The serial order and the temporal structure of sensory-motor sequences are tightly coupled, and interact in terms of their neural representation. This has been characterized behaviourally in the discrete sequence processing (DSP) task, in which key-press sequences of 4-6 elements are learned over repeated trials. In this task subjects adopt a temporal segmentation structure in which an initial set of elements is learned as a contiguous group, separated by a pause from the second contiguous group. The placement of the pause can emerge spontaneously in learning, or can be imposed in the training protocol. We propose that individual movement elements are represented in the primary motor cortex and combined into short movement segments by the SMA. Crucially, these short segments are then selectively activated in the correct sequential order via the cortico-striato-thalamo-cortical (CSTC) motor circuit. We thus predict that during learning of a sensory-motor sequence, the presence of a temporal segmentation cue will strongly solicit the CSTC network in establishing the neural representation of the compositional structure of the sequence.
To test this hypothesis we performed an event related fMRI experiment in which subjects fulfilled the DSP task in 4 different conditions: Learning new sequences With (LearnWp) and WithOUT (LearnWOUTp) the imposed pause, and Performing previously learnt sequences With (PerfWp) and WithOUT (PerfWOUTp) the pause. We used SPM5 to perform a random effect analysis over the fMRI images with uncorrected p < 0.001 and 25 voxels clusters. Comparison of conditions LearnWp vs LearnWOUTp revealed a significant activation of the visuo-spatial CSTC system, with bilateral activation of dorsolateral prefrontal cortex, caudate nucleus, putamen and thalamus. This activation was significantly greater in the LearnWp vs LearnWOUTp condition. Its profile is consistent with our hypothesis that learning temporal segmentation structure significantly solicits the CSTC system. This thus extends the role of the CSTC system in sensory-motor sequence learning, by emphasizing the importance of the role that the temporal structure plays.