Section for Physiology, Department of Integrative Medical Biology, Umeå University, SE-90187 Umeå, Sweden
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ABSTRACT |
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Jenmalm, Per, Seth Dahlstedt, and Roland S. Johansson. Visual and Tactile Information About Object-Curvature Control Fingertip Forces and Grasp Kinematics in Human Dexterous Manipulation. J. Neurophysiol. 84: 2984-2997, 2000. Most objects that we manipulate have curved surfaces. We have analyzed how subjects during a prototypical manipulatory task use visual and tactile sensory information for adapting fingertip actions to changes in object curvature. Subjects grasped an elongated object at one end using a precision grip and lifted it while instructed to keep it level. The principal load of the grasp was tangential torque due to the location of the center of mass of the object in relation to the horizontal grip axis joining the centers of the opposing grasp surfaces. The curvature strongly influenced the grip forces required to prevent rotational slips. Likewise the curvature influenced the rotational yield of the grasp that developed under the tangential torque load due to the viscoelastic properties of the fingertip pulps. Subjects scaled the grip forces parametrically with object curvature for grasp stability. Moreover in a curvature-dependent manner, subjects twisted the grasp around the grip axis by a radial flexion of the wrist to keep the desired object orientation despite the rotational yield. To adapt these fingertip actions to object curvature, subjects could use both vision and tactile sensibility integrated with predictive control. During combined blindfolding and digital anesthesia, however, the motor output failed to predict the consequences of the prevailing curvature. Subjects used vision to identify the curvature for efficient feedforward retrieval of grip force requirements before executing the motor commands. Digital anesthesia caused little impairment of grip force control when subjects had vision available, but the adaptation of the twist became delayed. Visual cues about the form of the grasp surface obtained before contact was used to scale the grip force, whereas the scaling of the twist depended on visual cues related to object movement. Thus subjects apparently relied on different visuomotor mechanisms for adaptation of grip force and grasp kinematics. In contrast, blindfolded subjects used tactile cues about the prevailing curvature obtained after contact with the object for feedforward adaptation of both grip force and twist. We conclude that humans use both vision and tactile sensibility for feedforward parametric adaptation of grip forces and grasp kinematics to object curvature. Normal control of the twist action, however, requires digital afferent input, and different visuomotor mechanisms support the control of the grasp twist and the grip force. This differential use of vision may have a bearing to the two-stream model of human visual processing.
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INTRODUCTION |
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Most manipulative
tasks require precise coordination of fingertip actions for grasp
stability. To avoid accidental slips, subjects apply large enough grip
forces normal to the grip surfaces in relation to destabilizing
tangential loads, while they avoid excessive grip forces. Potentially
destabilizing loads include time-varying linear load forces but also
torque loads, i.e., torque tangential to the grip surfaces. Due to
gravitational and inertial reaction forces, torque loads typically
develop in manipulation of objects whose center of mass does not lie on
the grip axis, i.e., the axis between the centers of the
grip surfaces of the tips of the thumb and the index finger during a
precision grip. These torque loads tend to rotate the object around the
grip axis. The grip forces required to prevent rotational slips
increase linearly with the torque load with a slope that depends on the friction of the grasp (Kinoshita et al. 1997). Subjects
automatically increase and decrease the grip force in parallel with the
changes in torque load (Goodwin et al. 1998
;
Johansson et al. 1999
; Wing and Lederman
1998
) just as they do for linear loads (Flanagan and
Wing 1993
; Johansson and Westling 1984
).
Thus the sensorimotor mechanisms that control the grip force in
manipulatory actions predict the destabilizing effects of
self-generated linear and torque loads tangential to the grip surfaces.
Most objects that we handle have curved surfaces. We have recently
demonstrated that the surface curvature has modest effects on grip
force requirements for grasp stability under linear load forces
(Jenmalm et al. 1998), whereas the effects are profound under torque loads (Goodwin et al. 1998
). With
spherically curved convex surfaces, the minimum grip force required to
prevent rotational slip under torque load increases with curvature. In
agreement with established principles concerning the adaptation of
fingertip forces to other object properties (for overviews, see
Johansson 1996
, 1998
), adaptation to surface curvature
for grasp stability under torque load is achieved by parametric
adjustments of the balance between the grip force and the fingertip
load (Goodwin et al. 1998
). That is, for a given torque
load, subjects scale their grip force to curvature, keeping an adequate
safety margin against rotational slips. However, the sensory
information that underlies this parametric adaptation to surface
curvature is unknown, both concerning sensory modality and use of
sensory information for possible anticipatory control of force-torque
coordination and for compensatory adjustments (cf. Johansson
1996
; Wing 1996
). In the study by Goodwin
et al. (1998)
, the subjects grasped, lifted, and held an object
in air under linear load forces before rotating it such that torque
load developed. That is, subjects could have used visual as well as
tactile cues related to surface curvature during any phase of the trial
according to any control policy.
When subjects lift objects with flat grip surfaces that are tapered
upward or downward, they use visual cues for feedforward control such
that the ratio between the grip force and the linear load force is
adapted to object shape before the execution of the motor commands
(Jenmalm and Johansson 1997). When vision is not
available, subjects use tactile information obtained at touch to update
parametrically the coordination to a change in object shape. However,
this scheme may not necessarily apply for the adaptation of the motor
output to object curvature under torque loads. Objects that have
tapered flat grip surfaces may be particularly suitable for forward
control based on visual geometric cues because the surface angle
directly relates to the required coordination of grip and load forces.
Moreover, the fact that the effects of surface curvature on the grip
force requirements strongly depend on the type of load suggests a
different control scheme. With curved surfaces under torque load,
tactile information may be critical despite visual cues. High-fidelity
tactile information about surface curvature is indeed available once
the object is contacted, i.e., at very low contact forces
(Goodwin et al. 1991
, 1997
). Furthermore when the torque
load develops, tactile information related to localized and overt
rotational slips may update memory systems for parametric control of
force-torque coordination, similar to the manner by which subjects use
tactile signals for adaptation of the grip-load force coordination to
the frictional condition of the grasp under linear load
(Johansson and Westling 1984
, 1987
).
