Dynamic Illusion Effects in a Reaching Task: Evidence for Separate Visual Representations in the Planning and Control of Reaching

Transcription

1 Journal of Experimental Psychology: Human Perception and Performance 2001, Vol. 27, No. 3, Copyright 2001 by the American Psychological Association, Inc. 0O /O1/J5.0O DOI: //OO O Dynamic Illusion Effects in a Reaching Task: Evidence for Separate Visual Representations in the Planning and Control of Reaching Scott R. Glover and Peter Dixon University of Alberta The effects of an orientation illusion on perception and 2 different actions were investigated. An 8-cm X 2-cm cylindrical bar was placed in front of participants at various orientations. A background grating was used to induce an orientation illusion. In a perception task, the illusion affected participants' ability to align the bar with their sagittal planes. In one reaching task, a similar effect of the illusion was found on the choice between 2 possible grasping postures. In a second reaching task involving a single grasping posture, the orientation illusion affected the orientation of the hand at the beginning of the reach but not near its end. The authors argue that reaching trajectories are planned and initiated through a contextdependentrepresentationbut are corrected on-line through a context-independent representation. The relation of this model to a more general dichotomy between perception and action is discussed. Context-induced optical illusions have often been found to have smaller effects on motor performance than they have on perceptions. In the present article, we present evidence that, in at least one case, the effects of an illusion are actually found early in a movement trajectory but largely disappear by the time the target is reached. We propose that these results, as well as many others in the literature, can be explained by assuming that the visual context surrounding the target is considered in movement planning but that the context is largely ignored during the on-line control of movements. In particular, we propose that context-induced illusion effects on action may be weak or absent because on-line control mechanisms can correct for illusion effects that occurred during the planning of the movement. In the following, we discuss some of the evidence regarding the effects of context-induced illusions on action and elaborate on how a "planning-control" model would account for these results. This view is contrasted with a commonly held distinction between the visual representations underlying perception and action. Illusion Effects and Action The interaction of context-induced illusions and action has been studied in a variety of paradigms. For example, actions such as pointing, grasping, and eye movements have been found to be relatively insensitive to the Ebbinghaus size-contrast illusion (Aglioti, de Souza, & Goodale, 1995; Haffenden & Goodale, 1998; Scott R. Glover and Peter Dixon, Department of Psychology, University of Alberta, Edmonton, Alberta, Canada. This research was supported by a scholarship and research grant from the Natural Sciences and Engineering Research Council of Canada. We gratefully acknowledge the assistance of Maria Kotovych and Isaac Lank. We also wish to thank Howard Zelaznik and two anonymous reviewers for their helpful and insightful comments on earlier versions of this article. Correspondence concerning this article should be addressed to Scott R. Glover, Department of Psychology, University of Alberta, Edmonton, Alberta T6G 2E9, Canada. Electronic mail may be sent to glover ualberta.ca. but see Franz, Gegenfurtner, Bulthoff, & Fable, 2000; Pavani, Boscagli, Benvenuti, Rabuffetti, & Fame, 1999), the Muller-Lyer illusion of extent (Daprati & Gentilucci, 1997; Gentilucci, Chieffi, Daprati, Saetti, & Toni, 1996; Otto-de Haart, Carey, & Milne, 1999; Westwood, Heath, & Roy, 2000), and Roelef s induced motion effect (Bridgeman, Lewis, Heit, & Nagle, 1979; Bridgeman, Perry, & Anand, 1997; Wong & Mack, 1981). A common feature of these illusions is that the perception of a target stimulus is affected by the nature of the surrounding visual context. As such, these context-induced illusions can be considered to be distinct from other types of visual distortions such as exposure aftereffects (e.g., McCollough effect), saccadic suppression (e.g., Goodale, Pelisson, & Prablanc, 1986; Hansen & Skavenski, 1985), or system deficiency (e.g., color blindness). Although there is substantial evidence indicating that actions can be relatively unaffected by optical illusions, significant effects of illusions on actions have been found under some circumstances. For example, Smeets and Brenner (1995) found that the reaction times and movement times of a striking movement were affected by a velocity illusion. Brenner and Smeets (1996) found that lifting force was affected by a size illusion. Jackson and Shaw (2000) found that a size illusion also affected the force applied to the sides of an object in grasping it. Several studies (Bridgeman et al., 1997; Gentilucci et al., 1996; Westwood et al., 2000; Wong & Mack, 1981) have shown that illusion effects on actions were larger when the movements were performed without visual feedback 2 or more seconds after vision of the target was removed than when no such delay was imposed. A Planning-Control Model We suggest that this pattern of illusion effects and noneffects on action can be explained by distinguishing between the visual representations used to plan and control actions. The distinction between planning and control has been central in the history of research on motor performance (Jeannerod, 1988; Woodworth, 1899) and seems important to consider in an account of illusion effects on actions. In our approach, we assume that separate 560

2 DYNAMIC ILLUSION EFFECTS IN REACHING 561 processing modules underlie planning and control and that each module uses a distinct visual representation. Here we discuss how planning and control may operate in simple reaching and grasping movements, the behavior studied in the current research. We hypothesize that plans for the kinematics and trajectories of reaching and grasping movements are generated before movement initiation. This planning involves a visual representation that incorporates the visual array surrounding the target. Plans for reaching are based on a comparison of the current visual information with that from past experiences of reaching to similar targets in similar circumstances (for a computational model based on similar assumptions, see Rosenbaum, Loukopoulos, Meulenbroek, Vaughan, & Engelbrecht, 1995). Once a plan for the reach and grasp has been formed, a copy is sent to the control module. The control module uses this efference copy, along with proprioceptive and visual feedback, to monitor and adjust the reach on-line. Crucially, we assume that the visual information used by the control module is largely independent of the context. For example, the control module may use information concerning the distance and orientation of the target that is calculated without regard to the surrounding visual array. This context independence allows it to correct in flight many of the effects of context-induced illusions on actions. We also assume that the control module's representation of the target is limited to its spatial characteristics, such as its size, shape, and orientation. As a consequence, the control module is only able to correct aspects of actions related to the target's spatial characteristics (e.g., amplitude of reach and shaping of the hand). Conversely, the control module does not possess information regarding nonspatial characteristics of the target, such as its weight, fragility, or temperature. Thus, the control module is not able to correct aspects of action related to the target's nonspatial characteristics, for example the force applied in grasping an object (which will depend on its weight, texture, or hardness). Furthermore, we assume that the visual information used by the control module is short lived. As a consequence, illusion effects on action will remain uncorrected if the visual stimulus is removed 2 or more seconds before the initiation of the movement, because the context-independent representation of the control module will have decayed in the interval. A crucial difference between the visual information used