24 Dec The Nose Knows: How Our Brain Makes Sense of Odors
Medical Research: What is the background for this study?
Dr. Verhagen: The study explores how the part of the brain that encodes odors (the Olfactory Bulb, OB) works. It is much studied, but much remains to be learned about the Olfactory Bulb.
It used to be thought that odors were encoded by spatial patterns of activity across the Olfactory Bulb alone. Due to advances in the resolution in imaging, it has become clear that odor coding is a highly dynamic process. We learned that after each sniff a pattern of activity evolves across the Olfactory Bulb, some areas activating sooner, some later. This suggested that the odor code is a spatial AND a temporal map, in other words, more like playing a brief movie than showing a brief picture after each sniff.
We tested whether this was true by using a relatively new method called “Optogenetics”, which allows us to accurately control the activity across the Olfactory Bulb.
Medical Research: What are the main findings?
Dr. Verhagen: We asked if mice could tell the difference between simplified static or dynamic OB patterns that mimicked OB activity. If they couldn’t discriminate them, we’d conclude that the dynamics were unimportant. However, we found that they could detect even very fine dynamics, as small as 13ms. As the dynamics in the bulb are in the order of 100-200ms we concluded that they must play a role. To directly test this we then asked if mice could discriminate real dynamic patterns, imaged from the OB, from the same patterns but made static. Mice again discriminated them readily, and did so irrespective of when the mice actually sniffed. So, mice don’t need the their sniffing to be able to “decode” these spatio-temporal maps: the temporal information within the OB suffices.
The implication is that the Olfactory Bulb has access to much more information about odors than previously thought, which makes sense as there exist vast numbers of unique odor mixtures that animals need to recognize.
Medical Research: What should clinicians and patients take away from your report?
Dr. Verhagen: Even though this work is not directly investigating a disease, the sense of smell plays a big role in health and disease.
Smell guides us in our food choices, which is at least in part responsible for metabolic syndrome (obesity, diabetes, CHD). Dr. Shepherd recently published a very accessible book “Neurogastronomy” that describes all this very well. Accordingly, my lab and that of Dr. Shepherd explores how the brain underlies food flavor perception. Further, smell deteriorates very early on in Alzheimer’s disease –even before minor cognitive deficits appear. My lab is actively researching this too, using new mouse models for sporadic AD and new objective behavioral olfactory tests. So, the sense of smell plays major roles in health and disease.
Medical Research: What recommendations do you have for future research as a result of this study?
Dr. Verhagen: Now that we know that these dynamics are used by the brain to make sense of odors, we need to better understand how exactly it does so.
Is the order of activations encoded? How much information can be encoded? How is this temporal information carried downstream in the brain?
How does this differ for odors smelled from food in the mouth versus odors out in the environment?
With continued funding from NIH I will be able to address these issues. It’s sure to be another exciting time!