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Content uploaded by Yves Paulignan
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All content in this area was uploaded by Yves Paulignan
Content may be subject to copyright.
Exp Brain Res (1991) 87:407-420
E.m
Brain Research
9 Springer-Verlag 1991
Selective perturbation of visual input during prehension movements
2. The effects of changing object size
Y. Paulignan 1, M. Jeannerod 1, C. MacKenzie 2, and R. Marteniuk 2.
1 Vision et Motricit6, INSERM U94, 16 Avenue du Doyen L6pine, F-69500 Bron, France
2 Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada
Received February 27, 1991 / Accepted May 22, 1991
Summary.
1. Subjects were instructed to reach and grasp
cylindrical objects, using a precision grip. The objects
were
two concentric dowels made of translucent material placed
at 35 cm from the subject. The inner ("small") dowel was
10 em high and 1.5 cm in diameter. The outer ("large")
dowel was 6 cm high and 6 cm in diameter. Prehension
movements were monitored using a Selspot system. The
displacement of a marker placed at the wrist level was used
as an index for the transport of the hand at the location of
the object. Markers placed at the tips of the thumb and the
index finger were used for measuring the size of aperture of
the finger grip.
2. Kinematics of transport and grasp components were
computed from the filtered displacement signals. Move-
ment time (MT), time to peak velocity (TPV) and time to
peak deceleration (TPD) of the wrist, time to peak velocity
of grip aperture (TGV), time to maximum grip aperture
(TGA) were the main parameters used for comparing the
movements in different conditions. Spatial paths of the
wrist, thumb and index markers were reconstructed in two
dimensions. Variability of the spatial paths over repeated
trials was computed as the surface of the ellipses defined by
X and Y standard deviations from the mean path.
3. Computer controlled illumination of one of the dowels
was the signal for reaching toward that dowel. Blocks of
trials were made to the small dowel and to the large dowel.
Mean MT during blocked trials was 550 ms. The acceler-
ation phase of the movements (measured by parameter
TPV) represented 33% of MT. About half of MT (52%)
was spent after TPD in a low velocity phase while the hand
was approaching the object. This kinematic pattern was
not influenced by whether movements were directed at
small or large dowels.
4. Grip aperture progressively increased during transport
of the hand. TGA corresponded to about 60% of MT, that
is, maximum grip aperture was reached during the low
velocity phase of transport. Following TGA, fingers closed
around the object until contact was made. This pattern of
*
Supported by INSERM, Paris
Offprint requests to:
M. Jeannerod
grip formation differed whether the movement was direc-
ted at the large or the small dowel: TGA occurred often
earlier for the small dowel, and the size of the maximum
grip aperture was larger for the large dowel. Variability of
both the wrist and finger spatial paths was larger during
the first half of MT, and tended to become very low as the
hand approached the dowels.
5. Selective perturbations of dowel size were randomly
produced at the onset of prehension movements. Per-
turbations involved increase in object size (the illumina-
tion was suddenly shifted from the small to the large
dowel, S-L perturbation), or decrease in object size (from
the large to the small dowel, L-S perturbations). During S-
L perturbations MT was increased by 175 ms on average.
As TPV and TPD did not differ from control unperturbed
movements, increase in MT was entirely due to lengthe-
ning of the low velocity phase following TPD.
6. Grip formation was affected by the perturbation. Grip
aperture first peaked to the size corresponding to the small
dowel, then reinereased for accomodating the size of the
large dowel. The time where grip aperture reincreased (as
measured on the curve of grip velocity) occurred 329.8 ms
on average after movement onset. Variability of wrist and
finger spatial paths was increased with respect to controls,
but it remained low during the final phase of the move-
ment. L-S perturbations had similar effects, though at-
tenuated with respect to S-L perturbations.
7. This relatively long time taken to initiate corrections in
response to object size perturbations contrasts with the
short time (about 100 ms) for initiating corrections during
perturbations of object position. This difference suggests
some degree of independence of the mechanisms gener-
ating finger movements during grip formation from those
generating transport of the hand. In addition, the kine-
matic coupling of the two components (demonstrated here
by lengthening of the low velocity phase of the transport
during correction of finger grip size) suggests the existence
of a different mechanism subserving temporal coordi-
nation of the two components.
Key words: Motor control grasping - Human
408
Introduction
The posture that is assumed by the hand before contact
with an object represents the end result of a motor
sequence which starts well ahead of the action of grasping
itself. The fingers begin to shape during transport of the
hand to the object location and contribute to a final spatial
configuration termed opposition space by Arbib (1985; see
also Iberall et al. 1986), where the relative positions of the
hand and the object to be grasped are specified by object
properties and task requirements.
Preshaping of the hand is a highly stable motor
pattern. It first involves a progressive opening of the grip
with straightening of the fingers, followed by a closure of
the grip until its matches object size. The point in time
where grip size is the largest is a clearly identifiable
landmark which occurs during the deceleration phase of
the transport of the hand, that is, well before the fingers
come in contact with the object. The size of the maximum
grip aperture correlates with the size of the object (Jean-
nerod 1981, 1984; see also Wing et al. 1986, Wallace and
Weeks 1988, Marteniuk et al. 1990; Gentilucci et al. 1991).
These movements of the fingers during grasping are
largely predetermined by object-related visual input. In
normal subjects, they are correctly achieved in situations
where the hand remains invisible to the subject (Jeannerod
1984).
The existence of independent finger movements during
transport of the hand at the spatial location of the object,
however, implies that these two components of the action
of prehension have to be linked or coordinated with each
other, so that the time course of the finger movements
matches that of the transport. This is critical for antici-
patory closure of the finger grip to occur in tight syn-
chrony with approach of the object by the fingertips. Early
or late closure will both result in inaccurate grasps, with
the consequence of bumping and eventually breaking
fragile objects. The current model for the coordination
between the components of prehension specifies that the
mechanisms for achieving a correctly oriented and sized
grip operate in parallel with other mechanisms for hand
transport to object position in space. To help elucidate the
nature of the motor coordination implied by this model,
we designed a series of experiments where the visual input
related to either grip formation (e.g., object size), or
transport of the hand (object location) can be selectively
perturbed during the movement. This paradigm of pertur-
bing visual input in relation to movements has been widely
used in the recent years for probing the mechanisms
related to motor representation and preparation (Geo-
rgopoulos et al. 1981; Soechting and Lacquaniti 1983;
Goodale et al. 1986; Pelisson et al. 1986).
