Hand Path Priming in Manual Obstacle Avo (1)

Spatiotemporal Forms in Human Motor Control

Robrecht P.R.D. van der Wel Robin M. Fleckenstein

Pennsylvania State University Duke University

Steven A. Jax David A. Rosenbaum

Moss Rehabilitation Research Institute Pennsylvania State University

Previous research suggests that motor equivalence is achieved through reliance on effector-independent spatiotemporal forms. Here the authors report a series of experiments investigating the role of such forms in the production of movement sequences. Participants were asked to complete series of arm movements in time with a metronome and, on some trials, with an obstacle between 1 or more of the target pairs. In moves following an obstacle, participants only gradually reduced the peak heights of their manual jumping movements. This hand path priming effect, scaled with obstacle height, was preserved when participants cleared the obstacle with 1 hand and continued with the other, and it was modulated by future task demands. The results are consistent with the hypothesis that the control of movement sequences relies on abstract spatiotemporal forms. The data also support the view that motor programming is largely achieved by changing just those features that distinguish the next movement to be made from the movement that was just made.

Keywords: sequence production, motor equivalence, movement planning, obstacle avoidance, reaching

A key phenomenon of human perception and performance is Loukopoulos, and Vaughan (1996), who extended a general model motor equivalence , the capacity to achieve the same output

of motion planning (Rosenbaum, Engelbrecht, Bushe, & Louko- through different means (Lashley, 1930). One of the best known

poulos, 1993; Rosenbaum, Loukopoulos, Meulenbroek, Vaughan, examples of motor equivalence comes from handwriting, where an

& Engelbrecht, 1995; Rosenbaum, Meulenbroek, Vaughan, & individual’s graphic output is recognized to be his or hers regard-

Jansen, 2001) to writing and drawing. Following Berkenblit and less of the means by which it is generated—whether scrawled

Feldman (1988) and Keele, Cohen, and Ivry (1990), Meulenbroek across a blackboard or inscribed on a check, whether produced

et al. suggested that when people write or draw, they access with the preferred or nonpreferred hand, with the feet, or even with

abstract spatiotemporal forms. According to Meulenbroek et al., the pen clenched between the teeth. Researchers concerned with

writing and drawing are achieved by generating a series of move- motor equivalence have shown that written script is indeed ap-

ments, each of which goes through a via point and then to a goal proximately invariant with the effector used, the size of the gen-

point. The via points are points of maximal speed, whereas the erated script, and the orientation of the writing surface (Castiello &

goal points are points of minimal speed. Using computer simula- Stelmach, 1992; Lashley, 1942; Merton, 1972; Raibert, 1977;

tions rendered as stick-figure animations, Meulenbroek et al. Swinnen, 1991; Wright, 1993).

showed that it was possible, with these assumptions, to generate How is motor equivalence achieved? One attempt at answering

the same written output with different effectors on different planes this question came from Meulenbroek, Rosenbaum, Thomassen,

and with different sizes. Their model therefore instantiated a possible solution to the problem of motor equivalence.

Robrecht P.R.D. van der Wel and David A. Rosenbaum, Department of On what basis did Meulenbroek et al. (1996) argue that abstract Psychology, Pennsylvania State University; Robin M. Fleckenstein, De-

spatiotemporal movement forms are used to guide writing and partment of Physical Therapy, Duke University; Steven A. Jax, Moss

drawing, aside from the fact that their simulations worked reason- Rehabilitation Research Institute, Philadelphia, Pennsylvania.

ably well? These authors noted that people can learn the order in This work was supported by National Science Foundation Grant SBR-

which spatial targets are presented (Kagan, 1971; Keele et al., 94-96290, National Institute of Mental Health Grants KO2-MH0097701A1

1990), and they appealed to the fact that psychophysical experi- and R15 NS41887-01, and grants from the Social Science Research Insti-

ments have shown that people can mentally project images onto tute and the Office of Research and Graduate Studies, College of Liberal

different distal surfaces, even adjusting the size of the projected Arts, Pennsylvania State University. We thank Peter Strick for suggesting

the second experiment. We also thank Meesha Ahuja, Joshua Albert, Mike images if they wish (Kosslyn, 1980). These arguments supported

Iskoe, Christine Schiller, Allie Schubert, Mycheel Stubbs, Dana Voelker, the hypothesis that actors have access to effector-independent and Matthew Walsh for help with data collection.

spatiotemporal forms. However, the arguments do not necessarily Correspondence concerning this article should be addressed to Robrecht

prove that those forms play a role in online movement production. P.R.D. van der Wel or David A. Rosenbaum, Department of Psychology,

Evidence for the latter proposition has recently come from Jax Pennsylvania State University, University Park, PA 16802. E-mail:

and Rosenbaum (in press). In their experiments, participants sat at rxv906@psu.edu or dar12@psu.edu

a table and held a vertically oriented dowel that stood on a wide

VAN DER WEL, FLECKENSTEIN, JAX, AND ROSENBAUM

circular disk with felt on its bottom, allowing the manipulandum to positions were certain, that outcome would curtail the generaliz- slide smoothly from one position to another on the table. An

ability of Jax and Rosenbaum’s conclusions. OPTOTRAK motion tracking device registered participants’

