Task-Dependent Behavioral Dynamics Make the Case for Temporal Integration in Multiple Strategies during Odor Processing

The Wire Is Not the Territory: Understanding Representational Drift in Olfaction With Dynamical Systems Theory

Representational drift is a phenomenon of increasing interest in the cognitive and neural sciences. While investigations are ongoing for other sensory cortices, recent research has demonstrated the pervasiveness in which it occurs in the piriform cortex for olfaction. This gradual weakening and shifting of stimulus-responsive cells has critical implications for sensory stimulus-response models and perceptual decision-making. While representational drift may complicate traditional sensory processing models, it could be seen as an advantage in olfaction, as animals live in environments with constantly changing and unpredictable chemical information. Non-topographical encoding in the olfactory system may aid in contextualizing reactions to promiscuous odor stimuli, facilitating adaptive animal behavior and survival. This article suggests that traditional models of stimulus-(neural) response mapping in olfaction may need to be reevaluated and instead motivates the use of dynamical systems theory as a methodology and conceptual framework.

Robust odor identification in novel olfactory environments in mice

Relevant odors signaling food, mates, or predators can be masked by unpredictable mixtures of less relevant background odors. Here, we developed a mouse behavioral paradigm to test the role played by the novelty of the background odors. During the task, mice identified target odors in previously learned background odors and were challenged by catch trials with novel background odors, a task similar to visual CAPTCHA. Female wild-type (WT) mice could accurately identify known targets in novel background odors. WT mice performance was higher than linear classifiers and the nearest neighbor classifier trained using olfactory bulb glomerular activation patterns. Performance was more consistent with an odor deconvolution method. We also used our task to investigate the performance of female Cntnap2^-/- mice, which show some autism-like behaviors. Cntnap2^-/- mice had glomerular activation patterns similar to WT mice and matched WT mice target detection for known background odors. However, Cntnap2^-/- mice performance fell almost to chance levels in the presence of novel backgrounds. Our findings suggest that mice use a robust algorithm for detecting odors in novel environments and this computation is impaired in Cntnap2^-/- mice.

Testing effects of trigeminal stimulation on binary odor mixture quality in rats

Prior attempts at forming theoretical predictions regarding the quality of binary odor mixtures have failed to find any consistent predictor for overshadowing of one component in a binary mixture by the other. We test here the hypothesis that trigeminality contributes to overshadowing effects in binary mixture perception. Most odorants stimulate the trigeminal nerve in the nasal sensory epithelium. In the current study we test rats' ability to detect component odorants in four binary odor sets chosen for their relative trigeminality. We predicted that the difference in trigeminal intensity would predict the degree of overshadowing by boosting or suppressing perceptual intensity of these odorants during learning or during mixture perception. We used a two-alternative choice (TAC) task in which rats were trained to recognize the two components of each mixture and tested on a range of mixtures of the two without reinforcement. We found that even though odorant concentrations were adjusted to balance volatility, all odor sets produced asymmetric psychometric curves. Odor pairs with the greatest difference in trigeminality showed overshadowing by the odorant with weaker trigeminal properties. Odor sets with more evenly matched trigeminal properties also showed asymmetry that was not predicted by either small differences in volatility or trigeminality. Thus, trigeminal properties may influence overshadowing in odor mixtures, but other factors are also likely involved. These mixed results further support the need to test each odor mixture to determine its odor quality and underscore recent results at the level of olfactory receptor neurons that show massive and unpredictable inhibition among odorants in complex mixtures.

Smell-induced gamma oscillations in human olfactory cortex are required for accurate perception of odor identity

Studies of neuronal oscillations have contributed substantial insight into the mechanisms of visual, auditory, and somatosensory perception. However, progress in such research in the human olfactory system has lagged behind. As a result, the electrophysiological properties of the human olfactory system are poorly understood, and, in particular, whether stimulus-driven high-frequency oscillations play a role in odor processing is unknown. Here, we used direct intracranial recordings from human piriform cortex during an odor identification task to show that 3 key oscillatory rhythms are an integral part of the human olfactory cortical response to smell: Odor induces theta, beta, and gamma rhythms in human piriform cortex. We further show that these rhythms have distinct relationships with perceptual behavior. Odor-elicited gamma oscillations occur only during trials in which the odor is accurately perceived, and features of gamma oscillations predict odor identification accuracy, suggesting that they are critical for odor identity perception in humans. We also found that the amplitude of high-frequency oscillations is organized by the phase of low-frequency signals shortly following sniff onset, only when odor is present. Our findings reinforce previous work on theta oscillations, suggest that gamma oscillations in human piriform cortex are important for perception of odor identity, and constitute a robust identification of the characteristic electrophysiological response to smell in the human brain. Future work will determine whether the distinct oscillations we identified reflect distinct perceptual features of odor stimuli.