Our initial prediction was that, if the two components
of prehension operate independently, the reaction of the
prehension system to these perturbations should be lim-
ited to the perturbed component, leaving the other com-
ponent unaffected. In the first paper of this series, we
reported the effects of changing object position. Indeed,
this perturbation produced corrections of hand trajectory
in space, such that movements were adequately redirected
and rescaled. Movement time was increased by 100 ms on
average, and the first signs of correction (change in acceler-
ation of the ann) were observed within about 100 ms from
onset of the perturbation. However, in apparent contra-
diction to the model, the pattern of grip formation was also
found to be consistently affected by the change in object
position (Paulignan et al. 1990, 1991).
In the present paper, we present the complementary
experiment where we studied the corrective reponses to a
sudden visual change in object size, without alteration of
object position.
Methods
Recording technique
As in the previous experiment (Paulignan et al. 1991), prehension
movements were recorded bidimensionally by a Selspot system
camera equipped with a 50 mm lens, placed 3 m above the horizontal
working plane. The camera monitored the displacements of 5 active
markers (infrared emitting diodes, IREDS) placed on the right arm in
the following positions: 1, on the lateral lower corner of the index
finger nail; 2, on the medial lower corner of the thumb nail; 3, on the
dorsal aspect of the hand immediately proximal to the meta-
carpophalangeal joint of the index finger; 4, on the skin immediately
proximal to the styloid process of the radius at the wrist; and 5, on the
skin above the cubital head at the elbow. The wrist IRED was used to
measure the transport component of the prehension movements. The
elbow, wrist and metacarpophalangeal IREDS were used for com-
puting the wrist angle (not reported in this paper). Finally, the
fingertip IREDS were used to measure the displacements of the index
and thumb, respectively, and the size of the grip aperture (index-
thumb distance). Dynamic accuracy tests indicated that spatial
precision of the measures was 2 mm. Positions of the IREDS were
sampled at 200 Hz and stored on a PDP 11/73 computer.
Subjects
Subjects were five right handed adults (3 females, 2 males) ranging in
age between 25 and 35 years. Three of the subjects were naive with
regard to the purpose of the experiment. The other two had also
participated in the experiment reported in the previous paper.
Apparatus and procedure
During the experimental sessions, subjects were seated comfortably,
without restraint, facing the working surface in a dimly lit room. The
subject placed the right hand at the starting position in front of the
body midline. The hand rested on its ulnar edge, with the tip of the
index and the tip of the thumb in contact with each other, and the
lower part of the thumb pressed against a start switch used for
triggering perturbations (see below). The targets were two concentric
dowels made of translucent material, placed at 20 ~ to the right of
subject's midline, at 35 cm from the hand. The inner dowel was 10 cm
high and 1.5 cm in diameter; it will be referred to as the "small"
dowel. The outer dowel was 6 cm high and 6 cm in diameter; it will be
referred to as the "large" dowel. Computer controlled light emitting
diodes (LEDs) embedded into the transparent material covering the
working surface were used to transilluminate the dowels. One LED
was placed below the small dowel, two LEDs were placed below the
large one (Fig. 1),
The illumination of a dowel was the signal for the subject to start
the movement. The subject was requested to reach, grasp and lift the
dowel accurately and rapidly, using the distal pads of the thumb and
index finger. A few trials were used for instructing the subject. In the
first part of the experiment, two counterbalanced blocks of trials
409
surfaces in each condition were cumulated (Georgopoulos et al.
1981).
6 7
2
4
8 2 cm
Fig. 1. Target objects used in the experiment. 1: Small dowel; 2:
Large dowel. The Small dowel can be lifted independently from the
Large one, The large dowell can be removed from the opaque
support (3); 4: transparent working surface; 5: Light emitting diodes
(LEDs). The middle LED (7) illuminates the small dowel, the other
two LEDs (6 and 8) illuminate the large dowel
were run by illuminating either the small or the large dowel 10
consecutive times (Small and Large
blocked
trials, respectively). In
the second part, a single block of 100 trials was run. Each single trial
during this run started with illumination of either the small or the
large dowel. In 80% of the trials, no perturbation occurred (Small
and Large
control
trials). In 20% of the trials, the light was
unexpectedly shifted from the dowel illuminated first to the other one
(i.e., 10 trials from Small to Large and 10 trials from Large to Small,
S-L and L-S
perturbed
trials, respectively). The shift was produced by
the release of the start switch when the hand left its resting position.
Since the room was dimly lighted, the appearance was that of an
instantaneous change in dowel size. Corrective movements per-
formed during perturbed trials involved a small upward (in the L-S
trials) or downward (in the S-L trials) component. This component,
the maximum amplitude of which was about 4 cm (see Fig. 1), was
neglected in the present experiment.
Data processing
Data were processed as reported previously (Paulignan et al. 1991). A
second-order Butterworth dual pass filter (cutofffrequency, 8 Hz, see
Franks et al. 1990) was used for data processing. X and Y trajectories
of each IRED were computed after filtering. Tangential velocities
(e.g., for the wrist IRED) were also computed after filtering. Acceler-
ation data were derived by differentiating the tangential velocity
data. Movement time was measured as the interval between the onset
of the thumb IRED movement, and the time when the fingers came in
contact with the object (as seen from computation of the grip size).
Spatiotemporal variability of the wrist, thumb, and index trajec-
tories was quantified after time normalization of the data. First, with
respect to the spatial path, the standard deviations of the mean X and
Y positions of each IRED were calculated for each of the 100
normalized time frames. In addition, to obtain a global estimate of
variability (index of variability) for each IRED in each condition, the
surface areas of the ellipses defined by the standard deviations in X
and Y dimensions were computed (in cm2), and the values of these
Results
Blocked trials
Movement time.