Apart from rotational slips, torsional viscoelasticity of the fingertip
pulps may tend to destabilize precision grips in tasks that involve
tangential torque load. Although little is known about fingertip
mechanics in relation tangential torque load, the fingertip pulp shows
viscoelastic properties both when compressed (e.g., Pawluk and
Howe 1999; Serina et al. 1997
, 1998
;
Srinivasan 1989
; Srinivasan and Dandekar
1996
) and when subjected to tangential shear forces
(Nakazawa et al. 2000
). It seems therefore reasonable that viscoelasticity of the fingertip pulp would account for rotational compliance and creep that subjects would need to control for if required to precisely orient objects in space under torque load. Furthermore we hypothesized that any changes in surface curvature would
influence this control. This is because the rotational yield of the
grasp increases with surface convexity since the contact area between
the fingertips and the object decreases, i.e., the fingertips only
partially mold to a highly curved surface.
We devised a prototypical manipulatory task that involved dexterous application of torques around the grip axis to examine how subjects adapt the fingertip actions to changes in object curvature and which sensory information they use. The subjects were asked to use a precision grip to grasp an elongated object at one end and lift it while keeping it level. Due to the location of the object's center of mass in relation to the horizontal grip axis, gravitational and inertial forces gave rise to significant torque loads that required adaptation of the grip forces to surface curvature for grasp stability. We investigated the capacity of visual and tactile sensory information to mediate such adaptation by analyzing the behavior of subjects when vision, digital sensibility, or both were removed. We also examined the use of visual and tactile sensory information in predictive and reactive control. Finally we tested if subjects controlled for possible influences of surface curvature on the rotational yield of the grasp during torque loading and how sensory information about curvature might be integrated in this control.
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METHODS |
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Subjects and general procedure
Eight healthy women and men between 21 and 32 yr participated in the study. All gave their informed consent, and the local ethics committee had approved the experimental protocol. The subjects sat in a chair with the right upper arm parallel to the trunk and the forearm extending anteriorly. In this position, they were asked to grasp an elongated object at one end and lift it (Fig. 1). The test object was located on a tabletop about 8 cm below the hand. The length axis of the object was oriented in a sagittal plane, and the proximal end of the object, grasped by the subjects, was located about 10 cm in front of the hand. The object was grasped between the tips of right index finger and thumb, and subjects were instructed to lift the object vertically about 8 cm while keeping it level and maintaining its orientation in the horizontal plane. During the lift, the grasp was subjected to a torque load because the center of mass of the object was located distal to the grasped surfaces. That is, the line between the centers of the grasped surfaces, the grip axis, was horizontal and perpendicular to the length axis of the object. About 5 min before the experiments, the subjects washed their hands with soap and water.
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Apparatus
The test object was built of balsa and its total weight was 50 g (Fig. 1). It was equipped with two grip surfaces whose lateral parts were separated by 25 mm. The center of mass of the object was located 21 mm in front of the grip axis, which resulted in a nominal torque load of 10.3 milli-Newton-meters (mNm) when the object was held level in air. An electromagnetic position-angle sensor (FASTRAK, Polhemus, Colchester, VT) was attached to the object to measure its vertical position referenced to the midpoint between the centers of the grasped surfaces. This sensor also measured the elevation angle of the object in relation to the horizontal; elevation of the distal end of the object provided positive angles (Fig. 1B). Another sensor was attached to the nail to measure the elevation angle of the right index finger to assess possible rotations of the grasp about the horizontal grip axis during the lifting trials. That is, one likely way to compensate for rotational yield of the grasp under torque loading would be that subjects employed a radial flexion of the wrist to rotate the fingertips around the grip axis when holding the object. Furthermore by computing the difference between the elevation angles of the finger and the object, we assessed the rotational yield of the grasp under tangential torque load. In the environment of the sensors, the accuracy of the position measurement was better than 0.5 mm (resolution: 0.12 mm), and the accuracy of the angle measurement was better than 1° (resolution: 0.025°).
The grasped surfaces were coated with silicon carbide grains (50-100
µm) covered by a thin layer of cyanoacrylate. The surface contacted
by index finger could be changed between lifting trials. It was either
flat or spherically curved with a radius of 10 or 5 mm, i.e., the
convex curvature defined as the inverse radius was 0, 100, or 200 m1. This grip surface was
connected to the rest of the object via a six-axis force-torque sensor
(Nano F/T transducer, ATI Industrial Automation, Garner, NC) that
measured forces and torques applied by the index finger. We measured
the grip force along the grip axis (resolution of 0.05 N) and the
component of the load force orthogonal to the grip axis that was
vertical when the object was supported by the table (resolution of
0.025 N). These measurements provided an adequate estimate of the
vertical load force during the lifting trial because the deviations in
object elevation from level was relatively small (see
RESULTS). The time-varying torque load was measured as the
tangential torque about the grip axis (resolution: 0.125 mNm). Because
the surface contacted by the thumb was equipped with a ball bearing
mechanism, the index finger took up the entire torque load during the
lifting task. The curvature contacted by the thumb was constant at 200 m
1.
There were several reasons for the design of the test object: first, by
focusing the torque load of the grasp to one of the two digits, we
could directly relate the employed grip force to the torque load. We
have previously demonstrated that the partitioning of torque can vary
widely between the index finger and thumb when humans manipulate
objects with fixed grip surfaces (Goodwin et al. 1998;
Johansson et al. 1999
). Second, by using a strongly curved surface at the thumb, we promoted the use of grasp sites that
were close to the centers of the grasped surfaces. This is important
because otherwise subjects could have generated substantial coupled
tangential forces at the two digits to prevent the object from
spinning. Third, by keeping the mass of the object low we could safely
assume that the linear force loads observed in our experiments (less
than 1 N) had negligible influences on grip force requirements compared
with those imposed by the torques (Kinoshita et al.
1997
). Finally, with low load forces and the grasp sites
centered on the grasped surfaces, we reduced errors in torque
measurements because otherwise linear load forces (tangential to the
grip axis) could have contributed significantly to the measured torque.
In control experiments carried out on two subjects, we compared the
behavior with the present configuration of the test object and the
behavior with a similar object that had matching pairs of grip surfaces
that were both fixed. The pattern of the time-varying fingertip forces
and torques were the same across the two objects for all surface
curvatures. The only differences that we observed concerned the
distribution of the torque load between the two digits and the
magnitude of the grip forces. As expected, the partitioning of the
torque load varied between the two digits across trials (see
Goodwin et al. 1998). Furthermore the magnitude of the
grip force was adapted to the torque load of the digit that showed the
greater of the two torque loads.
