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Movement duration does not affect automatic online control
Human Movement Science 29 (2010) 871–881
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Human Movement Science journal homepage: www.elsevier.com/locate/humov
Movement duration does not affect automatic online control Erin K. Cressman a, Brendan D. Cameron b, Melanie Y. Lam b, Ian M. Franks b, Romeo Chua b,⇑ a b
School of Human Kinetics, University of Ottawa, Canada School of Human Kinetics, University of British Columbia, Vancouver, Canada
a r t i c l e
i n f o
Article history: Available online 30 August 2010 PsycINFO classification: 2300 2330 Keywords: Vision, Action Automatic guidance Movement time
a b s t r a c t Pisella et al. (2000) have shown that fast aiming movements are automatically modified on-line in response to a change in target position. Specifically, when a movement is less than 300 ms in duration the reach is completed to a target’s new location even when one never intended to respond to the target jump. In contrast, when movements are slower, the reach is completed according to instructions. At present, it is unclear if it is possible for one’s intentions to guide the initial stages of these slow movements. To determine if the intentional control mechanism can guide the initial stages of a slow aiming movement, participants aimed to targets that could jump at movement onset, with a slow and very slow movement time goal. In particular, participants were to point towards (‘‘pro-point”) or away from (‘‘anti-point”) the target jump, with a movement time goal of 500 or 1200 ms. Results showed that in the anti-point condition, movement trajectories first deviated in the same direction as the target jump, followed by a response in the intended (opposite) direction. This suggests that while movement outcome is controlled by the intentional system, even in these slow aiming movements the automatic system is engaged at movement onset. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Rapid aiming movements are modified on-line in response to a change in target position in order to bring the hand to the target’s new location (e.g., Bard et al., 1999; Chua & Enns, 2005; Cressman, Franks, Enns, & Chua, 2006; Day & Lyon, 2000; Desmurget & Prablanc, 1997; Desmurget et al., ⇑ Corresponding author. Address: School of Human Kinetics, University of British Columbia, 210-6081 University Boulevard, Vancouver, BC, Canada V6T 1Z1. Tel.: +1 604 822 1624; fax: +1 604 822 6842. E-mail address: [email protected] (R. Chua). 0167-9457/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.humov.2010.07.001
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1999; Fecteau, Chua, Franks, & Enns, 2001; Goodale, Pélisson, & Prablanc, 1986; Gritsenko, Yakovenko, & Kalaska, 2009; Komilis, Pélisson, & Prablanc, 1993; Magescas, Urquizar, & Prablanc, 2009; Paulignan, MacKenzie, Marteniuk, & Jeannerod, 1991; Pisella et al., 2000; Prablanc & Martin, 1992; Prablanc, Pélisson, & Goodale, 1986; Pélisson, Prablanc, Goodale, & Jeannerod, 1986; Soechting & Lacquaniti, 1983).1 It has been suggested that these adjustments in limb trajectory occur automatically (i.e., independent of one’s intentions). Specifically, Pisella et al. (2000) have argued that fast aiming movements are driven by an ‘‘automatic pilot”, which escapes intentional control. This proposal is based on their findings that for movements completed between 125 and 350 ms, the hand is adjusted and the movement completed to the new target position even when participants are instructed to interrupt their movements in response to a target jump. Similar adjustments are not observed for movements completed in less than 125 ms as, presumably, there is not enough time for the central nervous system to detect errors in the limb’s trajectory in relation to the new target location and correct the unfolding movement (Blouin, Teasdale, Bard, & Fleury, 1995). In support of Pisella’s claim of automatic visuomotor guidance, Day and Lyon (2000; see also Johnson, van Beers, & Haggard, 2002) have also shown that fast aiming movements are rapidly modified in the direction of a target jump, independent of instructions. In their paradigm, participants were required to respond as quickly as possible to a target jump by reaching in either the same (pro-point, PP) or opposite (anti-point, AP) direction. They found that all reaches, both PP and AP, were initially modified in the direction of the target jump, even though this was an incorrect response on the AP trials. Furthermore, these changes in trajectory occurred at a similar latency (125 ms into the movement) for both PP and AP trials. Accordingly, and in agreement with Pisella et al. (2000), Day and Lyon inferred that these adjustments were under ‘‘automatic” control. By examining movement trajectories, Day and Lyon (2000) were able to observe that in their AP task, participants modified their trajectories twice, first in the direction of the target jump (an incorrect modification) and then in the opposite direction (a correct modification). The second modification ensured that participants completed the movement according to their intentions and is similar to the control observed by Pisella and colleagues for slow aiming movements. Specifically, Pisella found that when participants completed slower aiming movements (>300 ms), they did not touch down at a new target position but stopped their movements in response to a target jump as instructed. Taken together, the results of Day and Lyon (2000) and Pisella et al. (2000) demonstrate that while the automatic pilot controls fast aiming movements initially, one’s intentions can control the latter portion of a movement if enough time is provided. This observation raises the question: Can intention guide a goal-directed aiming movement for its entirety (i.e., from start to finish), or is the automatic pilot always engaged at movement onset? Given (1) the differences in movement outcome observed by Pisella et al. (2000) for fast vs. slow movements (the latter never landed at the perturbed location) and (2) the fact that intentional control took over Day and Lyon’s trajectories only after sufficient time had elapsed, it is possible that if we allow participants to make slow movements, the automatic pilot will never be engaged, a proposal that has been put forth in the literature (see Pisella et al., 2000; Rosetti & Pisella, 2002) but never examined directly. This proposal implies that slow movements are prepared differently from fast ones, such that they do not deviate until a top-down process allows them to do so. This would be in contrast to fast movements, which, as we have described above, respond immediately and automatically to target perturbations. In the current study we determined if intention controls the initial stages of a slow aiming movement by examining trajectories to double-step targets. Specifically, we examined the trajectories of slow aiming movements (500 and 1200 ms) completed to double-step targets when participants were instructed to PP or AP in response to a jumping target. In our task, target jumps occurred at movement onset and were visible to participants. An AP movement was adopted as opposed to a stop instruction, as anti-pointing has the benefit of providing (in the form of a trajectory deviation) a clear indication of intentional control onset. Furthermore, given that some of our movements were completed with a very long movement time (MT) goal, we were concerned that if participants were instructed to stop
1 Each of these studies employed a double-step protocol, first introduced by Bridgeman, Lewis, Heit, and Nagle (1979), in which the initial target of a reach changes location at some point prior to or during the reach.
