Considerable progress was made on this project during the past reporting year. In a key aspect of this project, we studied neuronal activity as subjects discriminated relative durations and distances. Our analysis focused on the frontal cortex. In a relative duration task, subjects had to report whether a red stimulus or a blue stimulus had lasted longer, when these two stimuli could appear in either order. During a delay period between the two stimuli, many neurons reflected the absolute duration of the first stimulus and others encoded which stimulus, red or blue, had appeared first. During a delay period after the second stimulus, the largest population of neurons encoded whether the first or second stimulus had lasted longer, and a substantial population encoded which stimulus, red or blue, had lasted longer. In a relative distance task, an overlapping population of neurons encoded whether the first or second stimulus had appeared farther from the center of a video screen and/or whether the red or blue stimulus did so. These results show that frontal cortex neurons encode relative durations and/or distances, including both the stimuli (red versus blue) and the order that they appeared (first versus second). In another key aspect of this project, we completed the first study of the frontal pole cortex. The frontal pole cortex is the part of prefrontal cortex that expands most dramatically during primate evolution, and we studied its neuronal activity. In addition, we contrasted the properties of neurons in the frontal pole cortex with those of cells in the dorsolateral and orbital prefrontal cortex. The task we used, a strategy task, required that subject's remember their previous goal (left or right) until a cue appeared, which instructed one of two strategies: stay or shift. Stay cues required a saccadic eye movement to the previous goal; shift cues required a saccade to the other goal. One block of trials had visual strategy cues (vertical versus horizontal bars or yellow versus purple squares), whereas in a separate block of trials the strategy cue was one versus two drops of fluid. Feedback followed each saccade. In the visually cued strategy task, many frontal pole neurons encoded whether the monkeys had chosen the left or right goal, and they did so only at one time during the trial: just before and/or after feedback. Importantly, they did so for both correct and incorrect choices. Frontal pole cells did not encode the anticipation or delivery of fluid when it served as a cue, nor did they encode the visual cues, strategies, memory for previous goals, future goal choices or plans for future movements. Thus, during a brief period near feedback, the frontal pole cortex encodes which goal the subjects selected, a signal that lasts longer when they need to remember their own choices. Our findings indicate that the frontal pole cortex plays a role in monitoring self-generated choices, which could account for its dramatic expansion during human evolution. This project also involved the exploration of neuronal mechanisms involved in selecting future goals. In this part of the project, we studied the choice of a future goal based on a strategy task similar to the one described above. In the previous reporting year (Fiscal Year 2007), we found that one population of prefrontal neurons encodes future goals (F cells) and a nearly completely separate population encodes previous goals (P cells). In this reporting year (Fiscal Year 2008), we found that the prefrontal cortex encodes these goals and strategies whether the subject chooses a goal based on the correct or incorrect strategy. This result was reported in the European Journal of Neuroscience. To better understand the role of neuronal interactions in these cognitive functions, we also studied transient cross-correlations in cell pairs during the performance of this task. As a report to be published in the journal Cerebral Cortex explains, we found that F-P and F-F pairs showed many significant positive correlations, but P-P pairs did not. At the population level, both F-P and F-F pairs increased their correlations after cue onset, and these correlations remained relatively stable at least until cue offset. In F-F pairs, but not in F-P pairs, correlations remained stable until feedback arrived. Importantly, correlated F-F pairs usually shared the same goal preference, whereas F-P pairs did not. Further, both F-F and F-P pairs showed their highest correlations when monkeys had to reject a previous goal. These findings indicate that future goals are selected through interactions among F-P pairs, with one component (the P cell) retrospectively coding a previous goal and the other component (the F cell) prospectively coding a future goal, and usually a different one. Later, recurrent connections among F-F pairs support goal maintenance. This project is coming to a conclusion some time during Fiscal Year 2009.
| Genovesio, Aldo; Brasted, Peter J; Wise, Steven P (2006) Representation of future and previous spatial goals by separate neural populations in prefrontal cortex. J Neurosci 26:7305-16 |
| Genovesio, Aldo; Tsujimoto, Satoshi; Wise, Steven P (2006) Neuronal activity related to elapsed time in prefrontal cortex. J Neurophysiol 95:3281-5 |