by the planning and control modules is the dependence on context. The representation used by the planning module is assumed to be context dependent. In particular, the representation of spatial characteristics of the target in the planning module depends at least in part on the relationship between the target and the surrounding context. For example, size might be encoded in terms of the number of subtended texture elements (cf. Gibson, 1979), or orientation might be encoded relative to the horizon or other canonical features of the environment (Asch & Witkin, 1948). The effects of optical illusions arise because calculation of the characteristics of the target based on such relational considerations does not always produce veridical results (Gregory, 1968). In contrast, the representation of the target used by the control module is assumed to be largely independent of the context. As such, this representation of the target might be limited to those characteristics of the target that are relevant to the spatial precision of the movement and might be encoded in terms of the parameters of that movement. Because calculation of such a representation would not use the relationship between the target and the environment, it would be unaffected by variations in the background or surrounding objects in the visual field. In short, in the planning-control model, movements are planned and initiated through a context-dependent visual representation that is subject to optical illusions. During the execution of the movement, errors related to the spatial characteristics of the target can be corrected on-line through a combination of contextindependent visual information, an efference copy of the movement plan, and proprioceptive feedback. Thus, aspects of the action related to the spatial characteristics of the target (such as size) are relatively unaffected by illusions because the errors induced by illusions are corrected on-line. However, aspects of actions dependent on nonspatial target properties (such as weight) cannot be corrected by the control module and, as a consequence, are susceptible to large illusion effects. The Perception-Action Model In contrast to the planning-control model, several authors have argued that the relatively weak effects of optical illusions on motor performance are due to the characteristics of independent visual processing modules underlying perception and action (Aglioti et al., 1995; Bridgeman et al., 1997; Haffenden & Goodale, 1998; Milner & Goodale, 1995). This "perception-action" model is based on the separation of visual processing in the monkey cortex into dorsal and ventral streams (Mishkin, Ungerleider, & Macko, 1983) and their putative homologues in the human brain. The ventral stream is assumed to constitute a "perception" module that provides a context-dependent representation of the target. Presumably, such a representation aids in identifying objects and attaching significance to them. Conversely, the dorsal stream is assumed to constitute an "action" module that provides a representation of the target that is largely independent of the context. This contextindependent representation allows for accurate action production (Aglioti et al., 1995; Milner & Goodale, 1995). According to the perception-action model, most actions are less affected by contextinduced illusions because they are planned and controlled with less reference to the context. However, according to the perception-action model, illusions may affect actions when the perception and action modules interact. Such an interaction would be necessary, for example, when knowledge of the target's identity must be used to plan the movement. An interaction would also be necessary when actions are initiated 2 or more seconds after the offset of the visual stimulus because the visual representation underlying action decays quickly, and actions must rely on the more durable perceptual representation under such conditions (Milner & Goodale, 1995). Otherwise, however, both the planning and control of movements make use of the context-independent representation in the action module. Comparing the Planning-Control and Perception-Action Models Several similarities exist between the planning-control and perception-action models. Both are based on the assumption of two visual representations, one providing a context-dependent representation of the target and the other largely ignoring the context. In both models, illusions should affect actions that are

3 562 GLOVER AND DIXON mediated by nonspatial characteristics of the target, including aspects of actions dependent on the target's weight, the estimation of which requires an integration of size and density information. Furthermore, in both models illusion effects are expected to increase after a delay of 2 s or more. On the basis of these predictions, both models are consistent with a large number of findings in the literature (e.g., Aglioti et al, 1995; Brenner & Smeets, 1996; Bridgeman et al., 1997; Haffenden & Goodale, 1998; Jackson & Shaw, 2000). However, the models provide critically different views of how actions are planned. In the planning-control model, it is assumed that all aspects of a plan for an upcoming action are based on a context-dependent visual representation and are thus susceptible to illusions. Only after a movement has been initiated can the control module correct errors in planning caused by illusions, and this correction process can be applied only to aspects of the movement related to the spatial characteristics of the target. In contrast, proponents of the perception-action model make no such distinction between illusion effects on planning versus control. Rather, it is assumed that aspects of movement related to the spatial characteristics of the target are both planned and controlled through a context-independent visual representation and, as such, should be relatively immune to illusions. We believe that the planning-control model provides a better account of the pattern of illusion effects on action than does the perception-action model. For example, many indexes of action that might plausibly reflect planning processes, such as reaction times (Smeets & Brenner, 1995) and movement times (Gentilucci et al., 1996; Smeets & Brenner, 1995; van Donkelaar, 1999), are significantly affected by context-induced illusions. Furthermore, the availability of visual feedback of the hand and target results in smaller effects of illusions on action than in conditions in which visual feedback is not available (Gentilucci et al., 1996; Westwood et al., in press), suggesting that visual feedback is used in correcting illusion effects on movement trajectories. These findings would not be predicted on the basis of the perception-action model. However, many studies do not bear on the distinction between the planning-control and perception-action models because they are based on indexes of action that occur at or near the end of the movement. For example, measuring the final accuracy of a pointing movement (Bridgeman et al., 1979, 1997) leaves open the possibility that the context-induced illusion may have affected the planning of the movements but that these effects were corrected in flight. Similarly, measuring the maximum grip aperture (Aglioti et al., 1995; Daprati & Gentilucci, 1997; Franz et al., 2000; Haffenden & Goodale, 1998; Otto de-haart et al., 1999; Pavani et al., 1999; Westwood et al., 2000), a kinematic marker that occurs well into the second half of the movement (Jakobson & Goodale, 1991; Jeannerod, 1984), allows the possibility that participants compensated for context-induced illusion effects during the early part of the reach. Although some of these studies have eliminated visual feedback during the movement (Bridgeman et al., 1979, 1997; Haffenden & Goodale, 1998), this does not rule out the correction of illusion effects in flight. On-line corrections commonly occur in both the presence and absence of visual feedback (Goodale et al., 1986; Khan, Franks, & Goodman, 1998), indicating that corrections can also be based on mechanisms such as proprioception or efference copy. Overview of Experiments In the present research, we evaluated the effect of a contextinduced orientation illusion on reaching movements and attempted to disentangle the effects on the initial planning of the reach from the effects on the