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their movements in response to a target jump, the hand may drift to the target’s new position over time, even though the participant’s first response was a correct interruption of their movement. If we do not find deviations toward the jumping target in the anti-point condition in the slow aiming movements of the present study, it would indicate that intentional control is engaged from movement onset and overrides the automatic pilot. In contrast, if the hand initially deviates towards the target before being intentionally countermanded, it would suggest that the automatic pilot is engaged during aiming, regardless of movement duration, and controls the early portions of the movement until intentional processes take over. 2. Methods Seven right-handed university students (mean age = 25.4 ± 1.3 years) with normal or corrected-tonormal visual acuity volunteered to participate in this study.2 All participants gave informed consent and the study was conducted in accordance with the ethical guidelines set by the University of British Columbia. 2.1. Apparatus All stimuli were presented on a custom-built 18 24 in. display panel oriented 20° from horizontal. Stimuli consisted of red dots of light (5 mm in diameter) produced by light emitting diodes below a transparent Plexiglas sheet. Participants sat with their head in a chinrest and viewed the display panel from a distance of approximately 60 cm. The participants were positioned such that their midline was aligned with the vertical meridian of the panel and a circular home position (12 mm in diameter) located centrally at the bottom of the panel. The initial target was 27 cm directly above the home position. The target could jump 5 cm to the left or right at movement onset. 2.2. Motion analysis Participants pointed with their right-hand using a stylus. An infrared-emitting diode was placed at the tip of the stylus and 3D position was monitored using an OPTOTRAK (Northern Digital, Waterloo, ON, Canada) motion analysis system. The tip of the stylus was equipped with a microswitch that provided a logic signal indicating whether or not the stylus was in contract with the panel. The 3D stylus position and the microswitch status were sampled at 500 Hz. Raw OPTOTRAK data were converted into 3D coordinates and filtered using a low-pass Butterworth filter (2nd order, dual pass, 10 Hz cut-off). Start and end of the movement were determined by the microswitch status. 2.3. Procedure and design Participants completed all reaches in a dark room that minimized vision of the hand. In the first set of reaches, participants completed 25 practice trials to the center target. They completed these movements with a MT goal of 500 or 1200 ms and MT feedback was provided verbally after each trial. After ensuring that participants could perform the movement in the time allotted (i.e., 95% of the last 20 practice trials were completed within 400–600 ms for the 500 MT goal and between 1000 and 1400 ms for the 1200 ms MT goal), participants completed two testing blocks of 96 trials. In each testing block, a trial began with the appearance of the target. Participants were instructed that this was not a reaction time task and as such, they could begin their movement anytime following target onset. On 1/3 of the trials, the target would jump randomly either 5 cm to the left or right of its initial position at movement onset (32 perturbed trials: 16 left-jump + 16 jump-right trials). The perturbed trials were randomly interleaved with the 64 unperturbed (non-jump) trials within a testing block. In the ‘‘pro-point” (PP) testing block, participants were instructed to respond to the target jump by pointing to its new location. In the ‘‘anti-point” (AP) block, participants were to respond to a target jump by 2
Data from an eighth participant was collected but due to technical errors during collection, we were unable to analyze the data.
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reaching in the opposite direction to the jump and touching down at an imagined spot on the screen at the same distance as the target jump. MT feedback was provided on all unperturbed trials in order to aid participants in adhering to the MT goal. After completing the two testing blocks, participants performed all tasks a second time with the second MT goal. Thus, if participants began performing the task with a goal MT of 500 ms (e.g., PP500 and AP-500), they now completed a second round of 25 practice trials and the two testing blocks (PP and AP) with a MT goal of 1200 ms (e.g., PP-1200 and AP-1200). The MT goal and the order of the PP and AP testing blocks were counterbalanced across participants, such that if participants completed the PP block first within a given MT goal, they again performed the PP block first when pointing with the second MT goal. Two participants completed each MT-testing block sequence except for the PP-500, AP-500, PP-1200, AP-1200 sequence, which only one participant completed. All participants showed similar patterns of responses independent of testing order. 2.4. Data analyses In order to ensure that participants completed the task as instructed (i.e., unperturbed reaches landed at the center target position and were performed in the time allotted), we analyzed both mean lateral displacement errors and movement times achieved on unperturbed trials in a 2 MT goal (500 ms vs. 1200 ms) 2 Instruction (PP vs. AP) repeated measures analysis of variance (RM ANOVA). We then determined if participants adjusted their trajectories on the perturbed trials in response to a target jump by classifying pointing responses according to the target to which they were completed. A pointing response on a perturbed trial was classified as having been correctly modified if its endpoint fell more than 2.5 cm to the left or right of the center target position in the direction of the target jump (PP) or opposite the direction of the target jump (AP). For movements that were modified in response to the target jump and completed in the correct direction (886 out of a possible 896 movements), we next examined the corresponding trajectories in order to determine the direction in which participants first modified their reaches. In order to examine adjustments in participants’ trajectories, we first plotted all of the trajectories for each participant as illustrated in Fig. 1. We then derived average spatial trajectories from trials completed to the correct eccentric positions as shown in Fig. 1C and D. Average spatial trajectories were attained for each stimulus condition (left-jump, right-jump, unperturbed), by calculating spatial averages in the 2D plane of the movement. The 2D plane of movement was defined by display coordinates such that the target stimulus was initially located 27 cm forward (y-direction) with a lateral position of 0 (x-direction). The points in the average spatial trajectories were obtained by deriving the mean spatial position in the x–y plane from spatial coordinates in the corresponding trials at 2 ms intervals. From these plots it appeared that for all participants the hand was first pulled in the direction of the target jump, regardless of instruction or movement time goal. To quantify the time at which participants modified their movements in the perturbed trials, we derived the mean and standard deviation of lateral velocity profiles for each participant, and for each condition from corresponding displacement profiles. t-Tests were then used to compare each mean perturbed path with its corresponding mean unperturbed path at all 2-ms time intervals from 100 ms onward. We designated 100 ms as a minimum time constraint as visual examination of the mean paths revealed that velocity profiles did not start to diverge from each other until at least 110 ms into the movement. A mean perturbed path was designated as diverging significantly from the mean unperturbed path when the t-test was significant, provided that the difference between paths remained for at least the next 35 time intervals (70 ms). We found two points in time at which participants modified their movements on AP perturbed trials with respect to the unperturbed trials when the target jumped right. This was consistent with the observation that participants appeared to first change their trajectories in the direction of the target jump on AP trials, and then subsequently correct their movements to complete the task as instructed. The early time point signifies when participants modified their trajectories in the direction of the target jump
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Fig. 1. Examples of spatial trajectories. In (A) and (B) we show examples of trajectories for one participant when the movement time was 500 and 1200 ms, respectively. In (C) and (D) we show spatial trajectories averaged across all participants. Trajectories are plotted in two-dimensions such that the y values represent forward motion and the x values provide lateral position information. The mean unperturbed paths are represented by circles, the solid lines denote mean trajectories completed under the ‘‘pro-point” instructions (black line = right-jump trials and grey line = left-jump trials), and the dashed lines correspond to mean trajectories completed under the ‘‘anti-point” instructions (grey dashed line = right-jump trials and black dashed line = left-jump trials). Note that in (C) and (D) the trajectories of the perturbed anti-point trials appear to be completed before achieving the ‘‘target” position. This was not the case. In order to ensure that the full extent of the mean trajectory included data from all participants we had to cut the trajectories to the shortest MT achieved.
and the second time point indicates when trajectories were modified from the unperturbed profiles in the instructed direction. In contrast to the right-jump AP trials, analyses revealed significant deviations in trajectories on AP trials when the target jumped left for only about half of the participants. To determine if similar processes were engaged during early adjustments in the PP and AP conditions, we compared latencies of trajectory modifications when the target jumped right in a 2 MT goal (500 ms vs. 1200 ms) 2 Instruction (PP vs. AP) RM ANOVA. Finally, we compared final trajectory modifications across both left- and right-jump trials in a 2 MT goal (500 ms vs. 1200 ms) 2 Direction (left vs. right) RM ANOVA. Because the PP trials demonstrated only 1 early correction, we did not include these trials when examining final trajectory modifications.
