Timing and Force Components in Bilateral Transfer of Learning

Transcription

1 Brain and Cognition 44, (2000) doi: /brcg , available online at on Timing and Force Components in Bilateral Transfer of Learning Luis Augusto Teixeira School of Physical Education and Sport, University of São Paulo, São Paulo, Brazil Published online August 18, 2000 Bilateral transfer of perceptual and motor components in movement control was investigated through two experiments. In Experiment 1 a simple anticipatory timing task was practiced with either the preferred or the nonpreferred hand. After a short resting interval an additional set of trials was performed with the contralateral hand. In Experiment 2, the same experimental design was used to investigate bilateral transfer of fine force control in a wrist-flexion movement. Analysis of the results showed that bilateral transfer of learning took place for both anticipatory timing and force control, with more noticeable transfer of training for the former. Asymmetry in transfer was found for force control, with significant transfer only in the preferredto-nonpreferred direction. Transfer of anticipatory timing occurred similarly in both directions. These results indicated anticipatory timing as a powerful component for bilateral transfer, while force control showed to be more dependent on practice with the specific muscular system Academic Press INTRODUCTION Acquisition of motor skills involves two complimentary sides that determine the extent to which new movements can be performed proficiently: the specificity and the generalizability of learning. Experimental evidence has pointed out that some conditions of practice lead to specificity of learning so that remarkable decrease in performance is seen when the sensory information available during acquisition is modified (Proteau, Marteniuk, & Lévesque, 1992; Teixeira, 1995). On the other hand, different kinds of transfer of learning are known to occur after the acquisition of a motor task, ranging from transfer of relational to specific parameters of control (see Young & Schmidt, 1987, for review). Completely specific acquisition of motor skills or perfect transfer of learning, however, rarely are observed, as in most instances there are some components that are incorporated by a new control assembly and other ones that need to be specifically learned from the require- Address correspondence and reprint requests to Luis Augusto Teixeira, Escola de Educação Física e Esporte, Av. Prof. Mello Moraes, 65, Universidade de São Paulo, Butantã, S.P., Brazil lateixei@usp.br /00 $35.00 Copyright 2000 by Academic Press All rights of reproduction in any form reserved.

2 456 LUIS AUGUSTO TEIXEIRA ments of the new task. Studies on bilateral transfer of learning have usually shown an advantage of previous practice with the homologous contralateral limb in tasks such as drawing/writing (Hicks, 1974; Parlow & Kinsbourne, 1989; Thut et al., 1996), tactual recognition (Parlow & Kinsbourne, 1990; Sathian & Zangaladze, 1998), pointing under displaced vision (Choe & Welsh, 1974; Elliott & Roy, 1981; Kalil & Freedman, 1966), pursuit tracking (Hicks, Gualtieri, & Schroeder, 1983), mirror tracing (Cook, 1933a, 1933b), and maze tracking (Milisen & Riper, 1939; Wieg, 1932). These investigations have revealed an increased proficiency in performance with a resting body segment after practice with the contralateral homologous limb, but few investigations have provided information about what control parameters are effectively transferred to the contralateral side of the body. Considering transfer of learning in general, temporization of movements has been shown to be relatively independent of the effector system used to produce the response. Franz, Zelaznik, and Smith (1992) showed that intraindividual variability in repetitive timing tasks is significantly correlated between movements made with the index finger, forearm, and jaw, pointing out a common element responsible for movement synchronization (see also Keele, Pokorni, Corcos, & Ivry, 1985). Additionally, Matos, Teixeira, Lomônaco, Lima, and Sañudo (1997) have shown the transfer of anticipatory timing between tasks involving different levels of movement complexity. Because transfer of learning was observed to take place from a simple (button pressing) to a complex (hitting a foam ball with a racquet) anticipatory timing task as well as in the opposite direction, this study suggested anticipatory timing to be a generalizable element in movement control. On the other hand, transfer of learning in tasks characterized by reduced perceptual and strong motor components has shown to be less pronounced than in tasks that require greater perceptual resources. Karni et al. (1995) analyzed cortical and behavioral modifications during learning a serial fingertapping task and in subsequent transfer-tapping tasks performed with either the same or the contralateral hand. The results indicated that learning led to an increased cortical map only for the practiced motor task, and neither transfer to different tapping sequences performed with the same hand nor transfer between hands for the same sequence were found. Similar specificity of learning has been found by Teixeira (1992, 1993) in investigations on bilateral transfer of learning when a task requiring fine force control was used. In these studies, Teixeira employed a task of propelling a small plastic disk with the index finger from a home position to a horizontal target on a smooth surface and analyzed the gain in performance with the contralateral hand after practice. The results indicated no significant improvement for the contralateral hand in comparison to a condition without any previous practice. In general, these results are indicative that bilateral transfer of learning is task specific, with potential of transfer apparently being a function of the main control components required for proficient performance. Another as-