movements' subsequent control. Participants reached out and grasped a small bar lying on a table in front of them. Underneath the bar was a background grating. An orientation illusion was induced by misaligning the grating slightly with the participant's sagittal plane. This illusion is illustrated in Figure 1. When the grating was turned 10 clockwise from sagittal, the orientation of the bar appeared slightly more counterclockwise than it actually was, and vice versa. An illusion of this form wouldbe produced if the grating were used as an index of the orientation of the sagittal plane. This orientation illusion has a potential effect on the action of reaching and grasping the bar because the orientation of the hand must be aligned to some extent with the orientation of the bar. Two experiments were conducted with this paradigm. In Experiment 1, the bar was positioned in a range of orientations that encouraged participants to adopt one of two postures, depending on the orientation of the bar. The two postures are illustrated in Figure 2. When the bar is oriented 35 clockwise from the sagittal plane (left panel, Figure 2), it is more comfortable for right-handed participants to grasp the bar using a hand-abducted (thumb-right) posture. However, when the bar is oriented 5 clockwise from sagittal (right panel, Figure 2), it is more comfortable to grasp the bar using a hand-adducted (thumb-left) posture. In a critical range between these two extremes, the probability of using the handadducted posture increases as the bar is rotated further counterclockwise. In effect, posture choice as a function of the target orientation constitutes a psychophysical function in which the probability of using one posture increases (or decreases) as the orientation of the target is systematically changed. Such patterns of effects have been found in a variety of studies (Kelso, Buchanan, & Murata, 1994; Rosenbaum et al., 1990; Rosenbaum, Vaughan, Jorgensen, Barnes, & Stewart, 1992; Short & Caraugh, 1997; Stelmach, Castiello, & Jeannerod, 1994). By analogy to threshold psychophysics, then, the empirical question in Experiment 1 was whether the orientation illusion would affect the threshold at which Figure 1. The orientation illusion used in Experiments 1 and 2. On the left, the background grating is oriented at 10 clockwise from sagittal; on the right, the grating is oriented at 10 counterclockwise from sagittal. Both bars are oriented at 0, yet the bar on the left is perceived to be rotated slightly counterclockwise from vertical, and the bar on the right is perceived to be rotated slightly clockwise from vertical.

4 DYNAMIC ILLUSION EFFECTS IN REACHING 563 Hand Abducted Hand Adducted Two possible outcomes are of interest. According to the planning-control model, the illusion should have comparable effects on the posture choice and adjustment tasks because both perceptual judgment and the planning of postures are based on context-dependent visual representations that are affected by context-induced illusions. Conversely, according to the perceptionaction model, because the action module both plans and controls aspects of movements related to the spatial characteristics of the target (including its orientation), the choice of grasping posture should reflect the operation of the action module using a contextindependent visual representation. As a consequence, according to the perception-action model, posture choice should be unaffected by the illusion. Method Figure 2. The two grasping postures allowed in Experiment 1. On the left, a hand-abducted (thumb-right) posture is being used to grasp a bar oriented at 35 clockwise from sagittal. On the right, a hand-adducted (thumb-left) posture is being used to grasp a bar oriented at 5 clockwise from sagittal. participants switch postures from hand abduction (thumb right) to hand adduction (thumb left). In Experiment 2, the orientation of the hand was measured while participants reached to grasp the bar. The orientation of the hand is normally dependent on the orientation of the target, even early in the trajectory (Desmurget et al., 1996). Consequently, hand orientation provides an index of the effect of the illusion on action throughout the reach and can be used to assess the time course of such effects. In both experiments, the magnitude of the orientation illusion in perceptual judgments was assessed in a separate task that did not involve grasping the bar. To anticipate, both experiments provided results that support the planning-control model but would be difficult to incorporate within simple versions of a perception-action model. Experiment 1 In Experiment 1, we asked whether the orientation illusion would affect the posture participants used in picking up the bar. We anticipated that the choice of posture would be made before movement initiation and that it would be relatively awkward to change in flight. Therefore, we reasoned that the choice of posture would provide an index of planning that was relatively uncontaminated by on-line control processes. For comparison, we also measured the effects of the orientation illusion on perceptual judgments using an adjustment task in which participants aligned the bar with their sagittal planes. Participants. Ten University of Alberta undergraduates participated in the study in exchange for course credit. Each participant performed both the posture choice task and the adjustment task, with the order of presentation counterbalanced across participants. All 10 participants were righthanded, had normal or corrected-to-normal vision, and were naive with respect to the research hypotheses. Apparatus. The arrangement of the experimental apparatus is shown in Figure 3. The bar and the background grating were arranged on a tabletop 43 cm from the participant's edge of the table. The participants viewed the stimulus display through a two-way mirror. The mirror was mounted in a frame that restricted their view to a rectangular area surrounding the target bar. The visible region measured 21.5 cm in width and 15 cm in depth, corresponding to approximately 23 horizontal and 16 vertical of visual angle. The experimenter controlled the participant's ability to see the stimulus display by illuminating either the participant's side of the twoway mirror or the tabletop. The latter was illuminated by four floodlights positioned at the corners of the table (see Figure 3). When the tabletop was visible, its illumination was 0.9 cd/m 2 (measured from the participant's side of the two-way mirror). An adjustable chair and a headrest were used to keep the participant's eyes 42 cm above the platform and 65 cm from the target. When the tabletop was not visible, participants saw only the reflection of the white ceiling in the glass of the two-way mirror. The illusion-inducing background was a 15-cm paper disc on which a high frequency square-wave grating was printed. The grating frequency was 3.92 cycles/cm, or 4.19 cycles/degree at the participant's viewing distance. The target bar was an 8-cm X 2-cm white wooden dowel. The bottom of the bar was flattened slightly to prevent it from rolling,, Adjustment task procedure. For the adjustment task, a sheet of clear plastic film, cut to the size of the grating disk, was interleaved between the grating and the bar. A 12-cm plastic strip extended from the right edge of the sheet and was used as a handle to rotate the sheet and the bar resting on it around their center. A trial began when the experimenter illuminated the tabletop, making the two-way mirror transparent. The participant's task was to align the bar with his or her sagittal plane using the handle. Once the participant was satisfied that the bar was aligned, the experimenter reversed the lighting to once again block the participant's view through the mirror, measured and recorded the orientation of the bar, and set up the next trial. Two grating orientations were used. On half of the trials, the grating was oriented 10 clockwise from the participant's sagittal plane (grating +10 condition); on the other half, it was oriented 10 counterclockwise (grating 10 condition). For each grating orientation, the bar was placed initially at one of 10 different orientations, ranging from 25 to 25 in 5 steps but excluding 0. Each participant completed one trial for each combination of bar and grating orientation in a random order (a total of 20 trials). Posture choice procedure. The task was to pick up the bar using only the pads of the right thumb and forefinger with either an abducted (thumbright) or adducted (thumb-left) hand posture. Participants were instructed to start each trial in the same position: The hand was to be loosely clenched and positioned so that the wrist was just supported by the tabletop and the knuckles pointed in the direction of the target bar. A pencil outline of the participant's hand was drawn on the tabletop to promote consistency. Participants could see their wrist and about half of their hand in this position under the lower edge of the mirror mounting frame. A trial began when the experimenter illuminated the tabletop, allowing the participant a view of the stimulus display. Two seconds after the table had been illuminated, a tone was sounded. The participant was allowed to move anytime after the tone. Speed was not emphasized. Vision of the moving