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3. Results 3.1. Unperturbed trials Participants were very accurate on unperturbed trials with respect to lateral pointing errors. In Fig. 1 we show examples of all trajectories completed to the center target for one participant when the movement time goal was 500 ms (Fig. 1A) and 1200 ms (Fig. 1B). These unperturbed profiles demonstrate that the participant made consistent, straight movements to the target. On average, participants completed reaches to the center target with a lateral constant error of 2.5 mm (SD 2.5, range = 1.8–6.8 mm) from the middle of the target, in a movement time of 504 ms (SD 13) when the MT goal was 500 ms and 1172 ms (SD 29) when the MT goal was 1200 ms. RM ANOVA revealed that spatial accuracy was independent of MT goal and Instruction (MT goal: F(1, 6) < 1, Instruction: F(1, 6) < 1, MT goal Instruction: F(1, 6) = 1.213, p = .313). Mean MT on unperturbed trials was also independent of Instruction (Instruction: F(1, 6) < 1, MT goal Instruction: F(1, 6) < 1).
3.2. Perturbed trials: automatic responses Even though we used a conservative criterion for establishing if a movement had been updated in response to a target jump, given the high accuracy achieved during unperturbed trials, only 10 perturbed trials in total failed to meet our criterion. All of these error trials occurred when the MT goal was 500 ms and on the majority of these trials (7 trials) participants touched down at the wrong position in the AP-500 condition (an example of one such trial is shown in Fig. 1A). Specifically, participants adjusted their trajectory in the direction of the target jump and completed their movement to the actual target location. These error trials are similar to the results obtained by Pisella et al. (2000), when their participants completed very fast movements (200–300 ms) and incorrectly touched down at a new target position, raising the possibility that even relatively slow aiming movements of 500 ms are automatically captured by a target jump. Fig. 1 illustrates this proposed hand capture in more detail. Example trajectory profiles are shown in Fig. 1A and B for one participant and pointing trajectories averaged across participants for all stimulus conditions are shown in Fig. 1C and D when the MT goal was 500 and 1200 ms, respectively. As noted previously, on unperturbed paths (middle path designated by circles in Fig. 1C and D) the hand moved in a fairly straight line toward the center target. The trajectories on perturbed trials also appear to have been initiated towards the center target. However, these trajectories were then modified in the direction of the target jump, regardless of instruction or MT goal. In the PP condition, these initial adjustments in movement were carried to completion and participants touched down at the new location. In contrast, in the AP condition, these early adjustments were countermanded and the limb further adjusted to a position opposite the direction of the target jump. To determine the latency of the trajectory modifications on the perturbed trials, we compared perturbed vs. unperturbed lateral velocity profiles at different time points using t-tests. In accordance with the displacement data, the velocity profiles indicated that perturbed trajectories were first modified in the direction of the target jump when the target jumped right, even for the AP trials (see Fig. 2). Specifically, for all participants, analyses of the AP trials revealed that the hand was first modified in the direction of the target before reversing direction when the target jumped right. On the PP trials, the hand was also modified in the direction of the target but, in contrast to the AP trials, no lateral velocity profiles exhibited reversals in direction. The latency range of the initial modifications in trajectory across all target jump-right conditions was 140–276 ms (mean latency = 202 ms (SD 34)). As observed in Fig. 2A and B (see also 2C), RM ANOVA (2 MT goal 2 Instruction) revealed that movements were adjusted at a similar latency in the direction of the target jump regardless of MT goal (F(1, 6) = 2.423, p = .171) or instruction (F(1, 6) < 1). On AP trials in which the target jumped left, trajectories deviated significantly in the direction of the target jump for approximately half of the participants (four participants when the MT goal was 500 ms and two participants when the MT goal was 1200 ms). Latencies of these initial corrections were within the range obtained for trials in which the target jumped right (range: 188–276 ms, mean
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Fig. 2. Examples of mean lateral velocity profiles for one participant when the target jumped right: (A) 500 ms MT goal and (B) 1200 ms MT goal. Velocity profiles are plotted such that time (up to 600 ms) is represented on the vertical axis and velocity magnitude on the horizontal axis. The mean unperturbed paths are represented by grey lines. Solid lines denote mean trajectories completed under the ‘‘anti-point” instructions when the target jumped right. The dashed lines indicate the latencies at which the mean perturbed velocity profiles diverge from the unperturbed paths, first in the direction of the target jump and then in the instructed direction. (C) Mean latencies (ms) for early trajectory modifications when the target jumped right. Results are displayed as a function of instruction (‘‘pro-point” vs. ‘‘anti-point”) and movement time goal. Error bars denote standard deviations.
latency = 224 ms (SD 33)). A 2 MT (500 vs. 1200) 2 Direction (left-jump vs. right jump) RM ANOVA revealed that there were no differences in the latency of corrections on PP trials when the target jumped right vs. left (F(1, 6) < 1). 3.3. Perturbed trials: intentional responses As shown in Figs. 1 and 2, early, incorrect, limb adjustments in the AP trials were followed by a later, final correction that brought the limb to the intended position. RM ANOVA indicated that this adjustment occurred earlier when the MT goal was 500 ms compared to 1200 ms (F(1, 6) = 17.565, p = .006), suggesting that the latency of late, intentional corrections scaled to MT (see Fig. 3). In addition, adjustments were earlier in response to a target jumping left vs. right (F(1, 6) = 7.627, p = .033). 4. Discussion The goal of the present experiment was to investigate visuomotor processing during slow aiming movements. In particular, we examined if intentional (volitional) or automatic (unintended) motor responses control the initial stages of a slow aiming movement. To address this question we had
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Fig. 3. Mean latencies (ms) for final trajectory modifications under the ‘‘anti-point” instructions. Results are displayed as a function of target jump direction (left vs. right) and movement time goal. Error bars denote standard deviations.
participants make slow aiming movements and adapted experimental protocols that have previously been shown to distinguish between intentional and automatic motor responses during movement execution (Cressman et al., 2006; Day & Lyon, 2000; Johnson et al., 2002; Pisella et al., 2000). Participants pointed either in the direction of the displaced target (PP) or in the opposite direction (AP) with a MT goal of 500 and 1200 ms. These slow aiming movements were chosen specifically as previous work has demonstrated that the outcome of these movements is controlled by intention and suggested that voluntary control may guide the movement for its entirety (i.e., they do not engage the same automatic visuomotor processes as fast aiming movements; see Pisella et al. (2000) and Rossetti and Pisella (2002)). Similar to previous results, we found that the latter portion of a slow aiming movement was controlled by an intentional mechanism that ensured the movement was completed according to instructions. However, in contrast to our expectations, we found that automatic processes guided the movement initially, even for very slow movements. This was shown by the finding that for both slow (500 ms) and very slow (1200 ms) aiming movements, the limb was first pulled to the right when the target jumped right, even when participants were instructed to move in the opposite direction of the displaced target. Furthermore, the latency at which the hand was captured by the target (approximately 200 ms) was not influenced by MT goal or instruction suggesting that the same automatic visuomotor processes, leading to adjustments in limb trajectory, are recruited regardless of the temporal demands of the task. The engagement of automatic visuomotor processes is further revealed if we compare our estimates of the time of change in limb trajectory to previous work examining early adjustments to double-step targets when participants reached with fast movement time goals (Cressman et al., 2006; Day & Lyon, 2000; Paulignan et al. 1991). While we observed slightly later adjustments in the current study compared to previous results, this delay in movement correction is due to the fact that we defined the latency of correction in the present study as the time when trajectory profiles significantly diverged from each other. After trajectory modifications are initiated it takes some time for a significant separation between the unperturbed and perturbed profiles to be achieved, creating the longer latencies observed in the present results. If we were to determine the time at which the perturbed profiles for each participant initially diverged from corresponding unperturbed profiles through visual inspection (as done by Day & Lyon, 2000), we find that the latency at which the hand was captured by the target was approximately 130 ms. This estimate is similar to the latencies observed in previous work examining early adjustments to double-step targets during fast aiming movements (Cressman et al., 2006; Day & Lyon, 2000; Paulignan et al. 1991). Thus, given the temporal consistency in trajectory changes across experiments with different movement time goals, it appears that the same automatic visuomotor processes are engaged during fast and slow movements.