Lift series
Each subject performed two different types of lift series. In
the first kind of series, we wanted to stress the need of adapting the
fingertip actions to changes in surface curvature. This series consisted of 37 lifts with unpredictable changes of the surface curvature between lifts. To facilitate analysis of possible effects of
surface curvature in the previous lift, each of the three curvatures (0, 100, or 200 m1) was
preceded by one of the other two curvatures or the same curvature at
least four times during a single series. Five seconds after the object
was initially touched, an auditory cue (1 kHz, 0.1 s) told the
subjects to replace the object on the table while maintaining the
object level. The interval between trials was 5 s. To assess the
importance of vision and digital sensibility, all eight subjects
carried out this lift series under normal sensory conditions, i.e.,
with vision and normal digital sensibility; with normal digital
sensibility but without vision; with normal vision but with digital
anesthesia; and when both vision and digital sensibility were removed
(see table in Fig. 1C). To block vision, the subjects wore
electronic shutter glasses that occluded vision at grip forces above
0.1 N. Thus the subject could reach for the object guided by vision but
once the object was touched vision was removed. Furthermore we
prevented the subjects from ever seeing the surface contacted by the
index finger by using a small horizontally oriented paper shield
attached to the top of object (see Fig. 1A, dashed
rectangle). We removed digital sensibility by blocking the digital
nerves of the index finger by injecting a mixture of bupivacain (3 ml
5% solution) and prilocain (1.5 ml 10% solution) about 2 cm distal to
the metacarpophalangeal joint. We considered the procedure successful
when the subjects failed to report light touch, heavy touch, pinprick
and squeezing of the distal phalanx of the index finger. Four of the
subjects completed the same lifting series one additional time during
digital anesthesia. This time we prevented the subjects from seeing the
right grip surface by using the shield, but they could see the test
object (and the hand) throughout the lifting trials ("Restricted
vision" in Fig. 1C). If the manipulandum accidentally
rotated due to slippage during any lift series, the subject resumed the
series by repeating the current trial.
The second type of lift series was designed primarily for measurements
of rotational friction of the grasp. Rather than replacing the
manipulandum while keeping it level, the subjects ended the trials by
decreasing the grip force when the object was held in air to allow it
to rotate under the influence of gravity until its distal tip touched
the table (cf. Goodwin et al. 1998). An auditory cue (1 kHz tone for 0.1 s) that occurred 5 s after the object was
initially touched (defined at a grip force level of 0.1 N) signaled the
commencement of this "rotational slip test." Following the
rotational slip, the subjects fully replaced the object on the table.
This lift series consisted of 24 trials delivered in six blocks of 4 trials in which the curvature of the right grip surface was held
constant. We varied the curvature of the grasped surface between blocks
and the sequence of presentation of curvatures varied among the
subjects. The subjects were free to use vision of the object and its
surface curvature. To analyze whether rotational friction of the grasp
was influenced by digital anesthesia, we ran this series with normal
digital sensibility and after we had anesthetized the right index finger.
INSTRUCTIONS TO THE SUBJECTS.
Before data collection started, the experimenter demonstrated the task
and the subject performed four practice trials with each of the three
different surface curvatures (0, 100, and 200 m1). We specifically
instructed each subject to grasp the surfaces at their centers, which
we pointed out, and to lift the object vertically while avoiding any
tilt. We carefully observed the subject during the practice trials to
ensure that they complied with the instructions. Importantly, the
subjects were given no instructions regarding which fingertip forces to
use, and they always lifted the object at their own preferred speed.
Data collection and analysis
A flexible data-acquisition and -analysis system (SC/ZOOM, Section for Physiology, IMB, Umeå University) was used to digitize and store the transducer signals. Signals were recorded with a resolution of 12 bits and at 400 samples/s from the force-torque sensor and 14 bits and at 60 samples/s from the two position-angle sensors. Rate of change in grip force was assessed from the first time differential of the force signal using a ±5-point numerical differentiation (±12.5-ms window).
All measurements obtained for quantitative analyses were taken from single trials. We defined the torque-loading phase as the period during which the torque load increased until object liftoff. The start, midpoint, and end of this phase were defined as the times at which the torque load reached 10, 50, and 90% of the static torque load recorded during the hold phase when the subjects held the object still in air (see Fig. 2A). Measurements taken during the hold phase are refereed to as "static" measures and were obtained by averaging the relevant signals during a 1-s interval that commenced 4 s after the object was first contacted. We defined the point of initial contact with the object as the time when the grip force exceeded 0.1 N.
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ROTATIONAL SLIP COEFFICIENT.
The onset of the slips evoked in the rotational slip test was
determined off-line by visual inspection of the object elevation, grip
force, and torque load records as described by Kinoshita et al.
(1997). Thus the onset of a rapid decrease in torque load followed by a decrease in object elevation indicated the commencement of the rotational slip. The rotational slip force, i.e., the
minimum grip force required to prevent rotational slip, was defined as the grip force value at the onset of such rapid decreases in torque. The difference between the static grip force and the rotational slip
force represented the safety margin against rotational slips. We also
computed the rotational slip coefficient, i.e., the ratio between the
grip force and torque load at the onset of rotational slip. With normal
finger sensibility this coefficient was 0.22 ± 0.03 (SD)
mm
1 (for all subjects and
trials) for the flat surface and 0.31 ± 0.07 and 0.63 ± 0.14 mm
1 for the 100 and
200 m
1 surfaces,
respectively. These coefficients are similar to those reported in a
previous study for the same curvatures (Goodwin et al.
1998
). Anesthesia of the index finger did not appreciably influence the rotational slip coefficients. They were 0.24 ± 0.04, 0.33 ± 0.08, and 0.65 ± 0.16 mm
1 for the three
surfaces when the index finger was anesthetized.
