Hippocampal network oscillations coordinate the firing of neuronal ensembles. They can be either driven by external stimuli to form new memory traces or be reactivated by intrinsic mechanisms to reactivate and consolidate old memories. One important question is how the topology, i.e. the functional connectivity of neuronal networks supports their desired function. This topology of a network can be determined if certain properties of its assemblies are assessed.

To do this, we performed Ca²⁺ imaging using the chemical fluorescent Ca²⁺ indicator Oregon Green BAPTA 1-AM in acute hippocampal slices that were recorded in a Haas-type interface chamber. To faithfully reconstruct firing patterns of multiple neurons in the field of view, we optimized deconvolution-based detection of action potential associated Ca²⁺ events. Our analysis outperformed currently available detection algorithms by its sensitivity and robustness.

Applying it in combination with subsequent sophisticated network analysis, we were able to show that acute hippocampal slices contained a median of 9 neuronal assemblies with a median size of 4 neurons. This low number of assembly members, is due to the recording restriction to one optical plane and a small field of view. The number of identified assemblies is independent on these restrictions. Relationships between assembly size, assembly strength, Shannon entropy and other parameters provide evidence that CA1 neuronal assemblies have a scale-free topology. That ascribes them ‘small world’ properties, with very local connectivity and super-connected hubs with long-range connections. Thus, the scale-free topology in CA1 neuronal assemblies would ensure cost-efficient signal transfer with avoidance of signal jamming and/or interference.