Dynamic synchrony of firing in the monkey prefrontal cortex during working-memory tasks

Synchronized firing among neurons in the working brain is inferred to reflect coding by cell assemblies, which dynamically change their sizes and functional connections to encode various information. It therefore follows that, if synchronized firing reflects cell-assembly coding, it should show dynamic changes that depend on the tasks and events being processed and on the distance between the neurons. By using unique spike-sorting and multi-neuronal recording methods, we investigated such dynamics of synchrony in the prefrontal cortex of monkeys while they were successively performing two tasks in which working memory for either stimulus duration or color was required. Forty-eight percent of 1405 neuronal pairs showed firing synchrony during the performance of the tasks. Almost half of such neuronal pairs showed fixed synchrony and constantly fired together in both tasks. However, some neuronal pairs showed task-dependent synchrony that appeared in only one of the tasks. Moreover, the other neuronal pairs showed event-task-dependent synchrony that appeared during stimulus or retention periods in the tasks, but the periods showing synchrony varied between the tasks. Fixed synchrony and task-dependent synchrony were mostly observed among neighboring neurons and showed little variation of spike timings; the event-task-dependent synchrony, in contrast, was more often detected among distant neurons with larger variation of spike timings than the other two types of synchrony. These results suggest that some closely neighboring neurons have dynamic and sharp synchrony to represent certain situations (tasks), whereas some distant neurons show more dynamic and unstable synchronous firing to represent quickly changing events being processed in working memory.