TWIST OF GRASP, ROTATIONAL YIELD, ROTATIONAL COMPLIANCE, AND ROTATIONAL CREEP. We estimated the twist of the grasp around the grip axis as the change in fingertip elevation from the start of the torque-loading phase until the end of the hold phase. (In illustrations including the twist of the grasp, it has been set to 0 at the start of the torque-loading phase.) We also assessed the amplitude of the twist during the first 200 and 400 ms after the start of the torque-loading phase measured as the change in fingertip elevation over these intervals. Rotational yield of the grasp was computed as the time varying difference between twist of the grasp and object elevation angle that occurred after the start of the torque-loading phase. Rotational compliance of the grasp was assessed as the slope of the relationship between the rotational yield and the torque during the torque-loading phase established by linear regression based on the stream of sampled data points obtained in single trails. During this phase, there was an approximately linear relationship between the rotational yield and the torque load. For instance, the 192 correlation coefficients obtained from all trials in the second type of lift series (8 subjects × 3 curvatures × 8 trials) ranged from 0.85 to 1.0 (median = 1.0; data from all three curvatures included). Rotational creep during the hold phase was defined as the change in the rotational yield over a 3-s window that ended at the sound signal that instructed the subject to terminate the lift.
STATISTICAL ANALYSIS.
Repeated-measures ANOVAs were used to evaluate the possible influence
of surface curvature (0, 100, 200 m1), lift series (2 levels; e.g., normal and impaired digital sensibility or sighted and
blindfolded) and surface curvature in the previous trial (0, 100, 200 m
1). Details are
provided in RESULTS. One-way interactions were analyzed
and, unless reported in RESULTS, they were insignificant. The level of probability selected as statistically significant was
P < 0.05. Unless otherwise stated, the following
procedure was used when pooling data across subjects: for each subject, the trials in which the experimental parameters were identical were
combined providing a subject mean for each measurement. These average
values were used in the ANOVA analyses, and in many figures subject
means and SE (n = 8) are presented.
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RESULTS |
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We first describe principal features of our prototypical lifting task that were common for trials in all experimental conditions. Second, we explore effects of the curvature of the grasped surface on the development of the grip force, on the torque load, and on the twist of the grasp that subjects performed to counteract rotational yield of the grasp. We based these analyses on lifts performed under optimum sensory conditions, i.e., with vision and normal digital sensibility. Finally, we study the importance of somatosensory and visual information for the adaptation of fingertip actions by analyzing the performance of the subjects during restriction of vision and of digital sensibility.
General structure of the trials
Figure 2A shows an example trial that illustrates the
general features of subjects' behavior when asked to lift and hold the test object in air while keeping it level. The subject had vision available and had normal digital sensibility, and the surface curvature
was 200 m1. At the start
of the trial, there was a brief increase in grip force as the grip was
established. After this initial "preload phase" (Johansson
and Westling 1984
), the vertical load force and grip force
increased together. When the vertical load force became large enough,
the proximal end of the object lifted off while the distal end of the
object remained on the table. Thus the moment of proximal liftoff was
readily observed in single trial records by a simultaneous onset of an
increase in vertical position and a decrease in object elevation
("proximal liftoff" in Fig. 2A). Following the proximal
liftoff, the torque load increased while the elevation angle of the
object decreased (object pointing downward). During the torque-loading
phase, the grip force and the vertical load force continued to increase
together with the torque, and eventually the distal part of the object
lifted off ("total lift-off" in Fig. 2A). Thus the time
of total lift-off coincided with the moment the torque load first
exceeded the torque load recorded during the hold phase; the object
tilt at lift-off was too small to account for any appreciable
discrepancy in this respect.
Before the start of the torque-loading phase, subjects typically applied a small, negative, torque load (less than 2 mNm) that developed soon after the finger contacted the object (Fig. 2A). This torque resulted from a twist of the grasp in the negative angular direction (for sign of twist see Fig. 1) due to ulnar-flexion of the wrist when subjects reached for the object partly from above. This twist ceased before the torque-loading phase, in which the subjects instead applied a twist around the grip axis in the opposite (positive) direction by an arm movement that included a radial flexion of the wrist (Fig. 2A). This positive twist during the torque-loading phase contributed to the development of torque and to maintaining the object close to level despite the rotational yield of the grasp. However, the object elevation angle still declined during the torque-loading phase because the applied twist did not fully compensate for the rotational yield although subjects were instructed to keep the object level during the lifting. The rotational yield, represented in Fig. 2A as the difference between the finger elevation and the object elevation angles, was approximately proportional to the torque load during the torque-loading phase (Fig. 2B).
The object elevation angle continued to fall for some 40 ms before the object rotation reversed although the torque load at total liftoff closely matched the torque required to hold the object level in the air; this was due to the inertia of the object and the rotational compliance of the grasp. This reversal implied a rotational acceleration of the object and was thus associated with an "overshoot" in torque load above that required to counterbalance the static torque (see the hatched zone of the torque signal in Fig. 2A). Driven by a continuing twist of the grasp, within some 0.5 s after total lift-off, the object was raised close to level.
COORDINATION OF GRIP FORCE AND FINGERTIP LOAD.
During the precision lifting task the grip force increased smoothly
with an approximately unimodal rate profile from the moment the subject
touched the grip surface until the object was held in air (see grip
force rate in Fig. 2A). However, the torque-loading phase
typically did not start until the grip force reached its peak rate and
grip force was quite high. Because of the sequential development of
grip force and torque load, these variables showed a markedly curved
relationship rendering high grip force-to-load ratios at low torques
and the lowest ratios when the object was in the air (Fig.
2C). Importantly, the high grip forces early during the
trial could not be explained by stability requirements imposed by the
linear load force; throughout the trial, the grip force was at least an
order of magnitude larger than the load force. The grip force
development instead anticipated requirements induced by the upcoming
torque load. Although the vertical load force contributed only to a
minor degree to the total fingertip load, it played a crucial role for
the kinematic structure of the task. The smooth vertical lifting
movement of the object indicated that the development of the vertical
load force was adequately programmed for the mass of the object
(Johansson and Westling 1988a) as well as for the
desired height and speed of the lift (Kinoshita et al.
1993
). Furthermore the increase in load force determined the
onset of the torque load increase (proximal lift-off) and together with
the twist and rotational compliance of the grasp, it determined the
further development of the torque load. When the subjects replaced the
test object on the tabletop, the motor output largely mirrored that
observed during the lifting phase illustrated in Fig. 2A.
Adaptation of fingertip actions to changes in surface curvature during normal sensory conditions
In this section, we describe the adaptation of the fingertip
actions to changes in the curvature of the grasped surface based on
lifts performed with vision and normal digital sensibility. Unless
stated otherwise, the analyses are based on the first lift series in
which we changed the surface curvature (0, 100, or 200 m1) in an unpredictable
order between lifts. Because object mass and mass distribution were
constant, changes in surface curvature influenced neither the final
torque load nor the vertical load force.