3 TRANSFER OF TIMING AND FORCE 457 pect of bilateral transfer of learning related to the specificity of the task is the direction of transfer. In the cross-activation model (Parlow & Kinsbourne, 1989), direction of bilateral transfer of learning has been proposed to be related to relative cerebral specialization of function. When the dominant neuromotor system is used, engrams are formed in both cerebral hemispheres, while the use of the nondominant neuromotor system leads to the formation of a single-sided engram in the nondominant hemisphere. As a corollary, bilateral transfer is hypothesized to take place from the dominant to the nondominant system but not in the opposite direction. Because the left cerebral hemisphere is supposed to play the main role in the sequential organization of movements and the right hemisphere to be the main responsible for visuospatial analysis (Goodale, 1990), the direction of transfer would be determined by the main component required for the control of action. In the callosal access model (Taylor & Heilman, 1980), on the other hand, it has been proposed the formation of a neural representation only in the dominant left hemisphere. When the dominant side of the body is used there is a direct access to this representation. When a nondominant limb is used, control activities of the nondominant hemisphere are carried out with the leading participation of the dominant contralateral hemisphere through the corpus callosum. Thus, as a function of bilateral cerebral activation in the performance with the nondominant (but not with the dominant) limb, transfer of learning from the nondominant to dominant hand is expected to take place. The research strategy employed here to investigate the issues raised above was to select a task composed of strong perceptual and weak effector components (Experiment 1) and then of the opposite characteristics, i.e., strong effector and weak perceptual components (Experiment 2) and to analyze the amount of bilateral transfer of learning after practicing one of these tasks with either the preferred or the nonpreferred hand. EXPERIMENT 1 In Experiment 1 a strong perceptual/weak effector component task was used to assess the bilateral transferability of learning. The task selected to represent this category involved pressing a switch coincidentally with the end of the displacement of a luminous stimulus. The main skill developed during the acquisition of this task was to make a simple movement in a narrow temporal window; that is, to learn to anticipate the correct time to produce the response to make two events occur simultaneously. Utilization of the contralateral hand after the task acquisition was expected to lead to a similar level of performance in relation to the practiced hand, since this short and simple movement requires a minimum of both spatial and force accuracy. As the perceptual component was maintained unchanged in the transfer condition, one could expect to find a high index of bilateral transfer to the untrained hand.

