Motor Learning

Even the simple acts of motor production described previously (e.g., singing an isochronous series of a single pitch or tapping an isochronous sequence on a single key) require the execution of a learned motor program or an integrated sequence of movements. More complex sequences that follow a precise temporal plan and involve multiple vocal or instrumental pitches and variable timing and intensity are integral to musical expression. Such sequential movements performed as a unit require advance programming prior to their execution.

In the experiment discussed previously (see Section III.B), Perry, Zatorre, and Petrides asked their subjects to listen to a two-note sequence drawn from a major scale and either imagine it in their heads, or hum it out loud. When they hummed out loud, in contrast to just listening to the sequences, activation was seen not only in motor cortex, SMA, and the right putamen as seen during imaging but also in the left putamen, cerebellum, right primary auditory region, and more extensively in motor cortex bilaterally. Thus, this simple act of musical motor programming activated the same areas thought to be involved in the execution of movement sequences generally (i.e., primary and premotor cortex, SMA, basal ganglia, and the cerebellum).

The SMA activation fell anterior to that seen during simple, repetitive singing of a single pitch. Rather than falling in "SMA proper,'' as defined by Nathalie Picard and Peter Strick in their cross-species neuroa-natomical analysis, it fell in pre-SMA, a cortical area thought to be important for producing organized sequences of movements. Thus, more complex sequential singing resulted in additional activation of pre-motor cortex, pre-SMA, and the basal ganglia (putamen).

Justine Sergent and associates carried out a study of right-handed piano performance using PET. When playing a scale was contrasted to listening to scales, CBF increases were seen in contralateral primary motor cortex, SMA proper, and the ipsilateral cere-bellum.When sight-reading an unfamiliar Bach melody was contrasted to reading a score of it while listening to a recorded performance, additional activation was seen in contralateral premotor cortex and in pre-SMA. Although the increase in motor programming complexity between playing scales and sight-reading a melody would seem to be much greater than that between singing a single pitch and singing two-note sequences, the results are highly similar. Both comparisons emphasize the activation of premotor cortex and pre-SMA during the expression of more complex and variable motor programs.

Although these two studies measured actual musical expression, few others have done so. However, others have studied quasi-musical tasks helpful for isolating components of musical expression.

In the study of rhythm production described in Section IV.A, Penhune and Zatorre asked subjects to reproduce sequences with elements of variable durations, either short (250 msec) or long (750 msec), again cued either visually or auditorily. In one condition, subjects reproduced a previously learned sequence repetitively. When contrasted to the isochronous condition, additional activation during sequence reproduction was seen in the cerebellum and in the contralateral primary motor cortex only for the visually cued condition. In another condition, they reproduced novel sequences. In contrast to the repeated sequence condition, additional activation was seen across tasks in the cerebellar vermis and bilaterally in the cerebellar hemispheres. Additional activation was seen in pre-SMA and the basal ganglia for the auditory condition only.

At least for the auditorily cued condition, these results for rhythm production parallel those described previously for singing and piano performance: activation of SMA proper for simple, predictable movements and of pre-SMA for more complex and less predictable movement sequences. The activation of the basal ganglia by the task placing more demands on motor programming and execution is also consistent for both the vocal pitch and manual rhythm reproduction tasks.

Christian Gerloff and associates used repetitive trains of transcranial magnetic stimulation (TMS) that temporarily disrupt neural activity to examine its effects on the performance of finger sebquences on a (silent) piano keyboard. The sequences differed in complexity, and TMS was applied over the contralateral primary motor cortex or over the SMA. Subjects thoroughly learned the 16-item sequences first. The simplest sequence consisted of 16 repeated index finger presses of a single key; a scale-like sequence consisted of alternating 4-item ascending and descending subsequences; and the most complex was a variable 16-item sequence. Stimulation over the primary motor cortex induced errors in both the complex and scalelike sequences and, with sufficient intensity, in the simple repetitive sequence. Stimulation over the SMA disrupted only the complex sequence.

These TMS results are consistent with activation studies that demonstrated increased activation of primary motor cortex and pre-SMA with increased demands for motor sequence planning. Although the role of primary motor cortex was once thought to be restricted to the execution of simple voluntary movements by individual muscles, increasing evidence indicates its involvement in the learning and execution of more complex movement sequences.

Norihiro Sadato and colleagues measured CBF increases during the execution of sequential finger movements (touching fingers to thumb) of varying complexity using PET. Each sequence (4-16 items) was thoroughly learned before the PET scan, and the rate was determined by following a metronome. Activation in one set of motor-related areas did not vary with the complexity of the sequences: bilateral primary sensor-imotor cortex, contralateral ventral premotor cortex, SMA proper, contralateral putamen, and ipsilateral cerebellum. Four areas showed a linear increase in CBF with increasing sequence length: right dorsal premotor cortex, right superior parietal cortex, left thalamus, and the cerebellar vermis. The association of sequence complexity with premotor cortex activation agrees with results of subtractive studies mentioned previously. However, the basal ganglia were activated equivalently by all tasks and activation of pre-SMA was not demonstrated.

