Alan Carleton’s team from the Neuroscience Department at the University of Geneva (UNIGE) Faculty of Medicine has just shown that the representation of an odor evolves after the first breath, and that an olfactory retentivity persists at the central level.

The phenomenon is comparable to what occurs in other sensory systems, such as vision or hearing. These movements undoubtedly enable the identification of new odors in complex environments or participate in the process of odor memorization. This research is the subject of a publication in the latest online edition of the journal PNAS.

This image is a  coronal section through the main olfactory bulb of an adult male mouse.

By observing the brain of an alert mouse, researchers recorded the electrical activity emitted by the olfactory bulb of animals inhaling odors. This image shows a coronal section through the main olfactory bulb of an adult male mouse. The image was taken with a confocal microscope visualizing all cell nuclei with stain TOTO-3. False color applied with photoshop to indicate the three main anatomical layers: Blue – Glomerular layer containing periglomerular (juxtaglomerular cell bodies), Red – External Plexiform layer and Mitral cell layer containing cell bodies of mitral and tufted cells (and granule cells resident in mitral cell layer), Green – Internal plexiform and granule cell layer containing cell bodies of immature migrating neuroblasts, and mature granule cells.

Rodents can identify odors in a single breath, which is why research on sense of smell in mammals focuses on that first inhalation. Yet we must remember that from a neurological standpoint, sensory representations change during and after the stimuli. To understand the evolution of these mental representations, an international team of researchers led by Professor Alan Carleton at the University of Geneva (UNIGE) Faculty of Medicine conducted the following experiment: by observing the brain of an alert mouse, the neuroscientists recorded the electrical activity emitted by the olfactory bulb of animals inhaling odors.

They were surprised to find that in mitral cells, some representations evolved during the first inhalations, and others persisted and remained stable well after the odor ceased. The cohort subjected to these analyses revealed that the post-odor responses contained an odor retentivity—a specific piece of information about the nature of odor and its concentration.

Will odor memory soon be understood?

Using cerebral imaging, researchers discovered that the majority of sensory activity is visible only during the presentation of odors, which implies that retentivity is essentially internal to the brain. Therefore, odor retentivity would not be dependent upon odorous physicochemical properties. Finally, to artificially induce retentivity, the team photostimulated mitral cells using channelrhodopsin, then recorded the persistent activity maintained at the central level. The strength and persistence of the retentivity were found to be dependent on the duration of the stimulation, both artificial and natural.

In summary, the neuroscientists were able to show that the representation of an odor changes after the first breath, and that an olfactory retentivity persists at the central level, a phenomenon comparable to what occurs in other sensory systems, such as vision and hearing. These movements undoubtedly enable the identification of new odors in complex environments or participate in the process of odor memorization.

Notes about this neuroscience and odor memory research

Contact: Alen Carleton – Université de Genève
Source: Université de Genève press release
Image Source: The olfactory bulb image is credited to Matt Valley and has been released into the public domain.
Original Research: Abstract for “Odor representations in the olfactory bulb evolve after the first breath and persist as an odor afterimage” by Michael Andrew Patterson, Samuel Lagier, and Alan Carleton in PNAS. Published online August 5 2013 doi:10.1073/pnas.1303873110