Consistent Shifts of Stimulus Modality Induce Chunking in Sequence Learning

The ability to learn event sequences is crucial to human behavior because it enables us to interact adaptively with our environment. The sensory environment is essential in guiding the acquisition of these sequences, so the role of the stimulus modality in sequence learning is of high relevance. The present study examined structured stimulus modality shifts in sequence learning using the serial reaction time task (SRT). Participants had to respond to numbers that were presented either in the visual or in the auditory modality. Importantly, the numbers, as well as the stimulus modality, followed a fixed pattern. We found better performance in sequenced than in random blocks, indicating sequence learning. Moreover, the performance was better when the stimulus modality remained the same than when the modality changed between successive trials (the modality shift effect, MSE). However, sequence learning facilitated performance primarily in modality repetitions, so that the MSE became progressively larger in the sequenced blocks, while it was small in the random blocks, and this pattern was particularly pronounced for the participants who showed a high recall level for the response sequence. To account for this effect, we assume that consistent modality shifts induce parsing of the sequence into chunks. Because chunk retrieval at chunk boundaries incurs RT costs, the acquired sequence knowledge is more efficiently expressed in modality repetitions (i.e., within chunks). Together, the data suggest that the formation of explicit knowledge enhances chunking in sequence learning.