A brief history of calcium imaging

8 10 2008

A few months ago I threw together a short presentation on the history of calcium imaging for a journal club here at Janelia. It is incomplete. It lacks notes. It is technical. It focuses much attention on early genetically-encoded indicators. However, calcium imaging is so intertwined with the work of Roger Tsien, my Ph.D. thesis advisor, and since he just won the Nobel Prize, I thought it might be of interest to some of the audience of Brain Windows. It does provide a little bit of background for some of the more recent developments chronicled on this site.


Journal Club : GFP Reconstitution Across Synaptic Partners (GRASP)

25 03 2008

This week, I’m guest presenting a recent Neuron paper over at the Nature Network’s neuroscience journal club. The authors propose a clever new technique, Genetic Reconstitution Across Synaptic Partners, to track the locations of selected synaptic connections backed by an impressive set of in vivo proof-of-principal experiments in C. elegans. The gist of the strategy is to split a fluorescent marker into two non-functional components and then distribute each half on different sides of circuit’s connection. Only at synaptic connections would the two components be close enough to undergo trans-complementation and reconstitute a functional marker.

Check out the full presentation and join the discussion.

Journal Club Content

20 02 2008

I’m going to try adding an additional type of content. Notes from journal clubs I attend. These will be more of a quick data dump format than a strict write-up, and topics will be broader than the specific field of brain imaging. The text formatting will be normal text = presented material, italics = audience interjections, bold = presenter responses. Journal Club entries can be accessed specifically by the top navigation bar. Hopefully this can expand our audience and perhaps promote additional discussion.

The first Journal Club discussion is here :

Sparse representation of sounds in the unanesthetized auditory cortex.

Hromádka T, Deweese MR, Zador AM.

How do neuronal populations in the auditory cortex represent acoustic stimuli? Although sound-evoked neural responses in the anesthetized auditory cortex are mainly transient, recent experiments in the unanesthetized preparation have emphasized subpopulations with other response properties. To quantify the relative contributions of these different subpopulations in the awake preparation, we have estimated the representation of sounds across the neuronal population using a representative ensemble of stimuli. We used cell-attached recording with a glass electrode, a method for which single-unit isolation does not depend on neuronal activity, to quantify the fraction of neurons engaged by acoustic stimuli (tones, frequency modulated sweeps, white-noise bursts, and natural stimuli) in the primary auditory cortex of awake head-fixed rats. We find that the population response is sparse, with stimuli typically eliciting high firing rates (>20 spikes/second) in less than 5% of neurons at any instant. Some neurons had very low spontaneous firing rates (<0.01 spikes/second). At the other extreme, some neurons had driven rates in excess of 50 spikes/second. Interestingly, the overall population response was well described by a lognormal distribution, rather than the exponential distribution that is often reported. Our results represent, to our knowledge, the first quantitative evidence for sparse representations of sounds in the unanesthetized auditory cortex. Our results are compatible with a model in which most neurons are silent much of the time, and in which representations are composed of small dynamic subsets of highly active neurons.