Neuronal ensembles: Building blocks of neural circuits

Abstract

Neuronal ensembles, defined as groups of neurons displaying recurring patterns of coordinated activity, represent an intermediate functional level between individual neurons and brain areas. Novel methods to measure and optically manipulate the activity of neuronal populations have provided evidence of ensembles in the neocortex and hippocampus. Ensembles can be activated intrinsically or in response to sensory stimuli and play a causal role in perception and behavior. Here we review ensemble phenomenology, developmental origin, biophysical and synaptic mechanisms, and potential functional roles across different brain areas and species, including humans. As modular units of neural circuits, ensembles could provide a mechanistic underpinning of fundamental brain processes, including neural coding, motor planning, decision-making, learning, and adaptability.

Keywords: assembly; attractor; neural networks; synfire.

Figure 1:. Original models of ensembles.

(A) Cajal’s drawing of neuronal activity progressing through a putative cortical module (Courtesy of the Cajal Institute, “Cajal Legacy”, Spanish National Research Council (CSIC), Madrid, Spain). (B) Lorente’s representation of a reverberating chain. Adapted with permission from. (C) Hebb’s representation of activity flow (arrows) through a neuronal assembly. Adapted with permission from. Note recurrent connectivity. (D) Pattern completion: after neurons are activated by a synchronous input (left), connections between neurons are generated, or strengthened, (middle), and stimulation of one neuron triggers the rest. Adapted with permission from. (E) Attractor landscape generated by a recurrently connected neural network. Population activity is represented as a ball that rolls over to the lowers energy point, where a group of neurons is coactive, defining an attractor. Adapted with permission from.

Figure 2:. Hippocampal ensembles.

(A) Top: Extracellular electrophysiological recordings in vivo. Bottom left: Raster plots of spiking during spatial exploration arranged in order of physical position along CA1 pyramidal layer (vertical lines indicate troughs of theta wave). Bottom right: Spike raster rearranged to highlight synchrony demonstrate ensembles, with repeatedly synchronous firing of neuronal subpopulations (circled). Adapted with permission from. (B) Top left: Two-photon calcium imaging of CA1 neurons in head-fixed mice in vivo. Top right: Contour map of imaged neurons shows spatially-intermingled ensembles (color coded according to ensemble membership). Bottom: Rasterplot displaying neuronal activation, color-coded according to ensemble affiliation (4 ensembles labeled from A to D). Ensembles are re-activated in the same order during sequences.

Figure 3:. Neocortical ensembles and pattern completion.

(A) Cortical ensembles activated spontaneously (top) or by naturalistic visual stimuli (bottom) in mouse primary visual cortex in vivo. Red cells are members of an ensemble and green are those active in both conditions. Note similarity between visually-evoked and spontaneous cortical ensembles. Adapted with permission from. (B) Ensemble imprinting and pattern completion. Left: Two-photon optogenetic activation of a group of neurons (red). Middle: Many stimulated neurons become spontaneously active, forming an ensemble (light red). Right: Ensemble is reactivated by optogenetic photostimulation of one neuron (arrow), demonstrating pattern completion. Adapted with permission from.

Figure 4:. Neocortical ensembles can trigger behavior and are long-lasting.

(A) Ensemble activated by Go stimulus in a visual discrimination licking task (green neurons). This ensemble can be reactivated by two-photon optogenetic stimulation of two cells (red). (B) Behavior induced by recalling a Go ensemble in absence of visual stimuli. Raster plot of activity of ensemble neurons during holographic stimulation of those two pattern completion cells (blue lines). Vertical red lines indicate optogenetic photo-stimulation. Red marker shows successful recall of Go ensemble and licking behavior. Black marker shows partial recall with no behavior associated. C. Behavioral performance evoked by recalling Go ensemble by optogenetic stimulation in the absence of visual stimuli (right) is significantly higher than performance in partially recalled trials (left). Adapted with permission from. (D) Long-term stability of ensembles. Rasterplots of spontaneous (left) and visually-evoked (right) neuronal activity over 46 days in mouse visual cortex in vivo. Neurons sorted based on ensemble identity (color-coded; right columns illustrate ensemble assignment). Adapted with permission from.

Figure 5:. Stepwise model of ensemble development.

Cartoon schematizing three steps of cortical ensemble development taking place before the start of the critical period: proto-ensembles connected through electrical synapses (1) are followed by highly synchronous population activity (2) and sparse, synaptically-connected ensembles, constrained by somatic inhibition (3), resulting in two ensembles.

Figure 6.. Neuronal ensembles in non-mammalian species.

(A) Calcium imaging of the activity of the entire nervous system of Hydra vulgaris reveals ensembles of coactive neurons (color-coded). (B) Neural ensembles in distinct divisions of the body of the jellyfish Clytia hemisphaerica display synchronized calcium signals (color-coded). (C) Ensembles of neurons in anatomically distinct zones of juvenile zebrafish forebrain exhibit highly correlated ongoing calcium activity (color-coded). (D) Electrophysiological recordings of sequential activation of neurons in high vocal center of songbird during song generation reveals a stereotyped activity pattern. Figures adapted from^,,,.

Figure 7:. Potential ensembles in human neocortex.

(A) Top: Example of human brain organoids expressing transgenic calcium indicators. Bottom” spontaneous activity demonstrate coordinated population events. (B) Top: Spatially targeted multi-neural recording electrodes during neurosurgery to record the activity of large neural populations of human neurons, in vivo. Bottom: Synchronized activity in neuronal populations in hippocampus. (C) Multicellular extracellular recordings from human brain slices of tissue resected during brain surgery demonstrates synchronized activity of neuronal populations. Figures are adapted from^–.