Neural basis of deduced ordinal judgments in the primate prefrontal cortex

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Inferring the ordinal relationship between two objects based on earlier acquired information is a form of deductive reasoning called transitive inference (TI). For instance, if A>B and B>C, it can be inferred that A>C. Despite the importance of ordinal information and TI for intelligent behaviors, its neuronal mechanisms remain largely unexplored. In this project, the prefrontal mechanisms of a rule-based deduction capability will be studied in the realm of ordinal numerical competence. Rhesus monkeys will be trained to deduce and memorize the ordinal position of pictures that are exemplars of five categories, and decide based on a rule cue whether to order the categories according to lower or higher rank. This task requires working-memory, flexible assignment of novel pictures to the respective visual categories, as well as a grasp of ordinality to allow decision-making. In line with core hypothesis 1, we suspect that prefrontal neuron ensembles of the behaving monkeys become dynamically coordinated during different epochs of the task to flexibly encode the inferred and relative rank of novel category members. The contributions of putative excitatory pyramidal neurons and inhibitory interneurons will be assessed via extra-cellular recordings to learn about the micro-circuitry giving rise to TI processes. By assessing local field potentials in combination with single-unit recordings, the role of neuronal synchrony in assigning neurons to different ensembles as a putative mechanism for flexibility during ordinality judgments will be explored. To explore core hypotheses 2 and 3, simultaneous recordings from potential cortical input structures to the PFC, such as the posterior parietal cortex, and potential output areas, such as the premotor cortex, will help to resolve input- and output-defined ensembles during flexible behavior. The data will be instrumental in deciphering the prefrontal mechanisms and circuits for cognitive flexibility during nonsymbolic logical reasoning.