Abstract
In this review article we focus on research methodologies for detecting the actual activity of cell assemblies, which are populations of functionally connected neurons that encode information in the brain. We introduce and discuss traditional and novel experimental methods and those currently in development and briefly discuss their advantages and disadvantages for the detection of cell-assembly activity. First, we introduce the electrophysiological method, i.e., multineuronal recording, and review former and recent examples of studies showing models of dynamic coding by cell assemblies in behaving rodents and monkeys. We also discuss how the firing correlation of two neurons reflects the firing synchrony among the numerous surrounding neurons that constitute cell assemblies. Second, we review the recent outstanding studies that used the novel method of optogenetics to show causal relationships between cell-assembly activity and behavioral change. Third, we review the most recently developed method of live-cell imaging, which facilitates the simultaneous observation of firings of a large number of neurons in behaving rodents. Currently, all these available methods have both advantages and disadvantages, and no single measurement method can directly and precisely detect the actual activity of cell assemblies. The best strategy is to combine the available methods and utilize each of their advantages with the technique of operant conditioning of multiple-task behaviors in animals and, if necessary, with brain–machine interface technology to verify the accuracy of neural information detected as cell-assembly activity.
Highlights
The hypothesis of cell assemblies, functional groups of neurons, was first proposed by the psychologist Hebb (1949)
This study indicates that the technique is effective to observe the transformation occurred over time in ensemble activity of neurons that are part of the same cell assembly
Studies of neuronal activities and oscillation of electroencephalograms, in which articles have been explosively increasing over the past decade, are often considered in relation to cell assemblies (e.g., Buzsáki and Draguhn, 2004)
Summary
The hypothesis of cell assemblies, functional groups of neurons, was first proposed by the psychologist Hebb (1949). Vetere et al (2017) reported that in vivo chemogenetic silencing of different network nodes (vertices) impaired fear memory consolidation in mice This method can reveal specific influences for molecularly defined cell types that are often intermingled with neuronal populations having different functions (Sternson and Roth, 2014), suggesting that it could be applied to detect and activate specific cell assemblies in behaving animals. This study indicates that the technique is effective to observe the transformation occurred over time in ensemble activity of neurons that are part of the same cell assembly Another recent work (Wilson et al, 2016) using two-photon imaging of individual neurons in primary visual cortex has revealed that summation of synaptic inputs can predict the given neuron’s orientation selectivity; cannot accurately or reliably predict differences across neurons. Systematically cover every possible pattern using the power-oftwo-based permutation logic (Figure 8A)
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