Since subjects were instructed to move
accurately and rapidly, large interindividual variations in
movement time (MT) occurred. Mean MT ranged from
390 ms in one subject to 650 ms in another one. The grand
mean was around 550 ms. No significant differences in MT
were found (using an unpaired t test) between movements
directed at small or at large dowels (Table 1).
Transport component
Kinematic analysis.
The wrist IRED was used for ana-
lysing kinematically the transport component of the move-
ments. As in the experiment reported in the previous
paper, wrist movements had a single peak, bell-shaped,
velocity profile (Fig. 2). The peak value of resultant
velocity (parameter APV) was 1203 mm/s for Small trials
and 1128 mm/s for Large ones. The acceleration phase of
the movement (between movement onset and the time of
the velocity peak, parameter TPV) represented 33% of
MT. The deceleration phase was marked by a sharp
deceleration, which peaked at about 290 ms following
movement onset (parameter TPD). Thus, about 52% of
movement time was spent during the low velocity phase
following peak deceleration.
Inspection of Table 1 reveals that none of the para-
meters used for describing wrist kinematics (TPV, APV,
TPD) was influenced by the size of the dowel. Because
TPV was similar in both Small and Large trials, it was
Table l. Kinematic characteristics of prehension during Small (S)
and Large (L) blocked trials (Intersubject mean and SD)
Transport MT TPV APV TPD
S 548.7 183.4 1203.3 283.6
(108) (32) (158) (40)
L 555.7 189.3 1128.6 295.5
(104) (39) (124) (32)
Grasp TGV TGA AGA
S 195.4 319.3 92.2
(58) (54) (11)
L 188.4 385.2 124.9
(43) (106) (7)
MT: Movement Time (ms), TPV: Time to Peak Velocity (mm/s),
APV: Amplitude of Peak Velocity (mm), TPD: Time to Peak
Deceleration (ms), TGV: Time to maximum Grip Velocity (ms),
TGA: Time to maximum Grip Aperture (ms), AGA: Amplitude of
maximum Grip Aperture (mm)
1400
S
E
g
.~ 700
o~
>
._
410
600
200 400
Time
(ms)
15000
7.
7500
o
0 ~
3
3
-7500
200 400
Time
(ms)
1400
700
L
15000
7500
0
-7500
600
Subject: GC
-- Velocity
-- Acceleration
Fig. 2. Kinematic profile of the wrist
displacement during blocked trials. Velocity
and acceleration curves are shown as a
function of time in two representative trials
directed at the Small dowel
(S, left)
and the
Large dowel (L,
right)
respectively
considered unnecessary to tabulate the values of time to
peak acceleration (parameter TPA) in this experiment.
Spatial path and variability of the wrist trajectory.
As
shown by the averaged trajectories from one subject in Fig.
3a and b, transport components during blocked trials had
a rectilinear spatial path. The variability of the spatial
paths over repetitions of the same movement is indicated
by vertical and horizontal bars placed on the trajectory at
the location corresponding to each of the 100 frames. The
horizontal bar represents variability in the X dimension,
and the vertical bar, in the Y dimension. The mean index of
variability, which cumulates the measures over the 100
frames, was 291 cm 2 for the Small trials, and 273 cm 2 for
the Large ones. Variability, however, was not evenly
distributed over the trajectory. It increased from the
starting position up to a point representing about half of
target distance, and then decreased until contact with the
object. Distribution of variability in movement time is
shown in Fig. 3c, d. The time at which variability was the
largest corresponded roughly to 35% of MT, that is, it was
close to the time of the velocity peak.
The temporal frames corresponding to maximum ve-
locity (TPV) and to maximum deceleration (TPD) have
been marked on the wrist paths of Fig. 3. This confirms
graphically the asymmetry of wrist movement kinematics,
where more than 50% of movement duration was spent in
the low velocity phase within the last one fourth of target
distance.
Grasp component
The grasp component was analysed by recording the
distance between the tip of the index finger and the tip of
the thumb. The index-thumb distance corresponds to grip
size, a measure which accounts for the combined move-
ments of the two fingers.
Kinematic analysis.
As in all previous studies of the grasp
component during prehension, grip size was found to
increase during transport of the hand, up to a maximum
aperture, before enclosing the object. As shown by Table 1,
the mean amplitude of this maximum grip aperture (para-
meter AGA) was related to dowel size: it was 92.2 mm for
the Small trials, and 124.9mm for the Large trials
(P < 0.0005) (see Fig. 4 for two representative trials).
The time at which maximum grip aperture occurs
(parameter TGA) is also an important parameter, because
it is indicative of how the grasp component is coordinated
in time with the transport component. TGA corresponded
to 58% and to 69% of movement time for Small and Large
trials, respectively (Table 1), that is, in both cases it
occurred after deceleration of the wrist, during the low
velocity phase near the object. The difference between
Small and Large trials was highly significant (P < 0.0005).