4 458 LUIS AUGUSTO TEIXEIRA Method Subjects. University students (n 40), ages 18 to 33 years old (M 22 years), volunteered to participate in this study. Thirty-seven subjects declared right-handedness for fine motor skills and three of them declared left-handedness. Apparatus and task. A commercial Bassin Anticipation Timer (Lafayette Instrument) was used to produce the displacement of a luminous signal in the direction of the subject. This apparatus is made up of a metallic runway 152 cm long, which holds 32 light-emitting diodes (LEDs) arranged in line on its mid-longitudinal axis. Adjacent LEDs are spaced by intervals of 4.5 cm, which gives rise to a displacement distance of 144 cm. The sequenced lightening of the LEDs produce the perception of a single stimulus moving from one end of the runway to the other. The runway was placed on the mid-sagittal plane in a horizontal position a few centimeters from the subject, with the stimulus moving toward the subject at a constant velocity of 4.4 m/s. At the end of the runway there was a manual switch, which was to be pressed with the thumb coincidentally with the lightening of the last LED of the sequence. The performance was analyzed as a function of the unsigned temporal difference between the synchronizatory movement and the actual arrival of the luminous stimulus at the criterion position (absolute error) and consistency of response (variable error). Design and procedures. Subjects sat facing the apparatus, which was positioned on a workbench, aligning the mid-sagittal axis of their bodies with the line formed by LEDs on the runway. After assessing subjects preferred hand by verbally inquiring about the preferred hand used to perform fine motor tasks such as writing and painting, they were familiarized with the task and experimental procedures by performing three trials in the condition of the following practice. Immediately after familiarization, 60 practice trials were performed in a single series, with intertrial intervals of approximately 7 s. On each trial a verbal warning signal was followed by the lightening of the first LED of the runway (for 1 s), and then the luminous stimulus began to move toward the subject. The subject s practice hand was kept in a constant place, positioned on the workbench beside the last LED of the sequence. Subjects were pseudorandomly assigned to one of two groups, maintaining the number of subjects and the number of males and females equivalent between them. One group carried out practice trials with their preferred hand. After this acquisition phase, they remained engaged on a cognitive task for 10 min and then performed five trials with the nonpreferred hand (P-NP group). The other group underwent the opposite treatment, practicing with the nonpreferred hand and being transferred to the preferred hand (NP-P group). Knowledge of results was provided both during acquisition and transfer by verbally informing the subject with the directional timing error (in milliseconds) after every trial. RESULTS AND DISCUSSION Absolute error in the acquisition were grouped in blocks of six trials and analyzed through a two-way 2 (Group) 10 (Block) ANOVA with repeated measures on the second factor. The results showed only a significant main effect for Block [F(9, 342) 7.73, p.0001], indicating improvement in accuracy during acquisition. Absence of a significant main effect for Group [F(1, 38).35, p.1] and interaction [F(9, 342) 1.13, p.1] showed equivalent levels of accuracy and homogeneous improvement throughout acquisition in the performance with the preferred and nonpreferred hand (Fig. 1). In the comparison between transfer and control trials, a descriptive analysis was made by calculating the index of transfer, based on the proportional

5 TRANSFER OF TIMING AND FORCE 459 FIG. 1. Temporal absolute error for blocks of six trials during the acquisition phase for the preferred (P) and nonpreferred (NP) hands. gain in performance in the transfer condition in relation to the opposite group s initial five acquisition trials (same hand, intergroup comparison) and the index of performance decline, representing the proportional deterioration of performance from the last five acquisition trials in relation to transfer (different hand, intragroup comparison). For the preferred hand, the experimental condition had a 36% gain in performance, while its index of performance decline was 27% (Fig. 2a). A similar index of transfer was achieved with the nonpreferred hand, in which there was a 41% gain in relation to the control condition and an index of decline equal to 29% (Fig. 2b). Further inferential analysis was conducted for accuracy scores through a three-way 2 (Group) 3 (Phase: beginning of acquisition end of acquisition transfer) 5 (Trial) ANOVA with repeated measures on the last two factors. The results indicated significant main effects for Phase [F(2, 76) 30.61, p.001] and Trial [F(4, 152) 6.81, p.001]. Significant interactions were observed for Group Phase [F(2, 76) 4.08, p.05], Group Trial [F(4, 152) 2.67, p.05], and Phase Trial [F(8, 304) 3.07, p.005]. Newman Keuls post hoc procedures pointed out significant superior performance of the experimental conditions over their controls, i.e., in the comparison between transfer trials of one group with initial trials of the other group, except for the last trial when similar performance was observed. These results were due to a significant improvement of performance throughout trials in the beginning of acquisition while no significant modifications in temporal accuracy were observed for transfer trials. Additionally, in the analysis of the transition from acquisition to transfer no significant differences were found, which indicates no disruption in performance by the transfer task. Analysis of variable error was conducted through a two-way 2 (Group) 3 (Phase) ANOVA with repeated measures on the second factor. The results

