Sensory maps in the olfactory cortex defined by long-range viral tracing of single neurons

Scent tracking In the mouse, glomeruli in the olfactory bulb receive projections from single classes of olfactory neurons, thereby forming an odour map. Information from the glomeruli is then relayed to the cortex but the projection patterns from individual glomeruli are not known. Three papers now examine the details of this projection. Luo and colleagues use a combination of genetics and retrograde mono-trans-synaptic rabies virus labelling. They trace the presynaptic connections of individual cortical neurons and find no evidence of connections supporting a stereotyped odour map in the cortex, but see systematic topographical differences in amygdala connectivity. The lack of stereotypical cortical projection is corroborated, both at the level of bulk axonal patterning and in projections of individually labelled neurons, by two papers — one from the Axel laboratory, and one from the Baldwin laboratory — that examine the anterograde projections from individual glomeruli. Together, these findings provide anatomical evidence for combinatorial processing of information from diverse glomeruli by cortical neurons and may also reflect different functions of various areas in mediating innate or learned odour preferences.