One cell's response
(Brief methods: To look at whether a given breath is responsive, I segmented the recordings into breaths, and fit each breath to a standard breath length (if a given breath was longer than the average breath length, I deleted all spikes after the end of the standard breath; if a given breath was shorter, I assumed the rest of the time included no spikes). To quantify whether breaths were "responsive," I compared a breath's tonic firing rate to the control, pre-odor breaths (using ANOVA with p<0.05, and Tukey's post-hoc testing); and I tested whether the "phase" or timing of the breath differed from the pre-firing rate (using a Kolmogorov Smirnov test; here I used p<0.02 as the threshold for significance as using p<0.05 yielded many false positives when comparing different control breaths). And when I looked at the data, it was obvious that some cells had strikingly different codes for the first breath versus later breaths.)
One example is shown below (this is the same neuron-odor pair from the previous post, albeit different trials). The top panel shows the PSTH of the cell's response to amyl acetate, with 40ms bins. You can see that before odor presentation, the neuron fired irrespective of phase. During the first sniff of the odor, there was a strong, transient burst of activity in the middle of the breathing cycle. However, in the subsequent sniffs, the cell was inhibited.
To look at the phasic changes, I plotted (for ten trials) the cumulative spike times for control breaths and breaths during the odor (bottom panel, above). Before the odor, the spikes occur without phase bias (black line), while during the first breath you can see that most spikes come between 150-200ms of the breathing cycle. However, on the second breath, the phasic nature of the response has begun to dissipate.
Population changes in the odor code
The transformation between the first and second breath can take many forms. The example above shows a neuron that switches from a strong, phasic, excitatory response to an inhibitory response. Many other neurons are inhibited during the first breath but not afterward. Below is a more subtle example, where the neuron does not respond to ethyl butyrate during the first sniff. However, on the subsequent sniffs, the timing of the response shifts to earlier in the breathing cycle.
|This cell appears to not respond during the first breath, but has a phasic response during later breaths.|
When I showed this to my boss he was not impressed, and said they had already showed this in a previous paper. And indeed, buried in three panels of Fig. 4, they did show something similar (below). There are some significant differences, though. First, those experiments were in anesthetized animals, rather than awake animals. Second, I've shown that individual cells use strikingly different codes between breaths. Third, they did not create their population vector to consider a cell's firing as a whole. This could change the interpretation of the results. In any case, asking around the lab, no one seemed to remember this was even in the paper.
|The velocity of the population representation is highest during the first odor and post-odor breaths. A. The population vector contains the firing of each cell in a given time-bin. B. Cross-correlation and distance for the population during a given odor. C. The velocity of the population vector (how much the distance changes) is highest at the beginning and end of odor presentation.|
From Bathellier, et al, 2008.
These results also gave me an idea for an experiment to test whether the difference in coding is perceptually important. It is now possible to stimulate the olfactory epithelium via Channelrhodopsin while mice sniff (using an OMP-ChR2 line), which makes it possible to mask an odor response with olfactory white noise. To test how important different sniffs are to perception, you would start by establishing the detection threshold for an odor. Then you could measure the detection threshold while masking either the first or second sniff with the olfactory noise. There are a few possible results. First, the threshold might not change at all, as both the first and second sniff contain sufficient information to detect an odor. Second, the sensitivity could be equally decreased when either sniff is blocked. This would also imply their is equal information in each sniff. The third possibility is that masking the first sniff would decrease sensitivity far more than masking the second sniff (which is what I expect). It has been shown that mice and humans can detect odors in a single sniff. And in daily life, no odor is as strong as its first whiff. The difference in odor coding between the first and second sniff might be one step towards explaining why.
While this is pretty basic analysis, I had to perform this en route to doing more sophisticated comparisons while trying to measure a form of plasticity in the odor code. This is also the first complete data I've shown from this lab. I would appreciate any feedback on this, as it's always useful to get a perspective outside the insular confines of a lab. Were the figures legible? The analysis convincing? Or is this entirely un-novel?
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