The rate of change of grip aperture (represented
graphically in Fig. 4 as the curve of the first derivative of
grip size over time) was marked by an early peak followed
by a deceleration. This deceleration was marked by an
inflexion which corresponded to the plateau in grip aper-
ture before grip closure. The time at which aperture
change was the fastest (parameter TGV) was similar in
Small and Large trials (Table 1). This is congruent with
both the smaller AGA and the earlier
occurrence
of TGA
in Small trials: grip size increased at the same rate in Small
and in Large trials, but stopped to increase earlier in the
Small trials. One subject (PG), however, adopted a differ-
ent strategy. In this subject, TGA had the same value in
Small and Large trials (330 ms), whereas TGV was shorter
(by 46 ms) in Large trials than in Small ones. Thus, for this
A
I ~~~iii D 290 ms
i // T 20 ms
C
mm
o ~ sol
"o i
>-
+
x
0 ,
0 25 50 75 100
Normalized lime (%)
B
T
D
W
_ TPV 202 ms Y
o
8cm
Subject: CU
mm
60 84
l,
0 0 25 50 75
100
Normalized time
(%)
411
Fig. 3A-D. Spatial path and variability of
prehension movements in the Blocked trials
condition. In A, B are represented the
normalized and averaged spatial paths of the
wrist (W), the tip of the thumb (T) and the tip of
the index finger (I), in 2 blocks of 10 trials
directed at the small dowel (A) and the large
dowel (B), respectively. Movements are shown in
2 dimensions (X, Y) as seen from above. The Y
coordinate was aligned with body axis. The
hand starting position is at the bottom part of
the drawings. Numbers on the wrist spatial
paths are the mean values of time to peak
velocity (TPV) and time to peak deceleration
(TPD). Horizontal and vertical bars on the
averaged spatial paths represent the amplitude
of one standard deviation in X and Y
dimensions, respectively. Each pair of bars
represents one of the 100 time frames of the
normalized movements. C, D Amplitude of X
and Y standard deviations (computed as the
square root of Sd X 2 + Sd y2) as a function of
time for wrist, thumb and index finger spatial
paths. Due to normalization, time is represented
in % of total movement time
S
140
E
g
.p
0
175 550 525 700
Time (ms)
- 800
400 G')
.~.
<
o
o
-0 ~-
3
3
-400 v
- -800
140
70
0 0
L
175 550
Time (ms)
800
400
-0
--400
----800
525 700
Subject CU
-- Grip aperture
-- Grip velocity
Fig. 4. Kinematic profile of grip formation
during Blocked trials. Grip aperture
(distance between tip of thumb and tip of
index finger) and grip velocity are shown as
a function of time in two representative
trials directed at the Small dowel
(S, left)
and the Large dowel
(L, right),
respectively.
Note larger aperture in the Large trial
412
400
-
S
E 300-
E
200 -
~5
X lOO
0H
, , ,
0 25 50 75 100
Normalized time (%)
400
300 9
200
100
L
25 50 75 100
Normalized time (%)
Subject: CU
Fig. 5. Mean displacement in the
X dimension of wrist (W), tip of
the thumb (T) and tip of the
index finger (I) as a function of
normalized time in Blocked trials.
S, trials directed at the Small
dowel, L, at the Large dowel.
Note relative invariance of thumb
position with respect to wrist
position
one subject, the rate of increase in grip size was faster in
Large trials, hence allowing for a larger opening within the
same amount of time.
Spatial path and variability of the finger trajectories.
Spatial
path of the tip of the thumb was rectilinear, and paralleled
that of the wrist. This means that, as suggested by previous
authors (e.g., Wing and Fraser 1983), the position of the
thumb remained invariant with respect to the wrist during
the movement. By contrast, the spatial path of the index
finger showed a marked curvature which tended to sur-
round the dowel. This difference in spatial path between
the thumb and the index finger is shown both in Figs 3a
and b and in Fig. 5. In the latter Figure, the path of the
thumb in the X dimension can be seen to closely follow
that of the wrist, which is not the case for that of the index.
The variability of the finger trajectories was larger than
for the wrist (Fig. 3c, d); this shows that variability of finger
movements did not merely reflect that of wrist
movements. Variability amounted to 484/432 cm z for the
thumb and 665/668 cm z for the index, in Small and Large
trials, respectively. The variability of the finger trajectories
was also larger at the time where velocity of the wrist was
maximum, and tended to become very low before the
fingers contacted the dowel. The low terminal variability
indicates that the fingers tended to contact the object at
the same points on each repetition of the movement,
irrespective of the size of the object.
Control trials
Control trials corresponded to the cases (80%) where the
same dowel (large or small) remained illuminated through-
out the trial and where no perturbation occurred. Move-
ment times during these trials were found to be shorter
than during the corresponding blocked trials. Mean MT
was 505 ms, that is, about 50 ms less than in blocked trials
(compare Small and Large control trials in Tables 2 and 3,
respectively, with blocked trials in Table 1).
Although MT was shorter, the time values of kinematic
landmarks for the wrist (parameters TPV and TPD), as
well as the amplitude value of APV, were not different
from those of blocked trials. This was the case for both
Small and Large control trials. Thus, the shorter MT
found for control trials was due to shortening the low
velocity phase of the movement at the vicinity of the target.
The grasp component was also similar in control and
blocked trials, except for the difference in time to grip
aperture (TGA) between Small and Large trials. This
Table 2. Kinematic characteristics of prehension during Small con-
trol trials (S) and Small to Large perturbed trials (S-L) (Intersubject
mean and SD)
Transport MT TPV APV TPD
S 507.8 189.3 1203.7 286.8
(82) (29) (120) (34)
S~L 683.9 188.3 1159.8 292.9
(91) (26) (109) (34)
Grasp TGV TGA AGA TGA2 AGA2
S 194.9 308.6 95.4
(46) (47) (8.3)
S--*L 179.4 293.6 97.8 475.2 121.6
(41) (33) (13.9) (86) (10.4)
MT: Movement Time (ms), TPV: Time to Peak Velocity (ram/s),
APV: Amplitude of Peak Velocity (mm), TPD: Time to Peak
Deceleration (ms), TGV: Time to maximum Grip Velocity (ms),
TGA: Time to maximum Grip Aperture (ms), AGA: Amplitude of
maximum Grip Aperture (mm), TGA2: Time to Second peak in Grip
Aperture (ms), AGA2: Amplitude of Second peak in Grip Aperture
(ram)
Table 3. Kinematic characteristics of prehension during Large con-
trol trials (L) and Large to Small perturbed trials (L-S) (Intersubject
mean and SD)
Transport MT TPV APV TPD
L 501.3 179.1 1191.2 280.4
(71) (28) (134) (39)
L~S 598.8 185.2 1188.0 278.1
(94) (30) (136) (39)
Grasp TGV TGA AGA TGA2
L 176.2 314.4 126.4
(34) (43) (8)
L~S 179.2 322.8 121.3 392.1
(35) (57) (9) (106)
MT: Movement Time (ms), TPV: Time to Peak Velocity (ram/s),
APV: Amplitude of Peak Velocity (mm), TPD: Time to Peak
Deceleration (ms), TGV: Time to maximum Grip Velocity (ms),
TGA: Time to maximum Grip Aperture (ms), AGA: Amplitude of
maximum Grip Aperture (mm), TGA2: Time to Second peak in Grip
Aperture (ms), AGA2: Amplitude of Second peak in Grip Aperture
(mm)
difference, which amounted to 65 ms in blocked trials, no
longer existed in control trials. This result seems to be
explained by the strategy adopted by the subjects of making
faster movements in the situation where perturbations
occurred. Indeed, the rate of opening of the grip, as
measured by parameter TGV, was faster in Large control
trials than in Small ones, which accounts for the fact that
TGA occurred at the same time in both cases. The
difference in TGV between Small and Large control trials
was significant (P < 0.025).