Coordination between grip force and torque load
Surface curvature strongly influenced the grip force throughout the trials. Figure 3, A and B, shows examples of single trials with the three surfaces performed by two subjects (aligned in time on touch), and Fig. 7A shows data averaged across all subjects (trials synchronized at the start of the torque-loading phase). Right from the beginning of the trial, the grip force was higher with a more curved grip surface. The peak rate of grip force increase was scaled to the curvature (P < 0.0006; Figs. 3, A and B, and 7A), and so were the grip forces at both the start of the torque-loading phase (P < 0.0007; Fig. 4A) and at the end of the hold phase (P < 0.0001; Fig. 5). The surface curvature also influenced the rate of the torque load increase (Fig. 3, A and B), i.e., the curvature influenced the duration of the torque-loading phase (Fig. 4B; P < 0.0001). With the flat contact surface, the rate of torque increase was very fast with a typical duration of the torque-loading phase of less than 0.1 s. The duration increased with increasing surface curvature. Due to the effects of surface curvature on the grip force and the rate of torque load increase, changes in surface curvature efficiently changed the balance between the grip force and the torque load during the trial; the grip force subjects used at any given torque load increased with the surface curvature (Fig. 3, C and D). The surface curvature, however, only modestly influenced the temporal coordination of force and torque development before the torque-loading phase. Curvature did not influence the time between the peak grip force rate and the start of the torque-loading phase, but it did influence the time between object contact and the start of the torque-loading phase (P < 0.003), which increased with a more curved surface (Fig. 4C).
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SAFETY MARGINS AGAINST FRICTIONAL SLIPS. All subjects maintained an adequate safety margin against accidental rotational slips in the sense that there were no discernible slips during any phase of any trial even though we changed the surface curvature between trials in an unpredictable order. We estimated the subject's grip force safety margin against rotational slips defined as the difference between grip force measured in the hold phase and the corresponding minimum grip force required to prevent rotational slip ("rotational slip force"). For this estimation, we used data from the lift series with slip force measurements. The behavior of the subjects in this series was essentially indistinguishable from that observed during the lift series with unpredictable changes in surface curvature. Indeed, a repeated-measures ANOVA with surface curvature and lift series (3 × 2) as factors failed to indicate an effect by lift series on the static grip forces.
Both the grip force and the rotational slip force during the hold phase increased with increasing surface curvature (Fig. 5). Thus subjects adjusted their grip force to guard against rotational slippage. The grip force safety margin (Fig. 5, shaded area) varied with curvature (P < 0.0001), i.e., it increased with increasing curvature. However, if expressed as a fraction of the grip force, the relative safety margin tended to be rather constant across curvatures, being on the average 43%. The observed safety margins were comparable to those previously reported in other manipulatory tasks involving torque loads (Goodwin et al. 1998TWIST OF GRASP. The surface curvature also influenced the twist of the grasp. A stronger twist was applied with a more curved surface (Figs. 3, A and B, and 7A). This curvature effect on the twist was significant during the initial 200-ms epoch following the start of the torque-loading phase (P < 0.0001; Fig. 4D), and it increased during the progress of the trial (Fig. 4E), being most pronounced at the end of the trial (Fig. 4F). Although subjects actively generated a stronger twist, the dip in object elevation at the end of the torque-loading phase was deeper with a more curved surface (Figs. 3, A and B, and 7A; P < 0.00001). With the flat surface, it was 1.3 ± 0.6° and with the most curved one it was 4.8 ± 1.5° (mean ± SD for individual trails by all subjects). On average, the elevation angle differed between the flat and the most curved surface by 3.5 ± 0.7° (n = 8). Interestingly, later during the trial the curvature appeared to influence the object elevation less; for the hold phase the corresponding difference was 2.1 ± 1.8° (see Fig. 8H).
ROTATIONAL COMPLIANCE AND CREEP.
That the dip in object elevation angle was deeper with a more curved
surface despite an increased twist indicates that surface curvature
markedly influenced the rotational yield of the grasp. Indeed surface
curvature reliably influenced the rotational compliance measured as the
slope of the relationship between the rotational yield and the torque
load during the torque-loading phase (Fig. 6, A and C;
P < 0.0001). Furthermore there was a slow rotational yield when the subjects held the object in air under constant torque
load. Regardless of curvature, this "rotational creep" occurred at
an approximately constant angular velocity (Fig. 6B). The
rotational creep was influenced by surface curvature (P < 0.0001), and it increased with curvature in a manner similar to the
rotational compliance (Fig. 6D). Although there was a
statistically significant positive correlation between the rotational
compliance and the rotational creep, the correlation was not impressive
(r values ranged between 0.32 and 0.80 for individual
subjects). This suggests that the underlying mechanical factors
differed. The rotational creep that took place during the hold phase
accounted for 35 ± 20, 32 ± 10, and 33 ± 10%
(mean ± SD for all subjects and trials) of the overall rotational
yield with the 0, 100, and 200 m1 surface curvatures,
respectively. Because the effect of curvature on object elevation
declined during the course of the trial, we conclude that the twist of
the grasp that occurred after the torque-loading phase was scaled by
the effect of surface curvature on the rotational compliance as well as
the rotational creep.
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Sensory factors in adaptation of fingertip actions to changes in surface curvature
We have demonstrated that the curvature of grasped surfaces scales the coordination between grip force and torque load for grasp stability as well as the twist drive that helped to keep the object reasonably level. Thus subjects used sensory information pertaining to surface curvature to control the fingertip actions. In this section, we investigate the contribution of vision and digital sensibility by analyzing the behavior of the subjects when vision, digital sensibility, or both were removed. Importantly, the basic coordination of the grip force, vertical load force, torque load, and twist of the grasp in all experimental conditions was similar to that observed with vision and normal digital sensibility. Furthermore with the exception of the condition in which we impeded both vision and digital sensibility, subjects maintained a robust safety margin in the sense that there were no noticeable accidental slips during any phase in any of the trials. Importantly, we observed no effect of digital anesthesia on rotational compliance (P = 0.15), rotational creep (P = 0.27) or the rotational slip coefficient (P = 0.24; ANOVA with curvature and digital sensibility as factors).