To address this concern, in the present experiments we used a movements. Participants saw a computer-generated stick-figure

procedure in which participants had full knowledge of targets and image of their right arm on a TV screen. This stick-figure image

obstacles before interacting with them. We asked participants to moved as a participant’s right arm did, with no noticeable delay

hold a dowel, using a power grip, and to tap the base of the dowel after the participant moved. Besides seeing an image of his or her

on each of a series of targets in time with a metronome (see Figure own right arm, the participant also saw targets for movement and,

1). The targets were all fully visible before each trial began and in some conditions, obstacles. The targets were displayed in the

remained fully visible while the trial was underway. The targets context of a center-out movement task. A circle appeared in the

were arranged in an arc on a table. In the control conditions, there middle of the screen, and the participant brought his or her hand

was no obstacle between any targets, but in the experimental trials, marker into the circle, whereupon a circle appeared at some point

an obstacle (a vertical piece of cardboard) stood between a pair of along the rim of an imaginary circle around the center circle. The

targets, again in full view of the participant before and during the participant’s task was to move the hand marker to the target and

trial. When an obstacle was present, participants were asked to then to return it to the center circle as quickly as possible.

carry the dowel over the obstacle in time with the metronome, In one control condition of Jax and Rosenbaum’s (in press)

tapping the targets on either side of it in time with the metronome. experiments, no obstacle ever appeared between the center circle

The question was what would happen to the jumps from target to and target circle. In another control condition, an obstacle always

target after, and also before, the obstacle was cleared. appeared between the center circle and a target circle. In the latter

If the hand path priming effect generalizes to situations in which condition, the participant was expected to make circuitous move-

there is no uncertainty about targets and obstacles, one would ments around the obstacle. Of greatest interest were the experi-

expect to see the effect in the present experiment. In particular, mental conditions. Here, an obstacle sometimes appeared between

jump heights between targets after obstacles are cleared should be the center circle and target, but the obstacle’s appearance was

higher than jump heights between those same targets when no unpredictable. If an obstacle appeared, it always appeared at the

obstacles are cleared. The latter outcome would be expected if moment the target came on and always stood midway between the

hand path priming involves abstract spatiotemporal forms that are center circle and the target (as was the case in the control condi-

carried over in successive movements. The abstractness of the tion, in which an obstacle always appeared).

forms would be supported by the fact that the successively en- The question of primary interest was what would happen on

countered targets occupied different locations in the workspace trials in which an obstacle was possible but did not appear. The

and were reached with different limb positions and different mus- answer, as Jax and Rosenbaum (in press) discovered, was that on

cle groups.

those trials, participants made movements whose curvature ex- Regarding what would happen to the jumps between targets ceeded the curvature of movements made when obstacles never

before obstacles were cleared, if participants anticipated forthcom- appeared. Jax and Rosenbaum called this phenomenon the hand path priming effect.

Of special importance to the claim that there are abstract spa- tiotemporal forms for movement, the hand path priming effect generalized over the workspace. It was not necessary to repeat the same target on successive trials to get the effect. Instead, greater than normal curvature was observed for movements to obstacle- free targets that were removed from the last target tested. On the basis of this observation, Jax and Rosenbaum (in press) argued that their participants relied on abstract spatiotemporal forms (i.e., hand paths that were not tied to particular spatial positions in the workspace nor to specific muscles). Jax and Rosenbaum also suggested that an advantage of relying on these abstract spatio- temporal forms is that they help eliminate the need for planning of movements from scratch. They proposed that the spatiotemporal form of one movement could be applied to, or retained for, the plan of the next movement to come.

A question that can be raised about Jax and Rosenbaum’s (in press) study concerned the generalizability of its conclusions. Jax and Rosenbaum obtained the hand path priming effect when target positions and obstacle positions were uncertain. (Recall that these appeared suddenly on a computer screen, with the participant not

Figure 1. Overview of the experimental setup (rendered in MATLAB; knowing from trial to trial where a target would appear or whether MathWorks, Natick, MA). Participants held a dowel with the right hand and moved it back and forth from target to target and from left to right or

it would be accompanied by an obstacle.) Thus, it is possible that right to left, tapping the targets in time with a metronome. The gray the effect depended critically on such uncertainty. If that were the

rectangle between two of the targets depicts an obstacle. The obstacle case, one might not expect the effect when target positions and

occupied all possible intertarget positions in the experimental trials. Three obstacle positions are known in advance. If the hand path priming

infrared-emitting diodes (IREDs) near the top of the dowel recorded the effect failed to materialize when target positions and obstacle

dowel position. The IREDs are not depicted in this figure.

1119 ing jumps, one would expect to see the jump heights change as the

HAND PATH PRIMING

Before the start of the experiment, participants practiced moving obstacles were approached. Finding such an anticipatory effect,

in time with the metronome but with no obstacle present. Data which was not possible in the method used by Jax and Rosenbaum

collection began when the participant reported feeling comfortable (in press), would add to the list of anticipatory phenomena in

with the task and when the experimenter judged the participant’s perceptual–motor skills.

performance to comply with the instructions. This usually occurred within 1–2 min.

