Zeidman, P. | Search bioRxiv

bioRxiv2018

Thalamocortical dynamics underlying spontaneous transitions in beta power in Parkinsonism

Parkinsons disease (PD) is a neurodegenerative condition in which aberrant oscillatory synchronization of neuronal activity at beta frequencies (15-35 Hz) across the cortico-basal ganglia-thalamocortical circuit is associated with debilitating motor symptoms, such as bradykinesia and rigidity. Mounting evidence suggests that the magnitude of beta synchrony in the parkinsonian state fluctuates over time, but the mechanisms by which thalamocortical circuitry regulates the dynamic properties of cortical beta in PD are poorly understood. Using the recently developed generic dynamic causal modelling framework, we recursively optimised a set of plausible models of the thalamocortical circuit (n=144) to infer the neural mechanisms that best explain the transitions between low and high beta power states observed in recordings of field potentials made in the motor cortex of anesthetized Parkinsonian rats. Bayesian model comparison suggests that upregulation of cortical rhythmic activity in the beta-frequency band results from changes in the coupling strength both between and within the thalamus and motor cortex. Specifically, our model indicates that high levels of cortical beta synchrony are mainly achieved by a delayed (extrinsic) input from thalamic relay cells to deep pyramidal cells and a fast (intrinsic) input from middle pyramidal cells to superficial pyramidal cells. We therefore hypothesize that beta synchronisation at the cortical level could selectively be modulated via interventions that are capable of finely regulating cortical excitability in a spatial (delivered to either the superficial or deep cortical laminae) and time specific manner.

neuroscience↗

bioRxiv2017

Differentiable processing of objects, associations and scenes within the hippocampus

The hippocampus is known to be important for a range of cognitive functions including episodic memory, spatial navigation and future-thinking. Wide agreement on the exact nature of its contribution has proved elusive, with some theories emphasising associative processes and another proposing that scene construction is its primary role. To directly compare these accounts of hippocampal function, we devised a novel mental imagery paradigm where different tasks were closely matched for associative processing and mental construction, but either did or did not evoke scene representations, and we combined this with high resolution functional MRI. The results were striking in showing that differentiable parts of the hippocampus, along with distinct cortical regions, were recruited for scene construction or non-scene-evoking associative processing. The contrasting patterns of neural engagement could not be accounted for by differences in eye movements, mnemonic processing or the phenomenology of mental imagery. These results inform conceptual debates in the field by showing that the hippocampus does not seem to favour one type of process over another; it is not a story of exclusivity. Rather, there may be different circuits within the hippocampus, each associated with different cortical inputs, which become engaged depending on the nature of the stimuli and the task at hand. Overall, our findings emphasise the importance of considering the hippocampus as a heterogeneous structure, and that a focus on characterising how specific portions of the hippocampus interact with other brain regions may promote a better understanding of its role in cognition.\n\nSignificance statementThe hippocampus is known to be important for a range of cognitive functions including episodic memory, spatial navigation and future-thinking. Wide agreement on the exact nature of its contribution has proved elusive. Here we used a novel mental imagery paradigm and high resolution fMRI to compare accounts of hippocampal function that emphasise associative processes with a theory that proposes scene construction as a primary role. The results were striking in showing that differentiable parts of the hippocampus, along with distinct cortical regions, were recruited for scene construction or non-scene-evoking associative processing. We conclude that a greater emphasis on characterising how specific portions of the hippocampus interact with other brain regions may promote a better understanding of its role in cognition.\n\nAbbreviations

neuroscience↗