Asymmetric effective connectivity between primate anterior cingulate and lateral prefrontal cortex revealed by electrical microstimulation.

The dorsal anterior cingulate cortex (dACC) and lateral prefrontal cortex (lPFC) of the non-human primate show neural firing correlations and synchronize at theta and beta frequencies during the monitoring and shifting of attention. These functional interactions might be based on synaptic connectivity that is equally efficacious in both directions, but it might be that there are systematic asymmetries in connectivity consistent with reports of more effective inhibition within the dACC than lPFC, or with a preponderance of dACC projections synapsing onto inhibitory neurons in the lPFC. Here, we tested effective dACC-lPFC connectivity in awake monkeys and report systematic asymmetries in the temporal patterning and latencies of effective connectivity as measured using electrical microstimulation. We found that dACC stimulation-triggered evoked fields (EFPs) were more likely to be multiphasic in the lPFC than in the reverse direction, with a large proportion of connections showing 2-4 inflection points resembling resonance in the 20-30 Hz beta frequency range. Stimulation of dACC → lPFC resulted, on average, in shorter-latency EFPs than lPFC → dACC. Overall, latencies and connectivity strength varied more than twofold depending on the precise anterior-to-posterior location of the connections. These findings reveal systematic asymmetries in effective connectivity between dACC and lPFC in the awake non-human primate and document the spatial and temporal patchiness of effective synaptic connections. We discuss that our results suggest that measuring effective connectivity profiles will be essential for understanding how asymmetries in local synaptic efficacy and connectivity translate into functional neuronal interactions during adaptive, goal-directed behavior.