5 564 GLOVER AND DIXON hand was occluded by the apparatus for roughly two thirds of the distance from the starting position to the target bar. Six practice trials were run before the test trials to ensure that participants understood the task. The posture choice task involved the same two grating orientations as the alignment task (±10 ). The bar was oriented at one of seven orientations ranging from 5 to 35 clockwise from sagittal in 5 steps. Each participant completed two blocks of 84 trials separated by a short rest. In each block, there were 6 trials with each combination of grating and bar orientation in a random order. Half of the participants performed the posture choice task before the adjustment task; the other half performed the adjustment task first, followed by the posture choice task. A Latin square design was used to determine task order. Data analysis. For the adjustment task, the dependent variable was the orientation of the bar (in degrees clockwise from sagittal) selected by the participant on each trial. The effect of the orientation illusion was assessed by comparing the fit of two linear models: a null model that included only a factor for participants and an alternative model that also included an effect of grating orientation. The relative fit of the two models was evaluated by computing the maximum-likelihood ratio, that is, the likelihood of the data given one model divided by the likelihood of the data given the other model. The likelihood ratio provides an index of the strength of the evidence for one model relative to the other (Goodman & Royall, 1988). The likelihood of the data for a linear model with normally distributed error is closely related to the residual variance not predicted by the model, and the likelihood ratio, A, can be computed as follows (Dixon & O'Reilly, 1999): A = [(1 - /Jg)/(1 - R 2 )]"' 2, where R 2, and Rj reflect the fit of the null and alternative models, respectively, and n is the number of independent observations. For the repeated measures design used here, n = s df, where s is the number of participants and df is the total degrees of freedom among the conditions. In a design such as the present one, the likelihood ratio thus obtained is closely related to the p value found by testing the hypothesis of no effect of background (Dixon, 1998), and, if one were to do null hypothesis significance testing, the null hypothesis would be rejected if the likelihood ratio was 10 or greater. The present approach to comparing models was adopted because of the many well-known logical flaws and interpretational difficulties involved in the more common practice of null hypothesis significance testing (cf. Cohen, 1994; Loftus, 1993). For example, one can reject the null hypothesis on the basis of effects of a trivial magnitude when the power is very Figure 3. Two views of the experimental setup in Experiment 1. i high (e.g., Thompson, 1993). On other hand, when power is weak to moderate, one can easily fail to reject the null hypothesis even when the magnitude of the effect is substantial. Thus, rejecting or failing to reject a null hypothesis, by itself, does not provide an unambiguous indication of whether an interesting effect has been found. It is also sometimes argued that, in many situations, the null hypothesis is known to be technically false a priori, without the collection of any data at all (e.g., Carver, 1978); in this case, the conceptual exercise of rejecting the null hypothesis is meaningless. The present approach, in which likelihood ratios are used simply to describe the strength of the evidence provided by the data, avoids these problems because no artificial "accept-reject" decision is implied. In the posture choice task, the dependent variable was the frequency of using an abducted (thumb-right) or an adducted (thumb-left) posture. These frequencies were fit with log-linear models, a common technique for analyzing categorical data of this sort. A log-linear model predicts the log odds of the response frequencies with a linear function of the independent variables. The best fit is found through an iterative procedure that maximizes the log likelihood of the observed response frequencies under the given model. In this case, we compared a model that predicted posture choice as a function of bar orientation with a model that also included an effect of background grating. The likelihood ratio for this comparison was computed directly from the log likelihoods resulting from the log-linear fit: A = e L^~ u \ where L o and L l are the maximum log likelihoods under the two models. However, because the observations were derived from a repeated measures design, the responses for each participant were not independent. To take this dependence into account, the models also included participants as a variable, and the likelihoods were adjusted by the factor (c l)/c, where c = 14 is the number of conditions. Results A clear effect of grating orientation was found in the adjustment task. When the grating was oriented in one direction, the bar appeared to the participants to be oriented further in the opposite direction than it actually was. The difference between the final adjusted orientation of the bar in the two grating conditions was The likelihood ratio (A) comparing a null model with one that incorporated an effect of background was above 1,000. Thus,

6 DYNAMIC ILLUSION EFFECTS IN REACHING 565 the data were more than 1,000 times more likely given an effect of background than without it. Table 1 lists the data from the adjustment task as a function of grating orientation and starting position. There was no clear evidence of either an effect of starting position or an interaction between starting position and background. The data were fit with a linear model that included different monotonic trends over starting position for each background condition, as well as the overall effect of background. The likelihood ratio comparing this model with one that included only the background effect was This is substantially below the likelihood ratio of 10 that Dixon (1998; Dixon & O'Reilly, 1999; see also Goodman & Royall, 1988) suggested as indicating clear evidence for an effect. An effect of background grating was also apparent in the posture choice task. Figure 4 shows the percentage of trials on which a hand-abducted posture was chosen. As can be seen, when the grating was oriented at -10, the psychophysical function was shifted to the right (more abduction choices) than when it was oriented at 10 clockwise from sagittal. The fit of two models was compared: one that incorporated only effects of participants and bar angle and one that also included an effect of background. The likelihood ratio comparing the fits was Thus, the data were more than 60 times as likely on the assumption that posture choice is affected by the background (as would be predicted by the planning-control model) than on the assumption that posture choice is independent of the background (as would be predicted by the perception-action model). The size of the effect of grating orientation for the posture choice task was calculated by dividing the log-linear model coefficient for the background variable by that for the linear effect of bar angle. The effect of grating orientation in the posture choice task (1.89 ) was quite similar to the effect of grating orientation in the adjustment task (2.01 ). Discussion