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4.1. The influence of the target jump direction on automatic visuomotor processes So far, we have discussed automatic deviations of slow aiming movements when the target jumps right. But what happens when the target jumps left? Is the automatic pilot still engaged? In the current study, individual displacement profiles revealed that the hand was pulled in the direction of the target on AP trials when the target jumped left. Moreover, PP trials deviated to the left at a similar latency as PP trials deviated to the right, suggesting engagement of automatic visuomotor processes even on trials in which the target jumped left. While results indicate that automatic visuomotor processes were engaged when the target jumped left, trajectory adjustments did differ depending on the direction of the target jump. In particular, statistical analyses revealed significant deviations in the leftward direction on AP left-jump trials for only about half of our participants, compared to all participants when the target jumped right and earlier adjustments on AP trials in the intended direction when the target jumped left (i.e., adjustments to the right) than when the target jumped right. These findings suggest that participants may have been able to counteract automatic responses earlier on left-jump trials compared to right-jump trials. However, given that we did not instruct participants on how to correct their movements if they found themselves reaching in the incorrect direction, this proposal is put forth as a hypothesis. Previous work examining reaches to stationary and perturbed targets in ipsilateral and contralateral space relative to body midline has demonstrated that movement kinematics are dependent on a target’s location (Castiello, Bennett, & Chambers, 1998; Fisk & Goodale, 1985; Prablanc, Echallier, Komilis, & Jeannerod, 1979). In particular Castiello et al. (1998) have shown that while reaches to jumping targets are initiated at a similar latency regardless of whether the target jumps left or right relative to body midline, differences are seen in later kinematic markers. Moreover, Fisk and Goodale (1985) have shown that the velocity of the limb is dependent on the spatial position of the target relative to the body axis. A greater peak and average velocity is achieved in movements made to targets in ipsilateral space compared to targets in contralateral space. It has been suggested that these differences in reaches arise due to the necessity of transferring information across hemispheres from visual to motor structures in contralateral reaches. This interhemispheric transfer is not present in ipsilateral reaches (Prablanc et al., 1979). As well, differences may arise due to the isometric contraction time of different muscles controlling hand position (Prablanc et al., 1979) or the natural synergies between elbow and shoulder joints. Regardless of the source of kinematic differences between ipsilateral vs. contralateral reaches, based on previous work, we would expect a lower lateral velocity to be achieved in the incorrect direction when the target jumped left compared to right. It is hypothesized that this lower lateral velocity would then be more easily overcome, resulting in earlier intentional corrections. However, as suggested above, given that we did not instruct participants on how to correct their movements, or control for the extent or speed of trajectory adjustments, we are unable to examine this proposal directly with the current data. Future research is required in order to determine if one’s ability to overwrite automatic adjustments after they have been initiated is dependent on direction of movement. For now, the results from the perturbed trials indicate that participants’ movements were initially guided by an automatic pilot, regardless of the direction of the target jump. It was only later, after enough time had passed, that movements came under intentional control. 4.2. The relevance of the target jump to automatic visuomotor processes The results from the current study raise the question: Are there circumstances under which the automatic pilot is not engaged at all? In the current study we looked for automatic adjustments in trajectories while manipulating the MT goal. In all of our conditions the target jump was relevant to the task and dictated how participants were to adjust their movements (i.e., PP or AP). Given that there was ample time for participants to correct their movements after they first moved in the incorrect direction, perhaps it is not surprising that the automatic pilot was engaged. Recent work examining the engagement of the automatic pilot has also focused on manipulating instructions but, instead of changing the temporal demands, Cameron, Cressman, Franks, and Chua (2009) and Streimer, Yukovsky, and Goodale (2010) explicitly instructed participants to ignore the target jump. In these cases the target jump was not relevant to the task and results demonstrated that the
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influence of the automatic pilot was reduced when one was told to ignore the target jump and complete the movement to the target’s original position. Thus, in contrast to the present results, the results of Cameron et al. (2009) and Streimer et al. (2010) indicate that there are situations in which the automatic pilot can be disengaged. However, given the circumstances under which the automatic pilot becomes disengaged and the results of the present study, we suggest that if the jump is relevant to the task (which is typically the case when reaching to a target) and there is no explicit instruction indicating that one is not to follow the target, the hand is susceptible to being captured by the jumping target, regardless of the movement’s duration. Acknowledgments This research was supported by the Natural Sciences and Engineering Research Council of Canada. References Bard, C., Turrell, Y., Fleury, M., Teasdale, N., Lamarre, Y., & Martin, O. (1999). 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What the hand can’t tell the eye: Illusion of space constancy during accurate pointing. Experimental Brain Research, 162, 109–114. Cressman, E. K., Franks, I. M., Enns, J. T., & Chua, R. (2006). No automatic pilot for visually guided aiming based on colour. Experimental Brain Research, 171, 174–183. Day, B. L., & Lyon, I. N. (2000). Voluntary modification of automatic arm movements evoked by motion of a visual target. Experimental Brain Research, 130, 159–168. Desmurget, M., Epstein, C. M., Turner, R. S., Prablanc, C., Alexander, G. E., & Grafton, S. T. (1999). Role of the posterior parietal cortex in updating reaching movements to a visual target. Nature Neuroscience, 2, 563–567. Desmurget, M., & Prablanc, C. (1997). Postural control of three-dimensional prehension movements. Journal of Neurophysiology, 77, 452–464. Fecteau, J. F., Chua, R., Franks, I. M., & Enns, J. T. (2001). Visual awareness and the on-line modification of action. 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Magescas, F., Urquizar, C., & Prablanc, C. (2009). Two modes of error processing in reaching. Experimental Brain Research, 193, 337–350. Paulignan, Y., MacKenzie, C., Marteniuk, R., & Jeannerod, M. (1991). Selective perturbation of visual input during prehension movements I. The effects of changing object position. Experimental Brain Research, 83, 502–512. Pélisson, D., Prablanc, C., Goodale, M. A., & Jeannerod, M. (1986). Visual control of reaching movements without vision of the limb II: Evidence of fast unconscious processes correcting the trajectory of the hand to the final position of a double-step stimulus. Experimental Brain Research, 62, 303–311. Pisella, L., Gréa, H., Tilikete, C., Vighetto, A., Desmurget, M., Rode, G., et al. (2000). An ‘‘automatic pilot” for the hand in human posterior parietal cortex: Toward reinterpreting optic ataxia. Nature Neuroscience, 3, 729–736. Prablanc, C., Echallier, J. F., Komilis, E., & Jeannerod, M. (1979). Optimal response of eye and motor systems in pointing at a visual target. Biological Cybernetics, 35, 113–124. Prablanc, C., & Martin, O. (1992). Automatic control during hand reaching at undetected two-dimensional target displacements. Journal of Neurophysiology, 67, 455–469. Prablanc, C., Pélisson, D., & Goodale, M. A. (1986). Visual control of reaching movements without vision of the limb I. Role of retinal feedback of target position in guiding the hand. Experimental Brain Research, 62, 293–302. Rosetti, Y., & Pisella, L. (2002). Several ‘‘vision for action” systems: A guide to dissociating and integrating dorsal and ventral functions. In W. Prinz & B. Hommel (Eds.), Common mechanisms in perception and action. Attention and performance XIX. Oxford University.