6 460 LUIS AUGUSTO TEIXEIRA FIG. 2. Temporal absolute error for the initial trials of the P-NP group (P-Bgn), final (NP-End) and transfer (P-Exp) trials of the NP-P group (a), initial trials of the NP-P group (NP- Bgn), and final (P-End) and transfer (NP-Exp) trials of the P-NP group (b). indicated only significant main effects for Phase [F(2, 76) 24.61, p.001]. Post hoc comparisons pointed out significant differences between beginning of acquisition and the other two phases, with no difference between end of acquisition and transfer. These results showed that consistency developed by both groups during practice was maintained in the transfer trials, resulting in a reduced variability of performance in relation to the beginning of acquisition (Fig. 3). Overall, these results indicated that anticipatory timing performance transfers bilaterally in a symmetric way. Comparing the gain of the contralateral nonpracticed hand over the control condition without previous practice, both P-NP and NP-P groups showed a significant improved performance. Considering the intergroup index of transfer, the similarity between the proportional gain in relation to initial trials for both groups indicates that bilateral transfer was rather symmetric, with the same amount of transfer from the preferred to nonpreferred hand as in the opposite direction. Low intragroup indexes of performance decline and absence of significant differences in the transition from acquisition to transfer, both in accuracy and consistency, pointed

7 TRANSFER OF TIMING AND FORCE 461 FIG. 3. trials. Temporal variable error for P-NP and NP-P groups in the initial, final, and transfer out a powerful bilateral transfer. Such findings reveal the subjects capacity to maintain similar levels of performance in the transfer to the contralateral homologous effector system in relation to the end of acquisition. In agreement with previous findings (Franz et al., 1992; Keele et al., 1985; Matos et al., 1997), the results presented here support the proposition of timing as a motor function relatively independent of the effector system, which can be incorporated into different sensorimotor maps without necessity of specific practice with a particular limb in order to maintain the level of performance achieved in the acquisition of a primary timing task. Additionally, such bilateral transfer achievement was observed on the very first trials, indicating that incorporation of the timing component by the contralateral hand structure of control was remarkably efficient. EXPERIMENT 2 Experiment 2 was designed to test transferability of learning in a task with strong effector and weak perceptual components. As tasks involving control of force in a stable environment are representative of this category of skills, the task consisted of launching a cursor through a linear runway to a specified target position. Because the launching of the cursor was made with a wristflexion movement, good performance at this task required refined control over the force produced to propel the cursor with the preferred or nonpreferred hand, without considerable load on perceptual abilities. As the main requirement for accurate performance at this task is controlled application of force with manual muscular groups, a limited bilateral transfer of learning was expected to be found.