Experiment 1 In each trial, the experimenter asked the participant to start either on the leftmost or rightmost target. The starting target was

Method counterbalanced across trial blocks. The participant was invited to start moving when he or she felt that he or she had internalized the

Participants. Thirty-six Pennsylvania State University stu- beat. If the participant started on the left, he or she moved from dents (12 male, 24 female) from an introductory psychology class

target to target in the rightward direction, all the way to the farthest participated for class credit. They ranged in age from 18 to 22

target on the right, and then back to the left, tapping all the targets years. The Penn State Institutional Review Board approved this

in between, whereupon he or she returned toward the right again, and all of the other experiments reported here. The rights of all the

and so on. If the participant started on the right, the sequence was participants were protected.

reversed. Participants performed this back-and-forth sequence five Apparatus, procedure, and design. Participants sat at a table

times on each trial without interruption. The experimenter told (122 cm wide, 61 cm deep, 78 cm high) with six target positions

participants to keep moving until they heard the verbal instruction evenly spaced around a semicircle whose radius was 41 cm (see

“Stop.” The experimenter also told the participants not to worry Figure 1). The targets were red foam dots (7 cm in diameter and

about making a few extra jumps after hearing the “Stop” com-

0.2 cm thick) that lay flat on the table 28.5 cm apart (center of one mand. The experimenter counted the back-and-forth movements target to center of the adjacent target). Participants held a wooden

and issued the “Stop” command after the participant hit the first dowel (20.2 cm high and 3 cm in diameter, weighing 99 g) with the

target in the sixth cycle.

right hand, using a power grip and keeping the little finger as close Each participant completed 20 trials, with 10 obstacle-absent as possible to the base of the dowel. Participants transported the

control trials and 10 obstacle-present trials starting on the right and dowel from target to target using a “jumping” movement such that

on the left, randomized across participants. The experimenter told they lifted the dowel off the target and made an arcing movement

the participant to take a break whenever he or she wanted, but that led to the dowel’s impact with the next target.

preferably not in the midst of a trial. After every 10th trial, the In the experimental conditions, an obstacle was placed between

experimenter mandated a break.

any given pair of targets, equidistant between them. The height of To evaluate the influence of moving over an obstacle, we the obstacle was varied between participants (12 participants per

subtracted the peak movement heights in the control (obstacle- obstacle height). The short obstacle was 7.5 cm high, the medium

absent) trials from the peak movement heights in the experimental obstacle was 15.0 cm high, and the tall obstacle was 22.5 cm high.

(obstacle-present) trials. The subtraction was done on a The three obstacles were made of sturdy pieces of cardboard

participant-by-participant, target-pair-by-target-pair, obstacle- attached to the vertical edge of a metal bookend that was secured

position-by-obstacle-position, and movement-direction-by- to the table in the position being tested.

movement-direction basis. Thus, for each participant, we estab- The experimenter asked participants to carry out the movements

lished a difference score in peak height for the movement from one in time with a metronome, which clicked every 0.60 s (1.67 Hz).

target a to the adjoining target b by subtracting the mean peak The instructions emphasized timing, so the main concern, as

height for the jump between targets a and b in the control condition expressed to the participants, was that they tap the base of the

from the mean peak movement height for the jump between targets dowel on the series of targets in time with the metronome. The

a and b in the obstacle-present condition. The resulting values experimenter also asked the participants to move over the obstacle

indicated a difference in peak movement height between the ex- rather than around it. To accommodate differences in participants’

perimental and control conditions, with positive values indicating arm lengths, the experimenter asked each participant to adjust his

higher movements, negative values indicating lower movements or her sitting position so his or her right arm was fully extended in

(which were theoretically possible), and 0 indicating no difference. the forward direction while holding the dowel between the two

We defined the peak height value for each intertarget movement central targets.

as the highest position of the dowel during a movement between a To record the dowel position, we attached three infrared-

target pair. We excluded trials if the OPTOTRAK failed to record emitting diodes (IREDs) around the top portion of the dowel so the

the position of any of the three dowel IREDs, if the participant hit dowel would always be in view of the OPTOTRAK 3020 motion

the obstacle, or if the participant failed to hit a target circle. tracking system (Northern Digital, Inc., Waterloo, Ontario, Can- ada) used to track participants’ movements. We also attached a

Results

fourth IRED to the top of the obstacle to record whether partici- pants collided with the obstacle at any time during a trial, in which

Peak movement heights. As shown in Figure 2, the peak case the trial was rerun. The experimenter taped the wires of the

movement heights after clearing the obstacle only gradually de- IREDs to the participant’s right arm with athletic tape. The wires

creased back to baseline in the postobstacle movements. In addi- were affixed to the participant’s arm in a way that allowed the

tion, the gradual decrease scaled with obstacle height, such that participant to move the arm freely within the confines of the

clearing a higher obstacle led to higher successive movements. workspace. The OPTOTRAK sampled the positions of the IREDs

To analyze these effects, we evaluated jump heights using a 3 at 100 Hz.

