My work of the last two years has just been published in PNAS. I know you, dear reader, are busy, so here are four key panels from the paper.
(With this publication, I am on the post-doc job market. While I am applying to a few specific labs, I'm interested in seeing how this blog could influence my career. If any American labs are looking to hire someone to do chronic, in-vivo recordings using optogenetics, send me an e-mail. If you want to know what I think like, this blog is probably an accurate representation. Here is my CV.)
Mitral cells have an odor afterimage
So I was recording the responses of mitral cells in the olfactory bulb while mice smelled odors, and I noticed something odd. Some cells continued to respond to odors after the end of the odor, and this post-odor response could last for a few seconds:
|PSTH from a mitral cell responding to ethyl butyrate. The cell was excited during the odor, but inhibited afterwards.|
To see whether these post-odor responses were odor specific, we performed some population analyses, and found that the post-odor responses were unique for each odor. However, as you can see above, these post-odor responses were not simply recapitulation of the odor responses. Given that these post-odor responses are different from the odor response, but contain odor information, we dubbed these responses odor afterimages.
Most glomeruli do not have afterimages
Some labs have previously reported that olfactory sensory neurons in drosophila can have long-lasting "super sustained" or "ultra prolonged" responses, and recordings from the olfactory nerve of rats also sometimes have off responses. To see whether our mitral cells' odor afterimages were simply the result of sustained input from the receptors, Sam (the 2nd author) performed calcium imaging of glomeruli, which contain the terminals of olfactory sensory neurons. While 5-10% of cells could have sustained activity, most did not:
|Response of three glomeruli to acetophenone. Some glomeruli did have a post-odor response, but most did not.|
Olfactory bulb activity in the absence of sniffing
Since we found that glomerular activity was not as strong as mitral cell activity during the post-odor, we investigated what types of feedback there could be during the post-odor. To look at this, we did my favourite crazy experiment: we recorded from an olfactory bulb while blocking the nostril. In this condition, all the activity in the bulb with the blocked nostril (ipsilateral), would have to come from the contralateral, unblocked nostril, via cortical connections. When we recorded from the bulb with the nostril blocked, almost all spiking activity was gone, as has been seen previously. However, we did find LFP activity:
Ringing in the olfactory bulb
Since we did not see strong input from the glomeruli into the olfactory bulb, we hypothesized that the odor afterimage was maintained centrally rather than from the periphery. This gave us the idea that perhaps we could generate an "artificial afterimage." So we took some Thy1-ChR2 mice, which express ChR2 in mitral cells, stimulated the heck out of the olfactory bulb, and found that mitral cells could continue firing for up to ten breaths after the end of photostimulation:
|PSTH of firing in a mitral cell in a Thy1-ChR2 mouse during and after photostimulation at 20Hz. It was difficult to record spikes during photostimulation, perhaps due to inhibition. Afterwords, the cell had rebound activity for a few seconds.|
So those are my favourite four figures of the paper. There are a lot more interesting tidbits scattered throughout, and I hope the paper is readable to a general audience. Hopefully my blogging over the last few years has improved my writing style. I suggest you check it out.