8 462 LUIS AUGUSTO TEIXEIRA Method Subjects. University students (n 40), ages 18 to 28 years old (M 21 years), participated as subjects in this experiment. Thirty-seven subjects asserted preference for the right hand to perform fine motor skills and three used the left hand. None of these subjects participated in Experiment 1. Apparatus and task. A commercial Linear Positioning Apparatus (Lafayette Instrument) was used to produce a force-control task. This instrument is composed of a 1-m-long tubular runway supported by a wooden frame, with a parallel scale in millimeters. A low-friction moving cursor can be displaced along this runway. Its position is shown on a digital display, indicating the discrepancy between the intended and the actual end position. At both ends of this runway there are cursor displacement limiters, which can be fixed at desired points along the runway to determine the start position of the movement. To yield a task requiring force control, subjects were asked to launch the cursor to a target 50 cm away from the initial position, using a wrist/fingers-flexion movement. Right-hand movements were made from the right to the left side, while left-hand movements were performed in the opposite direction. The cursor was contacted with the index and middle fingers, and the movement was initiated with the elbow and wrist lined up with the shoulder on the sagittal plane. In this initial position, the wrist and fingers were kept hyperextended in order to assume a favorable angle to propel the cursor. During the movement the elbow was maintained static and flexed at approximately 120 and the propelling of the cursor was performed by flexing in a continuous way the wrist and the two fingers contacting the cursor. Design and procedures. The experimental design was similar to that used in Experiment 1. Subjects were pseudorandomly assigned either to the P-NP group (i.e., practicing the task with the preferred hand and transferring to the nonpreferred hand) or to the NP-P group (practicing with the nonpreferred hand and transferring to the preferred hand). After determining subjects preferred hand for fine motor skills through verbal inquiry, they sat facing the apparatus. They had their shoulder, corresponding to their practice hand, aligned with the start position of the cursor. A limiter was used to establish the initial position for a distance of 50 cm from the target place. The target was indicated by a vertical arrow right under the criterion position along the runway. To prevent a whole-arm movement, a vertical paddle was positioned close to the wrist, limiting the movement in the direction of the target. Subjects were given three familiarization trials to assess their comprehension of the task and procedures. Afterward they began the acquisition phase, in which after every trial the experimenter recorded the discrepancy between the criterion and the actual final position of the propelled cursor and signaled to the subject to return the cursor to the initial position using their opposite hand. In order to prevent fatigue in the proximal muscles of the arm, the subjects were instructed to uphold their practicing arm on the table supporting the apparatus in the intertrial intervals. Because intrinsic knowledge of results was available from the apparatus scale during movement execution and cursor displacement, no augmented feedback was given. The 60 acquisition trials were performed in sequence with intertrial intervals of 7 8 s. After an active resting interval (the same activity as for Experiment 1), subjects changed their position in relation to the apparatus to perform five transfer trials. In the transfer task they had their shoulder, elbow, and hand of the resting arm lined up with the contralateral initial position and performed the 50-cm launching task with the opposite hand, also with the displacement of their wrist in the direction of the target limited by a paddle. Results and Discussion The results were analyzed in the same way as for Experiment 1, using spatial accuracy (absolute error) and consistency (variable error) as depen-

9 TRANSFER OF TIMING AND FORCE 463 FIG. 4. Spatial absolute error for blocks of six trials during the acquisition phase for the preferred (P) and nonpreferred (NP) hands. dent variables. The two-way 2 (Group) 10 (Block) ANOVA with repeated measures on the second factor, employed for analysis of absolute error in the acquisition phase, revealed only a significant main effect for Block [F(9, 342) 27.44, p.0001], indicating a homogeneous improvement in accuracy for both P-NP and NP-P groups (Fig. 4). The index of transfer showed an advantage of 11% for the experimental preferred hand in relation to its control, and an index of decline of 115% in relation to the performance with the contralateral practice hand in the end of acquisition (Fig. 5a). The experimental nonpreferred hand achieved an index of transfer equal to 35% and an index of performance decline of 65% (Fig. 5b). The three-way 2 (Group) 3 (Phase) x 5 (Trial) ANOVA with repeated measures on the last two factors for absolute error indicated significant main effects for Phase [F(2, 76) 33.35, p.001] and Trial [F(4, 152) 8.76, p.001]. Significant interactions were found for Group Phase [F(2, 76) 4.08, p.05], Phase Trial [F(8, 304) 2.86, p.05], and Group Phase Trial [F(8, 304) 2.21, p.05]. Newman Keuls procedures indicated that a significant gain in transfer compared to initial acquisition trials took place only on the first trial for the P-NP condition, while no significant gain was found for the NP-P group. In the comparison between end of acquisition and transfer, significant decline of performance was observed only on the first trial in the NP-P condition, while no significant differences were observed in the opposite direction of transfer. The combined effect of transfer for the P-NP group and decline of performance for the NP-P group on the first trial of transfer led to a significantly superior performance with the nonpreferred hand on this trial, while no significant between-hand differences were found afterward. Additionally, there were no between-trial significant differences in the performance with the nonpreferred hand in the

