BACKGROUND/INTRODUCTION Motor learning plays a central role in daily life and in neurorehabilitation. Several forms of motor learning have been described, among which motor skill learning, i.e. reaching a superior level of performance (a skill) through a shift of the speed/accuracy trade-off. During the first stage of learning a visuomotor skill, we observed differential patterns of evolution of the speed/accuracy trade-off in normal subjects. Half of the subjects rapidly achieved successful motor skill learning with an early shift of the speed/accuracy trade-off leading to a superior level of performance (shift pattern). The other subjects attained only minimal global improvement due to a converse evolution of speed and accuracy (i.e. a respect of the speed/accuracy trade-off: fit pattern). Functional magnetic resonance imaging (fMRI) was used to explore the neural substrates underlying these differential patterns during the first stage of motor skill learning in normal subjects. METHODS Twenty right-handed normal subjects performed an implicit visuomotor learning task with their non-dominant hand. The task ("circuit game") consisted in learning to navigate a pointer along a circuit as quickly and accurately as possible using a fMRI-compatible mouse. Velocity, accuracy, and performance indexes were used to characterise the motor learning pattern (shift/fit) and to perform fMRI correlation analysis in order to find the neural substrate associated with the shift and fit patterns during early motor skill learning. RESULTS Nine subjects showed a fit pattern (fitters), and eleven, a shift pattern ("shifters"). fMRI analyses at whole group level (ANOVA) and at sub-group level demonstrated that the supplementary motor area (SMA) was more activated in "shifters" than in the "fitters" groups and that the BOLD activation within the SMA correlated significantly with the on-line shift of the speed/accuracy trade-off in the "shifters" group. CONCLUSION Despite identical instructions and experimental conditions, during the first stage of motor skill learning normal subjects spontaneously adopted different patterns that can be differentiated based on distinct fMRI activation patterns. In this implicit visuomotor task, the SMA proper was the key area underlying the achievement of early successful motor skill learning, i.e. on-line shift of the speed/accuracy trade-off.