When transfer of learning relates to learning of the same task but with the contralateral limb, it is known as bilateral transfer, although it is sometimes referred to as intermanual transfer, cross-transfer, or cross-education. This well-documented phenomenon demonstrates our ability to learn a particular skill more easily with one hand or foot after we already have learned the skill with the contralateral hand or foot.
Experimental Evidence of Bilateral Transfer
Experiments designed to determine whether bilateral transfer does indeed occur have followed similar experimental designs. The most typical design has been the following:
| ||Pretest ||Practice Trials ||Posttest |
|Preferred limb ||X ||X ||X |
|Nonpreferred limb ||X || ||X |
bilateral transfer transfer of learning that occurs between two limbs.
Lab 13 in the Online Learning Center Lab Manual provides an opportunity for you to experience the bilateral transfer phenomenon.
This design allows the experimenter to determine if bilateral transfer to the nonpracticed limb occurred because of practice with the other limb. In the sample experimental design, note that the practice limb is the preferred limb. However, this does not need to be the case; the preferred limb/nonpreferred limb arrangement could be reversed. In either case, the researcher compares pretest-to-posttest improvements for each limb. Although the practiced limb should show the greater amount of improvement, a significant amount of improvement should occur for the nonpracticed limb, indicating that bilateral transfer has occurred.
Investigation of the bilateral transfer phenomenon was popular from the 1930s through the 1950s. In fact, the bulk of the evidence demonstrating bilateral transfer in motor skills can be found in the psychology journals of that period. One of the more prominent investigators of the bilateral transfer phenomenon during the early part of that era was T. W. Cook. Between 1933 and 1936, Cook published a series of five articles relating to various concerns of bilateral transfer, which they called cross-education. Cook terminated this work by asserting that the evidence was sufficiently conclusive to support the notion that bilateral transfer does indeed occur for motor skills.
Given such a foundation of evidence, very few experiments published since those by Cook have focused only on whether bilateral transfer occurs (e.g., Latash, 1999; Nagel & Rice, 2001; Rice, 1998; Weeks, Wallace, & Anderson, 2003). The bulk of the research literature since the 1930s has addressed several issues related to bilateral transfer. Among these are the direction of the greater amount of transfer and the reason bilateral transfer occurs, both of which will be discussed next.
Symmetry versus Asymmetry of Bilateral Transfer
One of the more intriguing questions about the bilateral transfer effect concerns the direction of the transfer. The question is this: Does a greater amount of bilateral transfer occur when a person learns a skill using one limb before learning it with the contralateral limb (asymmetric transfer), or is the amount of transfer similar when either limb is used first (symmetric transfer)?
Reasons for investigating this question are theoretical as well as practical. From a theoretical perspective, knowing whether bilateral transfer is symmetric or asymmetric would provide insight, for example, into the role of the two cerebral hemispheres in controlling movement. That is, do the two hemispheres play similar or different roles in movement control?
A more practical reason for investigating this question is that its answer can help professionals design practice to facilitate optimal skill performance with either limb. If asymmetric transfer predominated, the therapist, instructor, or coach would decide to have a person always train with one limb before training with the other; however, if symmetric transfer predominated, it would not make any difference which limb the person trained with first.
The generally accepted conclusion about the direction of bilateral transfer is that it is asymmetric. But there is some controversy about whether this asymmetry favors transfer from preferred to nonpreferred limb, or vice versa. The traditional view has been that there is a greater amount of transfer when a person practices initially with the preferred limb.
Although some controversy continues about this question (see Stückel & Weigelt, 2012, for an interesting analysis of how some of this controversy might be resolved), there is sufficient evidence to recommend that for most skill training and rehabilitation situations, the greater amount of transfer occurs from the preferred to the nonpreferred limb. This approach not only is consistent with the bulk of the research literature concerned with bilateral transfer, but also is supported by other factors that need to be taken into account, such as motivation. Initial preferred-limb practice has a greater likelihood of yielding the types of success that will encourage the person to continue pursuing the goal of becoming proficient at performing the skill with either limb.
A CLOSER LOOK An Example of Bilateral Transfer for Mirror Writing
Mark Latash (1999) reported a study in which students in his undergraduate class at Penn State University were required to learn a new skill as part of their class experience. The new skill was mirror writing, which involved handwriting a sentence on a piece of paper while looking in a mirror so that it would read correctly in the mirror, not on the paper. Note that this rearrangement of the familiar relation between a motor output and its perceptual consequences is exactly where we'd expect to see negative transfer. During a pretest the students wrote the sentence, "I can write while looking in the mirror," five times with one hand and then the other hand. For each trial, they timed how long it took to write the sentence and counted the errors they made. The students then practiced writing the sentence fifteen times a day, five days a week, for three weeks using the dominant hand only. At the end of the practice period, they did a posttest that required them to perform the task as in the pretest. By comparing the writing performance of the nondominant hand during the pre- and posttests, the students could determine if bilateral transfer resulted from the 225 practice trials with the dominant hand. The results showed that on the posttest for the nondominant hand, they wrote the sentence 40 percent faster than on the pretest, and their errors decreased by 43 percent.
