Odor representations in the olfactory bulb evolve after the first breath and persist as an odor afterimage
Proceedings of the National Academy of Sciences of the United States of America
Rodents can discriminate odors in one breath, and mammalian olfaction research has thus focused on the first breath. However, sensory representations dynamically change during and after stimuli. To investigate these dynamics, we recorded spike trains from the olfactory bulb of awake, head-fixed mice and found that some mitral cells' odor representations changed following the first breath and others continued after odor cessation. Population analysis revealed that these postodor responses
... ed odor-and concentration-specific information-an odor afterimage. Using calcium imaging, we found that most olfactory glomerular activity was restricted to the odor presentation, implying that the afterimage is not primarily peripheral. The odor afterimage was not dependent on odorant physicochemical properties. To artificially induce aftereffects, we photostimulated mitral cells using channelrhodopsin and recorded centrally maintained persistent activity. The strength and persistence of the afterimage was dependent on the duration of both artificial and natural stimulation. In summary, we show that the odor representation evolves after the first breath and that there is a centrally maintained odor afterimage, similar to other sensory systems. These dynamics may help identify novel odorants in complex environments. multielectrode recording | network dynamics | optogenetics S ensory systems, even when presented with fixed stimuli, use dynamic neural representations that change over time. These changes range from simple potentiation or adaptation to more complex temporal patterns (1, 2). These dynamics can persist even after the cessation of stimulus, which can take the form of an off-response or prolonged aftereffect. Aftereffects have been intensely studied in vision (1, 3) and also observed in audition (4, 5), touch (6, 7), taste (here the afterimages may be due to persistent ligand binding) (8, 9) , and insect olfaction (10, 11). In mammalian olfaction, the only reported aftereffect is a "persistent afterdischarge" following high concentration odors (12). Olfaction is an active process that is coordinated by breathing (2, 13). Odorants bind to odorant receptor neurons (ORNs) in the epithelium, which synapse onto mitral/tufted (M/T) cells in the olfactory bulb (OB) at precisely defined glomeruli. Direct recordings of ORNs in rats show that ORNs are excited by odors and that activity can last for seconds after odor cessation (14). Others have measured the output of ORNs, by imaging glomeruli, and found that the response is gated by each breath and that the amplitude decreases with time (13, 15). The importance of breath segmentation for M/T cells has recently been shown in awake subjects, where neurons respond with precise, phasic firing patterns (16-18). However, these recordings have focused on how information is represented during the first breath, as rodents can identify odors in a single breath (19-21). Activity in piriform cortex is also segmented by breaths, and neurons there respond sparsely (22, 23) , and with adaptation (24, 25). To date, no one has addressed, in the OB of awake rodents, how the odor representation evolves with each breath and how the odor representation changes after odor offset. Answering these questions will yield insight into how odor identity is maintained and how the olfactory network represents odors. To investigate olfactory dynamics, we recorded from M/T cells in awake, head-fixed mice. We found that, during the odor, the odor representation shifts significantly after the first breath and not via simple attenuation. Furthermore, we observed a subset of cells that responded in an odor-specific manner after odor cessation-an olfactory afterimage. We performed calcium imaging on sensory neuron terminals, photostimulated channelrhodopsinexpressing M/T cells, and determined that the afterimage is primarily maintained centrally rather than peripherally. In conclusion, we have found that the odor response is dynamic and that one must consider all breaths of the response during an odor and afterward to characterize a cell's "odor receptive field." The existence of an odor afterimage shows that afterimages may be a general, useful property of all sensory systems.