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Grasping an illusion
Neuropsychologia,Vol. 35, No. 12, pp. 1577-1582, 1997
~
Pergamon
~'; 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0028 3932/97 $17.00+0.00
PII: S0028 3932(97)00061M
Grasping an illusion ELENA
DAPRATI
and MAURIZIO
GENTILUCCI]"
Istituto di Fisiologia Umana, Universitfi di Parma, via Gramsci 14, 1-43100 Parma, Italy
(Received 23 July 1996; accepted 22 April 1997) Abstract--In the present study we attempted to determine the nature of the visual analysis that is performed on an object in order
to grasp it. We required eight healthy subjects to reach and grasp a wooden bar which was superimposed over the shaft of the Mt~ller-Lyer illusion. Vision of both the hand and the bar was allowed. Three different bar lengths were used. Two additional control tasks in which the subjects were required to reproduce the length of the shafts were carried out. The results showed that hand shaping while grasping the bar was influenced by the illusion configurations on which it was superimposed. However, this effect was smaller than that observed in the two tasks of length reproduction. These results support the notion that visual analysis performed on the object of a grasp movement is global and takes into account the object itself, as well as its relationships with surrounding cues. We propose, as suggested previously for reaching movements (Gentilucci, M. et al., Neuropsychologia, 1996, 34, 369-376), two partially independent stages during visuo-motor integration for grasping an object. In the first stage, the object is coded inside an objectcentred frame of reference. In the second stage it is transposed in an egocentric frame of reference, in which the spatial relations between object and agent are computed. In this second stage the influence of cues surrounding the target is minimized. © 1997 Elsevier Science Ltd. All rights reserved Key Words: reaching to grasp; Mtiller-Lyer illusion; grasping task; drawing task; matching task; kinematics; healthy humans.
grasping [4, 7, 13, 15, 17, 25]. Until recently, however, the question as to what processes are used to visually analyse an object in order to grasp it remains an open issue. It seems plausible that the first aim of the visual analysis is the selection of the appropriate opposition axis for the fingers on the object [12]. We can assume that, at first, by means of a rough analysis of the space occupied by the object, the opposition axis aligned along the initial finger orientation is selected. A more detailed analysis of the corresponding portions of the object's surface is then performed in order to determine whether a stable grasp is allowed. When this does not occur (e.g., the two opposite surfaces are not parallel), the adjacent opposition axes and the corresponding object's surfaces are progressively explored, until the appropriate points are located. Their position is computed in three-dimensional space with respect to the body (egocentric frame of reference) and grasping is achieved by carrying each finger to the appropriate contact point on the target object. According to this hypothesis, a global visual analysis of the object does not necessarily take place. Alternatively, we can suggest that a global visual analysis is performed on the object in order to grasp it. We define as global a process that implies visual analysis of the whole object, and the visual relations between the object and the surrounding scene. It is possible that information extracted from the knowledge of object's use and
Introduction
Although we perform it daily, reaching to grasp an object (prehension) is by no means a simple m o t o r act. For this m o t o r act to be successful, the hand must be carried to the appropriate location in space, and the fingers must be shaped according to the orientation, size and shape of the target object. On the basis of behavioural studies [14, 15] the act of prehension can be divided in two components: the transport (or reach) and the grasp. It has been hypothesized that, whereas the transport component is more concerned with visual computation of the spatial relationships between the target object and the body, the grasp component depends on intrinsic object properties, such as size and shape [2, 15]. Independent support for this hypothesis has come from single-neuron recording studies in the premotor [6, 22] and parietal cortices [23, 24]. In particular, the dependence of grasp planning on visual analysis of intrinsic object properties is demonstrated by numerous observations of a strong correlation between hand shaping and object size during
t Address for correspondence: Istituto di Fisiologia Umana, via Gramsci 14, 1-43100 Parma, Italy; fax: 521 291 304; e-mail: [email protected]. 1577
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E. Daprati and M. Gentilucci/Grasping an illusion
from relevant semantic features are also involved in this process. Accordingly, the object is primarily analysed inside an object-centred frame of reference, independently of the spatial relations between object and observer. In previous studies we attempted to determine the nature of the visual analysis used in order to localize the target of pointing movements. We reported that when asked to point to the distal vertex of the Mfiller-Lyer illusion [19], subjects misplaced the target according to the presented configuration. This effect was present in movements executed under visual control and it increased for memory-driven movements, as well as in conditions by which the efficiency of the egocentric system was reduced [8]. We argued that subjects acquired information on target location by means of a global analysis of the figure in allocentric coordinates (as demonstrated by the illusion effectiveness), and successively transposed the target in egocentric coordinates (as suggested by the modulation of the illusion effect). It has been shown previously that the visual analysis of the object in order to plan a grasping movement is different from and independent of that of the complementary reaching movement [7, 15]. In spite of this, a similar mechanism of visual elaboration can be employed to code the object of a grasping movement. That is, the object can be encoded in an object-centred frame of reference and successively transposed in an egocentric one. In order to test this possibility, we devised an experimental paradigm similar to that used previously [8], in which subjects were required to grasp a three-dimensional bar superimposed over the shaft of the Maller-Lyer illusion. We predicted that, if only a sequential analysis
of portions of the object coded in an egocentric frame of reference is performed in order to select the opposition points for the fingers, the grasping kinematic parameters should not be affected by the illusion. On the contrary, if at first a global analysis of the object is carried out, the illusion effect should affect the grasping movement. A successive transposition of the object in an egocentric frame of reference should reduce the effect of the illusion. Two control tasks were carried out in order to check the perceptual effectiveness of the illusion configurations.
Methods Eight right-handed subjects (two males and six females: age 20-31 years) participated in the present study. All were naive as to the purpose of the experiment. The subjects sat in front of a table with their right hand resting on its surface. The starting position was a disc located on the plane of the table along the subject's sagittal plane at 25 cm from his/her frontal plane. According to the task (see below), at the beginning of each trial the subjects placed on the starting position either the point of a pencil held in their right hand (drawing task), or their thumb and index finger, held in pinch position (grasping and matching tasks). Stimuli were displayed on an easel located on the plane of the table, 35 cm from the subject's frontal plane and inclined by 4 5 with respect to the plane of the table. Stimuli were the open and closed configurations of the M~iller-Lyer illusion (Fig. 1). A non-illusory configuration in which horizontal lines replaced either the open or the closed wings was used as a control condition. Lines were 1 cm wide and were drawn in black ink on the centre of white panels. The shaft of the configuration lay approximately on the subject's sagittal plane and could be either 5, 6 or 7 cm long. The ratio between wings and shaft length was 0.3. A black wooden bar
Fig. 1. Experimental set-up. The upper and lower rows show the apparatus for presentation of the closed and open configurations, respectively.