On the basis of the planning-control model, we predicted similar effects of the orientation illusion on the adjustment and posture choice tasks, and this is what we found. Given that it would be awkward and costly to change the choice of posture in midreach, the choice of posture would probably be determined by the plan- Table 1 Means and Standard Errors (in Parentheses) of Final Bar Orientation (in Degrees Clockwise from Sagittal) in the Adjustment Task of Experiment 1 Starting position + 10 Grating orientation -10 Grating Grating Degrees Degrees fl ff 0 Bar Orientation (degrees CW from sagittal) Figure 4. Percentage of trials on which participants chose the handabduction (thumb-right) posture in the posture choice task of Experiment 1 as a function of grating and bar orientation. The error bar represents the standard error of the log-linear effect of background condition, expressed as a proportion of the effect of bar orientation. CW = clockwise. ning module before the initiation of the movement. Thus, the choice of posture would be made through a context-dependent visual representation. The choice of posture presumably reflects a calculation of the most efficient and comfortable grasping posture based on previous experience and the perceived orientation of the bar. For example, in other tasks, posture choice has been found to correlate highly with reported comfort (Rosenbaum et al., 1990; Short & Caraugh, 1997). According to the perception-action model, on the other hand, one would have predicted a smaller effect of the orientation illusion in the posture choice task than in the adjustment task. Presumably, selecting a grasping posture is a function of the action module, and as such it should be based on a context-independent "action" representation of the target. The fact that the observed effect was comparable to that obtained in the adjustment task indicates that this interpretation of the perceptionaction model must be incorrect. At the very least, the results suggest that the perceptual system makes a substantial contribution to the choice of posture, even if the action module is responsible for the balance of the initial planning of the action as well as its subsequent control (0.71) (0.48) -2.10(0.43) -2.40(0.48) -1.90(0.59) (0.63) -1.80(0.63) -2.10(0.35) (0.63) (0.48) (0.47) (0.68) 0.10(0.31) 0.00 (0.47) (0.40) -0.10(0.46) (0.57) 0.00 (0.42) (0.45) (0.40) Experiment 2 Although the effects found in Experiment 1 are consistent with the planning-control model, a stronger test would be to measure the effect of the orientation illusion on the trajectory of the reaching movement itself. In particular, we reasoned that if planning was affected by the optical illusion, such effects should be apparent in the early part of the movement. However, given that the control module is relatively immune to the optical illusion, the

7 566 GLOVER AND DDCON illusion effects should diminish over the course of the movement. In contrast, a proponent of a perception-action model might expect the effect of the illusion to be consistently small throughout the course of the reach. Using the perception-action model, one would assume that the reaching trajectory is both planned and executed through a representation that is relatively unaffected by the optical illusion. Consequently, there would be little reason to expect a substantial effect of the illusion at any point during the reach, just as there was little reason to expect such an effect in the posture choice task of Experiment 1. To assess the effect of the illusion throughout the movement trajectory, we made use of the fact that the orientation of the hand during a grasp depends on the orientation of the target, even early in the reach (Desmurget et al., 1996). We reasoned that, to the extent that the orientation of the hand is tuned to the orientation of the bar, measuring the orientation of the hand throughout the reach would make it possible to obtain a continuous index of the effect of the illusion on the reaching trajectory. In particular, this method would allow us to assess the effects of the orientation illusion on both the planning and the control aspects of the action: The effects of the illusion on planning would be apparent in the initial portion of the reach, whereas the effects of the illusion on control would be apparent in the latter portion of the reach, as the hand approached the target. Method Participants in Experiment 2 performed the same adjustment task as in Experiment 1, as well as a reaching task in which they were required to pick up the bar with the hand abducted. A single posture was used to ensure that the hand orientation measurements were comparable across bar orientations. The apparatus, procedure, and conditions for the adjustment task were identical to those of Experiment 1. Participants. Ten University of Alberta undergraduates participated in Experiment 2 in exchange for course credit. (Data from 2 of these participants were not used because of equipment malfunction.) All participants were right-handed, had normal or corrected-to-normal vision, and were naive as to the hypothesis under investigation; none had taken part in Experiment 1. Apparatus. The stimulus display for the reaching task was generally the same as that for the posture choice task in Experiment 1, and the position of the participant and the use of the two-way mirror were the same as well. However, an additional apparatus was designed to record the orientation of the hand during the reach. Two infrared light emitting diodes were attached to the back of the hand. To maintain a consistent distance between the two diodes, they were mounted in arigid9-cm X 3-cm plastic case. The case was held to the back of the hand with a strip of elastic fabric that stretched from the ends of the case, around the hand, and across the participant's palm. Masking tape was used to secure it in place. The two infrared diodes were alternately lit at 60 Hz, and the position of the lit diode was captured by an infrared video camera mounted 90 cm above the tabletop. An Iscan eye monitor system, modified to detect the position of the infrared diodes, was used to analyze the video signal in real time. The Iscan system output the position of the lit diode at 60 Hz with a nominal resolution of 1.4 mm horizontally and 2.5 mm in depth; there were no measurable deviations from linearity over the monitoring region. To assess the precision of the system for monitoring movements, we used a method adapted from Haggard and Wing (1990). Two diodes were attached to a plastic card 8 cm apart. The card was swept systematically across different portions of the camera field of view at velocities comparable to those found during reaching (about 0.S m/s). The distance between the diodes was reconstructed from the system output with a standard deviation of 0.62 mm horizontally and 1.07 mm in depth. As a means of maintaining a consistent starting position across trials, participants wererequiredto pinch a starting bar between their thumb and all four fingers at the start of each trial. The starting bar was a 2-cm X 8-cm wooden dowel taped to the table directly in front of the participant, perpendicular to the direction of the reach, 23 cm from the center of the target bar and 21 cm from the participant's edge of the table. In the starting position, participants could see their wrist androughlya third of their hand under the lower edge of the mirror mounting frame. While reaching, participants could see their hand through the viewing aperture for about the final third of the reach only. Procedure. Each trial in the reaching task began when the tabletop was illuminated, allowing participants a view of the stimulus display. Participants were free to move immediately, reach out and pick up the target bar, and then place it down again anywhere on the half of the tabletop nearest to them. Speed was not emphasized. For consistency, participants were asked always to pick up the bar using an abducted (thumb-right) posture. Participants first performed six practice trials to ensure that they understood the task and then completed two blocks of trials separated by a short rest. The bar orientation ranged from 5 to 35 clockwise from sagittal in 5 steps. In each block, there were five trials with each combination of the seven bar orientations and two grating orientations (-10 and +10 ), for a total of 70 trials. The order of the trials was determined randomly for each participant. Data analysis. Data from the adjustment task were analyzed as in Experiment 1. For the reaching task, the recorded positions of the two diodes were first filtered to eliminate dropouts and other recording artifacts. Measurements were omitted when they implied a movement velocity of greater than 1.0 m/s or when the positions were outside the normal range of movement. Positions for these omitted observations were estimated by interpolation. The reaching movement itself was then identified as that portion of the recording during which the movement velocity was greater than 0.10 m/s. For each recorded diode position during this interval, the orientation of the hand was calculated by computing the angle between the position of the lit diode and the position of the unlit diode, interpolated from its position in the preceding and following measurements. Trials were excluded if either the calculated reaction time or movement time was less than 250 ms or greater than 1,500 ms; typically, these trials represented recording failures or artifacts. Finally, the orientation of the hand was estimated by interpolation at each of 21 equally spaced points in time during the movement, corresponding to the completion of 0%, 5%, 10%, and so on up to 100% of thereach.these hand orientations were averaged for each participant, background condition, and bar orientation. We included 93.3% of the trials from 8 participants in the final analysis. Nested linear models, incorporating effects of bar orientation, time, the interaction of time and bar orientation, and background, were fit to the results for hand orientation; likelihood ratios were used in evaluating the relative fits of these models. The sequence of model comparisons corresponded conceptually to the separate test of these effects in an analysis of Results Adjustment task. As in Experiment 1, there was a strong effect of background in the adjustment task. The final adjusted orientation of the bar was 2.09 more clockwise when the grating was rotated clockwise 10 than when it was rotated counterclockwise 10. This effect of background grating was quite similar to that obtained in the adjustment task of Experiment 1 (2.01 ). A null model incorporating only an effect of participants was compared with an alternative model in which background was an additional factor. The maximum-likelihood ratio (A) for the comparison was above 1,000; thus, the data were more than 1,000 times more likely given an effect of background on adjustments than given no such effect of background.

8 DYNAMIC ILLUSION EFFECTS IN REACHING 567 The final adjusted orientations are presented in Table 2 as a function of grating orientation and starting position. (The means are shifted relative to the values in Experiment 1 because a small shift had occurred in the experimenter's referents between experiments.) There was no clear evidence of either an effect of starting position or an interaction between starting position and grating orientation. The data were fit with a linear model that included different monotonic trends over starting positions for each background condition, as well as the overall effect of background. The likelihood ratio comparing this model with one that included only the background effect was This was below the criterion of 10 for clear evidence in favor of one model over another. Reaching task. As shown in Figure 5, there was a clear dependence of hand orientation on bar orientation, and the strength of this relationship increased over time. For all bar orientations, the hand was close to its final orientation by about 75% of the movement; only modest adjustments in hand orientation occurred in the last 25% of the movement. The range of hand orientation at the end of the reach was substantially lower than the range of bar orientations because participants used their finger and thumb joints as extra degrees of freedom, making it unnecessary to align the wrist joint precisely with the bar. The effect of bar orientation on hand orientation was evident even at the onset of the movement. Our interpretation of this aspect of the results is that participants adjusted their hand orientation slightly before moving their hand toward the bar. It was also surprising that the orientation of the hand at the beginning of the movement was not closer to 90, given that participants were instructed to begin each trial by grasping a start bar at that orientation. However, the starting position was directly in front of the participants' body, and they generally found that it was more comfortable to maintain contact with their fingers to the start bar but simultaneously orient their hands at about 55 clockwise from sagittal. As a consequence, the orientation of the hand was initially closer to sagittal than was required to pick up the bar, and participants had to rotate their hands about 10 clockwise during the reach, as shown in Figure 5. The central result, though, is that in addition to these reaching dynamics, there was a consistent relationship between the orientation of the hand and that of the bar throughout the movement. Table 2 Means and Standard Errors (in Parentheses) of Final Bar Orientation (in Degrees Clockwise from Sagittal) in the Adjustment Task of Experiment 2 Starting position (1.02) 0.00(1.30) 0.13 (0.97) (1.45) 1.75(1.13) (0.86) 1.50(1.05) -0.30(1.31) 0.75(1.31) 0.50(1.05) Grating orientation (1.15) 1.75(1.31) 1.88(1.17) 2.00(1.00) 2.38 (0.84) 2.25 (1.03) 2.88(1.01) 2.50 (0.85) 3.25 (0.75) 3.50(0.91) Normalized Time 1.00 Figure 5. Mean hand orientation (in degrees [deg.] clockwise [CW] from sagittal) in the reaching task of Experiment 2 as a function of time and bar orientation. The effect of background grating on hand orientation is shown in Figure 6. In this figure, hand orientation is graphed as a function of the angular discrepancy between the bar and the background grating, with separate panels for each 25% time interval over the course of the reaching movement. In this depiction of the results, an effect of background would be indicated by larger hand angles for the points in the -10 grating condition (those generally on the right of each panel) than those in the +10 grating condition (those generally on the left of each panel). If hand orientation were completely determined by the orientation of the bar relative to background, all of the points in each panel would fall on a single line. The results indicate that, overall, the hand was turned more in the clockwise direction in the 10 condition than in the +10 condition. This effect was superimposed on the movement effects discussed earlier: Hand orientation was generally dependent on the (veridical) orientation of the bar, and this dependence increased as the hand approached the bar. These data were modeled by assuming that hand angle was a linear effect of bar orientation and that this effect was different at each time slice during the reach. The effect of background grating was incorporated as a constant effect of bar-background discrepancy. The fit of this model is shown by the solid lines in Figure 6. This model was compared with several simpler models. When the effect of background orientation was omitted, the fit was worse and yielded a maximum-likelihood ratio (A) above 1,000. That is, the data were more than 1,000 times as likely given an effect of background than given no effect of background. The fit was also worse if the interaction between bar orientation and time was omitted, yielding a likelihood ratio above 1,000. In sum, these comparisons indicate that the data provide clear support for an

