Specificity of Internal Representations Underlying Grasping
IRAN SALIMI, IAN HOLLENDER, WENDY FRAZIER, AND ANDREW M. GORDON
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
Received 5 April 2000; accepted in final form 21 July 2000
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 weightrelated 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
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: [email protected]).
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.
INTRODUCTION
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0022-3077/00 $5.00 Copyright © 2000 The American Physiological Society
www.jn.physiology.org
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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 liftoff. 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.
SPECIFICITY OF REPRESENTATIONS UNDERLYING GRASPING
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).
METHODS
Subjects
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.
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.
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 II (OBJECT ROTATION).
FIG. 1. Schematic diagram of the grip apparatus with two parallel grip
surfaces (A) centered on top of an aluminum box (B). The grip surfaces
covered force sensors which measured the grip and load forces at each digit.
An electromagnetic position-angle sensor (C) measured the vertical 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.
When the weight was placed in the left or right compartment, the CM was
shifted 2 cm from the object’s center in either direction without changing the
visual appearance of the object. The CM positions are indicated by asterisks.
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
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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 naı̈ve to the purpose of the study.
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I. SALIMI, I. HOLLENDER, W. FRAZIER, AND A. M. GORDON
(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.
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
RESULTS
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 counterclockwise 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
FIG. 2. Grip force, load force, and load force rate
at each digit, object roll, and vertical position for a
representative subject when the CM was located on
the left and right side of the object. The horizontal
dotted traces represent the first lift with the CM in a
given location while the solid traces represent the fifth
lift with the CM in that location. Positive values for
the roll indicate counter-clockwise rotation while negative values indicate clockwise rotation. Vertical
dashed lines indicate lift onset. The traces are aligned
at the load force onset.
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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.
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).
SPECIFICITY OF REPRESENTATIONS UNDERLYING GRASPING
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FIG. 3. A: mean (⫾SE) load force rate (top) of the
thumb (unfilled) and the index finger (filled) and the
maximum roll (bottom) during 20 lifts (using the
right hand), when the center of mass was located in
the center (left column), left (middle column), and
right (right column) side of the object. B: grip force
(top) and the load force (middle) of the thumb (unfilled), and the index finger (filled), and the degree of
roll (bottom) during the static phase for all trials. Left,
center, and right refer to the location of CM. Positive
values for the roll indicate counter-clockwise rotation
while negative values indicate clockwise rotation.
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.
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
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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.
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).
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
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I. SALIMI, I. HOLLENDER, W. FRAZIER, AND A. M. GORDON
FIG. 4. Grip force, load force, and load force rate at
each digit, object roll, and vertical position for a representative subject when the CM was located on the left
and right side of the object, for the fifth lift (solid lines)
when the CM was located on the left and right side of
the object, and the first lift (dotted lines) following
rotation of the CM to those locations. Positive values
for the roll indicate counter-clockwise rotation while
negative values indicate clockwise rotation. Lift onset
is denoted by vertical dashed lines. The traces are
aligned at the load force onset.
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 differ-
ences 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.
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).
FIG. 5. Mean (⫾SE) load force rate (A) for the thumb (unfilled) and index finger (filled) and the maximum roll (B) for the
fifth lift with the right hand with the CM in a given location and
the first lift with the same hand after object rotation. Left and
right refer to the location of CM. Positive values for the roll
indicate counter-clockwise rotation while negative values indicate clockwise rotation.
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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.
SPECIFICITY OF REPRESENTATIONS UNDERLYING GRASPING
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FIG. 6. Mean (⫾SE) load force rate (A) for the thumb
(unfilled) and index finger (filled) and the maximum roll (B)
during the first lift with the left hand following translation or
both translation and rotation. Left and right refer to the
location of CM. Positive values for the roll indicate counterclockwise rotation while negative values indicate clockwise
rotation.
DISCUSSION
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 digitobject 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
FIG. 7. A: Mean (⫾SE) load force rate (top) and roll (bottom) for the thumb (unfilled) and index (filled) finger for the
fifth lift with the right hand with the CM in a given location
and the first lift with the same hand after both object and
subject translation. B: mean (⫾SE) load force rate (top) and
roll (bottom) for the thumb (unfilled) and index (filled) finger
for the fifth lift with the right hand and the first lift with the left
hand after subject translation. Positive values for the roll
indicate counter-clockwise rotation while negative values indicate clockwise rotation.
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Thus, simply translating the object did not diminish the anticipatory control during the subsequent lift.
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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I. SALIMI, I. HOLLENDER, W. FRAZIER, AND A. M. GORDON
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.
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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