Let It Linger

Prolonged responses to odors, called afterimages, may originate in the brain, rather than in the nose.

Oct 1, 2013
Kerry Grens

ODOR PATTERNS: These two panels show mouse olfactory bulb neurons responding to two different odors. In afterimages, patterns like these persist despite the odor being removed.COURTESY OF ALAN CARLETON


The paper
M.A. Patterson et al., “Odor representations in the olfactory bulb evolve after the first breath and persist as an odor afterimage,” PNAS, 110:E3340-49, 2013.

The finding
Afterimages are the lingering sensations of a stimulus that is no longer present. Taste, vision, hearing, and touch can all induce afterimages, and now, Alan Carleton of the University of Geneva and his colleagues have described the phenomenon in mouse olfaction. They showed that even after an odor is removed, some neurons persist in their odor-specific activity.

The details
Carleton’s group found little activity in olfactory glomeruli, where incoming sensory neurons terminate, once an odor was removed. However, activity continued in some olfactory neurons located downstream from the sensory neurons, called mitral/tufted (M/T) cells. When the researchers directly stimulated the M/T cells with light, using optogenetics, they observed lingering cellular activity in the form of neuronal firing, without invoking activity in the sensory neurons.

The location
The findings combine to implicate the brain, rather than the nose, as being responsible for the afterimages. “The post-odor response of mitral cells is not due to the odor molecules remaining in the nasal cavity, but presumably reflects the neuronal circuit activity generated centrally in the olfactory bulb and/or olfactory cortex,” Kensaku Mori of the University of Tokyo, who was not part of the study, said in an e-mail.

The function
Carleton said his next step is to understand whether there is any functional relevance of odor afterimages. “Maybe these afterimages can be useful to form traces or memories in the brain so that when you have [neural activity simulating] a longer odor presentation . . . the brain may memorize it better,” he said.