E. Daprati and M. Gentilucci/Grasping an illusion of length equal to the shaft, and height of 1 cm, was superimposed over each shaft and fixed to the easel by means of a magnet. Only the shaft was a three-dimensional object. Wings were two-dimensional figures in order to avoid them becoming an obstacle for the fingers during grasping movements. The distance of the centre of the shaft from the starting position was approximately 31 cm. Three experimental sessions were run, separated by intervals of at least I week. The tasks run in each session and the order of their presentation were as follows.
Session 1: drawing task Subjects were required to draw a vertical line of the same length as the three-dimensional bar presented on the easel. Inspection of the panel lasted for 5 sec, then the go signal was given by a sound controlled by a PC. Vision of the stimulus was allowed while drawing. On the contrary, vision of the hand and the sheet on which the line was drawn was prevented by a box covering them both.
Session 2. grasping task Subjects were required to reach and grasp the extremities of the three-dimensional bar presented on the easel with their right thumb and index finger (Fig. 1). The procedure was the same as in the drawing task, but full vision of both the stimulus and the hand was allowed. The subjects were instructed to move as naturally as possible, with the same velocity as during spontaneous movements. Removal of the bar from the easel was easy because the strength of the magnet was weak.
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the subject's sagittal plane 8 cm from the edge of the table and was used as a reference point. In the grasping task, the time course of the distance between the two markers positioned on the thumb and index finger was used in order to study the grasping component of the movement. Maximal finger aperture, time to maximal finger aperture, peak velocity of finger aperture and grasp time were the analysed parameters. The marker placed on the wrist was used to study the transport component. Transport time, peak velocity and deceleration time (expressed as percentage of total movement time) were measured. The distance between the markers positioned on the thumb and the index finger was used in order to measure finger aperture in the matching task. In both grasping and matching tasks, the distance between the two markers at the starting position was subtracted from that recorded at maximal finger aperture in the grasping task and at finger aperture in the matching task in order to obtain the distance between the finger palmar surfaces. All parameters measured in the three tasks were submitted to separate analyses of variance (ANOVAs) whose within-subject factors were bar length (5 cm vs 6 cm vs 7 cm) and configuration (open vs closed vs control configuration). We evaluated the task's effect on the visual analysis of the bar length by comparing the illusion effect on the following parameters: drawn line length (drawing task), maximal finger aperture (grasping task) and finger aperture (matching task). We measured variation of these parameters for presentation of the open and closed configurations with respect to the control configuration. These values were submitted to an ANOVA, whose withinsubject factors were task (drawing vs matching vs grasping), bar length (5 cm vs 6 cm vs 7 cm) and configuration (open vs closed configuration). The Newman Keuls test was used as post-hoe test in all analyses.
Results
Session 3: matching task Drawin 9 task Subjects were required to match the size of the three-dimensional bar presented on the easel by opening their right thumb and index finger. The subjects were allowed to lift their hand, but not to approach the object during the trial. The procedure was the same as in the grasping task. The aim of the first session was to establish whether the modified configurations of the Mt~ller-Lyer figures would be effective in inducing an illusion. The long interval between two successive sessions was chosen in order to minimize practice effects. Presentation order of the second and third task was selected in order to prevent the perceptual judgement performed in the matching task from influencing visuo-motor integration in the grasping task. In all tasks, the subjects were required not to pay attention to the two-dimensional drawing upon which the bar was superimposed. In each session, 84 trials were run. That is, seven trials for each stimulus length (5, 6, 7 cm) and configuration (open, closed, control) were presented in pseudo-random order. In the drawing task, the length of the drawn lines was manually measured by the experimenters at the end of the session. Resolution was approximately 2 mm. Finger aperture in matching the bar length and the kinematics of reaching~rasping movements were recorded using the ELITE system (B.T.S., Milan, Italy). Apparatus, movement reconstruction and data elaboration procedures are described elsewhere [10]. The spatial resolution of the ELITE system is 0.4mm [10]. Four markers were used. The first marker was placed on the styloid process of the radius at the wrist. The second and third markers were positioned on the nails of the thumb and the index finger, respectively. The fourth marker was placed on the table along
D r a w n lines were straight, and varied significantly a c c o r d i n g to the three b a r lengths [F(2,14)=142.28, P < 0 . 0 0 0 0 1 ; 31 . S m m vs 3 8 . 0 m m vs 45.0 mm]. H o w e v e r , subjects always u n d e r e s t i m a t e d the length o f the b a r (Fig. 2, u p p e r panel). D r a w n lines were clearly affected by the illusion effect [F(2,14) = 9.96, P < 0.002]. T h a t is, a longer line was d r a w n when the open c o n f i g u r a t i o n (39.4 mm) was presented with respect to the c o n t r o l (38.0 m m ) a n d the closed configurations (37.0 mm) ( P < 0.05).
Grasping task Subjects o p e n e d their fingers progressively to a maxim a l finger a p e r t u r e a n d then closed on the object (Fig. 3). Maximalfinger aperture was correctly scaled a c c o r d i n g to the length o f the b a r to be g r a s p e d [57.6 m m vs 64.7 m m vs 72.9 mm; F(2,14) = 201.20, P < 0.00001 ]. This p a r a m e t e r was significantly influenced by the illusion effect [ F ( 2 , 1 4 ) = 4 . 8 1 , P < 0 . 0 5 ] , a l t h o u g h the effect was small. Subjects o p e n e d their fingers wider when the open (65.5 m m ) a n d the c o n t r o l (65.2 m m ) configurations were p r e s e n t e d with respect to the closed c o n f i g u r a t i o n ( 6 4 . 5 m m ) (Fig. 2, lower panel, P < 0 . 0 5 ) . Peak velocity
E. Daprati and M. Gentilucci/Grasping an illusion
1580 D r a w i n g Task
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Fig. 2. Mean values of line length in the drawing task (upper panel), of maximal finger aperture in the grasping task (middle panel) and of finger aperture in the matching task (lower panel). CL5=shaft length 5cm, closed configuration; CO5=shaft length 5 cm, control condition; OP5 = shaft length 5 cm, open configuration; CL6=shaft length 6cm, closed configuration; CO6=shaft length 6cm, control configuration; OP6=shaft length 6 cm, open configuration; CL7 = shaft length 7 cm, closed configuration; CO7=shaft length 7cm, control condition; OP7 = shaft length 7 cm, open configuration ( I = S.E.).
offinyer aperture, like maximal finger aperture, was greater for presentation of the open (258.4 mm/sec) and the control (257.5 mm/sec) configurations with respect to the closed configuration (248.8 mm/sec). However, this effect did not reach significance. The remaining kinematic grasp and transport parameters were not influenced by the illusion.