Intracranial recordings from human olfactory cortex reveal a characteristic spectrotemporal response to odors, including theta, beta and gamma oscillations, and show that high-frequency responses are critical for accurate perception of odors.

Transfer of Odor Perception From the Retronasal to the Orthonasal Pathway

Although orthonasal odorants are often associated with the external environment, retronasal odorants are accompanied by consummatory behaviors and indicate an internal state of an animal. Our study aimed to examine whether the same odorants may generate a consistent perceptual experience when 2 olfactory routes potentiate variations in concentration in the nasal cavity and orosensory activation. A customized lick spout with vacuum removing odorants around the animal's nares was used to render a pure retronasal exposure experience. We found that pre-exposing rats to odorants retronasally with positive or negative reinforcers (sweet or bitter) lead to a significant learning rate difference between high- and low-vapor-pressure odorants. This effect was not observed for novel odorants, suggesting that odorants may generate similar perceptual quality in a volatility-dependent manner.

Plasticity of Sniffing Pattern and Neural Activity in the Olfactory Bulb of Behaving Mice During Odor Sampling, Anticipation, and Reward

The olfactory bulb (OB) is the first relay station in the olfactory system. In the OB, mitral/tufted cells (M/Ts), which are the main output neurons, play important roles in the processing and representation of odor information. Recent studies focusing on the function of M/Ts at the single-cell level in awake behaving mice have demonstrated that odor-evoked firing of single M/Ts displays transient/long-term plasticity during learning. Here, we tested whether the neural activity of M/Ts and sniffing patterns are dependent on anticipation and reward in awake behaving mice. We used an odor discrimination task combined with in vivo electrophysiological recordings in awake, head-fixed mice, and found that, while learning induced plasticity of spikes and beta oscillations during odor sampling, we also found plasticity of spikes, beta oscillation, sniffing pattern, and coherence between sniffing and theta oscillations during the periods of anticipation and/or reward. These results indicate that the activity of M/Ts plays important roles not only in odor representation but also in salience-related events such as anticipation and reward.

Primacy coding facilitates effective odor discrimination when receptor sensitivities are tuned

The olfactory system faces the difficult task of identifying an enormous variety of odors independent of their intensity. Primacy coding, where the odor identity is encoded by the receptor types that respond earliest, might provide a compact and informative representation that can be interpreted efficiently by the brain. In this paper, we analyze the information transmitted by a simple model of primacy coding using numerical simulations and statistical descriptions. We show that the encoded information depends strongly on the number of receptor types included in the primacy representation, but only weakly on the size of the receptor repertoire. The representation is independent of the odor intensity and the transmitted information is useful to perform typical olfactory tasks with close to experimentally measured performance. Interestingly, we find situations in which a smaller receptor repertoire is advantageous for discriminating odors. The model also suggests that overly sensitive receptor types could dominate the entire response and make the whole array useless, which allows us to predict how receptor arrays need to adapt to stay useful during environmental changes. Taken together, we show that primacy coding is more useful than simple binary and normalized coding, essentially because the sparsity of the odor representations is independent of the odor statistics, in contrast to the alternatives. Primacy coding thus provides an efficient odor representation that is independent of the odor intensity and might thus help to identify odors in the olfactory cortex.

Humans can identify odors independent of their intensity. Experimental data suggest that this is accomplished by representing the odor identity by the earliest responding receptor types. Using theoretical modeling, we here show that such a primacy code outperforms alternative encodings and allows discriminating odors with close to experimentally measured performance. This performance depends strongly on the number of receptors considered in the primacy code, but the receptor repertoire size is less important. The model also suggests a strong evolutionary pressure on the receptor sensitivities, which could explain observed receptor copy number adaptations. By predicting psycho-physical experiments, the model will thus contribute to our understanding of the olfactory system.