Without both vision and digital sensibility
In the absence of visual and digital sensory information, the
adaptation of fingertip actionsincluding grip force and grasp twist
to surface curvature was disrupted. The curvature no longer influenced the grip force development during the dynamic phase of the
trial (Fig. 7B). Neither the
peak rate of grip force nor the grip force at the onset of the
torque-loading phase was influenced by curvature (Fig.
8A). Subjects applied a grip
force that was strong enough for lifting the most curved surface (200 m
1) regardless of the
actual surface curvature. It seemed likely that these high grip forces
were the result of the subjects' experiences from rotational slips
that occurred with the most curved surface during the torque-loading
phase. During such slips, which were neither felt nor seen by the
subjects, the distal end of the object remained on the table while it
rotated around the grip axis during lifting attempts. When made aware
of the slips by the experimenter, subjects increased the grip force
during subsequent lifting attempts until they successfully lifted the
object. The subjects then continued to use the upgraded grip force
regardless of surface curvature. The magnitude of this aftereffect,
however, tended to decay over time; this, in part, may explain a
tendency to lower grip forces during the hold phase in trials with 0 and 100 than with 200 m
1
(Fig. 8B). This decay could eventually lead to a new episode of rotational slips with the most curved surface and a new voluntary upgrading of the grip force.
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Concerning temporal aspects of fingertip actions, the duration of the
torque-loading phase was the same as when the subjects had available
vision and digital sensibility (Fig. 8C), whereas the period
between touch and the start of the torque-loading phase was prolonged
(Fig. 8D; P < 0.01; ANOVA with sensory
condition and curvature as factors). Similar delays regarding load
application have previously been explained by an impaired verification
by tactile input that a stable contact with the object had been
established (Johansson and Westling 1984; see also
Collins et al. 1999
). This delay also contributed to the
overall high grip forces at the start of the torque-loading phase
during blindfolding and digital anesthesia (Fig. 8A). In
contrast to when subjects were sighted and had normal digital
sensibility, surface curvature did not reliably influence the period
from touch until start of the torque-loading phase (Fig.
8D).
The adaptation of the twist of the grasp to surface curvature was impaired in the absence of both vision and digital sensibility. The curvature did not influence the grasp twist measured during the two time bins after the start of the torque-loading phase, i.e., 0 to 200 ms and 0 to 400 ms (Figs. 7B and 8, D and E). However, the curvature did influence the overall twist from the start of the torque loading phase until the end of the hold phase (P < 0.003) but much less so than during the sighted condition with normal sensibility (Fig. 8G). We believe that the mechanism behind this influence was the same as that accounting for the effect of curvature on the grip forces during the hold phase as commented on in the preceding text (see Fig. 8B). Due to the impaired twist regulation, the curvature perturbed the object elevation angle during the lifting and during the hold phase (Fig. 8H) considerably more than in trials with vision and normal sensibility (Figs. 7, A and B, and Fig. 8H).
In sum, during blindfolding and digital anesthesia, subjects generated coordinated motor output, but they failed to predict the consequences of the prevailing curvature regarding both rotational friction and rotational yield. The subjects showed large errors in object elevation angle because of a poor compensation for the rotational yield of the grasp. In addition, subjects used excessive forces with flatter grip surfaces and rotational slips were common with the most curved surface. Thus we could safely conclude that the adaptation of the fingertip actions to changes in the surface curvature depended on the sensory modalities that we experimentally manipulated.
WITH VISION BUT WITHOUT DIGITAL SENSIBILITY. When the subjects saw the object, including the grasped surface, the curvature scaled the grip forces and grasp twist even when the digits were anesthetized (Fig. 9A). The curvature influenced grip force at all points of measurement, including the grip forces at the start of the torque-loading phase and during the hold phase as shown in Fig. 8, A and B (P < 0.001 in all instances, ANOVAs with curvature as factor). Furthermore there were no differences between the grip forces with and without digital anesthesia provided vision was available (Fig. 8, A and B; compare Figs. 9A and 7A; ANOVAs with digital sensibility and curvature as factors). Likewise, digital anesthesia influenced neither the duration of the torque-loading phase (Fig. 8C) nor the period between peak grip force rate and the start of the torque-loading phase. However, the period from touch to start of the torque-loading phase was slightly prolonged with digital anesthesia (Fig. 8D; P < 0.04).
|
PERFORMANCE WITH LIMITED VISION OF THE OBJECT DURING DIGITAL
ANESTHESIA.
Subjects thus efficiently use visual information to adapt the motor
output to surface curvature. However, we also wanted to know if vision
of the grasped surface whose curvature we changed was decisive for the
performance. We tested four of our anesthetized subjects when we
prevented the view of this surface but not of the rest of the test
object (see METHODS). The surface curvature still
influenced grip force and the twist of the grasp but not until
after the torque-loading phase. As can be seen in Fig.
9B, some 0.2 s after the proximal liftoff subjects used
a higher grip force with a more curved surface, although the grip
forces for the two smallest curvatures (0 and 100 m1) were only modestly
different. Furthermore after some additional 50 ms, the surface
curvature also influenced grasp twist. Thus subjects could use visual
cues other than vision of surface curvature for adjusting fingertip
actions. One obvious cue for the adaptation of the fingertip actions
was visual information related to the dip in object elevation that
occurred after the proximal liftoff. That subjects used this cue would
explain why the adjustments took place later than those observed with
vision and normal sensibility. Interestingly, the late influence on the
twist resembled that observed during digital anesthesia when the
subjects had full view of object, including the surface whose curvature
we changed (cf. Fig. 9, A and B). Thus in both
conditions, the subjects may have used visual cues related to object
movement after its proximal liftoff to adapt the twist.