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Fig. 3. Representative examples of time course of grasping the extremities of the closed configuration (dotted lines), of the control configuration (continuous lines) and of the open configuration (dashed lines) (subject CM.). One trial for each of the three shaft length conditions is represented.
Across-tasks comparisons As expected, a significant effect of the illusion was found [F(1,7)=18.12, P<0.005; closed configuration 1.4 ram, open configuration 0.9 ram]. In addition, there was a significant interaction between task and configuration [F(2,24) = 3.79, P < 0.05]. Post-hoc comparison showed that the effect of the closed configuration was significantly different in the matching task ( - 2 . 5 r a m ) with respect to the two other tasks (drawing task - 1 . 0 r a m , grasping task - 0 . 7 m m ; P<0.05). No significant difference was observed for the open configuration among the three tasks (drawing task 1.4 mm, grasping task 0.2 mm, matching task 1.1 ram).
Discussion
Matching task Finger aperture was approximately scaled according to the presented bar length [52.7 mm vs 61.7 mm vs 77.0 ram; F(2,14)=36.79, P<0.00001]. Moreover, it was affected by the illusion [F(2,14) = 6.85, P < 0.008]. Finger aperture was wider for presentation of the open (65.4mm) and the control (64.3 mm) configurations with respect to the closed configuration (61.7ram) (Fig. 2, middle panel, P<0.05).
The effect of the M/iller-Lyer illusion observed in both the drawing and the matching tasks was smaller than that observed in previous experiments in which perceptual judgement on the length of the shafts of the two configurations was required [5, l l, 20, 21]. In the present experiment, the three-dimensional shafts had large width and therefore they did not produce blurring effects at the vertices of the configurations [3]. Thus, the optical component of the illusion was removed and only effects
E. Daprati and M. Gentilucci/Grasping an illusion arising from visual analysis of figure shape could be elicited. It is known that we commonly use shape as an indicator of size. In the drawing task, the length of all figures was underestimated. This finding is likely a consequence of the lack of visual control while drawing. In order to compensate for this, it is possible that the length of the drawn line was judged on the basis of the time elapsed since movement began. Since it is known that hand movements without visual control of the hand are performed slowly [10, 16], a possible slowing down of the drawing could produce underestimation of the covered distance. The MMler-Lyer illusion influenced maximal finger aperture during grasping. Like finger aperture in the matching task, this parameter was enlarged for presentation of the open and control configurations with respect to the closed configuration. The particular orientation of the wings in the open configuration condition could be an obstacle for the grasping fingers, thus accounting for an increase in maximal finger aperture. However, since only the shaft was a three-dimensional object, whereas the wings of each configuration were two-dimensional figures, we can exclude this possibility. An effect of visual illusions on grasping movements was also found in a previous experiment [1]. In that experiment the two 'Titchener circle' configurations were presented simultaneously and subjects were required to pick up one of the two central discs according to a samedifferent judgement on their relative size. Such a task required that the subjects visually analysed the scene around the two potential target objects. This explains the small effect of the illusion found by the authors. However, due to the instruction given to the subjects, and to the presentation of two configurations at a time, this experiment does not prove whether an analysis of the entire target object and of the surrounding cues occurs for natural grasping movements. The aim of the present experiment was to determine the nature of the visual analysis that is performed in order to grasp an object. We considered two possibilities. The first possibility suggests that a serial analysis of adjacent portions of the object is performed in order to select a stable grasp. That is, the object is not analysed as a whole, but it is fragmented in order to select successive opposition axes, each computed with respect to the observer (egocentric frame of reference). The second possibility suggests that a global visual analysis, that takes into account cues surrounding the target, is first performed on the object. The findings of the present experiment support this second hypothesis. Indeed, not only was the distance between the two extremities of the three-dimensional shaft of the MMler-Lyer figures analysed, as suggested by the first hypothesis, but also the remaining figure (the two wings) responsible for the illusion. Interestingly, the configuration was considered as a whole, although part of it was presented as a twodimensional figure. In other words, a process of visual grouping, by which two-dimensional images were corn-
1581
bined with a three-dimensional object, was used in the visual analysis of the target. This was true even for the motor response. The second hypothesis may also be supported by kinematic data collected on a visual agnosic patient and a healthy control subject, picking up objects of different shapes [18]. In this study the finger opposition axes, recorded on the object at the end of the grasp, frequently passed through its centre of mass. The centre of mass of an object can be established only if the whole object is visually analysed. However, the opposite portions of the object's surface corresponding to the selected finger opposition axes were the only ones that were parallel to each other, allowing a stable contact of the fingers on the object. That is, these data may also support the first hypothesis, which suggests an analysis of opposite surface portions on the object. In the present study the effect of the illusion was smaller in the motor than in the reproduction tasks. This effect was clear when the grasping task was compared with the matching task, and less evident in the comparison with the drawing task. This latter result may be due to the more complex motor response required by the drawing task, since both proximal and distal arm segments are involved in drawing a figure. Such a complex motor response might require a more efficient use of the egocentric frame of reference. This, in turn, might induce greater correction of the illusion (see below). Another possibility is that the effect of the illusion may have been masked by the large underestimation of reproduction amplitudes. Taken together, these data suggest that, when visuomotor integration is performed, the effect of the illusion is partially corrected. In a previous experiment we found that, for pointing movements, the illusion effect is inversely correlated with the efficiency of the egocentric frame of reference [8]. We speculated that the effect of the illusion, due to coding the target in allocentric coordinates, was partially reduced by successive transformation of visual information into motor commands, that is while re-coding the object in egocentric coordinates. Although the visual analysis of the target has been hypothesized as different and independent for reaching and grasping movements [7, 15], the results of the present experiment suggest the possibility of there also being two stages in object visual analysis for planning grasp movements. We propose that in a first stage an object is coded in an object-centred frame of reference, whereas in a second stage it is transposed in an egocentric one. In a previous neuropsychological study [9], we suggested that the parietal lobe is involved in transposing the spatial position of the target of reaching movement from an allocentric to an egocentric frame of reference. On the basis of neurophysiological results [23], we can suggest that an analogous transformation for grasping movements is performed in the same parietal lobe. From the AlP (anterior intraparietal area), an area involved in planning grasp movements, neurons were recorded that
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were activated by object fixation without grasping it ('object' neurons). In addition, other neurons, called 'non-object' neurons, were activated by vision of interaction of the hand with the object. One can speculate that transformation of object coding from an object-centred frame of reference (likely used to code an object by 'object' neurons) to an egocentric frame of reference (used to code an object by 'non-object' neurons) can be performed by concurrent activity of the two types of neurons.