9 568 GLOVER AND DIXON t = 0.0 t = 0.25 Normalized Time t = 0.50 t = 0.75 t = - A0 0 o - o Grating Grating Degrees Degrees Bar-Background Discrepancy (degrees) Figure 6. Mean hand orientation (in degrees [deg.] clockwise [CW] from sagittal) in the reaching task of Experiment 2 as a function of time (t) and bar-background discrepancy. Points represent observed data; lines represent the predictions of a linear model described in the text. effect of bar orientation that changes over time, as well as an effect of background orientation. The best-fitting model incorporated an effect of background orientation that was constant over time, although it became progressively weaker relative to the effect of veridical bar orientation. To determine the size of the illusion effect in the reaching task, we first estimated the effect of bar orientation at each point in time by linear regression. The slope of this line indicated the extent to which the angle of the hand was dependent on the orientation of the bar at each time point. The difference in hand orientation between the two background conditions was then divided by this slope. The size of the illusion effect thus calculated for the reaching task is shown in Figure 7; the corresponding effects for each of the other tasks in Experiments 1 and 2 are presented for comparison. Although the adjustment and posture choice tasks produced effects of approximately 2, the effect in the reaching task was much larger early in the reach, decreasing to near zero by the end of the movement. The mean reaction and movement times for the reaching task are shown in Table 3. To evaluate whether there were any systematic effects of condition in these data, we fit two statistical models to the data: a null model that included only an effect of participants and an alternative model that also included a linear effect of bar angle, an effect of background, and the interaction. The alternative model fit the reaction times and movement times only slightly better than the null model, yielding likelihood ratios of 1.64 in the case of reaction times and 1.90 in the case of movement times. These likelihood ratios are both substantially below the value of 10 (corresponding roughly to a p value of.05) that Dixon and O'Reilly (1999) suggested as a criterion for clear evidence in favor - of a model. Thus, we conclude that there was little indication of any systematic effects of condition on these parameters. Discussion The results of Experiment 2 indicate that the orientation illusion affected the orientation of the hand during reaching. The illusion had a large effect early in the reach, but the magnitude of the effect decreased as the hand approached the bar. The presence of this Adjustment (Exp. 1 & 2) t= Posture Choice Hand Orientation (Exp. 1) (Exp. 2) Figure 7. Effect of the orientation illusion in the adjustment tasks (Experiments 1 and 2), the posture choice task (Experiment 1), and the reaching task (Experiment 2). t = normalized time; Exp. = experiment; deg. = degrees.

10 DYNAMIC ILLUSION EFFECTS IN REACHING 569 Table 3 Means and Standard Errors (in Parentheses) of Reaction Time and Movement Time (in milliseconds) in the Reaching Task of Experiment 2 Bar orientation Reaction time 678 (80) 694 (80) 675 (76) 707 (79) 680 (69) 675 (76) 699 (66) Movement time 553(35) 551(31) 560 (36) 540 (32) 548 (38) 551 (36) 541 (39) Grating orientation (88) 679 (67) 663 (69) 678 (66) 700 (70) 691 (86) 693 (64) 554 (35) 548 (36) 564 (35) 561 (32) 554 (39) 555 (39) 542 (43) dynamic illusion effect is consistent with the planning-control model. In this model, it is assumed that movements are planned through a context-dependent visual representation. After initiation of the movement, however, the control module uses a contextindependent representation to monitor and correct the trajectory of the movement. It is presumably this correction that leads to the decrease in the magnitude of the illusion effect over the course of the reach. In contrast, the data are inconsistent with a model based on a simple distinction between perception and action. Within such a framework, one would expect the trajectories to be planned through the action module and hence to be less affected by context-induced illusions than perceptual judgments. Despite the support found in the present study for the planningcontrol model, some aspects of the data are not easily explained. For example, the magnitude of the illusion effect on the initial trajectory of the reach was substantially larger than that in the adjustment and posture choice tasks. One possible reason for the apparent discrepancy is that reaches in Experiment 2 were made as soon as the stimulus was visible, whereas the other measures reflected performance after several seconds. For example, the adjustments took several seconds to complete, and the posture choice task was performed only after the stimulus was visible for 2 s. It is possible that the effect of the orientation illusion was minimized by compensatory strategies during these intervals. General Discussion The goal of the present study was to elucidate the relationship between the visual and motor systems and, in particular, the effects of a context-induced orientation illusion on reaching and grasping. Our approach differed from many previous investigations in that it involved measures of performance that were likely to index both the initial planning of the movement and its subsequent control. The results of Experiment 1 suggest that a context-dependent visual representation underlies the choice of grasping posture; the results of Experiment 2 suggest that a context-dependent representation was used to plan the reaching trajectory but that a context-independent representation was used to minimize the spatial error during execution. In sum, these results suggest that the planning and thus initial performance of an action are just as affected by context-induced illusions as perceptions and that accurate control of movements, independent of the context, emerges only over time. Evidence for a Planning Control Model Our distinction between action planning and control is closely related to traditional models of motor control in which movements are assumed to consist of an initial ballistic phase and a subsequent control phase (Meyer, Abrams, Kornblum, Wright, & Smith, 1988; Woodworth, 1899). During the initial phase, the trajectory of the movement is immutable and is determined entirely by planning processes before the onset of the movement. At some time after initiation of the movement, the control phase begins, and any error in the trajectory of the reach is corrected. This correction process produces movements that are more spatially accurate and responsive to changes in the position of the target or perturbations of the moving limb. This description of the control phase is consistent with the role of the control module hypothesized here. Although it is difficult to identify precisely when in the trajectory the control phase begins, the effects of visual and proprioceptive feedback can be detected early in a reach. For example, studies of motor performance have shown that visual feedback can influence actions in as little as ms (Paulignan, MacKenzie, Marteniuk, & Jeannerod, 1991; Zelaznik, Hawkins, & Kisselburgh, 1983, 1987; see Carlton, 1992, for a review). This is much faster than the time it takes to plan and initiate a movement, which is generally about 250 ms (Stark, 1968). However, the trajectory of a reach can also be modified without visual feedback of the reaching hand (Goodale et al., 1986; Khan et al., 1998; Pelisson, Prablanc, Goodale, & Jeannerod, 1986; Prablanc & Martin, 1992). These modulations must necessarily be based on proprioception (and possibly efference copy) and may occur within ms (see Jeannerod, 1988, for a review). The present results suggest an important role of proprioception or efference copy mechanisms, or both, during control. Because the orientation of the hand was occluded by the experimental apparatus over a substantial portion of the reach, visual feedback of the hand could not have been used to correct the trajectory until the final portion of the reach, yet the effect of the illusion began to decrease from the outset. We hypothesize that the control module begins to exert an influence on the movement almost as soon as the movement has been initiated, using whatever afferent or efferent mechanisms are available. The planning-control model can incorporate a wide range of results concerning the effects of illusions on actions. Such effects are predicted to be minimal when the movement is assessed with an index dependent on the spatial characteristics of the target (e.g., its size, orientation, or extent) and measured relatively late in the movement trajectory. In contrast, illusion effects are predicted to be more substantial when the dependent variable is measured early in the trajectory, when it pertains to aspects of the movement dependent on nonspatial characteristics of the target, or when the movement is performed after a delay.