Finally, the morphology of wrist and finger spatial
paths in control trials did not differ from that of blocked
trials. In Small trials, the index of variability was 197, 349,
468 cm 2 for wrist, thumb and index finger trajectories,
respectively, and for Large trials, 230, 401, and 645 cm z.
Perturbed trials
Movements during perturbed trials were compared with
movements during control trials starting with illumination
of the same dowel. Accordingly, movements during Small
to Large perturbed trials will be compared with move-
ments during Small control trials, and movements during
Large to Small pertubed trials, to Large control trials.
Perturbation from the Small to the Large dowels
Movement time.
The mean MT in Small to Large (S-L)
perturbed trials was increased by about 175 ms with
respect to the Small control trials (Table 2). In individual
subjects, this increase ranged from 74 ms in subject CU, up
to 255 ms in subject YR.
413
Transport component
Kinematic analysis.
No significant changes were found in
the time values of the wrist kinematic landmarks during S-
L perturbed trials, with respect to the Small control trials.
As shown by Table 2, parameters TPV and TPD were
within the same range in both cases. The amplitude of the
velocity peak (APV) was lower in perturbed trials by
44 mm/s (P > 0.05).
This result shows that the wrist kinematics were little
affected by the perturbation, at least during the first
300 ms or so following movement onset. The MT increase
in perturbed trials was therefore likely to be due to
lengthening the low velocity phase following peak deceler-
ation. Indeed, the acceleration phase of the movement
(reflected by parameter TPV), which in control trials
represented 37.2% of MT, represented only 27.5% in
perturbed trials. The low velocity phase was prolonged
and was marked by small changes in velocity which, in
some subjects, created the impression of secondary sub-
movements. Figure 6 shows a comparison between kine-
matic profiles of a movement during a Small control trial
(Fig.6, S) and a Small to Large perturbed trial (Fig. 6, S-L).
Spatial path and variability of wrist movement.
The S-L
perturbation had little effect on the spatial path of the
wrist, except that the movement was slowed down at the
vicinity of the object. Fig. 7 clearly shows that a higher
proportion of temporal frames were concentrated near the
object (compare the spatial positions of parameters TPV
and TPD in Small control trials, Fig. 7a and in S-E
perturbed trials, Fig. 7b).
Variability of wrist trajectories over repeated move-
ments was increased during S-L perturbed trials with
respect to Small control trials. The index of variability in
perturbed trials was 499 cm 2, as compared to 197 cm 2 for
Small control trials (P < 0.05).
Grasp component
Kinematic analysis.
In all subjects, the profile of change in
grip size during the movement in S-L perturbed trials was
marked by a discontinuity. In three subjects, grip size first
increased up to a first peak, then decreased and finally
reincreased up to a second peak before decreasing until
contact with the dowel. Kinematic analysis revealed that
the first peak in fact corresponded to the maximum grip
aperture observed in Small control trials. The time value of
this first peak was not significantly different from that of
parameter TGA in Small controls. Finally, the amplitude
of the first peak of grip amplitude was the same as AGA in
Small trials (Table 2). The second peak in grip size
occurred later in time (475 ms after movement onset), and
its amplitude corresponded to the size of grip observed in
Large control trials (compare the value of AGA2 in Table
2 with that of AGA for Large trials in Table 3). In the other
two subjects, the distinction between two peaks was less
easily made, and the grip aperture profile was only marked
by an inflexion. The example shown in Fig. 8 (S-L) belongs
to the latter category.
~14~176 I
1o5o]
g _
.#
0 700
_o
>
350-
S
414
1400-
15000
10000
0 175 350 525
Time (ms)
5000
O
0 ~
B
B
-5ooo
1050 10000
700
350
-10000
700
L
15000
5000
-0
--5000
-10000
700
0 175 350 525
Time
(ms)
S-L
1400 1 15000
10000
1050
/~ 5000
700
0
550 -5000
0 --~ -- -10000
175 350 525 700
Time (ms)
Velocity
-- Accele~tion
L-S
1400] 15000
10000
1050
5000
700
0
350- -5000
0 ,- , ~ -10000
0 175 550 525 700
Time (ms)
Su~e~: GC
Fig. 6. Kinematic profiles of the wrist
displacement during control and perturbed
trials. Four representative trials are shown. S:
Small control trial; S-L: Small to Large
perturbed trial; L: Large control trial; L-S:
Large to Small perturbed trial. Note longer
movement time and longer duration of low
velocity phase following peak deceleration in
perturbed trials. Legend as in Fig. 2
The double-peak pattern in grip size, however, was
clearly visible in all subjects on the curve of grip "velocity"
(e.g., Fig. 8 S-L). On this curve, the time occurrence of the
first velocity peak had the same value as for Small control
trials (parameter TGV). This first velocity peak was fol-
lowed by a second one corresponding to the reopening of
the grip. The time of the valley between the two velocity
peaks where grip size velocity was the lowest thus re-
presented the earliest consistent sign of corrective finger
movements aimed at grasping the large dowel. This im-
portant landmark was located at a mean time value of
329.8 ms following movement onset.
Spatial path and variability of finger trajectories.