(obstacle height) ! 5 (obstacle location) ! 8 (movement number)

VAN DER WEL, FLECKENSTEIN, JAX, AND ROSENBAUM

the height of the obstacle, such that a higher obstacle led to a larger constant increase in peak movement height.

The statistical tests of this effect took into account the possibil- ity that the overall increase in jump height prior to obstacles was not, in fact, only anticipatory but may have also reflected a gradual decrease in peak movement height after clearing a previous obsta- cle (i.e., a long-term perseveration effect). To test for anticipation without the possible contamination of long-term perseveration, we conducted an analysis of movements made before any obstacle was cleared within a trial, comparing those jump heights to the analogous intertarget jumps in the control condition. We included only the conditions in which the obstacle was between Targets 1 and 2 or between Targets 5 and 6 (see Figure 1), because in those conditions participants made the most movements (four of them) before confronting an obstacle for the first time on a trial. We omitted the very first movement from the analysis to avoid start-up effects.

The results of the 2 (obstacle presence) ! 2 (obstacle loca- tion) ! 3 (movement number) within-subject ANOVA with ob- stacle height as a between-subjects factor revealed a main effect of obstacle presence, F(1, 33) " 5.169, p # .05, such that participants made higher jumps in the obstacle-present conditions than in the obstacle-absent conditions (mean difference " 5.817 mm, SE " 2.559). The ANOVA revealed no other significant main effects or interactions (all ps $ .05). Thus, participants did in fact anticipate the obstacle from the start of the trials, as revealed by their higher

Figure 2. Peak movement heights above baseline for each obstacle po- jumps before an obstacle was encountered than in the comparable sition when clearing a low (ƒ), medium (▪), or high (‚) obstacle. The black

control conditions in which no obstacle would be encountered. bar indicates the positions of the obstacle, which are also shown on the

Movement times. To check that participants performed the task right. Movement number corresponds to {rem[(1:31),10] & 1}, such that

movement number " 1 contains movements 1, 11, and 21 of the sequence, in time with the metronome, we calculated the mean times between movement number " 2 contains movements 2, 12, and 22 of the sequence,

successive target landings, defining the moment of landing on the and so on. Error bars represent plus or minus 1 standard error.

target as the time when the velocity first fell below 15 mm/s in the vertical direction and the dowel was in the target area. The mean movement time for each obstacle height turned out to be 0.60 s (all

repeated measures analysis of variance (ANOVA). Obstacle height SE s ! 0.01), which was the same as the prescribed value. was treated as a between-subjects factor, and obstacle location and

To determine the influence of the independent variables on movement number were treated as within-subject factors. We

movement, we conducted a 3 (obstacle height) ! 5 (obstacle applied a log10 transformation to the data prior to the analysis to

location) ! 10 (movement number) repeated measures ANOVA correct for skew. We also excluded movements over the obstacle

with obstacle location and movement number as within-subject so as to avoid spuriously significant main effects or interactions

factors and obstacle height as a between-subjects factor. The due to those movements, which were by necessity much higher

ANOVA yielded an Obstacle Location ! Movement Number ! than movements for which no obstacle had to be cleared.

Obstacle Height interaction, F(7.706, 127.145) " 4.384, p # .01, The ANOVA, with Greenhouse–Geisser correction to the de-

and an Obstacle Location ! Movement Number interaction, grees of freedom where appropriate, revealed an Obstacle

F (3.853, 7.706) " 89.622, p # .01. Whereas movements over the Height ! Obstacle Location ! Movement Number interaction,

obstacle took longer than the prescribed duration, movements after

F (18.342, 302.643) " 4.720, p # .01. The effect of a preceding clearing the obstacle took less time than the prescribed duration. obstacle generalized over the workspace, as reflected in the Ob-

Movements in the obstacle-absent conditions did not depart from stacle Location ! Movement Number interaction, F(9.171,

the prescribed duration.

302.643) " 60.772, p # .01. The gradual decrease in peak move-

A follow-up analysis focused on the correlation between move- ment heights back to baseline scaled with obstacle height, such that

ment times and peak movement heights for each obstacle height clearing a higher obstacle led to higher successive movements.

separately but with movements over the obstacles removed. The This result was reflected in an Obstacle Height ! Movement

analysis revealed a significant correlation (r " %.160, p # .01) for Number interaction, F(8.885, 146.603) " 2.255, p # .05. The

the medium obstacle height but not for the low or high obstacle same qualitative pattern of results was found for every obstacle

heights ( ps $ .10).

location. Regarding preobstacle jump heights, the results gave little hint

Discussion

of a gradual increase before clearing an obstacle, although the results did indicate a constant, overall increase in peak movement

The results of Experiment 1 suggest that abstract spatiotemporal height when an obstacle was present as opposed to when no

forms are used during the production of movement sequences and obstacle was present. The magnitude of the increase depended on

that such forms carry over from one movement to the next.