11 570 GLOVER AND DIXON Alternative Models A variety of other accounts of the relationship between perception and action can be elaborated to account for the present findings. We discuss several of them here. Perception-action model. There are several ways in which the perception-action model might be extended to incorporate the present results. One possibility would be to assume that the perception and action modules interact before the action is planned. Such an interaction has been commonly invoked under other circumstances (see Milner & Goodale, 1995). To account for the present results, it would have to be assumed that such interactions are more pervasive than was originally claimed by Milner and Goodale (1995). For example, the results of both Experiments 1 and 2 suggest that the perception module provides a strong input to the action module even when the movement could conceivably be planned entirely on the basis of the immediately available visual information concerning the orientation of the target. Although extensive interaction of this sort would allow the present results to be incorporated in a perception-action model, it would seem to undermine the value of a simple dichotomy between perception and action. Allocentric-egocentric model. Gentilucci et al. (1996; Daprati & Gentilucci, 1997) proposed that actions are typically based on an egocentric representation that encodes the target's characteristics relative to the actor, whereas perception typically involves an allocentric representation that encodes the target's characteristics relative to the surrounding visual context. According to this analysis, movements are minimally affected by optical illusions because the egocentric representation of the target is independent of the context. However, when the action system must operate with reduced feedback or the imposition of a delay, the contextdependent, allocentric representation is more influential. In line with this hypothesis, Gentilucci et al. (1996) found small effects of the Muller-Lyer illusion on the accuracy of pointing movements when visual feedback was available, larger effects when visual feedback was removed, and larger effects still when a 5-s delay was imposed between target offset and movement onset. A distinction between allocentric and egocentric representations of the target can be used to explain many of the effects of illusions on action, particularly if one assumes that at least some aspects of actions (such as posture choice) rely exclusively on allocentric representations. However, such a distinction by itself would seem to offer little basis for predicting the dynamic illusion effect found in Experiment 2. This result could be predicted only if it were assumed that an allocentric representation was used to plan actions, but an egocentric representation was used to control them. However, in this case the model would be indistinguishable from the planning-control model. Task-demands model. Smeets and Brenner (1995; Brenner & Smeets, 1996) argued that many aspects of action rely on the position of the target relative to the viewer, whereas most aspects of perception require judgments of other object characteristics, such as velocity or size. These nonpositional characteristics are held to be calculated relative to other objects in the visual field and would thus be susceptible to the effects of context-induced optical illusions. According to this task-demands model, however, egocentric position information is unaffected by the context surrounding the target, and actions based on such information should also be relatively unaffected. Thus, according to Brenner and Smeets (1996), the Ponzo size illusion has little effect on maximum grip aperture because (accurate) position information is used to aim the thumb and finger at the edges of the target. On the other hand, the size illusion affects lifting force because the apparent size of the object (relative to its context) is used to estimate its weight. A related argument was made by Vishton, Rea, and Cutting (1999). They argued that some tasks require absolute judgments that are based on the target alone, whereas others require judgments of the target relative to the surrounding context. Actions tend to belong to the former category and thus are often unaffected by context-induced illusions. Vishton et al. showed that the horizontal-vertical illusion affected the placement of the fingers only when both the horizontal and vertical dimensions were relevant to grasping the target (a relative judgment); there was little effect of the illusion when only one dimension was relevant to the grasp (an absolute judgment). Similar to the position of Smeets and Brenner (1995; Brenner & Smeets, 1996), Vishton et al. suggested that illusions may or may not affect actions depending on the demands of the task. The results of the present study would be difficult to incorporate within a task-demands model without some additional assumptions. For example, a proponent of a task-demands model might assume that the choice of grasping posture and the orientation of the hand are calculated on the basis of the absolute egocentric positions of the edges of the bar. However, if this were the case, one would not expect an effect of the orientation illusion in the posture choice task of Experiment 1 or the reaching task of Experiment 2. An alternative conception is that the choice of posture and the initial orientation of the hand during reaching are determined in part by the orientation of the bar relative to the background, producing an effect of the orientation illusion. However, during reaching, egocentric or absolute position information concerning the edges of the bar is used to guide the thumb and forefinger to an accurate grip posture; this would explain why the effect of the illusion decreases over the course of the reach. Although this analysis allows the present results to be incorporated in a task-demands model, it would also seem to require an implicit distinction between the planning of a movement and its on-line control. Conclusion In the present research, we have demonstrated effects of an orientation illusion on a choice of posture and on the initial trajectory of a reach. Furthermore, the effect on the orientation of the hand decreased over the course of the movement. These findings seem inconsistent with the idea that actions are both planned and executed through a single visual representation unique to action production. Instead, a distinction between the planning of an action and its subsequent control seems to fit the results more naturally. Of course, it is possible that the effects observed here may be specific to the particular illusion that was used or other aspects of the experimental procedure. However, we have observed a similar dynamic effect of the Ebbinghaus size illusion on grip aperture when this was measured throughout the entire reach (Glover & Dixon, 2000; see also Franz et al., 2000; Pavani et al., 1999). The effect of the illusion on grip aperture was relatively large early in the reach but diminished as the hand approached the

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