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Odours are a fundamental part of the sensory environment used by animals to guide behaviours such as foraging and navigation 1 , 2 . Primary olfactory (piriform) cortex is thought to be the main cortical region for encoding odour identity 3 – 8 . Here, using neural ensemble recordings in freely moving rats performing an odour-cued spatial choice task, we show that posterior piriform cortex neurons carry a robust spatial representation of the environment. Piriform spatial representations have features of a learned cognitive map, being most prominent near odour ports, stable across behavioural contexts and independent of olfactory drive or reward availability. The accuracy of spatial information carried by individual piriform neurons was predicted by the strength of their functional coupling to the hippocampal theta rhythm. Ensembles of piriform neurons concurrently represented odour identity as well as spatial locations of animals, forming an odour–place map. Our results reveal a function for piriform cortex in spatial cognition and suggest that it is well-suited to form odour–place associations and guide olfactory-cued spatial navigation. Studies using neural ensemble recordings in rats show that cells in the piriform cortex carry a spatial representation of the environment and link locations to olfactory sensory inputs.

Background Key to curtailing the COVID-19 pandemic are wide-scale screening strategies. An ideal screen is one that would not rely on transporting, distributing, and collecting physical specimens. Given the olfactory impairment associated with COVID-19, we developed a perceptual measure of olfaction that relies on smelling household odorants and rating them online. Methods Each participant was instructed to select 5 household items, and rate their perceived odor pleasantness and intensity using an online visual analogue scale. We used this data to assign an olfactory perceptual fingerprint, a value that reflects the perceived difference between odorants. We tested the performance of this real-time tool in a total of 13,484 participants (462 COVID-19 positive) from 134 countries who provided 178,820 perceptual ratings of 60 different household odorants. Results We observe that olfactory ratings are indicative of COVID-19 status in a country, significantly correlating with national infection rates over time. More importantly, we observe indicative power at the individual level (79% sensitivity and 87% specificity). Critically, this olfactory screen remains effective in participants with COVID-19 but without symptoms, and in participants with symptoms but without COVID-19. Conclusions The current odorant-based olfactory screen adds a component to online symptom-checkers, to potentially provide an added first line of defense that can help fight disease progression at the population level. The data derived from this tool may allow better understanding of the link between COVID-19 and olfaction. Plain language summary From early on in the COVID-19 pandemic, a symptom associated with infection was rapid and often complete loss of the sense of smell. This rendered smell testing a potentially helpful tool in large-scale screening for SARS-CoV-2 infection. We built an online tool (smelltracker.org) that enables assessment of the sense of smell using commonly available household odorants. Initial use by 13,484 participants (462 COVID-19 positive) from 134 countries corroborated that SARS-CoV-2 infection is associated with impaired smell. Moreover, the tool detected infection in the absence of any other symptoms, including subjective loss in smell. Use of this tool may provide an added instrument for screening SARS-CoV-2 infection, and the data generated by the tool may provide for deeper understanding of the brain mechanisms involved with loss of smell associated with COVID-19. Snitz et al. develop a web-based olfactory screening tool for COVID-19, which relies on users smelling household odorants. Based on data from participants in 134 countries, the authors report that olfactory ratings are indicative of COVID-19 status.

Odors are a fundamental part of the sensory environment used by animals to inform behaviors such as foraging and navigation . Primary olfactory (piriform) cortex is thought to be dedicated to encoding odor identity . Here, using neural ensemble recordings in freely moving rats performing a novel odor-cued spatial choice task, we show that posterior piriform cortex neurons also carry a robust spatial map of the environment. Piriform spatial maps were stable across behavioral contexts independent of olfactory drive or reward availability, and the accuracy of spatial information carried by individual neurons depended on the strength of their functional coupling to the hippocampal theta rhythm. Ensembles of piriform neurons concurrently represented odor identity as well as spatial locations of animals, forming an \"olfactory-place map\". Our results reveal a previously unknown function for piriform cortex in spatial cognition and suggest that it is well-suited to form odor-place associations and guide olfactory cued spatial navigation.

Events in our external world are transformed into internal percepts and experiences. How can we begin to understand this transformation? It starts with the stimulation of peripheral sensory organs and ultimately requires the holistic synthesis of ensemble neural activity in the cortex. The cortex is composed of circuits connecting diverse types of neurons across various functional brain regions. In order to fully understand the cortical representation of an external stimulus, we must dissect out basic components of the circuit and characterize their stimulus response properties. We study neural circuits in primary olfactory (piriform) cortex of rodents using electrophysiological recordings in brain slices and in vivo . We examine the underlying synaptic mechanisms of odor representations, and how it can be modulated. First, we demonstrate an early developmental critical period for plasticity and structural changes in principal neurons of the olfactory cortex in response to sensory afferent activity. Next, we use in vivo recordings to demonstrate that synaptic inhibition is widely recruited by odor stimuli, whereas synaptic excitation is more selective in principal neurons. We show that the recruitment of both local interneurons as well as intracortical excitatory connections serve to shape odor-evoked synaptic activity. Synchronous beta (15-30 Hz) oscillations between the olfactory cortex and bulb are thought to be important for odor discrimination. We find that odor-evoked synaptic currents in principle cells of olfactory cortex are couple to beta frequency oscillations in the local field potential. A time window between oscillatory synaptic excitation and inhibition restricts the spike timing of odor-evoked spikes. Beta frequency oscillations have been shown to require a centrifugal feedback loop from olfactory cortex back to the bulb. We use an optogenetic approach to reveal that cortical feedback projections provide inhibition onto mitral cells in the olfactory bulb via activation of granule cells. This work establishes a framework to understand basic components of odor representations in olfactory cortex, and a role for cortico-bulbar feedback loop. In combination with work done in the visual, auditory, somatosensory and gustatory cortices, this study contributes to a more complete understanding of sensory information processing by cortical circuits.

Odors are a fundamental part of the sensory environment used by animals to inform behaviors such as foraging and navigation1, 2. Primary olfactory (piriform) cortex is thought to be dedicated to encoding odor identity3–8. Here, using neural ensemble recordings in freely moving rats performing a novel odor-cued spatial choice task, we show that posterior piriform cortex neurons also carry a robust spatial map of the environment. Piriform spatial maps were stable across behavioral contexts independent of olfactory drive or reward availability, and the accuracy of spatial information carried by individual neurons depended on the strength of their functional coupling to the hippocampal theta rhythm. Ensembles of piriform neurons concurrently represented odor identity as well as spatial locations of animals, forming an “olfactory-place map”. Our results reveal a previously unknown function for piriform cortex in spatial cognition and suggest that it is well-suited to form odor-place associations and guide olfactory cued spatial navigation.

Odors are a fundamental part of the sensory environment used by animals to inform behaviors such as foraging and navigation. Primary olfactory (piriform) cortex is thought to be dedicated to encoding odor identity. Here, using neural ensemble recordings in freely moving rats performing a novel odor-cued spatial choice task, we show that posterior piriform cortex neurons also carry a robust spatial map of the environment. Piriform spatial maps were stable across behavioral contexts independent of olfactory drive or reward availability, and the accuracy of spatial information carried by individual neurons depended on the strength of their functional coupling to the hippocampal theta rhythm. Ensembles of piriform neurons concurrently represented odor identity as well as spatial locations of animals, forming an olfactory-place map. Our results reveal a previously unknown function for piriform cortex in spatial cognition and suggest that it is well-suited to form odor-place associations and guide olfactory cued spatial navigation. Competing Interest Statement The authors have declared no competing interest.