Evidence for this interpretation comes from the strong hand path priming effect for movements made after clearing an obstacle. Participants made higher movements after clearing an obstacle than when they performed with no obstacle in the workspace. The postobstacle jump heights decreased with successive postobstacle movements and scaled with obstacle height. Because this effect extended over the workspace, it apparently reflected carryover of an abstract spatiotemporal form rather than a muscle-specific form.

A possible alternative explanation of the results can be based on the finding that movements over the obstacle took longer than the prescribed time interval, whereas the first and second movement after an obstacle sometimes took shorter than the prescribed time interval. In view of these observations about timing, one might venture the hypothesis that the observed scaling effects of obstacle height resulted from participants trying to minimize speed changes in successive jumps. To do so, they may have made higher jumps in postobstacle trials to lengthen the movement path so they would generate speeds similar to those needed for obstacle clearance.

This explanation cannot be the sole cause of the observed scaling effect, however, because the effect also emerged when participants closely followed the prescribed metronome rate for postobstacle movements. Furthermore, a significant correlation was found between movement times and peak movement heights only for the medium obstacle height, although the same pattern of peak heights emerged in the low- and high-obstacle conditions.

The results of Experiment 1 effectively replicate the earlier findings of Jax and Rosenbaum (in press), indicating that the hand path priming effect that they observed was not just an artifact of uncertainty about targets and obstacles in their experiments. Be- cause the hand path priming effect extended over the workspace in Jax and Rosenbaum’s study (in press) as well as in the present experiment, it seems to be a general phenomenon.

Experiment 2 Having considered one alternative explanation of the results of

Experiment 1 in the previous section, we now consider another alternative account. The alternative to be considered goes strongly against the abstract spatiotemporal view. According to this alter- native explanation, the results of Experiment 1 are attributable to biomechanical properties of muscles. This alternative account builds on the fact that the contraction time of muscles is much shorter than the relaxation time of muscles (Enoka, 1988). Thus, it could be that the muscles that contracted to help clear the obstacle could not return to their initial state quickly enough to permit normal, control-level jump heights. This hypothesis would not only explain why participants moved higher after clearing obsta- cles than when no obstacles had to be cleared. It would also explain why there was only a tiny jump-height increase before obstacles were cleared. The latter result would have stemmed from the time needed for muscles to contract, which is much shorter than the time needed for them to relax. Thus, there may have been no need to slowly prepare the muscles for the rapid contraction that would be needed to achieve a forthcoming leap over an obstacle.

We designed Experiment 2 to test whether the sequential effects in Experiment 1 could have been caused by slow muscle relax- ation. We asked participants to move over an obstacle with one hand and then continue moving between subsequent targets with the other hand. This manipulation eliminated the possibility that the muscles in the arm just used to clear the obstacle could have

contributed to the sequential effect. If slow relaxation of the muscles of the active arm caused the sequential effect in Experi- ment 1, then the effect would be eliminated when participants continued their movements with the other arm after clearing the obstacle. By contrast, if the sequential effect reflected the use of abstract spatiotemporal forms during sequence production, the effect would be preserved when participants continued their move- ments with the other arm.

Method Twenty Penn State undergraduates (13 male, 7 female) from an

introductory psychology class participated for class credit. They ranged in age from 18 to 23 years. In this experiment, we used only one obstacle (21.5 cm in height). During the hand-switch trials, participants held a dowel (20.2 cm in height and 3.4 cm in diameter, 140 g) in each hand. We asked participants to move over the obstacle with one hand and then continue with the other hand. We only used the most leftward and most rightward obstacle positions from Experiment 1, because those obstacle positions provided the most informative setup for the switch trials. In the switch trials, participants moved over the obstacle with one hand and continued moving with the other hand, starting at the next target. Thus, if the left hand jumped from Target 1 to 2, the next thing that happened was that the right hand jumped from Target 3 to 4. We used this design to prevent possible collisions between the two hands. We told participants that during the task, one of the two dowels had to be in contact with one of the targets at all times. The control trials were identical to the experimental switch trials except that no obstacle was present.

Each participant completed 20 trials, with 8 obstacle-absent trials and 12 obstacle-present trials. The order of these trials was randomized across participants. The rest of the procedure was identical to that of Experiment 1 except that participants completed three back-and-forth movement sequences instead of five. In all other respects, the method was the same as that in Experiment 1.

Results Peak movement heights. Figure 3 shows the data for each

obstacle location. To test whether the hand path priming effect of Experiment 1 still occurred when participants switched hands after clearing an obstacle, we conducted a 2 (start location) ! 2 (ob- stacle location) ! 6 (movement number) within-subject repeated measures ANOVA. We excluded the movements over the obstacle to avoid spurious main effects or interactions resulting from those movements. We again applied a log10 transformation to the data prior to the analysis to correct for skew. Note that the obstacle location factor incorporated hand switches because participants moved over the obstacle with the left hand and continued with the right hand when the obstacle was on the left, whereas participants moved over the obstacle with the right hand and continued with the left hand when the obstacle was on the right.