Why Does Bilateral Transfer Occur?
As we saw for explanations proposed to account for positive and negative transfer effects, cognitive and motor control explanations have been offered to answer the question of why bilateral transfer occurs.
The cognitive explanation of bilateral transfer. The cognitive explanation states that the basis for the positive transfer from a practiced to a nonpracticed limb is the important cognitive information related to what to do to achieve the goal of the skill, which we described in chapter 12 as an important characteristic of the first stage of learning. This information is relevant to performing the skill regardless of the limb involved. As a result of practice with one limb, the relevant cognitive information is acquired and stored in memory, which then makes it available when the skill is performed with the other limb.
We can relate the cognitive explanation of bilateral transfer to the "identical elements" theory Thorndike proposed, which we discussed earlier. This explanation gives strong consideration to those elements of a skill related to the performer's knowing "what to do." For example, we can consider the performance of a skill with one limb and then the other to be essentially two distinct skills. Throwing a ball at a target using the right arm is a different task from throwing a ball with the left arm. However, elements of these skills are common to both, regardless of which hand the thrower is using. Examples include the arm-leg opposition principle for throwing, the need to keep the eyes focused on the target, and the need to follow through. Each of these elements represents what to do to successfully throw the ball at a target and does not specifically relate to either arm.
Proponents of this view predict that if a person achieves proficiency at a skill using the right arm, the person does not need to relearn the common cognitive "what to do" elements when he or she begins practicing with the left arm. The person should begin performing with the left arm at a higher level of proficiency than he or she would have had if he or she had never practiced with the right arm.
asymmetric transfer bilateral transfer in which there is a greater amount of transfer from one limb than from the other limb.
symmetric transfer bilateral transfer in which the amount of transfer is similar from one limb to another, no matter which limb is used first.
A CLOSER LOOK Transfer across Domains
You may be surprised to learn that acquiring a new motor skill influences the development of nonmotor skills in addition to other motor skills. Joseph Campos and his colleagues have provided some of the best illustrations of this principle through their documentation of the profound psychological changes that occur after infants learn to crawl (see Campos et al., 2000, for a review). The onset of crawling heralds a psychological revolution, characterized by broad-scale changes in perception, perceptual motor coordination, spatial cognition, memory, and social and emotional functioning. Why Do These Changes Occur?
Changes in psychological function are driven by the pervasive set of new experiences that independent locomotion permits.
All of these experiences feed into a much deeper understanding of the self, the environment, and the relation between the self and the environment.
In this sense, crawling per se is not the critical factor in psychological change; rather, new experiences drive psychological development.
The relation between crawling and psychological development has enormous implications for children with physical disabilities that impede the development of motor skills.
Researchers are now beginning to think that at least some of the deficits in psychological function that are often seen in children with physical disabilities are a consequence of impoverished exploratory experiences stemming from the delayed or compromised development of motor skills (Anderson et al., 2013).
The motor control explanation of bilateral transfer. The motor control explanation for bilateral transfer incorporates the generalized motor program and the dynamical systems theories of motor control as well as our understanding of motor efference in the nervous system. According to the generalized motor program (GMP) theory, which we discussed in chapter 5, the muscles involved in the performance of a skill are not an invariant characteristic of the GMP. Rather, muscles are a parameter that the person adds to the GMP to allow the achievement of an action goal in a specific situation. As we discussed in chapter 1, action goal achievement can be attained for many motor skills by using a variety of movements. Thus the GMP does not develop as a muscle-specific program to control motor skill performance. This means that the GMP theory predicts that because practicing a skill with one limb establishes the development of a GMP with its invariant characteristics, the skill could be performed with the contralateral limb by applying to the GMP the muscles parameter for that limb.
The dynamical systems theory of motor control also provides a basis for bilateral transfer as noted in the previous discussions of intrinsic dynamics. This theory of motor control also states that what is learned is not specific to the limb used to practice the skill. The dynamical systems theory refers to skill learning as "effector independent," which means that when a motor skill is learned, coordination dynamics are learned without reference to the limb, or limbs, involved in practicing the skill. Rather, what is acquired is an abstract representation of the coordination dynamics. For example, as noted earlier, Kelso and Zanone (2002) showed that participants in an experiment who learned a novel relative phase with their arms or legs transferred those same relative phase characteristics to the nonpracticed pair of limbs. Similar results were reported by Camachon, Buekers, and Montagne (2004) for the transfer between walking and arm movements. It is important to note that although these experiments do not involve bilateral transfer as we have considered it, they demonstrate the effector independence of skill learning and how well people can transfer what is learned to a different set of effectors, which supports the motor control explanation for bilateral transfer.
