Abstract
Chaining movements into sequence is one of the most integral strategies to form complex behaviour. Many actions that we perform everyday are coordinated concatenations of motor primitives. Previously it has been shown that the visual system is involved in the preparation of fluent movement sequences by pre-selecting multiple future goal locations in advance of sequence initialisation. Here, we investigated in several experiments how the pre-selection of subsequent movement goals depends on the temporal dynamics of the planned motor sequence. Specifically, we tested the hypothesis that only in fluently executed movement sequences the motor primitives are chunked together such that visual attention splits in order to pre-select all subsequent movement goals simultaneously. In contrast, interrupted motor sequences may be prepared step-by-step and therefore visual attention is hypothesized to only select the immediate goal location.
In our experiments, participants were instructed to perform double-step eye- or hand movements in certain rhythms, i.e. to voluntarily interrupt the sequence production by predefined inter-movement delays. A secondary discrimination task served as a measure for the distribution of visual attention in the field during the initial motor preparation. The results show that subsequent goal locations were gradually better pre-selected the shorter the inter-movement delay was. Inter-movement delays of 300–500 ms constitute a threshold at which subsequent motor goals were no longer selected by visual attention in advance. This suggests that the visual preparation of intended movement-sequences crucially depends on the temporal dynamics of those motor units. Subsequent goals are only pre-selected if the inter-movement delay is too short for the full preparation of the second movement component.