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Abstract:
By combining concurrent electrophysiological recordings and fMRI, we recently demonstrated that the events known as hippocampal sharp wave-ripple complexes (SPW-R) are tightly associated with robust cortical activations that occur concurrently with a particularly intriguing strong inhibition of large portions of subcortical brain structures that are closely involved in neural plasticity, such as the basal ganglia (BG), the pontine region (PONS) and the cerebellar cortex (Logothetis, Eschenko et al. 2012, Nature 491:547-53). Particularly intriguing was the strong inhibition of large portions of subcortical brain structures that are closely involved in neural plasticity, such as the basal ganglia (BG), the PONS and the cerebellar cortex. In primates, the negative BOLD in the pontine region was systematically associated with inhibition of the lateral geniculate nucleus (LGN) and foveal V1 activity, despite the overall positive fMRI responses in peripheral V1 and all other primary sensory and associational cortices. The deactivation of PONS may therefore be due to a temporary suppression of cholinergic sites involved in local plasticity and synaptic consolidation, such as those underlying the generation-propagation of theta rhythm, and so-called ponto-geniculo-occipital (PGO) waves. PGO waves have been often associated with the consolidation of procedural memory or synaptic consolidation in general. To examine this hypothesis and better understand the global regulation of brain activity during memory consolidation we set out to employ the methodology of Neural-Event-Triggered fMRI (NET-fMRI), combining simultaneous electrophysiological recordings in the region of the parabrachial nucleus (PBn) and MR imaging in monkeys under opioid anesthesia. First, we established a structural-MRI and angiography-based site-localization approach to access various brainstem regions with long electrodes without potential complications due to vasculature-injury. We subsequently physiologically identified PBn, LGN and the Hippocampal CA1/CA3 fields, and conducted concurrent, uninterrupted multi-site physiological and fMRI recordings in a 4.7T magnet. PGO events were considered to be the large field deflections, with various temporal and repetition profiles that typically co-occur in PBn and LGN. In sharp contrast to isolated LGN or PONS events, the PGO-like events - co-occurring in both pontine and thalamic structures - yielded a robust and striking pattern of up/down modulation, suggesting the PGO events correlated with upregulation of subcortical centers concurrently with inhibition of activity in neocortex.
