Subjects were asked to pick up disks. The apparent size of the disks was manipulated by a visual illusion. Of the two aspects of the action which depend on the objects size, only one was affected by the illusion. We conclude that different aspects of an action are controlled independently, instead of being co-ordinated on the basis of one perceptual variable

Brenner, Eli

Jeroen.B.J., Smeets

English

CogPrints Cognitive Sciences Eprint Archive

Multiple Specification of an Objects Size for Picking it up.

Smeets, Jeroen B.J

Cognitive Sciences ePrint Archive

147
Multiple Specification of an Object’s Size for Picking it up
Jeroen B.J. Smeets & Eli Brenner
Vakgroep Fysiologie, Erasmus Universiteit Rotterdam
(SMEETS@FYS1.FGG.EUR.NL)
Abstract
Subjects were asked to pick up disks. The apparent size of the disks was
manipulated by a visual illusion. Of the two aspects of the action which depend on the
objects size, only one was affected by the illusion. We conclude that different aspects of
an action are controlled independently, instead of being co-ordinated on the basis of one
perceptual variable.
Introduction
One consequence of using a single perceptual invariant to control the execution of
an action is that the same visual information is used for different aspects of that action.
Thus, if catching an approaching ball is based on the invariant ‘tau’, the timing of both
the arm movement and the opening and closing of the hand can be expected to be based
on that invariant. For intercepting moving objects, however, we found that the direction
and the timing of the arm movement are based on different information on the object’s
motion (Smeets & Brenner, 1995). Position and time-to-contact are of course totally
different physical properties, so there may be good reasons for the corresponding
aspects of an action being based on different invariants. Can two aspects of an action
also be based on different information about a single physical property?
To answer this question, we examined how subjects pick up brass disks. Two
aspects of this action are directly coupled to one physical property: the size of the object
that is to be picked up. During the reaching part of the movement, the object’s size
determines the opening of the hand: the larger the object, the larger the opening
between thumb and finger (Jeannerod 1986). The size of the object also determines the
rate at which the force to lift the object increases: the larger the object, the faster the
force increases (Gordon et al., 1991). Are both aspects of picking up disks based on the
same analysis of the visual information (invariant), or is each aspect of the action based
on a different combination of the available visual information?
Visual illusions can be used to investigate whether different aspects of action are
based on different perceptual information. An appropriate background can change the
perception of specific aspects of the visual world (e.g. speed, direction, size), leaving
other aspects (e.g. position) unchanged. We used a variant of the Ponzo-illusion (see
Figure 1) to study the use of information on size when picking up a disk.
Methods
Eight subjects participated in the experiment. They sat behind a table, without any
restraint (Figure 1). Subjects started a trial with their hand on the right side of the table
with the thumb and finger 5cm apart. They were asked to pick up the brass disk in front
of them, and to put it smoothly and accurately on a 9cm diameter target position on the
left. Three brass disks were used, which differed slightly in size; on average they were
7cm diameter and weighed 0.9kg. We used a background with diverging lines to induce
148
Background left Background right
Figure 1 Experimental set-up in the two background conditions.
changes in the perceived size of the disk. With the background placed on the left side,
the disk appeared smaller than with the background placed on the right.
To study the visual information used during the grasping and lifting phase, we
extracted two parameters from each trial. These parameters were defined in a manner
that would reveal misjudgements in size, independent of the actual size of the disk. To
characterise the grasping phase, we used the parameter ‘extra grip’: the distance
between thumb and finger minus the disk size. This parameter was determined when the
hand was 2cm from the target, about 100ms before the fingers grasped the disk. To
characterise the lifting of the disk, we used the parameter ‘extra force’: the vertical
force exerted on the disk minus its weight. This parameter was determined 100ms after
the fingers grasped the disk.
An analysis of variance was performed to test whether the position of the
background had a significant effect on either of these two parameters.
Results
Figure 2 shows the effect of the visual illusion on the grasping and lifting of the
disk. The extra grip did not depend on the illusory information on size: the opening was
always about 2.2cm larger than the final grip. The difference between the two
conditions was not only not significant, but also very small: 0.04cm (about 0.5% of the
average disk size). Other measures of grip size (e.g. maximum opening) were also
independent of the illusion. We conclude that the different positions of the background
induced no change in the estimate of disk size used for grasping the disk.
The way the disk was lifted did depend on the illusory change in size. The extra
force was about 0.43N larger when the disk appeared to be larger. This corresponds to
an extra mass of 0.044kg. We conclude that the different positions of the background
induced a 5% change in the estimate of disk size used for lifting the disk.
Discussion
The results show that lifting and grasping use different information on the disk’s
size. This finding contradicts the idea that all aspects of an action are controlled by the
same visual information specifying the goal of the movement; it supports the idea that
different aspects of an action are controlled independently, each aspect on the basis of a
specific combination of visual information on physical properties of the environment.
149
0
0.2
0.4
0.6
0.8
1.0
1.2
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 (
N
)
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cm
)
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Background
Figure 2 Results. The grasping of the disk (extra grip) was not influenced by the
illusory change in the disk size (p>0.4), whereas the lifting (extra force) was
influenced: more force was  exerted to lift the disk when the disk seemed larger
(p<0.02).
The important question which remains is why different aspects of one goal-
directed action are controlled independently, instead of being co-ordinated on the basis
of a single perceptual variable (invariant)? A disadvantage of the latter approach is that
the nervous system has to determine an appropriate variable (which specifies the goal of
the action) separately for every new task. The alternative is to use a set of simple visuo-
motor units of action, which can be combined for a wide variety of tasks, without the
need for recallibration of the units for every new task. Such an approach implies that the
fundamental visuo-motor units of action are the same in different tasks.
For the task used in this study, one could describe the grasping phase as consisting
of two units: moving the thumb to a position on the surface of a disk and moving the
finger to another position. This would explain why the grasping phase is insensitive to
the size-illusion: these units do not use information on size. If moving the thumb and
moving the finger are indeed the fundamental units of action for grasping, one would
expect that the movement of a single finger towards a point on an oriented surface
shows some characteristics of the movement of a finger during grasping. This is indeed
what we have found for the curvature in the path of a pointing movement (Brenner &
Smeets, 1995).
This scheme is not yet a complete description of the grasping phase. We do not
know which visual information (invariant) specifies the affordance of touching the
surface of the object. Now we know that transport and grip are not the fundammental
units of action, we can start searching for the invariants specifying the results of these
movements.
References
Brenner, E. & Smeets, J.B.J. (1995) Moving one’s finger to a visually specified
position: target orientation influences the finger’s path. Experimental Brain
Research, 105:318-320.
Jeannerod, M. (1986) The formation of finger grip during prehension. A cortically
mediated visuomotor pattern. Behavioral Brain Research 19:99-116.
Gordon, A.M., Forssberg, H., Johansson, R.S., Westling (1991) Visual size cues in the
programming of manipulative forces during precision grip. Experimental Brain
Research, 85:226-229
Smeets, J.B.J. & Brenner, E. (1995) Perception and action are based on the same visual
information: distinction between position and velocity. Journal of Experimental
Psychology: Human Perception and Performance 21:19-31