10 464 LUIS AUGUSTO TEIXEIRA FIG. 5. Spatial absolute error for the initial trials of the P-NP group (P-Bgn), final (NP- End) and transfer (P-Exp) trials of the NP-P group (a), initial trials of the NP-P group (NP- Bgn), and final (P-End) and transfer (NP-Exp) trials of the P-NP group (b). transfer phase, which indicates stability of performance. Differently, the experimental preferred hand showed a similar pattern in relation to beginning of acquisition, with significant improvement of performance throughout trials. Analysis of variable error, conducted through a two-way 2 (Group) 3 (Phase) ANOVA with repeated measures on the second factor, indicated only a significant main effect for Phase [F(2, 76) 24.61, p.0001]. Posthoc contrasts indicated that all comparisons were significant, revealing an increased consistency of performance in transfer compared to initial trials in spite of the decline in the transition from acquisition to transfer (Fig.6). Previous studies on bilateral transfer of force control have failed to show significant effects in tasks involving fine manual musculature (Teixeira, 1992, 1993). Lack of bilateral transfer of control between distal body segments has been explained by absence of neural interhemispheric projections between homotopic primary motor and somatosensory areas representing the hands in the cerebral cortex (Thut et al., 1996). The present results, though, indicated significant improvement of the nonpreferred hand as a result of practice with the preferred hand, while the opposite direction of transfer did not show similar transfer effects. The inferential analysis was complemented

11 TRANSFER OF TIMING AND FORCE 465 FIG. 6. trials. Spatial variable error for P-NP and NP-P groups in the initial, final, and transfer by indexes of transfer and performance decline by showing that transfer from the preferred to nonpreferred hand produced a threefold higher index of transfer and a twofold lower index of performance decline in comparison with the opposite direction of transfer. Thus, bilateral transfer of force control was found to be asymmetric, with the main transfer from the preferred to the nonpreferred hand. GENERAL DISCUSSION In principle, bilateral transfer of learning may take place in different ways. It may occur through transfer of all relevant components for proficient performance or through transfer of only more generalizable, effector independent, control components. Yet, transfer may be partial, with decline in performance of one or more components in the transfer task, or it may be complete, so that no alteration in the performance is observed in the transfer to the contralateral limb for some components. Previous studies have shown that there is a third element to be considered, i.e., symmetry of transfer, with several findings of unidirectional transfer of learning either from the dominant to nondominant limb (e.g., Parlow & Kinsbourne, 1989) or in the opposite direction (e.g., Taylor & Heilman, 1980). The present results showed that both anticipatory timing and force control are transferable components in motor behavior. Comparison of the results of Experiment 1 and Experiment 2, though, reveals that bilateral transfer of learning occurred in qualitatively different ways for these two components in sensorimotor control. Transfer of force control between hands was found to be asymmetrical, reaching a significant level only from the preferred to nonpreferred hand direction. These results are in accordance with Parlow and Kinsbourne s (1989) proposition that bilateral transfer of learning occurs from the dominant to nondominant neuromotor system. Differently, transfer of anticipatory