Karnath and P. Thier. Springer, Berlin, 1997, pp. 339-354. 10. Gentilucci, M., Toni, I., Chieffi, S. and Pavesi, G., The role of proprioception in the control of prehension movements: a kinematic study in a peripherally deafferented patient and in normal subjects. Experimental Brain Research, 1994, 102, 483~494. 11. Gillam, G. and Chambers, D., Size and position are incongruous: measurements on the Mfiller-Lyer illusion. Perception and Psvchophysics, 1985, 37, 549556. 12. Iberall, T., Bingham, G. and Arbib, M. A., Opposition space as a structuring concept for the analysis of skilled hand movements. In Generation and Modu-
Acknowledgements--We thank Dr P. Dominey for valuable comments on the manuscript, and Drs M. Gangitano, G. Pagnoni and I. Toni for discussion of data. This work was supported by a Research Grant from the Human Frontier Science Program, by a grant from the European Neuroscience Program and by grants CNR (Centro Nazionale delle Ricerche) to the Institute of Human Physiology of Parma and MURST (Ministero dell'Universit/t e della Ricerca Scientifica e Tecnologica) to the Institute of Human Physiology of Parma and to M.G.
lation of Active Patterns, Evperimental Brain Research Series, Vol. 15, ed. H. Heuer and C. Fromm. Springer, Berlin, 1986, pp. 158-173. Jakobson, L. S. and Goodale, M. A., Factors affecting higher-order movement planning: a kinematic analysis of human prehension. Experimental Brain Research, 1991, 86, 199 208. Jeannerod, M., Intersegmental coordination during reaching at natural visual objects. In Attention and Performance, ed. J. Long and A. Baddeley. Erlbaum, Hillsdale, NJ, 1981, pp. 153-168. Jeannerod, M., The timing of natural prehension movements. Journal of Motor Behaviour, 1984, 16, 235-254. Jeannnerod, M., The Neural and Behavioural Organization of" Goal-directed Movements. Oxford University Press, Oxford, 1988. Marteniuk, R. G., Leavitt, J. L., MacKenzie, C. L. and Athenes, S., Functional relationships between grasp and transport component in a prehension task. Human Movement Science, 1990, 9, 149 176. Milner, A. D. and Goodale, M. A., The Visual Brain in Action. Oxford University Press, Oxford, 1995. M~Jller-Lyer, F. C., Dubloid-Reynolds Archive Ffir Anatomie und Physiologie (suppl.), 1889, pp. 263 270. Prinzmetal, W. and Gettleman, L., Vertical-horizontal illusion: one eye is better than two. Perception and Psvchophysics, 1993, 53, 81-88. Restle, F. and Decker, J., Size of the Maller-Lyer illusion as a function of its dimension: theory and data. Perception and Psychophysics, 1977, 21, 489 503. Rizzolatti, G., Camarda, R., Fogassi, L., Gentilucci, M., Luppino, G. and Matelli, M., Functional organisation of inferior area 6 in the macaque monkey. II. Area F5 and the control of distal movements. Experimental Brain Research, 1988, 71,491 507. Sakata, H., Taira, M., Murata, A. and Mine, S., Neural mechanisms of visual guidance of hand action in the parietal cortex of the monkey. Cerebral Cortex, 1995, 5, 429~438. Taira, M., Mine, S., Georgopoulos, A. P., Murata, A. and Sakata, H., Parietal cortex neurones of the monkey related to the visual guidance of hand movements. Experimental Brain Research, 1990, 83, 29 36. Wing, A. M. and Fraser, C., The contribution of the thumb to reaching movements. QuarterO' Journal of Experimental Psychology, 1983, 35A, 297-309.
