Implicit motor learning is preserved after stroke, but how the brain compensates for damage to facilitate learning is unclear. We used a random effects analysis to determine how stroke alters patterns of brain activity during implicit sequence-specific motor learning as compared to general improvements in motor control. Nine healthy participants and 9 individuals with chronic, right focal sub-cortical stroke performed a continuous joystick-based tracking task during an initial fMRI session, over 5 days of practice, and a retention test during a separate fMRI session. Sequence-specific implicit motor learning was differentiated from general improvements in motor control by comparing tracking performance on a novel, repeated tracking sequences during early practice and again at the retention test. Both groups demonstrated implicit sequence-specific motor learning at the retention test, yet substantial differences were apparent. At retention, healthy control participants demonstrated increased BOLD response in left dorsal premotor cortex (BA 6) but decreased BOLD response left dorsolateral prefrontal cortex (DLPFC; BA 9) during repeated sequence tracking. In contrast, at retention individuals with stroke did not show this reduction in DLPFC during repeated tracking. Instead implicit sequence-specific motor learning and general improvements in motor control were associated with increased BOLD response in the left middle frontal gyrus BA 8, regardless of sequence type after stroke. These data emphasize the potential importance of a prefrontal-based attentional network for implicit motor learning after stroke. The present study is the first to highlight the importance of the prefrontal cortex for implicit sequencespecific motor learning after stroke.
Fitts' law predicts that there is an essential trade-off between speed and accuracy during movement. Past investigations of Fitts' law have not characterized whether advance planning of upcoming fast and accurate movements impacts either behavior or patterns of brain activation. With an event-related functional magnetic resonance imaging (fMRI) paradigm, we investigated the neural correlates of advance planning and movement difficulty of rapid, goal-directed aimed movements using a discrete version of the classic Fitts' task. Our behavioral data revealed strong differences in response time, initial movement velocity, and end-point accuracy based on manipulation of both time to plan movements and response difficulty. We discovered a modulation of the neural network associated with executing the Fitts' task that was dependent on the availability of time to plan the upcoming movement and motor difficulty. Specifically, when time to plan for the upcoming movement was available, medial frontal gyrus (BA 10), pre-SMA (BA 6), putamen and cerebellar lobule VI were uniquely active to plan movements. Further, their activation correlated with behavioral measures of movement. In contrast, manipulating movement difficulty invoked a different pattern of brain activations in regions that are known to participate in motor control, including supplementary motor area (BA 6), sensory motor cortex (BA 4, 3, 2) and putamen. Our finding that medial frontal gyrus (BA 10) was important for discrete, fast and accurate movements expands the known role of this brain region, which in the past has been identified as a cognitive processing system supporting stimulus-oriented attending. We now extend this conceptualization to include motor functions such as those employed for processing for rapid, goal-directed aimed movements.
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