Due to the
fact that changes in grip size occurred late, the spatial path
of the fingers was little modified by the perturbation.
However, in order to accommodate for the larger size of
the object, both fingers contributed. First, the curvature of
the index finger path during enclosure was less marked
than in control trials. Second, the thumb tended to over-
extend in order also to increase grip size. This point is
demonstrated by Figs. 7 and 9. Figure 7b shows the
averaged finger paths during S-L perturbed trials. Figure
9b shows the lack of index finger flexion and the thumb
extension following the perturbation.
Variability of finger trajectories over repeated trials
was increased with respect to control trials. The mean
index of variability was 916 and 1337 cm 2 for thumb and
index finger, respectively. These values were significantly
greater than the corresponding values in the control trials
(thumb, P < 0.05, index, P < 0.025). Variability was more
pronounced during the early part of the movement, near
the time where velocity of the wrist was the highest.
However, in spite of the perturbation, variability remained
low during the low velocity phase of the movement near
the object. This finding confirms the tendency for the
fingers to come in contact with the object at the same
points (Fig. 7b, d).
A
S
9
/'4- TPD 294 ms
I T#~.- PV 207 ms
T
W
B
s L
~- ms
W 5 cm
415
C
60 mm
o
>
"o 30
t
X
0
0 25 50 75 100
Normalized time (%)
D
Subject: GC
r'nm
~176 1
3O
i
25 50 75 100
Normalized time (%)
Fig.
7A-D. Spatial path and variability
of prehension movements in the Small
control condition (A, C) and in the Small
to Large perturbed condition (B, D). Note
in this subject contributions of both
thumb and index to corrective increase in
grip aperture (B). Also note increased
variability in perturbed trials (D). Same
legend as in Fig. 3
Perturbation from the Large to the Small dowels.
Corrections in response to the L-S perturbation were
apparently generated more easily than corrections to S-L
perturbations. Movement time was increased by about
85 ms with respect to large control trials (P > 0.0005).
Transport component
No differences were found in time of occurrence of the
kinematic landmarks. Parameters TPV, TPA and TPD
had the same values as in Large control trials (Table 3).
The increase in MT (less marked than for the S-L per-
turbation) was therefore due to a lengthening of the low
velocity phase at the end of the movement (Compare Fig.
6, L with Fig. 6, L-S). The spatial path of the wrist was
similar to that of control trials. Its variability was not
significantly increased (t-test): the mean index of vari-
ability was 245 cm 2.
Parameters TGV, TGA and AGA had values very similar
to those in Large control trials (Table 3). The curve of grip
size as a function of time showed only one peak, as
confirmed by the curve of grip velocity (Fig. 8, L-S). The
main change with respect to control trials was the prolong-
ation of the enclosure phase of the grip size until it reached
the size of the small dowel (Fig. 8, L-S).
The spatial paths of the fingers were virtually identical
to those of Large control trials, except for the prolongation
of grip closure. Both fingers participated in this process. As
shown in Figs 9, L-S and 10b, closure was achieved by an
increase in index finger flexion, as well as by a flexion of the
thumb. Variability of finger trajectories over repeated
trials was not increased with respect to control trials. The
mean index of variability was 402 and 600 cm 2 for thumb
and index finger, respectively. These values were not
significantly different from those of the control trials
(unpaired t-test). Finally, due to the low variability during
the final phase of the movement, the fingers consistently
appeared to contact the object at the same points on each
repetition.
Grasp component
Discussion
The grasp component in L-S trials was unaffected with
respect to control trials until late in movement time. The main problem that we addressed in this paper was the
degree of independence of grasping with respect to the
S
140 1
E
105
E
70-~
g
._o. ss-
140
105
70
35
800
416
175 350 525
Time
(ms)
L
140
-
4oo
0
0 O.
3
-400
-800
700
800
105
70
35
140
400 105
0 70
-400 35
0 -80O
0 17s 3so s2s 700
Time
(ms)
S-L
175 350 525
Time
(ms)
800
- 400
-0
- -400
-
-800
700
L-S
9 800
- 400
0
-400
Grip aperture
Grip velocity '
-80o
175 .550 525 700
Time (ms)
Subject: CU
Fig. 8. Kinematic profile of grip formation
during control and perturbed trials. Four
representative trials are shown. S, Small
control trial; S-L, Small to Large perturbed
trial; L, Large control trial; L-S, Large to
Small perturbed trial. Same legend as in
Fig. 4
other components of prehension, such as the transport of
the hand at the location of the target object. The answer to
this question is not a simple one, as there are arguments
both in favor and in disfavor of independent, or parallel,
organization of these components.
Arguments in favor of independent visuomotor
subsystems
A first argument in favor of a relative independence of the
two components is the fact that in the present unperturbed
condition, the pattern of grip formation covaried with
object size, whereas the transport component remained
uninfluenced (at least within the limited range of object
sizes that we used in this experiment). The main effect of
object size on grip formation was the well-known increase
in maximum grip aperture with object size (Jeannerod
1981, 1984; Wallace and Weeks 1987; Marteniuk et al.
1990). Marteniuk et al. (1990) found that for each increase
of 1 cm in object size, the maximum grip aperture in-
creased by 0.77 cm. According to this result, the difference
in diameter between our two dowels (4.5 cm) would pre-
dict a difference in maximum grip aperture of 3.5 cm. The
difference of 3.2 cm that we found (Table 1) is very close to
that prediction. The subjects used two different strategies
for achieving this pattern. In the blocked trials, the grip
size increased at the same rate for both small and large
objects, with the consequence that maximum grip aperture
was reached earlier in movement time for a small object
than for a large object (see Marteniuk et al. 1990). By
contrast, in Control trials, the rate of increase was faster
for a large than for a small object, with the consequence
that grip size peaked nearly at the same time for both large
and small objects. This second pattern corresponds to that
described earlier by Jeannerod (1981). For each given
strategy, however, these object size related changes in grip
formation occurred without affecting movement time or
transport kinematics. It seems that the only determinant
for using either one of the two strategies was movement
duration, so that the subjects tended to equate the time to
maximum grip aperture for large and small objects when
the movement became faster. Finally, the fact that these
changes in grip formation were found to occur without
,~176 1
53~
200
0
Q.
