The Role Of Macaque Prefrontal Eye Filed And Dorsolateral Prefrontal Cortex During Sequential Saccades

Sequence motor learning is important for us to develop new skills.Everyday speaking,writing,piano playing,and driving are attributes to sequence motor learning.Abnormalities in motor sequence learning have been observed in some patients with functional encephalopathy,which negatively affects the quality of life.However,the neural substrate underlying sequence motor planning and execution are largely unknown.Previous studies have shown that the prefrontal cortex might be a key region for planning sequential behavior,but it needs to be verified.Using a novel task,we combined electrophysiological recording,microstimulation,and reversible inactivation to investigate the causal role of the frontal eye field（FEF）and dorsolateral prefrontal cortex（DLPFC）during sequential saccades.By recording the neural activities in FEF while monkeys were performing a "leftleft-right-right"（"LLRR"）sequential saccade task,we found that 41.1% of neurons changed the direction selectivity of the saccade.More than half of neurons showed sequence-related activity: 26.0% of neurons showed stronger firing at the first saccade（named “start cell”）or the last action of the sequence（“stop cell”）;28.3% showed stronger firing before/after subsequence switching（switch cells）.In addition,these sequence-related activities were influenced by different contexts,with different firing activities between visual-guided and memory-guided sequences.69.0% of neurons showed significant differences in response to the same motor element between two contexts.To further investigate the causal link between the FEF and sequential saccades,microstimulation was applied to observe how the sequential saccade behavior was changed.The microstimulation threshold to evoke saccade in FEF was increased than that during the fixation task.The evoked saccades can be integrated into the component of sequence when they were consistent with sequence execution but were ignored when they were inconsistent with sequence.Further inactivation of the whole FEF brain region severely affected the execution of the sequential saccade: the accuracy rate decreased significantly,the total duration of the sequence execution increased significantly,and the effect on the memory-guided sequential saccades task was greater than that on the visual-guided sequential saccades task.In addition,saccades in different order of sequences were affected differently,and the first contralateral saccades guided by memory were most heavily affected.These results revealed that the FEF encoded sequence information dynamically and played a key role in the planning and execution of sequential saccades.However,small clustered FEF neurons affected microstimulation could not change the structure of the entire sequence execution,suggesting that the sequence execution requires the involvement of other brain regions.FEF may be located at the bottom of the sequence execution brain network,closer to the behavior output.Since DLPFC is known as a brain area associated with advanced cognition and sends efferent tracts to FEF,we further investigated how DLPFC participates in sequential saccades.34.9% of neurons changed the direction selectivity of the saccade.63.2% of neurons showed LLRR sequence-related activity: 33.2% of neurons showed stronger firing at the start or end of the LLRR sequence（start/stop cells）,and 30.1%showed stronger firing before/after subsequence switching（switch cells）.Based on the LLRR sequence,the RRLL sequence was added as a control,so that neurons can be divided into sequence cells,subsequence cells,and element cells.The results showed that DLPFC neurons encoded sequential saccades at multiple levels: 14.0% were at the sequence level（specific order of sequence）,42.5% were at the subsequence level（beginning and end of subsequence）,and 32.9% were at the element level（specific direction or action）.Moreover,65.0% of sequence-related neurons showed stronger or weaker firing at the first saccade of sequence or subsequence,suggesting that DLPFC might be involved in encoding the initiation of sequence.Inactivation of the left or right DLPFC significantly increased the latency of the first saccade guided by memory,without affecting other saccades in sequences.It was further confirmed that DLPFC is involved in the initiation of sequence.Our result suggested that both FEF and DLPFC are involved in the dynamic encoding of sequential eye movements.Compared with DLPFC,FEF is likely at a low level of hierarchical control of sequence performance,and DLPFC is more involved in the controlling of sequence initiation.