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Soechting, J. F., & Lacquaniti, F. (1983). Modification of trajectory of a pointing movement in response to a change in target location. Journal of Neurophysiology, 49, 548–564. Streimer, C. L., Yukovsky, J., & Goodale, M. A. (2010). Can intention override the ‘‘automatic pilot”? Experimental Brain Research, 202, 623–632.
Contents lists available at ScienceDirect
Human Movement Science journal homepage: www.elsevier.com/locate/humov
Movement duration does not affect automatic online control Erin K. Cressman a, Brendan D. Cameron b, Melanie Y. Lam b, Ian M. Franks b, Romeo Chua b,⇑ a b
School of Human Kinetics, University of Ottawa, Canada School of Human Kinetics, University of British Columbia, Vancouver, Canada
a r t i c l e
i n f o
Article history: Available online 30 August 2010 PsycINFO classification: 2300 2330 Keywords: Vision, Action Automatic guidance Movement time
a b s t r a c t Pisella et al. (2000) have shown that fast aiming movements are automatically modified on-line in response to a change in target position. Specifically, when a movement is less than 300 ms in duration the reach is completed to a target’s new location even when one never intended to respond to the target jump. In contrast, when movements are slower, the reach is completed according to instructions. At present, it is unclear if it is possible for one’s intentions to guide the initial stages of these slow movements. To determine if the intentional control mechanism can guide the initial stages of a slow aiming movement, participants aimed to targets that could jump at movement onset, with a slow and very slow movement time goal. In particular, participants were to point towards (‘‘pro-point”) or away from (‘‘anti-point”) the target jump, with a movement time goal of 500 or 1200 ms. Results showed that in the anti-point condition, movement trajectories first deviated in the same direction as the target jump, followed by a response in the intended (opposite) direction. This suggests that while movement outcome is controlled by the intentional system, even in these slow aiming movements the automatic system is engaged at movement onset. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Rapid aiming movements are modified on-line in response to a change in target position in order to bring the hand to the target’s new location (e.g., Bard et al., 1999; Chua & Enns, 2005; Cressman, Franks, Enns, & Chua, 2006; Day & Lyon, 2000; Desmurget & Prablanc, 1997; Desmurget et al., ⇑ Corresponding author. Address: School of Human Kinetics, University of British Columbia, 210-6081 University Boulevard, Vancouver, BC, Canada V6T 1Z1. Tel.: +1 604 822 1624; fax: +1 604 822 6842. E-mail address: [email protected] (R. Chua). 0167-9457/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.humov.2010.07.001
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1999; Fecteau, Chua, Franks, & Enns, 2001; Goodale, Pélisson, & Prablanc, 1986; Gritsenko, Yakovenko, & Kalaska, 2009; Komilis, Pélisson, & Prablanc, 1993; Magescas, Urquizar, & Prablanc, 2009; Paulignan, MacKenzie, Marteniuk, & Jeannerod, 1991; Pisella et al., 2000; Prablanc & Martin, 1992; Prablanc, Pélisson, & Goodale, 1986; Pélisson, Prablanc, Goodale, & Jeannerod, 1986; Soechting & Lacquaniti, 1983).1 It has been suggested that these adjustments in limb trajectory occur automatically (i.e., independent of one’s intentions). Specifically, Pisella et al. (2000) have argued that fast aiming movements are driven by an ‘‘automatic pilot”, which escapes intentional control. This proposal is based on their findings that for movements completed between 125 and 350 ms, the hand is adjusted and the movement completed to the new target position even when participants are instructed to interrupt their movements in response to a target jump. Similar adjustments are not observed for movements completed in less than 125 ms as, presumably, there is not enough time for the central nervous system to detect errors in the limb’s trajectory in relation to the new target location and correct the unfolding movement (Blouin, Teasdale, Bard, & Fleury, 1995). In support of Pisella’s claim of automatic visuomotor guidance, Day and Lyon (2000; see also Johnson, van Beers, & Haggard, 2002) have also shown that fast aiming movements are rapidly modified in the direction of a target jump, independent of instructions. In their paradigm, participants were required to respond as quickly as possible to a target jump by reaching in either the same (pro-point, PP) or opposite (anti-point, AP) direction. They found that all reaches, both PP and AP, were initially modified in the direction of the target jump, even though this was an incorrect response on the AP trials. Furthermore, these changes in trajectory occurred at a similar latency (125 ms into the movement) for both PP and AP trials. Accordingly, and in agreement with Pisella et al. (2000), Day and Lyon inferred that these adjustments were under ‘‘automatic” control. By examining movement trajectories, Day and Lyon (2000) were able to observe that in their AP task, participants modified their trajectories twice, first in the direction of the target jump (an incorrect modification) and then in the opposite direction (a correct modification). The second modification ensured that participants completed the movement according to their intentions and is similar to the control observed by Pisella and colleagues for slow aiming movements. Specifically, Pisella found that when participants completed slower aiming movements (>300 ms), they did not touch down at a new target position but stopped their movements in response to a target jump as instructed. Taken together, the results of Day and Lyon (2000) and Pisella et al. (2000) demonstrate that while the automatic pilot controls fast aiming movements initially, one’s intentions can control the latter portion of a movement if enough time is provided. This observation raises the question: Can intention guide a goal-directed aiming movement for its entirety (i.e., from start to finish), or is the automatic pilot always engaged at movement onset? Given (1) the differences in movement outcome observed by Pisella et al. (2000) for fast vs. slow movements (the latter never landed at the perturbed location) and (2) the fact that intentional control took over Day and Lyon’s trajectories only after sufficient time had elapsed, it is possible that if we allow participants to make slow movements, the automatic pilot will never be engaged, a proposal that has been put forth in the literature (see Pisella et al., 2000; Rosetti & Pisella, 2002) but never examined directly. This proposal implies that slow movements are prepared differently from fast ones, such that they do not deviate until a top-down process allows them to do so. This would be in contrast to fast movements, which, as we have described above, respond immediately and automatically to target perturbations. In the current study we determined if intention controls the initial stages of a slow aiming movement by examining trajectories to double-step targets. Specifically, we examined the trajectories of slow aiming movements (500 and 1200 ms) completed to double-step targets when participants were instructed to PP or AP in response to a jumping target. In our task, target jumps occurred at movement onset and were visible to participants. An AP movement was adopted as opposed to a stop instruction, as anti-pointing has the benefit of providing (in the form of a trajectory deviation) a clear indication of intentional control onset. Furthermore, given that some of our movements were completed with a very long movement time (MT) goal, we were concerned that if participants were instructed to stop
1 Each of these studies employed a double-step protocol, first introduced by Bridgeman, Lewis, Heit, and Nagle (1979), in which the initial target of a reach changes location at some point prior to or during the reach.