The ANOVA, with Greenhouse–Geisser correction to the de- grees of freedom where appropriate, revealed a main effect of movement number, F(3.293, 55.977) " 7.830, p # .01. As in Experiment 1, peak movement heights after clearing the obstacle only gradually decreased back to baseline as more movements were made after the obstacle. Post hoc Sidak tests revealed that the first two movements after moving over the obstacle had signifi-

HAND PATH PRIMING 1121

VAN DER WEL, FLECKENSTEIN, JAX, AND ROSENBAUM

The ANOVA yielded an Obstacle Presence ! Obstacle Loca- tion ! Movement Number interaction, F(7, 98) " 3.368, p # .01, and an Obstacle Presence ! Movement Number interaction,

F (4.104, 57.450) " 6.023, p # .01. For the obstacle-absent con- ditions, the prescribed 0.60 s movement duration fell within the 95% confidence interval for each movement. For the obstacle- present conditions, movements over the obstacle generally took longer than the prescribed 0.60 s.

Because some of the movement times departed from the pre- scribed time, we again tested the correlation between movement times and peak movement heights, removing the jumps over the obstacle to avoid spurious results. The correlation between move- ment times and peak movement heights was not significant (r " %.019, p $ .10). Thus, the obtained results in peak movement heights for movements after clearing the obstacle did not depend on fluctuations in movement times.

Discussion

The results of Experiment 2 suggest that the sequential effect observed in Experiment 1 was not caused by residual activity in the muscles of the arm used to clear the obstacle. Instead, the results suggest that the effect originated from a more abstract source of information, as suggested by Lashley (1930) and the

Figure 3. Peak movement heights above baseline when clearing an other authors concerned with motor equivalence in handwriting obstacle with the left hand and continuing with the right hand (!) or vice

mentioned earlier (see also Harrington et al., 2000). versa ("). Movement number corresponds to {rem[(1:31),10] & 1}, such

Special note should be made of the fact that participants moved that movement number " 1 contains movements 1, 11, and 21 of the

over the obstacle with more clearance when using the left hand sequence, movement number " 2 contains movements 2, 12, and 22 of the

sequence, and so on. The top panel shows the height above baseline when than when using the right hand and also, especially, that the

participants moved over the obstacle. Error bars represent plus or minus 1 postobstacle jumps were then higher with the right hand than with standard error.

the left hand. The fact that larger obstacle clearances were ob- served for the left hand than for the right can be explained by saying that participants allowed for more variability with the nondominant hand than with the dominant hand, an idea that

cantly higher jump heights than subsequent movements. The re- accords with earlier suggestions by Worringham (1991) and Sabes sults revealed no other significant main effects or interactions.

and Jordan (1997). Interestingly, the greater clearance over the However, the data revealed a trend for movements to be higher

obstacle with the nondominant hand led to higher subsequent when the obstacle was positioned on the left side of the workspace

jumps with the dominant hand. This surprising outcome suggests than when the obstacle was positioned on the right side of the

a true carryover of the spatial form of the movement from one workspace, F(1, 17) " 3.724, p " .07. Thus, participants tended to

hand to the next.

move higher after clearing an obstacle with the left hand (on the left side of the workspace), continuing with the right hand, than

Experiment 3 after clearing an obstacle with the right hand (on the right side of

the workspace). The clearance over the obstacle differed between Experiment 3 was designed to provide further information about the hands, such that the movement over the obstacle was higher for

the role of anticipation in the current task. Recall that the analysis the left hand than for the right, F(1, 17) " 7.555, p # .05.

of the preobstacle moves of Experiment 1 showed that there was Movement times. Movements were again timed well with the

anticipation of forthcoming obstacles. The relevant finding was metronome. The mean movement time for each obstacle position

that jump heights were higher before the first obstacle was cleared was 0.60 s (all SEs ! .06), compared with the prescribed value of

than in the control condition, in which no obstacle was cleared.

0.60 s. This result indicated that anticipation affected jump heights. It is We conducted a 2 (obstacle presence) ! 2 (obstacle location) !

impossible to tell from the results of Experiment 1 or Experiment

2 whether the jump heights made after obstacles were cleared were to evaluate the influence of the independent variables on move-

8 (movement number) within-subject repeated measures ANOVA

only affected by their having been cleared some number of moves ment time for the switch trials. The obstacle location factor incor-

ago or whether there was also an effect of forthcoming obstacle- porated the hand combination for movements in the sequence

clearance demands.

because participants moved over the obstacle with the left hand In Experiment 3, we sought a more direct test of the role of and continued with the right hand when the obstacle was on the

anticipation. We introduced a second obstacle, reasoning that if left, whereas participants moved over the obstacle with the right

anticipation plays a role in the generation of movements before an hand and continued with the left hand when the obstacle was on

obstacle, jump heights after clearance of one obstacle preceding the right.

another would be different from jump heights made after clearance

1123 of the same obstacle when there was no other obstacle yet to be

HAND PATH PRIMING

cleared. Within this possibility, we considered two further, subordinate hypotheses. According to one, participants would try to minimize changes of jump heights between obstacles, in which case jump heights would be higher after an obstacle was cleared and another obstacle had to be cleared than after an obstacle was cleared and another obstacle did not have to be cleared. According to the other subordinate hypothesis, participants would try to minimize the energy expended over the series of jumps they made, in which case jump heights would be lower after an obstacle was cleared and another obstacle had to be cleared than after an obstacle was cleared and another obstacle did not have to be cleared. The third, null, hypothesis was that there would be no effect of a second obstacle on jump heights.

