Motor Cortex

The primary motor cortex, Brodmann's area 4, is an obvious starting point. It sends direct projections to the spinal cord and is unique among cortical areas in that it possesses reciprocal connections with all of the premotor regions that project to the spinal cord. Stimulation of this cortical area produces overt movement and the neural activity is more closely related to movement parameters than other cortical motor structures. For these reasons, the motor cortex has traditionally been considered an area associated with motor output rather than performing the higher level operations associated with motor learning.

However, there is strong evidence that the physiology of motor cortex changes as new motor skills are developed. Across a variety of sequence learning tasks, motor cortex activity has been shown to increase during the course of training. Interestingly, these changes appear to be strongest when the sequence knowledge is implicit; under conditions that favor the development of explicit knowledge, PET studies have failed to show learning-related changes in motor cortex.

One potential consequence of skill learning is that the motor fields of task-related neural ensembles are enhanced over the course of practice. Indeed, anatomical studies in the rat have shown a functional reorganization of the topographic map within motor cortex as a function of practice. The cortical region associated with the involved effectors becomes larger, paralleling learning-related changes that have been observed in various sensory cortices. The motor "receptive" field can also be measured in humans with transcranial magnetic stimulation (TMS). With this method, a large magnetic field is generated in a coil applied to the scalp and the underlying neural tissue is excited. When applied over motor cortex, discrete finger movements can be elicited. During SRT training, the region over which finger movements could be generated increased. Interestingly, as soon as the participants became aware of the sequence, the motor fields returned to their original size.

The previous results suggest an involvement of motor cortex during implicit sequence acquisition. However, these studies have all involved training that was restricted to a single session. Avi Karni and colleagues conducted a sequence learning study that spanned a much longer period. Participants were required to practice a series of finger movements over a period of several months. fMRI was used to measure activity within the motor cortex at various points during the training regimen, with the comparison made between epochs in which the participants produced the learned sequence and epochs in which the participants produced an alternative sequence. During the scanning sessions, the movements were paced so that the number of actual responses was equated for the two conditions. Nonetheless, after 3 weeks of practice (but not before), the trained sequence produced greater signals in primary motor cortex than an untrained control sequence. This increase was apparent despite the fact that the participants had full awareness of both the trained and control sequences.

There are at least two ways to reconcile the discrepancy between the results of this study and the SRT results. First, after many weeks of training, performance on the highly trained sequence may no longer depend on explicit representational systems. Indeed, at this point in training, the participants would show minimal cost on the sequencing task if required to concurrently perform a second, attention, demanding task. By this hypothesis, increases in motor cortex activity are again restricted to implicit sequence performance. Second, the increased signal may be a consequence of the pacing manipulation during the scanning sessions. By slowing down the production rate during the scanning session, motor cortex neurons may have remained active in anticipation of the next response. This latter hypothesis underscores a potential limitation with imaging studies of sequence learning. Neural activity may increase in an area not because there is a representational change within the area but because the input signal from an upstream neural region has become stronger.

Neurophysiological studies have also provided evidence for learning-related changes in motor cortex. Several groups of researchers have reported that the response properties of motor cortex neurons change as the animal learns to perform new tasks. These neurons may become more responsive to a given stimulus as the animal learns to associate it with a particular response, or they may alter their responsiveness to reflect the operation of a particular set of S-R mapping rules, regardless of the actual stimuli. Thus, it is unlikely that this region's computation is restricted to simple output properties.

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