WITHOUT VISION BUT WITH DIGITAL SENSIBILITY. When the subjects were blindfolded but had normal digital sensibility, they also adapted their fingertip actions to the prevailing surface curvature (Fig. 9C). In this condition, they used sensory information from the digits obtained after the grip surface was touched. This resulted in a delayed grip force adaptation compared with trials with vision (cf. Figs. 7A and 9C). Accordingly, in the blindfolded state, the prevailing curvature did not influence the development of the grip force at the start of the torque-loading phase (Fig. 8A). However, the curvature reliably influenced the grip force at the midpoint of the torque-loading phase and later during the trial (Figs. 9B and 8B; P < 0.0001 for both the midpoint of the torque loading phase and hold phase, ANOVAs with curvature as the factor). Blindfolding did not influence the period from touch to start of the torque-loading phase (Fig. 8, C and D; ANOVA with curvature and vision as factors). The duration of the period from the peak rate of the grip force to the start of the torque-loading phase was also unaffected by blindfolding (Fig. 9C). As with normal digital sensibility and vision, the surface curvature influenced the twist of the grasp right from the start of the torque-loading phase (Fig. 8E; P < 0.0003). The size of this influence increased during the trial (Fig. 8, F and G); the effect of the surface curvature on object elevation during the hold phase was indistinguishable from that in trials with vision (Fig. 8H).
We conclude that signals from digital mechanoreceptors provide information that can be used to adapt fingertip actions to surface curvature, including adaptation of the twist to the influences of surface curvature on rotational compliance and rotational creep of the grasp.Influences of curvature in the previous trial
Memories pertaining to physical properties of an object, obtained
during a previous interaction with it, play a central role for
predictive control during manipulative maneuvers. This applies to
object mass (Johansson and Westling 1988a), mass
distribution (Goodwin et al. 1998
; Johansson et
al. 1999
; Kinoshita et al. 1997
; Wing and
Lederman 1998
), viscous and elastic properties of objects
(Flanagan and Wing 1997
), friction in relation to the skin (Johansson and Westling 1984
), and object shape
(Jenmalm and Johansson 1997
). To assess whether such
memory systems also support predictive control for surface curvature,
we analyzed whether the curvature in the preceding trial influenced
grip force. For trials with occluded vision but with normal digital
sensibility, the previous curvature determined the grip force until
tactile information about the current curvature could be expressed in the motor output. That is, the curvature in the preceding trial had a
reliable effect on the grip force at the start of the torque loading
phase (P < 0.02; ANOVA with curvature and curvature in preceding trial as factors), whereas there was no effect of the current
curvature at this point. Later in the trial the influence of the
previous curvature diminished under the effect of the current surface
curvature. In contrast, in trials with vision, the curvature in the
previous trial did not reliably influence the grip force at the start
of the torque-loading phase. Finally, during blindfolding and digital
anesthesia, the motor output was strongly influenced by previous
experience of the object in the sense that knowledge about rotational
slips with the most curved surface scaled the fingertip actions as
described in the preceding text. Repeated-measures ANOVAs were used to
evaluate the possible influence of surface curvature in the previous
trial (0, 100, 200 m
1)
and current surface curvature (0, 100, 200 m
1) for each sensory condition.
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DISCUSSION |
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We analyzed the sensory control of precision grip when subjects contact and lift, in a continuous action, an object whose curvature changed unpredictably between trials. We found that the curvature of the grasped surface efficiently scaled the grip force for grasp stability and that humans can use both vision and digital sensibility for this scaling. We also found that the fingertip actions comprised a kinematic component by which subjects automatically counteracted the effects of rotational yield of the grasp by a twist of the grasp around the grip axis. Furthermore subjects scaled the size of this twist to the influence of the surface curvature on the rotational yield, again based on visual and digital afferent information. However, the adaptation of the grip force and the grip kinematics (twist) relied on differential use of sensory information. A normal early scaling of the twist action to changes in surface curvature required digital afferent input and different visuomotor mechanisms supported the control of the grip force and the grasp twist. The grip force control appeared to have special access to visual mechanisms that identify object shape whereas visual cues pertaining to actual movements of the object supported the control of the kinematics.
Rotational yield and twist of grasp
Our results demonstrate that rotational yield of the
fingertip pulps is an important biomechanical variable that is
controlled for during dexterous manipulation. That is, to obtain a
desired orientation of an object under torque load, subjects' actions of the digits included components that prevented rotational slips but
also actions that compensated for rotational compliance and rotational
creep at the fingertip pulps. Torsional elasticity of the fingertip
pulp accounted for the rotational compliance measured during the
torque-loading phase. Likewise, the decrease in effective contact area
engaged with a larger curvature would explain the effect of surface
curvature on rotational compliance; if the fingertip pulp is
approximated by a short elastic cylinder its torsional compliance will
decrease with the contact area (Cutkosky and Wright
1986). The viscoelastic properties of the fingertip pulp most
probably accounted for the substantial rotational creep that we
observed under constant torque load. It is well established that the
fingertip pulp show rate-dependent hysteresis and creep when subjected
to compression (e.g., Howe and Cutkosky 1996
;
Pubols 1982
; Serina et al. 1998
).
Moreover, the fingertips show viscoelastic properties when loaded by
forces in the tangential direction (Nakazawa et al.
2000
), which is a loading condition that mechanically relates to that of the torque load in the present experiment. Movements of
interstitial fluids within the fingertip as well as reorientation or
relaxation processes of collagen fibers and their attachments are
potential mechanisms of the creep that we observed (see Lanir 1987
; Purslow et al. 1998
; Wilkes et al.
1973
). Still we cannot exclude that also frictional creep
between the grasped surface and the digit could have contributed.
Because the normal pressure is smallest at the edges of the contact,
surface microslips presumably preferentially occur there (see
Johansson and Westling 1987
). Once slipping has
occurred, the region of slipping spreads from the periphery toward the
center such that creep would develop (see Cutkosky and Wright
1986
). A full understanding of the biomechanical events that
accounted for the rotational yield of the grasp in the present task
would, however, require further development of analytical models of
fingertip mechanics (cf. Cutkosky and Wright 1986
;
Howe and Cutkosky 1996
; Nakazawa et al.
2000
; Serina et al. 1998
; Srinivasan and
Dandekar 1996
).
Coordination of fingertip actions
In many manipulatory tasks, the activated sensorimotor programs
ensure grasp stability by maintaining an approximately linear relationship between the grip forces and self-generated linear load
forces (Blakemore et al. 1998; Flanagan and Wing
1993
; Flanagan et al. 1999
; Johansson and
Westling 1984
, 1988a
). Similarly, when subjects hold an object
in air and then rotate it such that torque loads develops with little
changes in linear force, an approximately linear coupling between the
grip force and tangential torque ensures grasp stability
(Goodwin et al. 1998
; Johansson et al.