12 466 LUIS AUGUSTO TEIXEIRA timing control showed a symmetric relationship, with both hands benefiting similarly from previous practice with the contralateral hand, as pointed out by the index of transfer, as well as demonstrated the same capacity to maintain approximately 70% of the level of performance achieved with the contralateral practiced hand, as was shown by the index of performance decline. Considered as a whole, these findings demonstrate that amount and symmetry of bilateral transfer of learning are dependent on the main components involved in movement control for a given task. Most studies on bilateral transfer of learning have used motor tasks in which there exists a complex combination of perceptual and motor components, or that even require different parameters of movement control, so that it is difficult to determine what control components were in fact transferred from one limb to the other. Previous findings by Teixeira (1992, 1993), employing a force-control task, have shown no significant gain in performance for the contralateral nonpracticed hand, which suggests acquisition of fine force control to be dependent on practice specifically with a particular neuromotor system. Somewhat different from these findings, the present results showed that for the wrist-flexion movement task used here the nonpreferred resting limb benefited from practice with the contralateral homologous limb. Scarce significant differences in relation to controls and high indexes of performance decline, however, indicate that the amount of bilateral transfer of force control was limited in relation to that observed for anticipatory timing. Such weak transfer of learning of force control may be related to the lack of neural interhemispheric projections between hand areas in the primary motor and somatosensory areas in the cerebral cortex (Pandya & Vignolo, 1971), which requires indirect neural pathways for interhemispheric communication between the main cortical areas responsible for controlling distal muscular systems. This characteristic of the brain architecture may dictate the increased dependence on specific practice for the acquisition of force control in manual movements, as observed here. Apparently contradictory to this is the fact that the anticipatory timing task involved a distal thumb movement and even so the transfer was rather robust in either direction. The anticipatory timing task, however, required neither accurate force nor another form of complex spatial specification, so that the critical aspect for proficient performance was initiating the movement at the right time. Under such circumstances, alteration of the effector system seems to lead to minimal disturbance on performance. These conclusions are in accordance with previous findings of significant correlations (Franz et al., 1992; Keele et al., 1985) and transfer of learning (Matos et al., 1997) between timing tasks of different levels of complexity in terms of the effector system used, shown by a timing capacity that can be used bilaterally, independent of the specific limb employed to perform the task. Previous conceptual models on bilateral transfer of learning have proposed the formation of single-sided engrams in the dominant cerebral hemi-

13 TRANSFER OF TIMING AND FORCE 467 sphere independent of the limb used during practice (Taylor & Heilman, 1980) or in the nondominant hemisphere when the nondominant hand is employed in the acquisition (Parlow & Kinsbourne, 1989). Additionally, Laszlo, Baguley, and Bairstow (1970) have proposed the formation of unilateral engrams on the basis of practice with the contralateral limb. All of these models propose a single direction of bilateral transfer of learning, which varies as a function of particular explanations for interhemispheric flow of information. In light of the present findings, no single model seems to provide a good fit for all the data. Symmetry between the preferred and nonpreferred hand observed in the performance on the anticipatory timing task suggests that the two cerebral hemispheres worked in a quite similar way, without asymmetries in the mode in which the task was acquired or in the amount of contralateral transfer of learning. Thus, formation of unilateral engrams, or asymmetrical transfer as a function of hemispheric dominance, turns out to be an inappropriate general conclusion. A conceptualization put forth to explain current and previous experimental findings is based on the conception of cerebral hemispheres as composed of a number of neural populations, which are responsible for specific functions in sensorimotor control. The assembly and activation of a group of such control units to perform a motor task in the hemisphere contralateral to the limb employed in the acquisition leads to activation of homologous units in the opposite hemisphere (cf. Tanji, Okano, & Sato, 1988). Power of interhemispheric coactivation is thought to depend on variables like the main sensorimotor components involved in the control of the movement and the strength of the connections among components of a neural population responsible for controlling a given sensorimotor operation. As a function of particular structural and anatomical characteristics of homologous limbs acquired throughout life-span development, motor components used in force control are dependent to some extent on specific practice with the criterion limb (Teixeira, 1992, 1993). Such practice with a particular body segment seems to hold an important function in proficient performance of strengthening the connections between control units and in molding them to conform with the idiosyncrasies of this particular effector system. Perceptual components like anticipatory timing, on the other hand, are capable of achieving better bilateral transfer. The fact that this component in a global sensorimotor map is a behavioral function that results from the collective work of a set of processing units, which are widely distributed throughout large regions in the associative area in both cerebral hemispheres, renders its connection to different effector systems easier to be implemented. This feature prevents greater disturbances in the response when transfer to a different action system is required. Bilateral transfer of learning, hence, is proposed to be a function of utilization of general-purpose control units and of well-defined neural groups of sensorimotor control. Similarity of performance between the preferred and

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