Subtle differences in task and experimental design are well-known to affect the outcome of imaging studies, and major differences exist between all these tasks. For example, Sadato's study required synchronization with an auditory metronome, perhaps resulting in activation of the basal ganglia across tasks. Future studies systematically varying such task parameters will further our understanding of the roles of each of these areas in movement sequencing.

Precise programs for movement sequences must first be learned. Then they can be retrieved and executed as one unit. Some studies have attempted to minimize learning by having subjects thoroughly learn the sequences beforehand (e.g., Gerloff, Sadato). Obviously, study of the brain mechanisms supporting learning as well as retrieval and execution of complex motor sequences that produce musical sound is central to a biological understanding of musical behaviors.

One study used TMS in a very different manner to investigate cortical changes associated with learning a motor sequence on a piano keyboard with auditory feedback. Alvaro Pascual-Leone and colleagues repeatedly mapped the contralateral motor cortex over a 5-day period during which subjects practiced a simple five-finger exercise on a computer-interfaced piano keyboard. They learned to play a 10-item scale-like sequence consisting of 5 ascending and 5 descending elements, with the goal of playing it evenly at a specified rate. None had any musical instrument-playing experience. Thresholds for the production of motor evoked potentials (MEPs) in response to single TMS bursts were measured from the contralateral hand (flexor and extensor muscles). Contour maps of the probability of inducing MEPs demonstrated increases in the cortical motor areas targeting the practiced hand and decreases in threshold. No such changes were seen for the untrained hand, and only modest gains were seen for subjects who simply played the piano at will for the same length of time (Fig. 7).

Figure 7 Piano exercise learning, trained vs untrained hand. Representative examples of cortical motor output maps for the long finger flexor and extensor muscles on Days l-5 from a subject who practiced 2hr/day (trained right and untrained left hand) and from one control subject who played random sequences for the same amount of time (trained right hand) (reproduced with permission from Pascual-Leone et al., 1995).

Figure 7 Piano exercise learning, trained vs untrained hand. Representative examples of cortical motor output maps for the long finger flexor and extensor muscles on Days l-5 from a subject who practiced 2hr/day (trained right and untrained left hand) and from one control subject who played random sequences for the same amount of time (trained right hand) (reproduced with permission from Pascual-Leone et al., 1995).

Other functional imaging studies of motor sequence learning are less directly related to musical expression. Nevertheless, they are relevant for understanding the motor sequence learning component, isolated from sound production. Though complex, the results suggest an expansion of the representation of the involved fingers within primary motor cortex. A. Karni and colleagues asked subjects to learn a particular sequence of finger-to-thumb opposition movements (e.g., 4-13-2-4) for 10-12 min a day over the course of 5 weeks. Once a week, cortical activity was measured during performance of the practiced sequence and during performance of an unpracticed sequence (e.g., 4-2-31-4) with fMRI.

After 3 weeks of practice, the extent of primary motor cortex activated by the learned sequence was greater than that activated by the unpracticed one.

When retested several months later, the skill and the increased extent of motor cortex activation persisted. Because the same fingers were used in both sequences, this increase is not in the cortical representation of particular fingers but in the representation of a particular sequential combination of finger movements.

William James' speculations regarding the brain mechanism underlying memory as ''paths'' within the brain's tissue can be applied not only to melodic memory but also to memory for the movement sequences used to produce them (i.e., as paths within somatotopically ordered cortical areas).

Other studies have shown activation of cerebellum, basal ganglia, thalamus, and dorsolateral frontal cortex during skill learning, decreasing with increasing acquisition. Plateaus in the increased activation within motor cortex during skill learning have been demonstrated, as have decreases after complete acquisition. The precise contributions of cortical and subcortical areas, including the basal ganglia, to the execution, learning, and retrieval of movement sequences remain controversial and a subject of active research.

Movement sequence learning in the service of musical expression adds further complexity by introducing the production of musical sound as the ultimate goal. It also introduces the possibility of sampling across wide variations in the degree of skill acquisition.

Extending his previous research, Pascual-Leone used single pulses of TMS and measurement of the consequent MEPs to map changes in cortical output maps during implicit and explicit sequence learning. Subjects rested four digits over four computer keys and pressed the keys sequentially in response to a visual cue. Unbeknownst to them, these cues corresponded to particular sequences of varying length. Even before becoming aware of this sequence, reaction times progressively declined (implicit learning) and rebounded when the order was changed. During this phase, the cortical motor output map to the involved contralateral muscles increased. Uninvolved muscles (i.e., the thumb) showed no change. After subjects became aware of the sequence, learning could proceed by both implicit and explicit means, and the maps tended to plateau. Finally, once the sequence was fully, explicitly learned and response times plateaued, there was a rapid reduction in the cortical output map toward baseline.

Using a different technique, trains of TMS were used to disrupt learning in the serial reaction time task during 60-sec blocks over dorsolateral frontal cortex or SMA. The effects of stimulation were inferred from task performance and were also tested during a final random block. Stimulation over the contralateral dorsolateral frontal cortex resulted in a complete disruption of implicit learning, indicating that it is not only activated during such learning but also critical to its success.

Motor sequence learning in musical context is always explicit, at least for the intended auditory result. Nevertheless, simultaneous implicit learning must play a crucial role, particularly for the actual sequence of fingers. Thus, these results are directly relevant to hypotheses for the motor learning underlying musical expression.

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