References
1. Aglioti, S., De Souza, J. F. X. and Goodale, M. A., Size-contrast illusions deceive the eye but not the hand. Current Biology, 1995, 5, 679-685. 2. Arbib, M. A., Perceptual structures and distributed motor control. In Handbook of" Physiology, Section 1, Vol. 2, Part 2, ed. V. B. Brooks. William & Wilkins, Baltimore, 1981, pp. 1449-1480. 3. Chaing, C. A., A new theory to explain geometrical illusions produced by crossing lines. Perception and Psychophysics, 1968, 3, 174-176. 4. Chieffi, S., Fogassi, L., Gallese, V. and Gentilucci, M., Prehension movements directed to approaching objects: influence of stimulus velocity on the transport and the grasp components. Neuropsvchologia, 1992, 30, 877-897. 5. Festinger, L., White, C. W. and Allyn, M. R., Eye movements and decrement in the Maller-Lyer illusion. Perception and Psychophysics, 1968, 3, 378382. 6. Gentilucci, M. and Rizzolatti, G., Cortical motor control of arm and hand movements. In Vision and Action: The Control of Grasping, ed. M. A. Goodale. Norwood, New Jersey, 1990, pp. 147-162. 7. Gentilucci, M., Castiello, U., Corradini, M. L., Scarpa, M., Umiltfi, C. and Rizzolatti, G., Influences of different types of grasping on the transport component of prehension movements. Neuropsychologia, 1991, 29, 361 378. 8. Gentilucci, M., Chieffi, S., Daprati, E., Saetti, M. C. and Toni, I., Visual illusion and action. Neuropsychologia , 1996, 34, 369 376. 9. Gentilucci, M., Daprati, E., Saetti, M. C. and Toni, I., On the role of the egocentric and allocentric frame of reference in the control of arm movements. In
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Pergamon
~'; 1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0028 3932/97 $17.00+0.00
PII: S0028 3932(97)00061M
Grasping an illusion ELENA
DAPRATI
and MAURIZIO
GENTILUCCI]"
Istituto di Fisiologia Umana, Universitfi di Parma, via Gramsci 14, 1-43100 Parma, Italy
(Received 23 July 1996; accepted 22 April 1997) Abstract--In the present study we attempted to determine the nature of the visual analysis that is performed on an object in order
to grasp it. We required eight healthy subjects to reach and grasp a wooden bar which was superimposed over the shaft of the Mt~ller-Lyer illusion. Vision of both the hand and the bar was allowed. Three different bar lengths were used. Two additional control tasks in which the subjects were required to reproduce the length of the shafts were carried out. The results showed that hand shaping while grasping the bar was influenced by the illusion configurations on which it was superimposed. However, this effect was smaller than that observed in the two tasks of length reproduction. These results support the notion that visual analysis performed on the object of a grasp movement is global and takes into account the object itself, as well as its relationships with surrounding cues. We propose, as suggested previously for reaching movements (Gentilucci, M. et al., Neuropsychologia, 1996, 34, 369-376), two partially independent stages during visuo-motor integration for grasping an object. In the first stage, the object is coded inside an objectcentred frame of reference. In the second stage it is transposed in an egocentric frame of reference, in which the spatial relations between object and agent are computed. In this second stage the influence of cues surrounding the target is minimized. © 1997 Elsevier Science Ltd. All rights reserved Key Words: reaching to grasp; Mtiller-Lyer illusion; grasping task; drawing task; matching task; kinematics; healthy humans.
grasping [4, 7, 13, 15, 17, 25]. Until recently, however, the question as to what processes are used to visually analyse an object in order to grasp it remains an open issue. It seems plausible that the first aim of the visual analysis is the selection of the appropriate opposition axis for the fingers on the object [12]. We can assume that, at first, by means of a rough analysis of the space occupied by the object, the opposition axis aligned along the initial finger orientation is selected. A more detailed analysis of the corresponding portions of the object's surface is then performed in order to determine whether a stable grasp is allowed. When this does not occur (e.g., the two opposite surfaces are not parallel), the adjacent opposition axes and the corresponding object's surfaces are progressively explored, until the appropriate points are located. Their position is computed in three-dimensional space with respect to the body (egocentric frame of reference) and grasping is achieved by carrying each finger to the appropriate contact point on the target object. According to this hypothesis, a global visual analysis of the object does not necessarily take place. Alternatively, we can suggest that a global visual analysis is performed on the object in order to grasp it. We define as global a process that implies visual analysis of the whole object, and the visual relations between the object and the surrounding scene. It is possible that information extracted from the knowledge of object's use and
Introduction
Although we perform it daily, reaching to grasp an object (prehension) is by no means a simple m o t o r act. For this m o t o r act to be successful, the hand must be carried to the appropriate location in space, and the fingers must be shaped according to the orientation, size and shape of the target object. On the basis of behavioural studies [14, 15] the act of prehension can be divided in two components: the transport (or reach) and the grasp. It has been hypothesized that, whereas the transport component is more concerned with visual computation of the spatial relationships between the target object and the body, the grasp component depends on intrinsic object properties, such as size and shape [2, 15]. Independent support for this hypothesis has come from single-neuron recording studies in the premotor [6, 22] and parietal cortices [23, 24]. In particular, the dependence of grasp planning on visual analysis of intrinsic object properties is demonstrated by numerous observations of a strong correlation between hand shaping and object size during
t Address for correspondence: Istituto di Fisiologia Umana, via Gramsci 14, 1-43100 Parma, Italy; fax: 521 291 304; e-mail: [email protected]. 1577
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from relevant semantic features are also involved in this process. Accordingly, the object is primarily analysed inside an object-centred frame of reference, independently of the spatial relations between object and observer. In previous studies we attempted to determine the nature of the visual analysis used in order to localize the target of pointing movements. We reported that when asked to point to the distal vertex of the Mfiller-Lyer illusion [19], subjects misplaced the target according to the presented configuration. This effect was present in movements executed under visual control and it increased for memory-driven movements, as well as in conditions by which the efficiency of the egocentric system was reduced [8]. We argued that subjects acquired information on target location by means of a global analysis of the figure in allocentric coordinates (as demonstrated by the illusion effectiveness), and successively transposed the target in egocentric coordinates (as suggested by the modulation of the illusion effect). It has been shown previously that the visual analysis of the object in order to plan a grasping movement is different from and independent of that of the complementary reaching movement [7, 15]. In spite of this, a similar mechanism of visual elaboration can be employed to code the object of a grasping movement. That is, the object can be encoded in an object-centred frame of reference and successively transposed in an egocentric one. In order to test this possibility, we devised an experimental paradigm similar to that used previously [8], in which subjects were required to grasp a three-dimensional bar superimposed over the shaft of the Maller-Lyer illusion. We predicted that, if only a sequential analysis
of portions of the object coded in an egocentric frame of reference is performed in order to select the opposition points for the fingers, the grasping kinematic parameters should not be affected by the illusion. On the contrary, if at first a global analysis of the object is carried out, the illusion effect should affect the grasping movement. A successive transposition of the object in an egocentric frame of reference should reduce the effect of the illusion. Two control tasks were carried out in order to check the perceptual effectiveness of the illusion configurations.