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their movements in response to a target jump, the hand may drift to the target’s new position over time, even though the participant’s first response was a correct interruption of their movement. If we do not find deviations toward the jumping target in the anti-point condition in the slow aiming movements of the present study, it would indicate that intentional control is engaged from movement onset and overrides the automatic pilot. In contrast, if the hand initially deviates towards the target before being intentionally countermanded, it would suggest that the automatic pilot is engaged during aiming, regardless of movement duration, and controls the early portions of the movement until intentional processes take over. 2. Methods Seven right-handed university students (mean age = 25.4 ± 1.3 years) with normal or corrected-tonormal visual acuity volunteered to participate in this study.2 All participants gave informed consent and the study was conducted in accordance with the ethical guidelines set by the University of British Columbia. 2.1. Apparatus All stimuli were presented on a custom-built 18 24 in. display panel oriented 20° from horizontal. Stimuli consisted of red dots of light (5 mm in diameter) produced by light emitting diodes below a transparent Plexiglas sheet. Participants sat with their head in a chinrest and viewed the display panel from a distance of approximately 60 cm. The participants were positioned such that their midline was aligned with the vertical meridian of the panel and a circular home position (12 mm in diameter) located centrally at the bottom of the panel. The initial target was 27 cm directly above the home position. The target could jump 5 cm to the left or right at movement onset. 2.2. Motion analysis Participants pointed with their right-hand using a stylus. An infrared-emitting diode was placed at the tip of the stylus and 3D position was monitored using an OPTOTRAK (Northern Digital, Waterloo, ON, Canada) motion analysis system. The tip of the stylus was equipped with a microswitch that provided a logic signal indicating whether or not the stylus was in contract with the panel. The 3D stylus position and the microswitch status were sampled at 500 Hz. Raw OPTOTRAK data were converted into 3D coordinates and filtered using a low-pass Butterworth filter (2nd order, dual pass, 10 Hz cut-off). Start and end of the movement were determined by the microswitch status. 2.3. Procedure and design Participants completed all reaches in a dark room that minimized vision of the hand. In the first set of reaches, participants completed 25 practice trials to the center target. They completed these movements with a MT goal of 500 or 1200 ms and MT feedback was provided verbally after each trial. After ensuring that participants could perform the movement in the time allotted (i.e., 95% of the last 20 practice trials were completed within 400–600 ms for the 500 MT goal and between 1000 and 1400 ms for the 1200 ms MT goal), participants completed two testing blocks of 96 trials. In each testing block, a trial began with the appearance of the target. Participants were instructed that this was not a reaction time task and as such, they could begin their movement anytime following target onset. On 1/3 of the trials, the target would jump randomly either 5 cm to the left or right of its initial position at movement onset (32 perturbed trials: 16 left-jump + 16 jump-right trials). The perturbed trials were randomly interleaved with the 64 unperturbed (non-jump) trials within a testing block. In the ‘‘pro-point” (PP) testing block, participants were instructed to respond to the target jump by pointing to its new location. In the ‘‘anti-point” (AP) block, participants were to respond to a target jump by 2
Data from an eighth participant was collected but due to technical errors during collection, we were unable to analyze the data.
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reaching in the opposite direction to the jump and touching down at an imagined spot on the screen at the same distance as the target jump. MT feedback was provided on all unperturbed trials in order to aid participants in adhering to the MT goal. After completing the two testing blocks, participants performed all tasks a second time with the second MT goal. Thus, if participants began performing the task with a goal MT of 500 ms (e.g., PP500 and AP-500), they now completed a second round of 25 practice trials and the two testing blocks (PP and AP) with a MT goal of 1200 ms (e.g., PP-1200 and AP-1200). The MT goal and the order of the PP and AP testing blocks were counterbalanced across participants, such that if participants completed the PP block first within a given MT goal, they again performed the PP block first when pointing with the second MT goal. Two participants completed each MT-testing block sequence except for the PP-500, AP-500, PP-1200, AP-1200 sequence, which only one participant completed. All participants showed similar patterns of responses independent of testing order. 2.4. Data analyses In order to ensure that participants completed the task as instructed (i.e., unperturbed reaches landed at the center target position and were performed in the time allotted), we analyzed both mean lateral displacement errors and movement times achieved on unperturbed trials in a 2 MT goal (500 ms vs. 1200 ms) 2 Instruction (PP vs. AP) repeated measures analysis of variance (RM ANOVA). We then determined if participants adjusted their trajectories on the perturbed trials in response to a target jump by classifying pointing responses according to the target to which they were completed. A pointing response on a perturbed trial was classified as having been correctly modified if its endpoint fell more than 2.5 cm to the left or right of the center target position in the direction of the target jump (PP) or opposite the direction of the target jump (AP). For movements that were modified in response to the target jump and completed in the correct direction (886 out of a possible 896 movements), we next examined the corresponding trajectories in order to determine the direction in which participants first modified their reaches. In order to examine adjustments in participants’ trajectories, we first plotted all of the trajectories for each participant as illustrated in Fig. 1. We then derived average spatial trajectories from trials completed to the correct eccentric positions as shown in Fig. 1C and D. Average spatial trajectories were attained for each stimulus condition (left-jump, right-jump, unperturbed), by calculating spatial averages in the 2D plane of the movement. The 2D plane of movement was defined by display coordinates such that the target stimulus was initially located 27 cm forward (y-direction) with a lateral position of 0 (x-direction). The points in the average spatial trajectories were obtained by deriving the mean spatial position in the x–y plane from spatial coordinates in the corresponding trials at 2 ms intervals. From these plots it appeared that for all participants the hand was first pulled in the direction of the target jump, regardless of instruction or movement time goal. To quantify the time at which participants modified their movements in the perturbed trials, we derived the mean and standard deviation of lateral velocity profiles for each participant, and for each condition from corresponding displacement profiles. t-Tests were then used to compare each mean perturbed path with its corresponding mean unperturbed path at all 2-ms time intervals from 100 ms onward. We designated 100 ms as a minimum time constraint as visual examination of the mean paths revealed that velocity profiles did not start to diverge from each other until at least 110 ms into the movement. A mean perturbed path was designated as diverging significantly from the mean unperturbed path when the t-test was significant, provided that the difference between paths remained for at least the next 35 time intervals (70 ms). We found two points in time at which participants modified their movements on AP perturbed trials with respect to the unperturbed trials when the target jumped right. This was consistent with the observation that participants appeared to first change their trajectories in the direction of the target jump on AP trials, and then subsequently correct their movements to complete the task as instructed. The early time point signifies when participants modified their trajectories in the direction of the target jump
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Fig. 1. Examples of spatial trajectories. In (A) and (B) we show examples of trajectories for one participant when the movement time was 500 and 1200 ms, respectively. In (C) and (D) we show spatial trajectories averaged across all participants. Trajectories are plotted in two-dimensions such that the y values represent forward motion and the x values provide lateral position information. The mean unperturbed paths are represented by circles, the solid lines denote mean trajectories completed under the ‘‘pro-point” instructions (black line = right-jump trials and grey line = left-jump trials), and the dashed lines correspond to mean trajectories completed under the ‘‘anti-point” instructions (grey dashed line = right-jump trials and black dashed line = left-jump trials). Note that in (C) and (D) the trajectories of the perturbed anti-point trials appear to be completed before achieving the ‘‘target” position. This was not the case. In order to ensure that the full extent of the mean trajectory included data from all participants we had to cut the trajectories to the shortest MT achieved.