Moving one’s finger to a visually specified position: target orientation influences the finger’s path.

The formation of finger grip during prehension. A cortically mediated visuomotor pattern.

Visual size cues in the programming of manipulative forces during precision grip.

Multiple specification of an object's size for picking it up.

Smeets, J.B.J.

Brenner, E.

VU Research Portal

Multiple specification of an object’s size for picking it up.

147
Multiple Specification of an Object’s Size for Picking it up
Jeroen B.J. Smeets & Eli Brenner
Vakgroep Fysiologie, Erasmus Universiteit Rotterdam
(SMEETS@FYS1.FGG.EUR.NL)
Abstract
Subjects were asked to pick up disks. The apparent size of the disks was
manipulated by a visual illusion. Of the two aspects of the action which depend on the
objects size, only one was affected by the illusion. We conclude that different aspects of
an action are controlled independently, instead of being co-ordinated on the basis of one
perceptual variable.
Introduction
One consequence of using a single perceptual invariant to control the execution of
an action is that the same visual information is used for different aspects of that action.
Thus, if catching an approaching ball is based on the invariant ‘tau’, the timing of both
the arm movement and the opening and closing of the hand can be expected to be based
on that invariant. For intercepting moving objects, however, we found that the direction
and the timing of the arm movement are based on different information on the object’s
motion (Smeets & Brenner, 1995). Position and time-to-contact are of course totally
different physical properties, so there may be good reasons for the corresponding
aspects of an action being based on different invariants. Can two aspects of an action
also be based on different information about a single physical property?
To answer this question, we examined how subjects pick up brass disks. Two
aspects of this action are directly coupled to one physical property: the size of the object
that is to be picked up. During the reaching part of the movement, the object’s size
determines the opening of the hand: the larger the object, the larger the opening between
thumb and finger (Jeannerod 1986). The size of the object also determines the rate at
which the force to lift the object increases: the larger the object, the faster the force
increases (Gordon et al., 1991). Are both aspects of picking up disks based on the same
analysis of the visual information (invariant), or is each aspect of the action based on a
different combination of the available visual information?
Visual illusions can be used to investigate whether different aspects of action are
based on different perceptual information. An appropriate background can change the
perception of specific aspects of the visual world (e.g. speed, direction, size), leaving
other aspects (e.g. position) unchanged. We used a variant of the Ponzo-illusion (see
Figure 1) to study the use of information on size when picking up a disk.
Methods
Eight subjects participated in the experiment. They sat behind a table, without any
restraint (Figure 1). Subjects started a trial with their hand on the right side of the table
with the thumb and finger 5cm apart. They were asked to pick up the brass disk in front
of them, and to put it smoothly and accurately on a 9cm diameter target position on the
left. Three brass disks were used, which differed slightly in size; on average they were
7cm diameter and weighed 0.9kg. We used a background with diverging lines to induce
Studies in ecological psycholgy: Proceedings of the Fourth European Workshop 
In: A.M.L. Kappers, C.J. Overbeeke, G.J.F. Smets, P.J. Stappers (eds.),
on Ecological Psychology(pp: 147-149). Delft: Delftse Universitaire Pers. 
148
Figure 1 Experimental set-up in the two background conditions.
changes in the perceived size of the disk. With the background placed on the left side,
the disk appeared smaller than with the background placed on the right.
To study the visual information used during the grasping and lifting phase, we
extracted two parameters from each trial. These parameters were defined in a manner
that would reveal misjudgements in size, independent of the actual size of the disk. To
characterise the grasping phase, we used the parameter ‘extra grip’: the distance
between thumb and finger minus the disk size. This parameter was determined when the
hand was 2cm from the target, about 100ms before the fingers grasped the disk. To
characterise the lifting of the disk, we used the parameter ‘extra force’: the vertical force
exerted on the disk minus its weight. This parameter was determined 100ms after the
fingers grasped the disk.
An analysis of variance was performed to test whether the position of the
background had a significant effect on either of these two parameters.
Results