Methods in Rodent Chemosensory Cognition

Olfactory information processing and learning are highly developed computational abilities of rodents. These attributes can be exploited to ask questions at several levels of complexity, from aspects of odorant binding by olfactory receptors to higher order learning about the predictive consequences of odorant stimulus presentation. Quantitative understanding of rodent odorant sampling patterns, both baseline nasal breathing and odorant-stimulated sniffing, is critical to elucidating mechanisms of olfactory information processing, from primary olfactory receptors to cortical centers that synthesize olfactory percepts from preprocessed multimodal inputs. This chapter outlines an innovative new method for measuring breathing and sniffing rates in unrestrained mice while the mice are performing odor-guided tasks in a computer controlled olfactometer.The method described here involves implantation of a wireless pressure sensor in the mouse that reports on thoracic pressure transients caused by breathing and sniffing. Recordings of pressure sensor outputs are made simultaneously with optically-sensed nose pokes by the mouse into an odor delivery port or a water delivery port. Odorant delivery timing and water reward delivery are also recorded simultaneously. This method allows for breathing and sniffing dependent thoracic pressure transients to be recorded with high temporal precision before, during, and after the mouse approaches an odor delivery port, samples the delivered odor, and obtains a water reward contingent on the identity of the odor that was presented and sampled.

A specific olfactory cortico-thalamic pathway contributing to sampling performance during odor reversal learning

A growing body of evidence shows that olfactory information is processed within a thalamic nucleus in both rodents and humans. The mediodorsal thalamic nucleus (MDT) receives projections from olfactory cortical areas including the piriform cortex (PCX) and is interconnected with the orbitofrontal cortex (OFC). Using electrophysiology in freely moving rats, we recently demonstrated the representation of olfactory information in the MDT and the dynamics of functional connectivity between the PCX, MDT and OFC. Notably, PCX-MDT coupling is specifically increased during odor sampling of an odor discrimination task. However, whether this increase of coupling is functionally relevant is unknown. To decipher the importance of PCX-MDT coupling during the sampling period, we used optogenetics to specifically inactivate the PCX inputs to MDT during an odor discrimination task and its reversal in rats. We demonstrate that inactivating the PCX inputs to MDT does not affect the performance accuracy of an odor discrimination task and its reversal, however, it does impact the rats' sampling duration. Indeed, rats in which PCX inputs to MDT were inactivated during the sampling period display longer sampling duration during the odor reversal learning compared to controls-an effect not observed when inactivating OFC inputs to MDT. We demonstrate a causal link between the PCX inputs to MDT and the odor sampling performance, highlighting the importance of this specific cortico-thalamic pathway in olfaction.

Task dependence of odor discrimination: choosing between speed and accuracy

Odor discrimination is a complex task that may be improved by increasing sampling time to facilitate evidence accumulation. However, experiments testing this phenomenon in olfaction have produced conflicting results. To resolve this disparity, Frederick et al. (Frederick DE, Brown A, Tacopina S, Mehta N, Vujovic M, Brim E, Amina T, Fixsen B, Kay LM. J Neurosci 37: 4416-4426, 2017) conducted experiments that suggest that sampling time and performance are task dependent. Their findings have implications for understanding olfactory processing and experimental design, specifically the effect of subtle differences in experimental design on study results.

The Physiological Foresight in Freeman's Work: Predictions and Verifications

Freeman's studies on the physiology of the mammalian olfactory system were based on his characterization of activity of neural masses, based on a sigmoid relationship at the mesoscopic scale between population spiking activity as a result of continuous inputs. His early development of computational models to describe oscillatory responses of neural masses allowed him to predict physiological and anatomical properties, some of which required decades of research to be confirmed. His models of neural masses therefore allow us to link between basic physiology and cognitive processes. Through the study of brain physiology at the mesoscopic level, we can understand how internally generated meaning-based responses to sensory input become action and how action leads to perception.

Decision-making behaviors: weighing ethology, complexity, and sensorimotor compatibility

Rodent decision-making research aims to uncover the neural circuitry underlying the ability to evaluate alternatives and select appropriate actions. Designing behavioral paradigms that provide a solid foundation to ask questions about decision-making computations and mechanisms is a difficult and often underestimated challenge. Here, we propose three dimensions on which we can consider rodent decision-making tasks: ethological validity, task complexity, and stimulus-response compatibility. We review recent research through this lens, and provide practical guidance for researchers in the decision-making field.