Chasing the cell assembly
Section snippets
Lighting the cell assembly
One of the biggest recent advances in the ability to record activity simultaneously from many neurons in vivo [48•], with single-cell and single-AP accuracy [49•], has come from multiphoton imaging [50•, 51] (for review see [52]). The main advantages here are the ability to record activity from all neurons within a local area on a trial-by-trial basis [53, 54, 55], known spatial location of all the recorded neurons [56, 57], the capacity to record activity from neurons that fire at low rates [
Imaging during behavior
Given the complexity of associating spikes with neuronal assemblies, recording in the primary sensory areas during a defined detection task limits the temporal window in which to search for spiking activity from assembly members. Because light penetration into tissue is less damaging than physical electrodes during movement (but see [80]), imaging allows recording neuronal activity in the awake animal where movements within the image can be successfully corrected offline, with very little data
Manipulation
Another set of tools in the optical armamentarium that has the potential to extend these behavioral studies a step further are light-activatable ion channels (for review see [84]). If one is capable of identifying behaviorally relevant functional cell assemblies using optical recording techniques, then it is logical to investigate the effect on the behavior of reversible activation or inactivation of assembly members. Channelrhodopsin 2 (ChR2), a cation specific, light-activated ion channel
The anatomy of the cell assembly
The vast number of neurons present in any cortical area combined with the dramatic complexity of their connectivity very much places the functional cell assembly as the needle in neurosciences’ haystack. While increasing the number of neurons from which activity is simultaneously sampled is one method for increasing the probability of finding evidence in support of the concept, another approach would be to identify a priori candidate assembly members. While transsynaptic tracing has been
Conclusion
The Hebbian cell assembly hypothesis was published 61 years ago, and since then a large amount of detailed knowledge about the neurons that make up the cortex has been gathered, but the next challenge is placing this knowledge back into the context of the behaving animal. The recent emergence of techniques that enable optical imaging of activity from large neuronal populations, optical manipulation of activity, and large scale reconstruction of neuronal circuits, offer new opportunities to
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgments
We would like to thank the researchers that supplied their original figures to us. We would also like to thank Winfried Denk for comments on an earlier version of this manuscript, and David Greenberg for providing the histology data, Verena Pawlak and Yvonne Grömping. This study was supported by the Max Planck Society.
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