1999
). However, in the present task, in which the subjects
contacted and lifted an object in a continuous action and the
dominating load was tangential torque, there was a markedly curvilinear
relation between the grip force and the load. The increase in grip
force greatly preceded the onset of the rather rapid increase in torque load in a manner resembling the preparatory grip force increase that
appear about 150 ms prior to a self-generated transient load force
increase. Such preparatory grip actions, employed to prevent slippage,
occur when subjects drop a weight from one hand into a receptacle held
by the other hand (Johansson and Westling 1988b
) and
when subjects move hand-held objects to collide with another object
(Turrell et al. 1999
). Subjects scale these preparatory grip forces to the frictional conditions of the grasp and to the peak
in load force at impact, which is influenced by the overall dynamics of
the task (Johansson and Westling 1988b
). Similarly in
the present task, subjects targeted the increase in the grip force for
the requirements imposed by the final torque load under the prevailing
surface curvature. Thus the grip force controller predicted the
destabilizing effects of the torque load well before it developed.
Deviations from a linear relationship between grip force and fingertip
load may also occur in self-paced tasks not involving transient load
increases. In particular, the grip force does not follow the
fluctuations in torque that subjects generate to produce smooth angular
movements during rapid object rotations (Goodwin et al.
1998
; see also Johansson et al. 1999
). In such tasks, the changes in grip force tend to be more smoothly coordinated to kinematics aspects of the task than to the changes in torque. Deviations from a stable linear relationship between grip force and
load also occur during self-generated linear load forces. Compared with
holding an object motionless, subjects generally employ higher
grip-to-load force ratios in tasks involving inertial forces. This
occurs when objects are moved along the grip axis (Werremeyer
and Cole 1997
) and during oscillatory movements tangential to
grasped surfaces, the depth of the modulation of the grip force with
load changes is reduced with increasing frequencies (see Blakemore et al. 1998
; Flanagan and Tresilian
1994
; Flanagan and Wing 1995
). Thus rather than
offering a precise moment-to-moment prediction of fingertip load, the
control of the grip force is influenced by a variety of intrinsic task
factors, including type of task, its phase and speed of execution. This
may have implications for using grip force to probe the capacity of the
CNS to predict various classes of loads and thus to probe both the
nature of various internal models proposed in motor control and their
functional relationships (cf. Kawato 1999
).
Although we instructed the subjects to keep the object level, there was a small, hardly visible, dip in object elevation during the lifting. To avoid this dip, the subjects should have applied a twist before object motion that produced a torque that exactly matched the torque required for a level liftoff. This would have required exact estimates of object mass and mass distribution and of the rotational yield with the prevailing surface curvature. These and related precision demands probably exceed the capacity of the sensorimotor system, or it may not be worth the CNS's effort to make the required predictions for a marginal improvement of the outcome.
Use of sensory information in the adaptation of fingertip actions to changes in surface curvature
A central role of sensory information in the control of dexterous
manipulation is to allow swift and efficient adaptation of the motor
output to critical physical properties of the target objects. The
present results demonstrate that sensory information related to
objects' curvature not only controlled the grip force but also the
action by which subjects counteracted the effects of rotational yield
of the grasp, i.e., the twist of the grasp around the grip axis.
Without vision and digital sensibility, subjects generated a
coordinated motor output apt to the task but that failed to predict the
consequences of the prevailing curvature regarding rotational friction
and yield of the grasp. In agreement with principles established in
previous studies concerning adaptation to other object properties
(Johansson 1996, 1998
), adaptation to surface curvature
took place by parametric adjustments of the motor commands, supported
by memory mechanisms for predictive control.
USE OF VISUAL CUES.
When subjects had full vision available, the motor output expressed the
prevailing surface curvature from the beginning of the grip force
attack even though the curvature changed unpredictably between trials.
Thus well before somatosensory information was available, subjects used
visual information to identify the target object in terms of grip force
requirements. This type of control has been described as anticipatory
parameter control based on internal memory representations pertaining
to critical properties of the environmental objects acquired during
previous manipulatory experience (Johansson 1998;
Johansson and Cole 1992
). The use of vision for
activation of such internal models for retrieval of relevant motor
command parameters has been demonstrated regarding other object
features, including aspects of object shape (Jenmalm and
Johansson 1997
), prediction of object weight based on object size (Gordon et al. 1991
; see also Flanagan and
Beltzner 2000
), and identification of common objects regarding
force requirements in lifting tasks (Gordon et al.
1993
). However, available data suggest that vision is of little
importance for anticipatory adjustments of the force output to
frictional conditions (Edin et al. 1992
); tactile
sensibility seems to be the principal modality in frictional adaptation
(Johansson and Westling 1987
).
USE OF DIGITAL AFFERENT INFORMATION.
With blindfolded subjects, in agreement with the anticipatory parameter
control policy, the grip force initially developed according to the
force requirements of the previous trial. Likewise after a change in
curvature, digital afferent information about the new curvature
influenced the grip force and the twist of the grasp about 0.1-0.2 s
after contact. This adaptation was associated with a change of the
internal models pertaining to the object properties because the grip
force in the subsequent trial initially developed according to the
requirement imposed by this "new" curvature. Thus the use of
somatosensory information in selecting or updating the relevant
internal models followed the discrete-event, sensor-driven control
policy previously described for manipulation concerning adaptation to
friction of the grasp, object shape, mass, and mass distribution (see
Johansson 1996, 1998
; Johansson and Cole
1992
; Johansson et al. 1999
). According to this
policy, recognition of a mismatch between the actual somatosensory
input and a predicted somatosensory input generated by the active
sensorimotor program (in conjunction with the efferent signals) is
critical to accomplish the relevant model changes. This scheme has
indeed elements in common with aspects of recent schemes that have
attempted to organize concepts of sensorimotor integration (e.g.,
Kawato 1999
; Merfeld et al. 1993
;
Miall and Wolpert 1996
; Prochazka 1993
;
Wolpert 1997
).
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ACKNOWLEDGMENTS |
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We thank Dr. G. Westling and A. Bäckström for technical support.
This study was supported by the Swedish Medical Research Council (project 08667) and the Göran Gustafsson Foundation for Research in Natural Sciences and Medicine.
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FOOTNOTES |
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Address for reprint requests: P. Jenmalm (E-mail: per.jenmalm{at}physiol.umu.se).
Received 5 June 2000; accepted in final form 14 August 2000.
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REFERENCES |
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