Learning of a covert sequence of effector movements: limits to its acquisition

Sequence learning in serial reaction time (SRT) tasks is an established, lab-based experimental paradigm to study acquisition and transfer of skill based on the detection of predictable stimulus and motor response sequences. Participants learn a stimulus sequence by associating their responses to the targets with the subsequently presented targets in the sequence. In the traditional paradigm, however, actions and targets are directly related. In contrast, the present study asked whether participants would demonstrate acquisition of a sequence of effector movements such as the left vs. right hand (e.g., hand sequence learning), whilst the actual targets and associated finger responses are unpredictable. Twenty-seven young adults performed a SRT task to visually presented characters with the index or middle fingers of both hands. While the specific fingers to respond with were randomly selected for each stimulus presentation, both hands followed a covert sequence. We asked whether participants would learn the underlying hand sequence as demonstrated by shortened response latencies and increased accuracy compared to a fully randomized hand sequence. The results show sequence-specific learning effects. However, categorization of hand responses depending on the previous response suggested that learning occurred predominantly for subsequent finger responses of the same hand but not between the hands. Nevertheless, a small effect was observed for predictable shifts between hands irrespective of the fingers involved. Our results thus suggest that humans are able to benefit from predictable hand repetitions but less so for predicted shifts between hands.