and the second time point indicates when trajectories were modified from the unperturbed profiles in the instructed direction. In contrast to the right-jump AP trials, analyses revealed significant deviations in trajectories on AP trials when the target jumped left for only about half of the participants. To determine if similar processes were engaged during early adjustments in the PP and AP conditions, we compared latencies of trajectory modifications when the target jumped right in a 2 MT goal (500 ms vs. 1200 ms) 2 Instruction (PP vs. AP) RM ANOVA. Finally, we compared final trajectory modifications across both left- and right-jump trials in a 2 MT goal (500 ms vs. 1200 ms) 2 Direction (left vs. right) RM ANOVA. Because the PP trials demonstrated only 1 early correction, we did not include these trials when examining final trajectory modifications.
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3. Results 3.1. Unperturbed trials Participants were very accurate on unperturbed trials with respect to lateral pointing errors. In Fig. 1 we show examples of all trajectories completed to the center target for one participant when the movement time goal was 500 ms (Fig. 1A) and 1200 ms (Fig. 1B). These unperturbed profiles demonstrate that the participant made consistent, straight movements to the target. On average, participants completed reaches to the center target with a lateral constant error of 2.5 mm (SD 2.5, range = 1.8–6.8 mm) from the middle of the target, in a movement time of 504 ms (SD 13) when the MT goal was 500 ms and 1172 ms (SD 29) when the MT goal was 1200 ms. RM ANOVA revealed that spatial accuracy was independent of MT goal and Instruction (MT goal: F(1, 6) < 1, Instruction: F(1, 6) < 1, MT goal Instruction: F(1, 6) = 1.213, p = .313). Mean MT on unperturbed trials was also independent of Instruction (Instruction: F(1, 6) < 1, MT goal Instruction: F(1, 6) < 1).
3.2. Perturbed trials: automatic responses Even though we used a conservative criterion for establishing if a movement had been updated in response to a target jump, given the high accuracy achieved during unperturbed trials, only 10 perturbed trials in total failed to meet our criterion. All of these error trials occurred when the MT goal was 500 ms and on the majority of these trials (7 trials) participants touched down at the wrong position in the AP-500 condition (an example of one such trial is shown in Fig. 1A). Specifically, participants adjusted their trajectory in the direction of the target jump and completed their movement to the actual target location. These error trials are similar to the results obtained by Pisella et al. (2000), when their participants completed very fast movements (200–300 ms) and incorrectly touched down at a new target position, raising the possibility that even relatively slow aiming movements of 500 ms are automatically captured by a target jump. Fig. 1 illustrates this proposed hand capture in more detail. Example trajectory profiles are shown in Fig. 1A and B for one participant and pointing trajectories averaged across participants for all stimulus conditions are shown in Fig. 1C and D when the MT goal was 500 and 1200 ms, respectively. As noted previously, on unperturbed paths (middle path designated by circles in Fig. 1C and D) the hand moved in a fairly straight line toward the center target. The trajectories on perturbed trials also appear to have been initiated towards the center target. However, these trajectories were then modified in the direction of the target jump, regardless of instruction or MT goal. In the PP condition, these initial adjustments in movement were carried to completion and participants touched down at the new location. In contrast, in the AP condition, these early adjustments were countermanded and the limb further adjusted to a position opposite the direction of the target jump. To determine the latency of the trajectory modifications on the perturbed trials, we compared perturbed vs. unperturbed lateral velocity profiles at different time points using t-tests. In accordance with the displacement data, the velocity profiles indicated that perturbed trajectories were first modified in the direction of the target jump when the target jumped right, even for the AP trials (see Fig. 2). Specifically, for all participants, analyses of the AP trials revealed that the hand was first modified in the direction of the target before reversing direction when the target jumped right. On the PP trials, the hand was also modified in the direction of the target but, in contrast to the AP trials, no lateral velocity profiles exhibited reversals in direction. The latency range of the initial modifications in trajectory across all target jump-right conditions was 140–276 ms (mean latency = 202 ms (SD 34)). As observed in Fig. 2A and B (see also 2C), RM ANOVA (2 MT goal 2 Instruction) revealed that movements were adjusted at a similar latency in the direction of the target jump regardless of MT goal (F(1, 6) = 2.423, p = .171) or instruction (F(1, 6) < 1). On AP trials in which the target jumped left, trajectories deviated significantly in the direction of the target jump for approximately half of the participants (four participants when the MT goal was 500 ms and two participants when the MT goal was 1200 ms). Latencies of these initial corrections were within the range obtained for trials in which the target jumped right (range: 188–276 ms, mean
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Fig. 2. Examples of mean lateral velocity profiles for one participant when the target jumped right: (A) 500 ms MT goal and (B) 1200 ms MT goal. Velocity profiles are plotted such that time (up to 600 ms) is represented on the vertical axis and velocity magnitude on the horizontal axis. The mean unperturbed paths are represented by grey lines. Solid lines denote mean trajectories completed under the ‘‘anti-point” instructions when the target jumped right. The dashed lines indicate the latencies at which the mean perturbed velocity profiles diverge from the unperturbed paths, first in the direction of the target jump and then in the instructed direction. (C) Mean latencies (ms) for early trajectory modifications when the target jumped right. Results are displayed as a function of instruction (‘‘pro-point” vs. ‘‘anti-point”) and movement time goal. Error bars denote standard deviations.
latency = 224 ms (SD 33)). A 2 MT (500 vs. 1200) 2 Direction (left-jump vs. right jump) RM ANOVA revealed that there were no differences in the latency of corrections on PP trials when the target jumped right vs. left (F(1, 6) < 1). 3.3. Perturbed trials: intentional responses As shown in Figs. 1 and 2, early, incorrect, limb adjustments in the AP trials were followed by a later, final correction that brought the limb to the intended position. RM ANOVA indicated that this adjustment occurred earlier when the MT goal was 500 ms compared to 1200 ms (F(1, 6) = 17.565, p = .006), suggesting that the latency of late, intentional corrections scaled to MT (see Fig. 3). In addition, adjustments were earlier in response to a target jumping left vs. right (F(1, 6) = 7.627, p = .033). 4. Discussion The goal of the present experiment was to investigate visuomotor processing during slow aiming movements. In particular, we examined if intentional (volitional) or automatic (unintended) motor responses control the initial stages of a slow aiming movement. To address this question we had
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Fig. 3. Mean latencies (ms) for final trajectory modifications under the ‘‘anti-point” instructions. Results are displayed as a function of target jump direction (left vs. right) and movement time goal. Error bars denote standard deviations.
