No movement is executed in isolation. In real life, we produce a constant stream of actions– with the movement being already selected and planned while the current movement is still being executed. Many skills, such as typing, playing an instrument or tying a knot, rely on complex sequences of movements, sometimes practiced and rehearsed, sometimes flexibly assembled just in time.

How does the brain plan and execute movements at the same time? How do we become better at concatenating simple movements into a skillful whole? What neural representations underlie the production and learning of sequence movements? To answer such questions, we are combining carefully designed behavioral experiments, high-resolution functional magnetic resonance imaging (fMRI), representation analysis tools, and computational modelling.

Sequence representation in human cerebral cortex

Using a pattern classification we first established that we could decode the sequence identity from the fine-grained (~2.3mm3) fMRI activity patterns of participants performing several 5-item sequences after a few day of practice (Wiestler and Diedrichsen, 2013). In further studies we then asked how sequences are represented in different brain regions. For example, we have shown that the temporal (i.e., time intervals between presses) and ordinal (i.e., order of fingers in a sequence) features of sequences are stored in partly separated areas including PMd, SMA, and SPL (Kornysheva and Diedrichsen, 2014). We also were able to show that the representation in rostral PMd reflect the instructional cue, while more caudal areas represent the actual finger movement (Wiestler et al., 2014).

More recent works from the lab addressed the question of hierarchical sequence organization. In the mid-twentieth century, Karl Lashley, George Miller, and others proposed that the actions are organized hierarchically by recursively grouping several simpler movements into a higher-order functional unit, sometimes called "chunks". Activation of a chunk automatically generates a set movement elements in specific order. Recently, we designed a new behavioral paradigm to stably introduce a 3-level hierarchy (single finger, chunk, and entire sequence), and to study these levels in brain activity patterns (Yokoi and Diedrichsen, 2019). Our data clear shows that M1 represents only individual finger movements (Yokoi et al., 2018). Within premotor and parietal cortices, chunk and sequence representations partially overlapped.

Future directions

Work from our and other labs show that sequential behaviours are controlled by a tight interaction between action selection, planning, and motor execution. How does the brain coordinate these processes without catastrophic interference? What type of solutions emerge in artificial neuronal networks trained to perform sequential tasks? How do these representations evolve over long periods of training?

Relevant papers