Coding of time with non-linear mixed selectivity in Prefrontal Cortex ensembles

Previous work has identified stimulus specific time cells as a potential mechanism for working memory maintenance. It has been proposed that populations of stimulus specific sequences of cells could support memory for many items in a list over long periods of time. This would require information about one stimulus to persist after the presentation of subsequent stimuli. However, it is not known if sequences triggered by one stimulus persist past the presentation of additional stimuli. It is possible that each new stimulus terminates preceding sequences, making memory for multiple stimuli impossible. To investigate this question, we utilized a data set originally published by (Warden & Miller, 2010), studying the firing of monkey prefrontal neurons during short lists of stimuli. We were able to decode "what happened when" throughout the list, using linear discriminant analysis. Additionally, we were able to decode the first stimulus after the presentation of the second stimulus. Furthermore, we found that stimulus modulated sequences of cells, with discrete temporal fields, continue after the second item was presented. Much of the information about the previous item was carried by neurons that responded to conjunctions of stimuli and the timing of late-firing cells was synchronized to the firing of the second stimulus rather than the first. These properties falsify a simple linear model of sequential time cells. These results suggest that non-linear mixed selectivity extends to continuous variables such as time, but that in this experiment at least, only the timing of the most recent stimulus was explicitly maintained in ongoing firing.