Concentration coding in insects
When I walk into our behaviour room and smell something malodorous, I can identify the smell as mouse shit, even though the smell is less intense than when I'm working closely with a mouse (pardon the example, but mouse shit is salient for me now that I'm doing a bit of behaviour). The fact that you can identify the odor across different concentrations gives the odor it's "odor identity." Yet, in the brain, faint mouse shit and strong mouse shit evoke different responses. For example, the weak smell activates fewer ORNs, less strongly.
The best study so far to investigate how odors are encoded at different concentrations was done in locusts by Stopfer, Jayaraman and Laurent. I have covered that paper previously on this blog, but I will summarize the findings.
Stopfer and Jayaraman recorded from projection neurons (PNs) in the locust antennal lobe while presenting odors at different concentrations over three orders of magnitude. They calculated the phase PN spikes compared to the LFP, and found that the phase of firing was the same at all concentrations. They then looked at how neurons responded over the course of the odor, and found that neurons responded similarly to different concentrations of an odorant. For example, PN 8 below has the same response to most concentrations of gerianol. There were some some subtleties, however. Some neurons changed the timing of their firing at different concentrations, like PN 3 which fired earlier at lower concentrations of hexanol. And sometimes neurons responded completely differently to neighboring concentrations, like PN 8's response to gerianol at the highest concentration.
|From Stopfer et. al., 2003.|
|From Stopfer et. al., 2003.|
Concentration coding in mammals
For one figure of my manuscript, I recorded mitral/tufted (M/T) cells in awake mice while presenting odors at three concentrations (0.1-2%). However, the manuscript is not about concentration and odor identity, so I will present that data here. In total, I recorded from 105 cells from two mice, and presented three odors, for 315 cell-odor pairs.
What I observed is similar to what Stopfer found: many neurons respond to different concentrations of the same odorant in a similar fashion. However, unlike Stopfer, M/T cells' firing was generally weaker at lower concentrations of an odorant.
|An M/T cell's response to 3-hexanone over the range of concentrations. This cell fired phasically in response to the odor, but at lower magnitude for lower concentrations.|
|This M/T cell is inhibited by the odor more strongly at lower concentrations of odorant. (Also note that post-inhibitory excitation at the highest concentration reflecting the dynamics of the odor response.)|
In contrast to the Rinberg lab, most of the neurons that I record do not have millisecond precision, but more like tens of millisecond precision. However, I did find one remarkable cell that responded with a short burst of spikes in response to amyl acetate. And indeed, if you look at the spike timing for this burst, the burst arrives ~10 ms later for lower concentrations of odorant.
In summary, neurons in the mouse olfactory bulb respond to different concentrations of the same odorant with similar spike trains. Some (but not all) neurons fire earlier in the sniff for higher concentrations. The similarity in responses provides a neural basis for encoding "odor identity," while the slight changes in magnitude and timing of the responses may allow the mouse to discriminate concentration.