Elsevier

Biosystems

Volume 183, September 2019, 103978
Biosystems

Integrate-and-fire network model of activity propagation from thalamus to cortex

https://doi.org/10.1016/j.biosystems.2019.103978Get rights and content

Abstract

The thalamus plays a crucial role in modulating the cortical activity underlying sensory and cognitive processes. In particular, recent experimental findings highlighted that the thalamus does not merely act as a binary gate for sensory stimuli, but rather participates to the processing of sensory information. Clarifying such thalamic influence on cortical dynamics is also important as the thalamus is the target of therapies such as DBS for Tourette patients. In this perspective, various computational models have been proposed in the last decades. However, a detailed description of the propagation of thalamic activity to the cortex is missing. Here we present a simple computational model of thalamocortical connectivity accounting for the propagation of activity from the thalamus to the cortex. The model includes both the single-neuron scale and the mesoscopic level of Local Field Potential (LFP) signals. Numerical simulations at both levels reproduce typical thalamocortical dynamics which are consistent with experimental measurements and robust to parameters changes. In particular, our model correctly reproduces locally generated rhythms as spindle oscillations in the thalamus and gamma oscillations in the cortex. Our model paves the way to deeper investigations of the thalamic influence on cortical dynamics, with and without sensory inputs or therapeutic electrical stimulation.

Introduction

Complex biological systems such as cells, organisms and neural networks are characterized by outstanding abilities of self-organization and information processing (Kauffman, 1993). In the field of neuroscience, the possibility to understand mechanisms underlying encoding and transmission of sensory information across brain structures such as the thalamus and the cortex is a challenging perspective.

The thalamocortical system constitutes the vast majority of mammalian brain. It is composed by the thalamus, a symmetrical structure located near the center of the brain, and the cerebral cortex, in particular the primary sensory areas, which are direct recipient of thalamic outputs. It contributes to several cognitive functions such as consciousness, sleep, decision making, alertness and so on (Sherman, 2016). In particular, the thalamocortical system is a processing center of the information coming from the external world. Most of sensory inputs coming from the peripheral system are projected to the thalamus, which in turn sends them to specific cortical areas (Yoshioka et al., 1994). Therefore, it is important to investigate the role of the thalamus in the mechanisms of transmission and processing of sensory information. For decades, the thalamus has been considered simply a relay station lacking mechanisms of computing and processing of sensory inputs (Sherman and Guillery, 2001). Instead, recent experimental and theoretical studies have revealed that the thalamus performs a pre-processing of information and modulates cortical areas through thalamocortical projections even without sensory inputs (Steriade et al., 1993a, Constantinople and Bruno, 2013). In fact, the thalamus behaves as a gatekeeper selecting which information to project to the cortex. This is consistent with many experiments confirming thalamic influence in attention and in the transition between sleep and wakefulness states (Sherman, 2001). Moreover, several thalamic nuclei have the key role of transmitting information between different cortical areas (Sherman and Guillery, 2002). A deeper investigation on thalamic functions is therefore needed to understand the pathways that external information crosses throughout the thalamocortical system.

Detailed experimental descriptions of thalamic activity are infrequent due to the difficulty of performing large scale recordings of single units in this area. Given this limitation, computational and numerical analyses are important because they offer a detailed perspective of neural network dynamics. Therefore, consistent and robust models of the thalamocortical system are crucial for connecting experimental findings with numerical results. During the last 20 years, several thalamic models have been developed for different applications (Golomb et al., 1996, Traub et al., 2005, Vijayan and Kopell, 2012). Cortical networks, in turn, have been widely studied and various mechanisms of cortical activity are now known in detail. However, little has been done regarding the interplay between these two systems. In particular, models able to describe dynamics at single neuron scale together with mesoscopical collective phenomena are necessary for a complete comprehension of the thalamocortical system. Here we present a novel computational model focusing on the description of the connectivity from the thalamus to the cortex and accounting for the spread of thalamic activity into the cortex. Our model takes into account single neuron dynamics by describing evolution of state variables for every neuron in the network model. Furthermore, from this microscopic dynamics we derived mesoscopic variables which we specifically related to experimentally measured LFPs. During the last twenty years, LFP has gained increasing attention because it provides an insight into a wide range of coordinated cellular activities (Harris Bozer et al., 2017). Our scaled computational model describes mesoscopic activity of both thalamus and cortex through simulated LFP signals. In particular, we were interested in investigating the propagation of mesoscopic rhythms of the thalamus into the cortex due to thalamocortical connections.

Section snippets

Methods

We considered a thalamic T = {TTC, TRE} and a cortical Γ = {ΓPY, ΓINT} neural networks, both composed by an excitatory and an inhibitory population (respectively pyramidal (PY) neurons/thalamocortical relay (TC) neurons and interneurons (INT)/reticular (RE) neurons for cortical/thalamic network). We based our work on two previous papers, Refs. Mazzoni et al. (2008) and Barardi et al. (2016), describing a cortical and thalamic network model, respectively. We extended these works by adding

Results

The presence of two different dynamical regimes in the thalamus is known (Steriade et al., 1993b). However, how these thalamic activities affect cortical dynamics is still an open issue. We developed our model for an efficient numerical and theoretical analysis of thalamocortical dynamics. We studied the simulated dynamics exhibited by our model during both asleep and awake states. We expected different types of rhythmic activities at the mesoscopical level due to excitation-inhibition

Conclusion

To achieve a better understanding of information routing through the mammalian brain, we presented a novel model of a local thalamocortical circuitry. Our model is able to reproduce typical dynamics of the thalamocortical system. We focused on a detailed description of the system at the single-neuron level, by means of both well-known and recently observed features of cortical and thalamic cells. Differently from rate or neural mass models, spiking network models allowed us to compute

Conflict of interest

All authors declare that they have no conflict of interest in this research.

Acknowledgements

J.G.O. was supported by the Spanish Ministry of Economy and Competitiveness and FEDER (project FIS2015-66503-C3-1-P and Maria de Maeztu Programme, project MDM-2014-0370), the Generalitat de Catalunya (project 2017 SGR 1054), and the ICREA Academia programme.

E.C. was supported by the grant “PANACEE” (Prevision and analysis of brain activity in transitions: epilepsy and sleep) of the Regione Toscana – PAR FAS 2007–2013 1.1.a.1.1.2 – B22I14000770002.

A.M. was supported by internal funds of Scuola

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