Method Thirty-two Penn State undergraduates (18 male, 14 female)

from an introductory psychology class participated for class credit. They ranged in age from 17 to 25 years. The method of Experi- ment 3 was the same as that in Experiment 1 except where noted below. In the one-obstacle trials, the obstacle (21 cm high) stood between two of the targets. In the two-obstacle trials, a second obstacle (also 21 cm high) stood between another pair of targets. We also included a no-obstacle control condition to obtain a baseline measure.

Each participant completed 32 trials made up of 2 obstacle- Figure 4. Peak movement heights above baseline for each obstacle po- absent trials, 10 one-obstacle trials, and 20 two-obstacle trials. We

sition when clearing two (‚) or one (ƒ) obstacles. The black bars indicate randomized the order of the 32 trials across participants. Partici-

the positions of the two obstacles. Movement number corresponds to pants completed three back-and-forth movement sequences, as in

{rem[(1:31),10] & 1}, such that movement number " 1 contains move- ments 1, 11, and 21 of the sequence, movement number " 2 contains

Experiment 2. movements 2, 12, and 22 of the sequence, and so on. Only the movements that are relevant for the comparisons (see text for details) are included.

Results

Error bars represent plus or minus 1 standard error. Peak movement heights. To compare the peak movement

heights after clearing the obstacle in the one-obstacle condition with the peak movement heights after clearing the obstacle in the

ment height, F(1.689, 52.366) " 31.455, p # .01; an Obstacle two-obstacle condition, we first subtracted the peak movement

Number ! Movement Number interaction, F(1.443, 44.724) " heights of the baseline trials from each of the experimental con-

59.777, p # .01; an Obstacle Number ! Movement Direction ditions. Then we subtracted the baseline-relative heights in the

interaction, F(1, 31) " 20.562, p # .01; and a Movement Direc- one-obstacle condition from the corresponding heights in the two-

tion ! Movement Number interaction, F(1.509, 46.771) " 11.202, obstacle condition. We focused on the conditions in which one of

p # .01. The peak height of the first movement after clearing an the obstacles was between Targets 1 and 2 and the other obstacle

obstacle was similar for the one- and two-obstacle conditions. was between Targets 5 and 6, because those conditions had the

However, the results indicated significantly higher peak movement most intermediate steps between the two obstacles. However, we

heights for the second and third movement after clearing a first also looked at jumps between obstacles when there were fewer

obstacle when a second obstacle had to be cleared than when no intermediate steps between them.

second obstacle had to be cleared. This tendency was stronger for Figure 4 shows the data for the relevant experimental condi-

leftward movements than for rightward movements. tions. We conducted a 2 (obstacle number) ! 2 (movement direc-

The latter results concerned cases in which participants made tion) ! 3 (movement number) repeated measures ANOVA to test

three jumps between obstacles. We decided to see whether the for differences in peak movement height. Movement number had

same findings would obtain when participants made two move- three levels corresponding to the three moves between the obsta-

ments between the first and the second obstacle. For the relevant cles. Thus, we compared the peak movement height of the nth

conditions, we conducted two 2 (obstacle number) ! 2 (movement movement after clearing a first obstacle in a one-obstacle condition

direction) ! 2 (movement number) repeated measures ANOVAs to the peak movement height of the nth movement after clearing a

to test for differences in peak movement height between the one- first obstacle in a two-obstacle condition. Again, we applied a

and two-obstacle conditions. Again, we applied a log10 transfor- log10 transformation to the data prior to the analysis to correct for

mation to the data prior to the analyses to correct for skew. For the skew.

first ANOVA, with Greenhouse–Geisser correction to the degrees The ANOVA revealed an Obstacle Number ! Movement Di-

of freedom where appropriate, we analyzed the one- and two- rection ! Movement Number interaction effect on the peak move-

obstacle conditions with the obstacles placed between the leftmost obstacle conditions with the obstacles placed between the leftmost

When the obstacles were located at the leftmost target pair and at the second-to-rightmost target pair, the ANOVA revealed a significant Obstacle Number ! Movement Number interaction effect on peak movement height, F(1, 31) " 6.171, p # .05, such that the first movement after clearing the obstacle was similar in the one-obstacle and two-obstacle conditions, but the second movement after clearing the first obstacle was higher in the two- obstacle condition. The results also revealed a Movement Direc- tion ! Movement Number interaction, F(1, 31) " 8.707, p # .01, such that the first movement after clearing the obstacle was higher for movements in the rightward direction than for movements in the leftward direction, but the second movement after the obstacle was higher for movements in the leftward direction than for movements in the rightward direction.