Methods Eight right-handed subjects (two males and six females: age 20-31 years) participated in the present study. All were naive as to the purpose of the experiment. The subjects sat in front of a table with their right hand resting on its surface. The starting position was a disc located on the plane of the table along the subject's sagittal plane at 25 cm from his/her frontal plane. According to the task (see below), at the beginning of each trial the subjects placed on the starting position either the point of a pencil held in their right hand (drawing task), or their thumb and index finger, held in pinch position (grasping and matching tasks). Stimuli were displayed on an easel located on the plane of the table, 35 cm from the subject's frontal plane and inclined by 4 5 with respect to the plane of the table. Stimuli were the open and closed configurations of the M~iller-Lyer illusion (Fig. 1). A non-illusory configuration in which horizontal lines replaced either the open or the closed wings was used as a control condition. Lines were 1 cm wide and were drawn in black ink on the centre of white panels. The shaft of the configuration lay approximately on the subject's sagittal plane and could be either 5, 6 or 7 cm long. The ratio between wings and shaft length was 0.3. A black wooden bar
Fig. 1. Experimental set-up. The upper and lower rows show the apparatus for presentation of the closed and open configurations, respectively.
E. Daprati and M. Gentilucci/Grasping an illusion of length equal to the shaft, and height of 1 cm, was superimposed over each shaft and fixed to the easel by means of a magnet. Only the shaft was a three-dimensional object. Wings were two-dimensional figures in order to avoid them becoming an obstacle for the fingers during grasping movements. The distance of the centre of the shaft from the starting position was approximately 31 cm. Three experimental sessions were run, separated by intervals of at least I week. The tasks run in each session and the order of their presentation were as follows.
Session 1: drawing task Subjects were required to draw a vertical line of the same length as the three-dimensional bar presented on the easel. Inspection of the panel lasted for 5 sec, then the go signal was given by a sound controlled by a PC. Vision of the stimulus was allowed while drawing. On the contrary, vision of the hand and the sheet on which the line was drawn was prevented by a box covering them both.
Session 2. grasping task Subjects were required to reach and grasp the extremities of the three-dimensional bar presented on the easel with their right thumb and index finger (Fig. 1). The procedure was the same as in the drawing task, but full vision of both the stimulus and the hand was allowed. The subjects were instructed to move as naturally as possible, with the same velocity as during spontaneous movements. Removal of the bar from the easel was easy because the strength of the magnet was weak.
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the subject's sagittal plane 8 cm from the edge of the table and was used as a reference point. In the grasping task, the time course of the distance between the two markers positioned on the thumb and index finger was used in order to study the grasping component of the movement. Maximal finger aperture, time to maximal finger aperture, peak velocity of finger aperture and grasp time were the analysed parameters. The marker placed on the wrist was used to study the transport component. Transport time, peak velocity and deceleration time (expressed as percentage of total movement time) were measured. The distance between the markers positioned on the thumb and the index finger was used in order to measure finger aperture in the matching task. In both grasping and matching tasks, the distance between the two markers at the starting position was subtracted from that recorded at maximal finger aperture in the grasping task and at finger aperture in the matching task in order to obtain the distance between the finger palmar surfaces. All parameters measured in the three tasks were submitted to separate analyses of variance (ANOVAs) whose within-subject factors were bar length (5 cm vs 6 cm vs 7 cm) and configuration (open vs closed vs control configuration). We evaluated the task's effect on the visual analysis of the bar length by comparing the illusion effect on the following parameters: drawn line length (drawing task), maximal finger aperture (grasping task) and finger aperture (matching task). We measured variation of these parameters for presentation of the open and closed configurations with respect to the control configuration. These values were submitted to an ANOVA, whose withinsubject factors were task (drawing vs matching vs grasping), bar length (5 cm vs 6 cm vs 7 cm) and configuration (open vs closed configuration). The Newman Keuls test was used as post-hoe test in all analyses.
Results
Session 3: matching task Drawin 9 task Subjects were required to match the size of the three-dimensional bar presented on the easel by opening their right thumb and index finger. The subjects were allowed to lift their hand, but not to approach the object during the trial. The procedure was the same as in the grasping task. The aim of the first session was to establish whether the modified configurations of the Mt~ller-Lyer figures would be effective in inducing an illusion. The long interval between two successive sessions was chosen in order to minimize practice effects. Presentation order of the second and third task was selected in order to prevent the perceptual judgement performed in the matching task from influencing visuo-motor integration in the grasping task. In all tasks, the subjects were required not to pay attention to the two-dimensional drawing upon which the bar was superimposed. In each session, 84 trials were run. That is, seven trials for each stimulus length (5, 6, 7 cm) and configuration (open, closed, control) were presented in pseudo-random order. In the drawing task, the length of the drawn lines was manually measured by the experimenters at the end of the session. Resolution was approximately 2 mm. Finger aperture in matching the bar length and the kinematics of reaching~rasping movements were recorded using the ELITE system (B.T.S., Milan, Italy). Apparatus, movement reconstruction and data elaboration procedures are described elsewhere [10]. The spatial resolution of the ELITE system is 0.4mm [10]. Four markers were used. The first marker was placed on the styloid process of the radius at the wrist. The second and third markers were positioned on the nails of the thumb and the index finger, respectively. The fourth marker was placed on the table along
D r a w n lines were straight, and varied significantly a c c o r d i n g to the three b a r lengths [F(2,14)=142.28, P < 0 . 0 0 0 0 1 ; 31 . S m m vs 3 8 . 0 m m vs 45.0 mm]. H o w e v e r , subjects always u n d e r e s t i m a t e d the length o f the b a r (Fig. 2, u p p e r panel). D r a w n lines were clearly affected by the illusion effect [F(2,14) = 9.96, P < 0.002]. T h a t is, a longer line was d r a w n when the open c o n f i g u r a t i o n (39.4 mm) was presented with respect to the c o n t r o l (38.0 m m ) a n d the closed configurations (37.0 mm) ( P < 0.05).
Grasping task Subjects o p e n e d their fingers progressively to a maxim a l finger a p e r t u r e a n d then closed on the object (Fig. 3). Maximalfinger aperture was correctly scaled a c c o r d i n g to the length o f the b a r to be g r a s p e d [57.6 m m vs 64.7 m m vs 72.9 mm; F(2,14) = 201.20, P < 0.00001 ]. This p a r a m e t e r was significantly influenced by the illusion effect [ F ( 2 , 1 4 ) = 4 . 8 1 , P < 0 . 0 5 ] , a l t h o u g h the effect was small. Subjects o p e n e d their fingers wider when the open (65.5 m m ) a n d the c o n t r o l (65.2 m m ) configurations were p r e s e n t e d with respect to the closed c o n f i g u r a t i o n ( 6 4 . 5 m m ) (Fig. 2, lower panel, P < 0 . 0 5 ) . Peak velocity
E. Daprati and M. Gentilucci/Grasping an illusion
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offinyer aperture, like maximal finger aperture, was greater for presentation of the open (258.4 mm/sec) and the control (257.5 mm/sec) configurations with respect to the closed configuration (248.8 mm/sec). However, this effect did not reach significance. The remaining kinematic grasp and transport parameters were not influenced by the illusion.
