Electrophysiological connections linking medial pulvinar, anterior nuclei of the thalamus and the hippocampus

Abstract The Papez circuit traditionally highlights the anterior nuclei of the thalamus (ANT) as the main relay of hippocampal (HPC) output to the cortex, a view that has shaped neuromodulation strategies in temporal lobe epilepsy (TLE). However, recent studies suggest that the medial subregion of the pulvinar (mPLV)—a thalamic nucleus that has undergone significant evolutionary expansion throughout the mammalian brain evolution—also forms functional connections with medial temporal lobe (MTL) structures, including the HPC. To date, however, there is a lack of causal evidence directly comparing the connectivity between the HPC and the two thalamic nuclei (mPLV and ANT) and between the two thalamic structures within the same brains. In this study, we investigated 41 patients with medial (mTLE, n = 22) and non-medial temporal lobe epilepsy (non-mTLE, n = 19) implanted with simultaneous depth electrodes in the HPC, ANT and mPLV. Repeated single-pulse electrical stimulations were applied to compare the causal electrophysiological connectivity of these regions within the same individuals. Our intra-subject analysis revealed that anterior HPC stimulation evoked strong responses in both ANT and mPLV, with mPLV responses occurring significantly later than those in the ANT [linear mixed-effect model (LMM), mean: 10.19 ms, 95% confidence interval (CI) (1.78, 18.59), false discovery rate (FDR)-corrected P = 0.040]. In contrast, stimulation of the posterior HPC resulted in stronger (and a trend for earlier) responses in mPLV compared with ANT [LMM: mean: 0.117, 95% CI (0.033, 0.201) (FDR-corrected P = 0.012)]. This finding suggests an anterio-posterior gradient of HPC connectivity. Furthermore, we found robust bilateral and bidirectional connectivity between ANT and mPLV. Stimulation of either elicited responses in the other, including the contralateral thalamus. This represents clear evidence for both intrathalamic and interthalamic connectivity within the human brain. Our findings offer new insights about the connectivity of human HPC with the thalamus and strong intrathalamic exchange of electrophysiological activity within the human brain.