When the obstacles were located at the second-to-leftmost target pair and at the rightmost target pair, the ANOVA revealed a significant Obstacle Number ! Movement Direction ! Movement Number interaction effect on peak movement height, F(1, 31) " 50.313, p # .01; an Obstacle Number ! Movement Number interaction, F(1, 31) " 68.821, p # .01; an Obstacle Number ! Movement Direction interaction, F(1, 31) " 12.156, p # .01; and

a Movement Direction ! Movement Number interaction, F(1,

31) " 7.096, p # .05. Qualitatively, the data indicated a sequential effect in both the one-obstacle condition and two-obstacle condi- tions such that the peak movement heights only gradually de- creased back to baseline after the clearing of an obstacle. Impor- tantly, participants tended to keep moving higher during the second movement after clearing an obstacle when a second obsta- cle needed to be cleared than when no second obstacle needed to

be cleared. The results also indicated that this tendency to antici- pate the upcoming obstacle was stronger when moving to the left than when moving to the right.

Movement times. As in the previous experiments, participants timed their movements well. The mean movement time was 0.60 s (SE " 0.01) for the obstacle-absent condition, 0.60 s (SE " 0.01) for the one-obstacle condition, and 0.60 s (SE " 0.01) for the two-obstacle condition. The ideal movement time, as specified by the metronome, was 0.60 s.

We conducted separate ANOVAs to compare the movement times between the same one-obstacle and two-obstacle conditions that we used to evaluate the peak movement heights (described above). In a first ANOVA, we compared movement times for the one- and two-obstacle conditions when the obstacles stood at the leftmost and/or rightmost positions. The 2 (obstacle number) ! 10 (movement number) repeated measures ANOVA revealed a sig- nificant Obstacle Number ! Movement Number interaction effect on movement times, F(2.784, 69.596) " 6.611, p # .01. Move- ments over the obstacle always took longer than the prescribed duration in the two-obstacle condition, whereas movements over the obstacle only took longer than the prescribed duration in the one-obstacle condition when the obstacle was positioned at the rightmost location. Additionally, the first two movements took slightly shorter than the prescribed duration after clearing an obstacle at the rightmost location in the two-obstacle condition, and the first movement after clearing an obstacle took slightly

shorter after clearing an obstacle at the leftmost location in the one-obstacle condition. The other movement times did not depart from the prescribed duration.

In a second ANOVA, we compared movement times for the one- and two-obstacle conditions when the obstacles stood at the leftmost and/or at the second-to-rightmost positions. The 2 (obsta- cle number) ! 10 (movement number) repeated measures ANOVA revealed a significant Obstacle Number ! Movement Number interaction effect on movement times, F(2.549, 68.827) " 12.912, p # .01. Movements over the obstacle always took longer than the prescribed duration in the two-obstacle condition, whereas movements over the obstacle only took longer than the prescribed duration in the one-obstacle condition when the obstacle was positioned at the leftmost location. Additionally, the first move- ment after clearing an obstacle took slightly less time than the prescribed duration in the two-obstacle condition, and the first movement after clearing an obstacle took slightly less time after clearing an obstacle at the leftmost location in the one-obstacle condition. The other movement times did not depart from the prescribed duration.

In a third ANOVA, we compared movement times for the one- and two-obstacle conditions when the obstacles stood at the second-to-leftmost and/or at the rightmost positions. The 2 (obsta- cle number) ! 10 (movement number) repeated measures ANOVA revealed a significant Obstacle Number ! Movement Number interaction effect on movement times, F(2.766, 69.162) " 11.275, p # .01. Departures from the prescribed duration did not follow a clear pattern and occurred only for movements between two of the target pairs in the two-obstacle condition and only for movements between three of the target pairs in the one-obstacle condition.

Because some of the movement times departed from the pre- scribed duration, we tested the correlation between movement times and peak movement heights (see Experiment 1 for the rationale behind this test). When the movements over the obstacle were removed from the analysis, the correlation analyses yielded no significant results for any of the obstacle combinations (all ps $ .10), suggesting that the obtained results in peak movement heights for movements after clearing the obstacle did not depend on the observed fluctuations in movement times.

Discussion The results of Experiment 3 are consistent with the hypothesis

that participants anticipated forthcoming obstacles. In accord with this hypothesis, jump heights were different following an obstacle when another obstacle had to be cleared than they were when no other obstacle had to be cleared.

The results are also consistent with the hypothesis that partici- pants tried to reduce the rate of change of the jump heights when

a second obstacle had to be cleared as compared with when no second obstacle had to be cleared. In accord with this hypothesis, postobstacle jump heights were higher in the two-obstacle condi- tion than in the one-obstacle condition. The latter outcome argues against the hypothesis that participants tried to conserve energy when they approached a second obstacle. If they had done so, one would have expected lower postobstacle jump heights in the two- obstacle case than in the one-obstacle case, not higher postobstacle jump heights, which is what we saw.

1124

VAN DER WEL, FLECKENSTEIN, JAX, AND ROSENBAUM

General Discussion In each of the three experiments reported here, we observed a

hand path priming effect after clearance of an obstacle. Peak heights for movements after obstacle clearance only gradually decreased with more moves away from the obstacle.