participants make slow aiming movements and adapted experimental protocols that have previously been shown to distinguish between intentional and automatic motor responses during movement execution (Cressman et al., 2006; Day & Lyon, 2000; Johnson et al., 2002; Pisella et al., 2000). Participants pointed either in the direction of the displaced target (PP) or in the opposite direction (AP) with a MT goal of 500 and 1200 ms. These slow aiming movements were chosen specifically as previous work has demonstrated that the outcome of these movements is controlled by intention and suggested that voluntary control may guide the movement for its entirety (i.e., they do not engage the same automatic visuomotor processes as fast aiming movements; see Pisella et al. (2000) and Rossetti and Pisella (2002)). Similar to previous results, we found that the latter portion of a slow aiming movement was controlled by an intentional mechanism that ensured the movement was completed according to instructions. However, in contrast to our expectations, we found that automatic processes guided the movement initially, even for very slow movements. This was shown by the finding that for both slow (500 ms) and very slow (1200 ms) aiming movements, the limb was first pulled to the right when the target jumped right, even when participants were instructed to move in the opposite direction of the displaced target. Furthermore, the latency at which the hand was captured by the target (approximately 200 ms) was not influenced by MT goal or instruction suggesting that the same automatic visuomotor processes, leading to adjustments in limb trajectory, are recruited regardless of the temporal demands of the task. The engagement of automatic visuomotor processes is further revealed if we compare our estimates of the time of change in limb trajectory to previous work examining early adjustments to double-step targets when participants reached with fast movement time goals (Cressman et al., 2006; Day & Lyon, 2000; Paulignan et al. 1991). While we observed slightly later adjustments in the current study compared to previous results, this delay in movement correction is due to the fact that we defined the latency of correction in the present study as the time when trajectory profiles significantly diverged from each other. After trajectory modifications are initiated it takes some time for a significant separation between the unperturbed and perturbed profiles to be achieved, creating the longer latencies observed in the present results. If we were to determine the time at which the perturbed profiles for each participant initially diverged from corresponding unperturbed profiles through visual inspection (as done by Day & Lyon, 2000), we find that the latency at which the hand was captured by the target was approximately 130 ms. This estimate is similar to the latencies observed in previous work examining early adjustments to double-step targets during fast aiming movements (Cressman et al., 2006; Day & Lyon, 2000; Paulignan et al. 1991). Thus, given the temporal consistency in trajectory changes across experiments with different movement time goals, it appears that the same automatic visuomotor processes are engaged during fast and slow movements.
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4.1. The influence of the target jump direction on automatic visuomotor processes So far, we have discussed automatic deviations of slow aiming movements when the target jumps right. But what happens when the target jumps left? Is the automatic pilot still engaged? In the current study, individual displacement profiles revealed that the hand was pulled in the direction of the target on AP trials when the target jumped left. Moreover, PP trials deviated to the left at a similar latency as PP trials deviated to the right, suggesting engagement of automatic visuomotor processes even on trials in which the target jumped left. While results indicate that automatic visuomotor processes were engaged when the target jumped left, trajectory adjustments did differ depending on the direction of the target jump. In particular, statistical analyses revealed significant deviations in the leftward direction on AP left-jump trials for only about half of our participants, compared to all participants when the target jumped right and earlier adjustments on AP trials in the intended direction when the target jumped left (i.e., adjustments to the right) than when the target jumped right. These findings suggest that participants may have been able to counteract automatic responses earlier on left-jump trials compared to right-jump trials. However, given that we did not instruct participants on how to correct their movements if they found themselves reaching in the incorrect direction, this proposal is put forth as a hypothesis. Previous work examining reaches to stationary and perturbed targets in ipsilateral and contralateral space relative to body midline has demonstrated that movement kinematics are dependent on a target’s location (Castiello, Bennett, & Chambers, 1998; Fisk & Goodale, 1985; Prablanc, Echallier, Komilis, & Jeannerod, 1979). In particular Castiello et al. (1998) have shown that while reaches to jumping targets are initiated at a similar latency regardless of whether the target jumps left or right relative to body midline, differences are seen in later kinematic markers. Moreover, Fisk and Goodale (1985) have shown that the velocity of the limb is dependent on the spatial position of the target relative to the body axis. A greater peak and average velocity is achieved in movements made to targets in ipsilateral space compared to targets in contralateral space. It has been suggested that these differences in reaches arise due to the necessity of transferring information across hemispheres from visual to motor structures in contralateral reaches. This interhemispheric transfer is not present in ipsilateral reaches (Prablanc et al., 1979). As well, differences may arise due to the isometric contraction time of different muscles controlling hand position (Prablanc et al., 1979) or the natural synergies between elbow and shoulder joints. Regardless of the source of kinematic differences between ipsilateral vs. contralateral reaches, based on previous work, we would expect a lower lateral velocity to be achieved in the incorrect direction when the target jumped left compared to right. It is hypothesized that this lower lateral velocity would then be more easily overcome, resulting in earlier intentional corrections. However, as suggested above, given that we did not instruct participants on how to correct their movements, or control for the extent or speed of trajectory adjustments, we are unable to examine this proposal directly with the current data. Future research is required in order to determine if one’s ability to overwrite automatic adjustments after they have been initiated is dependent on direction of movement. For now, the results from the perturbed trials indicate that participants’ movements were initially guided by an automatic pilot, regardless of the direction of the target jump. It was only later, after enough time had passed, that movements came under intentional control. 4.2. The relevance of the target jump to automatic visuomotor processes The results from the current study raise the question: Are there circumstances under which the automatic pilot is not engaged at all? In the current study we looked for automatic adjustments in trajectories while manipulating the MT goal. In all of our conditions the target jump was relevant to the task and dictated how participants were to adjust their movements (i.e., PP or AP). Given that there was ample time for participants to correct their movements after they first moved in the incorrect direction, perhaps it is not surprising that the automatic pilot was engaged. Recent work examining the engagement of the automatic pilot has also focused on manipulating instructions but, instead of changing the temporal demands, Cameron, Cressman, Franks, and Chua (2009) and Streimer, Yukovsky, and Goodale (2010) explicitly instructed participants to ignore the target jump. In these cases the target jump was not relevant to the task and results demonstrated that the
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influence of the automatic pilot was reduced when one was told to ignore the target jump and complete the movement to the target’s original position. Thus, in contrast to the present results, the results of Cameron et al. (2009) and Streimer et al. (2010) indicate that there are situations in which the automatic pilot can be disengaged. However, given the circumstances under which the automatic pilot becomes disengaged and the results of the present study, we suggest that if the jump is relevant to the task (which is typically the case when reaching to a target) and there is no explicit instruction indicating that one is not to follow the target, the hand is susceptible to being captured by the jumping target, regardless of the movement’s duration. Acknowledgments This research was supported by the Natural Sciences and Engineering Research Council of Canada. References Bard, C., Turrell, Y., Fleury, M., Teasdale, N., Lamarre, Y., & Martin, O. (1999). 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