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Fig. 3. Representative examples of time course of grasping the extremities of the closed configuration (dotted lines), of the control configuration (continuous lines) and of the open configuration (dashed lines) (subject CM.). One trial for each of the three shaft length conditions is represented.
Across-tasks comparisons As expected, a significant effect of the illusion was found [F(1,7)=18.12, P<0.005; closed configuration 1.4 ram, open configuration 0.9 ram]. In addition, there was a significant interaction between task and configuration [F(2,24) = 3.79, P < 0.05]. Post-hoc comparison showed that the effect of the closed configuration was significantly different in the matching task ( - 2 . 5 r a m ) with respect to the two other tasks (drawing task - 1 . 0 r a m , grasping task - 0 . 7 m m ; P<0.05). No significant difference was observed for the open configuration among the three tasks (drawing task 1.4 mm, grasping task 0.2 mm, matching task 1.1 ram).
Discussion
Matching task Finger aperture was approximately scaled according to the presented bar length [52.7 mm vs 61.7 mm vs 77.0 ram; F(2,14)=36.79, P<0.00001]. Moreover, it was affected by the illusion [F(2,14) = 6.85, P < 0.008]. Finger aperture was wider for presentation of the open (65.4mm) and the control (64.3 mm) configurations with respect to the closed configuration (61.7ram) (Fig. 2, middle panel, P<0.05).
The effect of the M/iller-Lyer illusion observed in both the drawing and the matching tasks was smaller than that observed in previous experiments in which perceptual judgement on the length of the shafts of the two configurations was required [5, l l, 20, 21]. In the present experiment, the three-dimensional shafts had large width and therefore they did not produce blurring effects at the vertices of the configurations [3]. Thus, the optical component of the illusion was removed and only effects
E. Daprati and M. Gentilucci/Grasping an illusion arising from visual analysis of figure shape could be elicited. It is known that we commonly use shape as an indicator of size. In the drawing task, the length of all figures was underestimated. This finding is likely a consequence of the lack of visual control while drawing. In order to compensate for this, it is possible that the length of the drawn line was judged on the basis of the time elapsed since movement began. Since it is known that hand movements without visual control of the hand are performed slowly [10, 16], a possible slowing down of the drawing could produce underestimation of the covered distance. The MMler-Lyer illusion influenced maximal finger aperture during grasping. Like finger aperture in the matching task, this parameter was enlarged for presentation of the open and control configurations with respect to the closed configuration. The particular orientation of the wings in the open configuration condition could be an obstacle for the grasping fingers, thus accounting for an increase in maximal finger aperture. However, since only the shaft was a three-dimensional object, whereas the wings of each configuration were two-dimensional figures, we can exclude this possibility. An effect of visual illusions on grasping movements was also found in a previous experiment [1]. In that experiment the two 'Titchener circle' configurations were presented simultaneously and subjects were required to pick up one of the two central discs according to a samedifferent judgement on their relative size. Such a task required that the subjects visually analysed the scene around the two potential target objects. This explains the small effect of the illusion found by the authors. However, due to the instruction given to the subjects, and to the presentation of two configurations at a time, this experiment does not prove whether an analysis of the entire target object and of the surrounding cues occurs for natural grasping movements. The aim of the present experiment was to determine the nature of the visual analysis that is performed in order to grasp an object. We considered two possibilities. The first possibility suggests that a serial analysis of adjacent portions of the object is performed in order to select a stable grasp. That is, the object is not analysed as a whole, but it is fragmented in order to select successive opposition axes, each computed with respect to the observer (egocentric frame of reference). The second possibility suggests that a global visual analysis, that takes into account cues surrounding the target, is first performed on the object. The findings of the present experiment support this second hypothesis. Indeed, not only was the distance between the two extremities of the three-dimensional shaft of the MMler-Lyer figures analysed, as suggested by the first hypothesis, but also the remaining figure (the two wings) responsible for the illusion. Interestingly, the configuration was considered as a whole, although part of it was presented as a twodimensional figure. In other words, a process of visual grouping, by which two-dimensional images were corn-
1581
bined with a three-dimensional object, was used in the visual analysis of the target. This was true even for the motor response. The second hypothesis may also be supported by kinematic data collected on a visual agnosic patient and a healthy control subject, picking up objects of different shapes [18]. In this study the finger opposition axes, recorded on the object at the end of the grasp, frequently passed through its centre of mass. The centre of mass of an object can be established only if the whole object is visually analysed. However, the opposite portions of the object's surface corresponding to the selected finger opposition axes were the only ones that were parallel to each other, allowing a stable contact of the fingers on the object. That is, these data may also support the first hypothesis, which suggests an analysis of opposite surface portions on the object. In the present study the effect of the illusion was smaller in the motor than in the reproduction tasks. This effect was clear when the grasping task was compared with the matching task, and less evident in the comparison with the drawing task. This latter result may be due to the more complex motor response required by the drawing task, since both proximal and distal arm segments are involved in drawing a figure. Such a complex motor response might require a more efficient use of the egocentric frame of reference. This, in turn, might induce greater correction of the illusion (see below). Another possibility is that the effect of the illusion may have been masked by the large underestimation of reproduction amplitudes. Taken together, these data suggest that, when visuomotor integration is performed, the effect of the illusion is partially corrected. In a previous experiment we found that, for pointing movements, the illusion effect is inversely correlated with the efficiency of the egocentric frame of reference [8]. We speculated that the effect of the illusion, due to coding the target in allocentric coordinates, was partially reduced by successive transformation of visual information into motor commands, that is while re-coding the object in egocentric coordinates. Although the visual analysis of the target has been hypothesized as different and independent for reaching and grasping movements [7, 15], the results of the present experiment suggest the possibility of there also being two stages in object visual analysis for planning grasp movements. We propose that in a first stage an object is coded in an object-centred frame of reference, whereas in a second stage it is transposed in an egocentric one. In a previous neuropsychological study [9], we suggested that the parietal lobe is involved in transposing the spatial position of the target of reaching movement from an allocentric to an egocentric frame of reference. On the basis of neurophysiological results [23], we can suggest that an analogous transformation for grasping movements is performed in the same parietal lobe. From the AlP (anterior intraparietal area), an area involved in planning grasp movements, neurons were recorded that
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were activated by object fixation without grasping it ('object' neurons). In addition, other neurons, called 'non-object' neurons, were activated by vision of interaction of the hand with the object. One can speculate that transformation of object coding from an object-centred frame of reference (likely used to code an object by 'object' neurons) to an egocentric frame of reference (used to code an object by 'non-object' neurons) can be performed by concurrent activity of the two types of neurons.
