Characteristics of sequential learning and memory in non-human primates

Sequential learning and memory are fundamental aspects of various daily tasks, including communication, navigation and executing a series of actions. Previous studies have demonstrated that monkeys can acquire motor sequence through trial-and-errors. However, the specific characteristics of how monkeys learn and maintain these sequences in memory across days remained unclear. In the current study, we focused on the learning and memory process involved in oculomotor sequences in non-human primates using a novel sequential saccade task. Two monkeys were trained to identify the 5-stages sequences by making consecutive saccades to one of the two targets in each stage. A 200ms blank screen period before each target’s onset was introduced to assess anticipatory saccades to the next future location in the sequence. Throughout the training, both monkeys exhibited clear signs of sequence acquisition. They gradually learned the correct sequence within hundreds of trials, evident in increasing correct rates, and decreasing target acquisition times. Repetitive training across days led to increased proficiency in directing oculomotor sequences, reflected in a plateaued correct rate and decreased target acquisition time. Examing the time for each order in the sequence suggested changes of the sequence representation. During the early training days, the target acquisition time increased with the steps within one sequence, implying a serial representation of each step within the sequence. Such increasing trend gradually decreased after a few days’ training, indicating the incorporation of the sequence as a whole. Anticipatory eye movements, signifying predictability of the next step in the sequence, emerged in most well-trained sequence. In summary, our results demonstrate the signs of sequence learning and memory during repetitive training. The study provides a paradigm with the potential to unveil the neural mechanisms underlying sequential learning and memory in non-human primates.