suggests that subjects were not able to scale the forces
based on the known (since they rotated the object themselves) location
of the CM, yet they did not employ the same forces which were used in
the previous lifts. Thus, our results cannot directly resolve this
issue. However, taken together with the findings from other work, we
speculate that there are representations of both. When subjects are
presented an object whose weight is varied in an unpredictable manner
between trials, but whose visual features do not change, the forces are
still influenced by the weight of the object during the previous lift,
albeit not to the same extent as when the weight is predictable
(Forssberg et al. 1992; Gordon et al.
1997
; Johansson et al. 1988
). Thus, despite the
fact that the subject knows the object's weight may differ, they are
unable to suppress the sensorimotor memory of the forces previously
employed. In contrast, when there are tangible features differentiating the objects (such as visual geometric cues), subjects are able to
immediately employ the appropriate motor commands associated with a
given object (Gordon et al. 1991
, 1993
). These findings may suggest that there are internal representations of both
the object's physical properties and the forces previously employed and that normally these representations require proper integration.
Conclusions
In conclusion, our findings suggest that the internal representation related to the object's CM is specific to the effector used to form it. We propose that multiple internal representations may be used during the anticipatory control of grasping, which include various object features and the forces used during previous manipulatory experiences. Understanding the nature of these internal representations is particularly important to understand the control of sophisticated tool use, since many tools have an uneven weight distribution and are not grasped at their center of mass.
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ACKNOWLEDGMENTS |
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We thank Drs. Sergei Aleshinsky, Ralf Reilmann, and Ashwini Rao for fruitful discussion of this work.
This project was supported by National Science Foundation Grant 9733679 and the VIDDA foundation (A. M. Gordon). I. Salimi was supported by a fellowship from the Medical Research Council of Canada.
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FOOTNOTES |
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Address for reprint requests: A. M. Gordon, Dept. of Biobehavioral Sciences, Box 199, Teachers College, Columbia University, 525 West 120th St., New York, NY 10027 (E-mail: ag275{at}columbia.edu).
Received 5 April 2000; accepted in final form 21 July 2000.
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REFERENCES |
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