Figure 2 shows the effect of the visual illusion on the grasping and lifting of the
disk. The extra grip did not depend on the illusory information on size: the opening was
always about 2.2cm larger than the final grip. The difference between the two
conditions was not only not significant, but also very small: 0.04cm (about 0.5% of the
average disk size). Other measures of grip size (e.g. maximum opening) were also
independent of the illusion. We conclude that the different positions of the background
induced no change in the estimate of disk size used for grasping the disk.
The way the disk was lifted did depend on the illusory change in size. The extra
force was about 0.43N larger when the disk appeared to be larger. This corresponds to
an extra mass of 0.044kg. We conclude that the different positions of the background
induced a 5% change in the estimate of disk size used for lifting the disk.
Discussion
The results show that lifting and grasping use different information on the disk’s
size. This finding contradicts the idea that all aspects of an action are controlled by the
same visual information specifying the goal of the movement; it supports the idea that
different aspects of an action are controlled independently, each aspect on the basis of a
specific combination of visual information on physical properties of the environment.
Background left Background right
149
0
0.2
0.4
0.6
0.8
1.0
1.2
e
x
tr
a
 f
o
rc
e
 (
N
)
e
x
tr
a
 g
ri
p
 (
cm
)
0
0.5
1.0
1.5
2.0
2.5
right
left
Background
Figure 2 Results. The grasping of the disk (extra grip) was not influenced by the
illusory change in the disk size (p>0.4), whereas the lifting (extra force) was
influenced: more force was  exerted to lift the disk when the disk seemed larger
(p<0.02).
The important question which remains is why different aspects of one goal-
directed action are controlled independently, instead of being co-ordinated on the basis
of a single perceptual variable (invariant)? A disadvantage of the latter approach is that
the nervous system has to determine an appropriate variable (which specifies the goal of
the action) separately for every new task. The alternative is to use a set of simple visuo-
motor units of action, which can be combined for a wide variety of tasks, without the
need for recallibration of the units for every new task. Such an approach implies that the
fundamental visuo-motor units of action are the same in different tasks.
For the task used in this study, one could describe the grasping phase as consisting
of two units: moving the thumb to a position on the surface of a disk and moving the
finger to another position. This would explain why the grasping phase is insensitive to
the size-illusion: these units do not use information on size. If moving the thumb and
moving the finger are indeed the fundamental units of action for grasping, one would
expect that the movement of a single finger towards a point on an oriented surface
shows some characteristics of the movement of a finger during grasping. This is indeed
what we have found for the curvature in the path of a pointing movement (Brenner &
Smeets, 1995).
This scheme is not yet a complete description of the grasping phase. We do not
know which visual information (invariant) specifies the affordance of touching the
surface of the object. Now we know that transport and grip are not the fundammental
units of action, we can start searching for the invariants specifying the results of these
movements.
References
Brenner, E. & Smeets, J.B.J. (1995) Moving one’s finger to a visually specified
position: target orientation influences the finger’s path. Experimental Brain
Research, 105:318-320.
Jeannerod, M. (1986) The formation of finger grip during prehension. A cortically
mediated visuomotor pattern. Behavioral Brain Research 19:99-116.
Gordon, A.M., Forssberg, H., Johansson, R.S., Westling (1991) Visual size cues in the
programming of manipulative forces during precision grip. Experimental Brain
Research, 85:226-229
Smeets, J.B.J. & Brenner, E. (1995) Perception and action are based on the same visual
information: distinction between position and velocity. Journal of Experimental
Psychology: Human Perception and Performance 21:19-31


Multiple Specification of an Objects Size for Picking it up.

Abstract

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CogPrints Cognitive Sciences Eprint Archive

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Cognitive Sciences ePrint Archive

CogPrints Cognitive Sciences Eprint Archive

Multiple Specification of an Objects Size for Picking it up.

Abstract

Similar works

Full text

Available Versions

CogPrints Cognitive Sciences Eprint Archive

Cognitive Sciences ePrint Archive

VU Research Portal

Cognitive Sciences ePrint Archive

CogPrints Cognitive Sciences Eprint Archive

Multiple Specification of an Objects Size for Picking it up.