Organization of cell assemblies in the hippocampus

Abstract

Neurons can produce action potentials with high temporal precision. A fundamental issue is whether, and how, this capability is used in information processing. According to the 'cell assembly' hypothesis, transient synchrony of anatomically distributed groups of neurons underlies processing of both external sensory input and internal cognitive mechanisms. Accordingly, neuron populations should be arranged into groups whose synchrony exceeds that predicted by common modulation by sensory input. Here we find that the spike times of hippocampal pyramidal cells can be predicted more accurately by using the spike times of simultaneously recorded neurons in addition to the animals location in space. This improvement remained when the spatial prediction was refined with a spatially dependent theta phase modulation. The time window in which spike times are best predicted from simultaneous peer activity is 10-30 ms, suggesting that cell assemblies are synchronized at this timescale. Because this temporal window matches the membrane time constant of pyramidal neurons, the period of the hippocampal gamma oscillation and the time window for synaptic plasticity, we propose that cooperative activity at this timescale is optimal for information transmission and storage in cortical circuits.

Time scales of memory, learning, and plasticity

“…However, in a quite similar experimental setup the blockage of LTP leads to no significant differences (Zamanillo 1999). Regarding the neural correlate of memory, potential memory items were found, for example, in songbirds during song and recapitulation as stereotypical sequences of spike burst (Hahnloser et al 2002) or during a spatial task, as spatiotemporal patterns (Harris et al 2003) in the hippocampus. Additionally, typical connectivity structures, so-called motifs, were measured in the cortex (Song et al 2005;Perin et al 2011).…”

Interaction between neocortical and hippocampal networks via slow oscillations

“…Mainly because of these features, coupled network oscillators comprise a natural substrate for information transfer. Synchronous activation of neurons on the fast time scale in local networks facilitates the formation of cell assemblies (Harris et al, 2003;Singer, 1993) and synaptic plasticity (Bliss and Lømo, 1973;Levy and Steward, 1983;Markram et al, 1997), whereas the timing of local events across structures can be coordinated by slower time scale oscillations. In the discussion below, we illustrate these principles by the interaction of thalamocortical and hippocampal oscillators.…”

A metric for space

“…A major reason for this development was the invention of technology for large-scale recordings in neuronal ensembles (Georgopoulos et al, 1986;Wilson and McNaughton, 1993;Harris et al, 2003), which made it possible to observe hippocampal population dynamics in a manner unprecedented at this time. Some of the key questions in these analyses were about the origin of the place signal.…”