A CLOSER LOOK Bilateral Transfer Training for Using an Upper-Extremity Prosthesis
For physical therapists who work with patients who will be fitted with, or who recently began wearing, a prosthetic limb on an amputated arm, an important goal is to facilitate the daily functional use of the prosthesis. Research by Weeks, Wallace, and Anderson (2003) provides a training option that is based on bilateral transfer. The training engages patients in the use of a prosthetic simulator with the intact limb. The researchers provided evidence of the effectiveness of this type of training by having non-amputees wear the prosthetic simulator shown in figure 13.2 and practice using it to perform three tasks.
The simulator: It included a figure-8 harness that was fitted around the shoulder contralateral to the prosthesis. It was attached to a cable that ran across the back and upper arm of the limb with the prosthesis. The cable inserted into the proximal end of the simulator and ran the length of the simulator to interface with a split-hook device that was identical to that of a regular prosthesis. The simulator split-hook device was a voluntary-opening device, which means that the wearer opened it by adjusting the tension of the cable with motions of the torso, shoulders, and arm.
The three tasks: Each task began from a common starting point, which was a microswitch button located at the person's midline and 20 cm from the table at which he or she sat. The tasks required the manipulation of various objects at different locations.
Toggle-switch task: The participant moved the prosthesis 25 cm forward and 20 cm upward to grasp and flip the paddle of a small toggle switch upward. Important in the performance of this task was to grasp the switch, not just flip it without grasping it first.
Fine-aiming task: The participant was given a stylus (10 cm long, 0.7 cm diameter) and had to place it in a hole directly in front of the start location to activate a microswitch located 2 cm under the task board.
Prehension task: The participant reached 20 cm laterally and 10 cm forward to grasp a 200 g metallic cylinder (4.2 cm high, 2.8 cm diameter, covered in fine-grain sandpaper) and transport it to the opposite side of the task board and place it in a 3.5-cm diameter target well with a lip 1.5 cm high.
Training: Before being seated at the task table, participants were helped with putting on the simulator. They then watched a video of a model wearing the simulator and demonstrating how to control it. During a second viewing of the video, participants imitated the control motions with the model. They then sat at the task table to begin the following testing protocol:
Pretest: Participants performed five trials of each task with the prosthesis on the transfer arm (i.e., nonpractice arm).
Practice: Participants practiced each task thirty times with the prosthesis on the arm not used for the pretest.
Posttest: Participants performed five trials of each task with the prosthesis on the arm used for the pretest.
Results: Practice benefited bilateral transfer. This was shown in the results by comparing the pretest and posttest performances (movement initial time and movement time) for the control group (performed the pretest and posttest but not the practice trials) with the two bilateral transfer groups (one practiced with the preferred arm, the other with the nonpreferred arm). The bilateral transfer groups showed a greater amount of performance improvement on the posttest with the nonpracticed arm, which demonstrated bilateral transfer from the practiced to the nonpracticed arm. Weeks et al. (2003) are not the only authors to advocate using bilateral transfer in a rehabilitation context. Nagel and Rice (2001) have also suggested that bilateral transfer could be used to improve the function of impaired limbs.
Upper-limb prosthetic simulator. Courtesy of Stephen A. Wallace.
© Courtesy of Marilyn Mitchell
Evidence of brain interhemispheric transfer of the motor components of tasks has also been used to support the motor control explanation for bilateral transfer (Hicks, Gualtieri, & Schroeder, 1983). One way researchers have demonstrated this mediation is by measuring the EMG activity in all four limbs when one limb performs a movement. When EMG activity occurs, it tells researchers that the central nervous system has forwarded commands to those muscles. In fact, research conducted as long ago as 1942 showed that the greatest amount of EMG activity occurs for the contralateral limbs (i.e., the two arms), a lesser amount occurs for the ipsilateral limbs (i.e., arm and leg on the same side), and the least amount occurs for the diagonal limbs (Davis, 1942).
Functional magnetic resonance imaging (fMRI) has also established a neural basis for bilateral transfer. For example, a series of experiments involving the learning of a 12 item finger sequence with the right hand found that the supplemental motor area (SMA) of the cortex had more activity when the skill was performed well with the left hand than when it was performed poorly (Perez, Tanaka, Wise, et al., 2007). In fact, in one of these experiments when SMA activation was blocked by the use of transcranial magnetic stimulation (TMS), no bilateral transfer occurred. Halsband and Lange (2006) have provided a particularly interesting discussion of the neural basis for bilateral transfer, with an emphasis on the asymmetries in brain activity that are associated with left-to-right and right-to-left transfer.
Which of the two explanations of bilateral transfer is correct? Research evidence indicates that both cognitive and motor factors are involved in bilateral transfer. There is no doubt that cognitive components related to "what to do" account for much of the transfer that results from practicing a skill with one limb. This is quite consistent with what we have discussed thus far in this book. For example, both the Fitts and Posner and the Gentile models of the stages of skill learning described in chapter 12 propose that determining "what to do" is a critical part of what a learner acquires in the first stage of learning. There is likewise no doubt that bilateral transfer involves a motor control basis as well. This is consistent with our discussion in chapter 5 of the control of coordinated action. It is also consistent with research evidence that there is some motor outflow to other limbs when one limb performs a skill.