Karnath and P. Thier. Springer, Berlin, 1997, pp. 339-354. 10. Gentilucci, M., Toni, I., Chieffi, S. and Pavesi, G., The role of proprioception in the control of prehension movements: a kinematic study in a peripherally deafferented patient and in normal subjects. Experimental Brain Research, 1994, 102, 483~494. 11. Gillam, G. and Chambers, D., Size and position are incongruous: measurements on the Mfiller-Lyer illusion. Perception and Psvchophysics, 1985, 37, 549556. 12. Iberall, T., Bingham, G. and Arbib, M. A., Opposition space as a structuring concept for the analysis of skilled hand movements. In Generation and Modu-
Acknowledgements--We thank Dr P. Dominey for valuable comments on the manuscript, and Drs M. Gangitano, G. Pagnoni and I. Toni for discussion of data. This work was supported by a Research Grant from the Human Frontier Science Program, by a grant from the European Neuroscience Program and by grants CNR (Centro Nazionale delle Ricerche) to the Institute of Human Physiology of Parma and MURST (Ministero dell'Universit/t e della Ricerca Scientifica e Tecnologica) to the Institute of Human Physiology of Parma and to M.G.
lation of Active Patterns, Evperimental Brain Research Series, Vol. 15, ed. H. Heuer and C. Fromm. Springer, Berlin, 1986, pp. 158-173. Jakobson, L. S. and Goodale, M. A., Factors affecting higher-order movement planning: a kinematic analysis of human prehension. Experimental Brain Research, 1991, 86, 199 208. Jeannerod, M., Intersegmental coordination during reaching at natural visual objects. In Attention and Performance, ed. J. Long and A. Baddeley. Erlbaum, Hillsdale, NJ, 1981, pp. 153-168. Jeannerod, M., The timing of natural prehension movements. Journal of Motor Behaviour, 1984, 16, 235-254. Jeannnerod, M., The Neural and Behavioural Organization of" Goal-directed Movements. Oxford University Press, Oxford, 1988. Marteniuk, R. G., Leavitt, J. L., MacKenzie, C. L. and Athenes, S., Functional relationships between grasp and transport component in a prehension task. Human Movement Science, 1990, 9, 149 176. Milner, A. D. and Goodale, M. A., The Visual Brain in Action. Oxford University Press, Oxford, 1995. M~Jller-Lyer, F. C., Dubloid-Reynolds Archive Ffir Anatomie und Physiologie (suppl.), 1889, pp. 263 270. Prinzmetal, W. and Gettleman, L., Vertical-horizontal illusion: one eye is better than two. Perception and Psvchophysics, 1993, 53, 81-88. Restle, F. and Decker, J., Size of the Maller-Lyer illusion as a function of its dimension: theory and data. Perception and Psychophysics, 1977, 21, 489 503. Rizzolatti, G., Camarda, R., Fogassi, L., Gentilucci, M., Luppino, G. and Matelli, M., Functional organisation of inferior area 6 in the macaque monkey. II. Area F5 and the control of distal movements. Experimental Brain Research, 1988, 71,491 507. Sakata, H., Taira, M., Murata, A. and Mine, S., Neural mechanisms of visual guidance of hand action in the parietal cortex of the monkey. Cerebral Cortex, 1995, 5, 429~438. Taira, M., Mine, S., Georgopoulos, A. P., Murata, A. and Sakata, H., Parietal cortex neurones of the monkey related to the visual guidance of hand movements. Experimental Brain Research, 1990, 83, 29 36. Wing, A. M. and Fraser, C., The contribution of the thumb to reaching movements. QuarterO' Journal of Experimental Psychology, 1983, 35A, 297-309.
References
1. Aglioti, S., De Souza, J. F. X. and Goodale, M. A., Size-contrast illusions deceive the eye but not the hand. Current Biology, 1995, 5, 679-685. 2. Arbib, M. A., Perceptual structures and distributed motor control. In Handbook of" Physiology, Section 1, Vol. 2, Part 2, ed. V. B. Brooks. William & Wilkins, Baltimore, 1981, pp. 1449-1480. 3. Chaing, C. A., A new theory to explain geometrical illusions produced by crossing lines. Perception and Psychophysics, 1968, 3, 174-176. 4. Chieffi, S., Fogassi, L., Gallese, V. and Gentilucci, M., Prehension movements directed to approaching objects: influence of stimulus velocity on the transport and the grasp components. Neuropsvchologia, 1992, 30, 877-897. 5. Festinger, L., White, C. W. and Allyn, M. R., Eye movements and decrement in the Maller-Lyer illusion. Perception and Psychophysics, 1968, 3, 378382. 6. Gentilucci, M. and Rizzolatti, G., Cortical motor control of arm and hand movements. In Vision and Action: The Control of Grasping, ed. M. A. Goodale. Norwood, New Jersey, 1990, pp. 147-162. 7. Gentilucci, M., Castiello, U., Corradini, M. L., Scarpa, M., Umiltfi, C. and Rizzolatti, G., Influences of different types of grasping on the transport component of prehension movements. Neuropsychologia, 1991, 29, 361 378. 8. Gentilucci, M., Chieffi, S., Daprati, E., Saetti, M. C. and Toni, I., Visual illusion and action. Neuropsychologia , 1996, 34, 369 376. 9. Gentilucci, M., Daprati, E., Saetti, M. C. and Toni, I., On the role of the egocentric and allocentric frame of reference in the control of arm movements. In
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