._~
"0 I00
X
S
40o ] I
i
L
3oo 1
!
200 4
25 50 75
Normalized time
(%)
L
100
100
0 25 50 75 1 O0
Normalized time (%)
400
300
200
100
S-L
25 50 75
Normalized
time (%)
100
L-S
4OO
I
300
/
200 W ~
T
I00# T
O~
0 25 50 75 1 O0
Normalized time (%)
Subject: GC
417
Fig. 9. Mean displacement in the X
dimension of the wrist (W), tip of the
thumb (T) and tip of the index (I) in
Control and Perturbed trials. S, Small trial;
S-L, Small to Large perturbed trial; L,
Large control trial; L-S, Large to Small
perturbed trial. Note in this subject the
large contribution of the thumb to the
corrections in grip aperture. Legend as in
Fig. 5
affecting transport kinematics is also suggestive of an
independence of the visuomotor control systems for finger
movements and for arm movements. Previous authors
(Wing et al. 1986; Von Hofsten and Ronnqvist 1988;
Wallace and Weeks 1988; Marteniuk et al. 1990), however,
had found faster movements for larger objects.
A second, and more elaborate, argument as to this
point stems from the comparison between the corrections
observed in response to the different types of perturba-
tions. In the experiment reported earlier, where the per-
turbation affected the spatial position of the object,
changes in wrist acceleration could already be detected
within about 100 ms following the perturbation (Paulig-
nan et al. 1990, 1991). This early correction sharply
contrasts with the effects of perturbations in object size
studied in the present experiment. Our results demonstrate
that the earliest change produced in prehension move-
ments by these perturbations did not occur before about
330 ms, when the closure of the grip was interrupted and
the sign of grip velocity began to revert. No sign of
correction could be detected before this time on the
velocity profile of the wrist. In accordance with Jeannerod
(1981), who reported a similar result from a preliminary
experiment, the latest considered wrist landmark (time to
peak deceleration, TPD) was unchanged with respect to
control unperturbed trials. This difference between the two
types of corrections is a somewhat counterintuitive find-
ing, since the inertial properties (e.g., the musculoskeletal
mass) of the systems respectively involved in correcting for
position or size perturbations would lead to the expecta-
tion that fingers should react at least as fast as the arm. The
fact that this did not occur means that the limiting factor
for the speed of corrections to size perturbations must be
looked for at the central stage of visuomotor processing,
rather than at the execution level.
The parallelist hypothesis would thus be a likely one
for explaining the timing differences in responses to posi-
tion and size perturbations. In neurophysiological terms, it
could be speculated that the slower response to size
perturbations relates to the higher degree of complexity of
the visuomotor pathways for controlling distal move-
ments. This pathway involves processing the visual at-
tributes of objects which relate to object identification and
recognition. Such processing, which seems to involve
418
A
axis
,
i,f
,~ - TPD 305 ms
I ~T ~ V 197 ms
w
B
L-S
I
W 5 cm
C mr'l
6O
o
"5
-8
30
X
I
0 25 50 75 100
Normalized time (%)
D
Subject: GC
60 mm
30
01 ....
0 25 50 75 100
Normalized time (%)
Fig. 10A-D. Spatial path and variability of
prehension movements in the Large control
condition (A, C) and in the Large to Small
perturbed condition (B, D). Same legend as in
Fig. 3
cortical mechanisms (e.g., Jeannerod 1986), would likely be
time consuming and would be compatible with the ob-
served correction delay of 330 ms. By contrast, the path-
way for processing spatial localization and controlling
proximal movements would be simpler and have a shorter
time constant. In this regard, a tectospinal pathway with
only a few synapses has been proposed by Alstermark et al.
(1990, in the cat) for explaining the short correction delays
during perturbation of target position.
A hypothesis for coordination of the two components
Other results obtained in these perturbation experiments,
however, are in apparent disfavor of independent visu-
omotor subsystems and could lead to a different inter-
pretation of the organization of prehension. In the pre-
vious paper dealing with perturbation of object position
we showed that the two components became kinematically
coupled during the corrective responses. The alteration of
the wrist trajectory for reorienting the movement was
immediately followed (within about 50 ms) by a brief
interruption of grip aperture, unrequired by the situation
since object shape and size remained unchanged
(Paulignan et al. 1990, 1991). This finding was recently
replicated by Haggard and Wing (1991). In their situation,
the subject's arm was suddenly pulled back by a mechan-
ical device during approach to the object. This perturba-
tion triggered a rapid correction of the transport compon-
ent, such that the arm was reaccelerated in order to reach
for object position. In about 70% of perturbed trials, the
perturbation applied to the arm also provoked a reversal
of grip aperture which occurred some 70 ms later than the
change in transport.
In the present experiments involving perturbation of
object size, the correction was also not limited to the
affected component, since it was made at the expanse of
movement duration or, more specifically, at the expanse of
the low velocity phase of the transport component. These
results, which show that components of prehension are
mutually influenced, do not necessarily speak against
independence of the two components. Instead, they sug-
gest the existence of a coordination mechanism.
The general hypothesis that we would like to propose
for explaining this coordination is that, in addition to the
separate parametrization of transport and grasp, there
would exist another mechanism for encoding the resultant
goal of the complete action. Separate parametrization
implies, for each of the involved visuomotor channels,
selection of the proper muscles and calibration of the
419
motor commands applied to these muscles. Encoding the
resultant goal implies controlling the timing of these
commands and the kinematics of the resulting movements
in order to precisely coordinate arrest of the reach and
closure of the fingers at contact with the object. This idea
of a central coding of the "desired" position of an effector
system has already been proposed by several authors for
the control of various kinds of movements (e.g., speech
movements, MacNeilage 1970; Abbs and Gracco 1984;
arm movements, Bizzi et al. 1984; finger movements, Cole
and Abbs 1987).
Some speculation can be offerred on the nature of the
coordination mechanism and on the way it could operate
for synchronizing transport and grasp. The resultant goal
would be encoded as a temporal structure, comparable to
a music score where both the action of the instruments and
their relative timing are represented. At each step of the
movement this temporal structure would be used as a
reference to which incoming signals arising from execution
would be compared. The corresponding motor commands
would be modulated for minimizing the mismatch of the
segmental movements with respect to the reference. When
a perturbation would occur during execution of the move-
ment, a new parametrization of the output would be
produced at the level of the affected visuomotor channel.
The mechanism of comparison between incoming move-
ment-related signals and the (updated) reference would
operate for preserving temporal coordination between
components (see Prablanc et al. 1979; Jeannerod 1990; for
a computer model using the same principle, see Bullock
and Grossberg 1988). The rate at which the reorganization
and corrections can be generated following perturbations
would depend on the amount and the rate of visual
processing needed for detecting and analysing changes
occurring in object attributes. Our experimental results
suggest that the rate of processing differs according to
whether the channels dealing with object spatial or iconic
attributes are affected by the perturbation, i.e., it is longer
for processing a change in iconic attributes.
Previous suggestions, partly based on experimental
results, have been made for the temporal structure used as
a reference for maintaining coordination between trans-
port and grasp. One of these suggestions was that move-
ment duration would keep constant for prehension over a
wide range of conditions. In this case the temporal struc-
ture would be preserved, only the amount of contraction
to the arm muscles would be changed for projecting the
arm at different distances; or only the amount of grip
opening would be changed for objects of different sizes.
Data showing invariance of movement duration for differ-
ent amplitudes have been reported during prehension by
Jeannerod (1984) and during writing by Viviani and
Terzuolo (1980). In the experiments reported in the present
paper, the time to maximum grip aperture (TGA) was also
maintained constant for different objects sizes (at least in
the control trials, see Tables 2 and 3), thus indicating that
not only movement duration, but also the all temporal
structure of coordination can be preserved while para-
metrization of the segmental movements is changed.
Temporal invariance in the strict sense, however, can-
not be considered as a universal way of maintaining
coordination. First, this mechanism could not hold for all
the conditions of prehension, which can differ widely from
case to case. Second, under certain conditions, changes in
object size can affect movement duration (Marteniuk et al.
1990) and timing of the grip (Marteniuk et al. 1990, and
present results, blocked trials). These data are thus more
compatible with a relative invariance of kinematic land-
marks within a changing movement duration (kinematic
scaling). Among these kinematic landmarks, one, the onset
of the low velocity phase of the movement (our parameter
TPD), seems to be preserved in nearly all the investigated
conditions. Indeed, the duration of the low velocity phase
represents a constant proportion of total movement time.
In the present experiment, the mean proportion of move-
ment time spent in the low velocity phase was between
43% and 48% in blocked and control trials (see also
Jeannerod 1981, 1984; Wallace and Weeks 1988, Von
Hofsten and Ronnqvist 1988). Remarkably, the onset of
this low velocity phase (TPD) corresponds to the time
(TGA) where the fingers begin to enclose the object. This is
also the time where variability of the wrist and finger
spatial paths is at its lowest. Temporal coordination
between TPD and TGA might also explain the coupling
between transport and grasp that we observed during the
corrections of both types of perturbations. In perturbation
of object position, the deceleration which stopped the first
wrist movement in the wrong direction was consistently
associated with finger closure. In the present experiment
where finger closure was prolonged because of changes in
object size, the deceleration of the wrist was also pro-
longed.
The hypothetical mechanism outlined above for co-
ordination between transport and grasp postulates the
existence of a central representation of the action of
prehension, which permanently monitors movement-re-
lated signals and compares them with the ongoing efferent
commands. There are several possible neural structures
that could fulfill this function of an on-line comparator.
One of these structures could be the C3-C4 propriospinal
neurons described by the Lundberg group (see Alstermark
et al. 1990). These neurons are under the influence of upper
level, including cortical, structures, they are likely to
receive proprioceptive signals generated by several seg-
ments of the same limb and they control the activity of the
corresponding motoneuron pools. One prediction arising
from this suggestion is that peripherally deafferented
subjects should loose the temporal coordination between
transport and grasp and, in addition, would be unable to
correct their prehension movements in response to per-
turbations of either object position or size. Although this
prediction, to our present knowledge, has never been
tested directly, there are indirect arguments against such a
pure spinal mechanism. Prehension movements were
examined by Jeannerod et al. (1984) in one patient with a
complete anaesthesia (including loss of position sense) of
one hand and forearm, due to a lesion of parietal cortex.
While in this patient kinesthetic input was spared at the
spinal level, her prehension movements were deeply disor-
ganized, and coordination between components was lost.
This result indicates that mechanisms for temporal co-
ordination between motor components during prehension
420
should lie upstream with respect to spinal cord, possibly at
the cortical level.
A number of experimental arguments in humans and
animals point to the posterior parietal cortex as a possible
site for this mechanism. Lesions of posterior parietal
cortex produce a profound disorganization of prehension.
First, the accuracy of reaching toward the object is im-
paired, the kinematics of the transport component are
altered, with increase in movement duration due to
lengthening of the deceleration phase, decrease in peak
velocity, and occurrence of several secondary velocity
peaks during deceleration. Second, grip formation is im-
paired, with exaggerated grip opening, incomplete or
absent grip closure, inaccurate posturing of the fingers
(Faugier-Grimaud et al. 1978 in monkey; Jeannerod 1986
and Perenin and Vighetto 1988 in humans). This massive
impairement in object-oriented action suggests that poste-
rior parietal cortex might be involved in building the
representation of the desired final configuration of opposi-
tion space during prehension. Indeed, this region of ce-
rebral cortex contains neurons, the activity of which is
selective in the configuration and/or the orientation of the
object for intended manipulation (Taira et al. 1990). These
neurons are likely to play a role in matching the pattern of
hand movement with the spatial characteristics of the
object to be manipulated.
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