Olfactory perception as a compass for olfactory neural maps

Responses of Chemosensory Perception to Stimulation of the Human Brain

OBJECTIVE

Significant advances have been made in our understanding of the neural substrates of human chemosensory processing, involving the piriform cortex, insula, and orbitofrontal cortex. However, the important and challenging issues are to localize the brain regions with high anatomic precision that can causally produce chemosensory perception and further delineate the topography of different classifications of chemosensory perception.

METHODS

We quantitatively measured subjective responses of chemosensory perception to intracranial electrical stimulation over the brain in neurosurgical patients (n = 302) with medically refractory epilepsy.

RESULTS

The chemosensory perceptions including olfaction, gustation, and chemesthesis were elicited in 21 of 302 patients (7%). Chemosensory responses were evoked in 53 (0.2%) of 21,661 stimulated sites. The highest response rate (1.8%) was in the insula (37/2,051 stimulated sites from 15/163 patients). The chemosensory perception emerged predominantly during stimulation of the insula along the central sulcus axis. Notably, there existed a distinct pattern that the anteroventral insula predominately represented orthonasal olfaction, whereas different chemosensory modalities converged in the mid-dorsal insula.

INTERPRETATION

This study provided a detailed characterization of chemosensory perception across the brain, especially in the insula. These results suggest that the cortex along the banks of the central sulcus of the insula may play a role in producing the supramodal sensation of flavor. It also indicates that dysfunction of the central insula should be considered during the evaluation of chemosensory-related epileptic seizures. ANN NEUROL 2023;93:175-183.

Decoding olfactory EEG signals for different odor stimuli identification using wavelet-spatial domain feature

BACKGROUND

Decoding olfactory-induced electroencephalography (olfactory EEG) signals has gained significant attention in recent years, owing to its potential applications in several fields, such as disease diagnosis, multimedia applications, and brain-computer interaction (BCI). Extracting discriminative features from olfactory EEG signals with low spatial resolution and poor signal-to-noise ratio is vital but challenging for improving decoding accuracy.

NEW METHODS

By combining discrete wavelet transform (DWT) with one-versus-rest common spatial pattern (OVR-CSP), we develop a novel feature, named wavelet-spatial domain feature (WSDF), to decode the olfactory EEG signals. First, DWT is employed on EEG signals for multilevel wavelet decomposition. Next, the DWT coefficients obtained at a specific level are subjected to OVR-CSP for spatial filtering. Correspondingly, the variance is extracted to generate a discriminative feature set, labeled as WSDF.

RESULTS

To verify the effectiveness of WSDF, a classification of olfactory EEG signals was conducted on two data sets, i.e., a public EEG dataset 'Odor Pleasantness Perception Dataset (OPPD)', and a self-collected dataset, by using support vector machine (SVM) trained based on different cross-validation methods. Experimental results showed that on OPPD dataset, the proposed method achieved a best average accuracy of 100% and 94.47% for the eyes-open and eyes-closed conditions, respectively. Moreover, on our own dataset, the proposed method gave a highest average accuracy of 99.50%.

COMPARISON WITH EXISTING METHODS

Compared with a wide range of EEG features and existing works on the same dataset, our WSDF yielded superior classification performance.

CONCLUSIONS

The proposed WSDF is a promising candidate for decoding olfactory EEG signals.

Developmental Profile of Brain Neprilysin Expression Correlates with Olfactory Behaviour of Rats

A neuropeptidase, neprilysin (NEP), is a major amyloid (Aβ)-degrading enzyme involved in the pathogenesis of Alzheimer's disease (AD). The olfactory system is affected early in AD with characteristic Aβ accumulation, but data on the dynamics of NEP expression in the olfactory system are absent. Our study demonstrates that NEP mRNA expression in rat olfactory bulbs (OB), entorhinal cortex (ECx), hippocampus (Hip), parietal cortex (PCx) and striatum (Str) increases during the first postnatal month being the highest in the OB and Str. By 3 months, NEP mRNA levels sharply decrease in the ECx, Hip and PCx and by 9 months in the OB, but not in the Str, which correlates with declining olfaction in aged rats tested in the food search paradigm. One-month-old rats subjected to prenatal hypoxia on E14 had lower NEP mRNA levels in the ECx, Hip and PCx (but not in the OB and Str) compared with the control offspring and demonstrated impaired olfaction in the odour preference and food search paradigms. Administration to these rats of a histone deacetylase inhibitor, sodium valproate, restored NEP expression in the ECx, Hip and PCx and improved olfaction. Our data support NEP involvement in olfactory function.

A Masked Aversive Odor Cannot Be Discriminated From the Masking Odor but Can Be Identified Through Odor Quality Ratings and Neural Activation Patterns

Odor masking is a very prominent problem in our daily routines, mainly concerning unpleasant sweat or toilet odors. In the current study we explored the effectiveness of odor masking both on a behavioral and neuronal level. By definition, participants cannot differentiate a fully masked unpleasant odor from the pleasant pure odor used as a masking agent on a behavioral level. We hypothesized, however, that one can still discriminate between a fully masked odor mixture and the pure masking odor on a neuronal level and that, using a reinforcing feedback paradigm, participants could be trained to perceive this difference. A pleasant, lemon-like odor (citral) and a mixture of citral and minor amounts of an unpleasant, goat-like odor (caproic acid) were presented to participants repeatedly using a computer-controlled olfactometer and participants had to decide whether two presented stimuli were the same or different. Accuracy of this task was incentivized with a possible monetary reward. Functional imaging was used throughout the task to investigate central processing of the two stimuli. The participants rated both stimuli as isopleasant and isointense, indicating that the unpleasant odor was fully masked by the pleasant odor. The isolated caproic acid component of the mixture was rated less pleasant than the pleasant odor in a prior experimental session. Although the masked and pure stimuli were not discriminated in the forced-choice task, quality ratings on a dimensional scale differed. Further, we observed an increased activation of the insula and ventral striatum/putamen for the pure in contrast to the fully masked odor, hence revealing a difference in neuronal processing. Our hypothesis that perceptual discrimination and neuronal processing can be enhanced using a reinforcing feedback paradigm is not supported by our data.

Chemosensory Perception: A Review on Electrophysiological Methods in “Cognitive Neuro-Olfactometry”

Various brain imaging techniques are available, but few are specifically designed to visualize chemical sensory and, in particular, olfactory processing. This review describes the results of quantitative and qualitative studies that have used electroencephalography (EEG) and magneto-encephalography (MEG) to evaluate responses to olfactory stimulation (OS). EEG and MEG are able to detect the components of chemosensory event-related potentials (CSERPs) and the cortical rhythms associated with different types of OS. Olfactory studies are filling the gaps in both the developmental field of the life cycle (from newborns to geriatric age) and the clinical and basic research fields, in a way that can be considered the modern “cognitive neuro-olfactometry”.

Response Patterns of GABAergic Neurons in the Anterior Piriform Cortex of Awake Mice

Local inhibition by γ-amino butyric acid (GABA)-containing neurons is of vital importance for the operation of sensory cortices. However, the physiological response patterns of cortical GABAergic neurons are poorly understood, especially in the awake condition. Here, we utilized the recently developed optical tagging technique to specifically record GABAergic neurons in the anterior piriform cortex (aPC) in awake mice. The identified aPC GABAergic neurons were stimulated with robotic delivery of 32 distinct odorants, which covered a broad range of functional groups. We found that aPC GABAergic neurons could be divided into 4 types based on their response patterns. Type I, type II, and type III neurons displayed broad excitatory responses to test odorants with different dynamics. Type I neurons were constantly activated during odorant stimulation, whereas type II neurons were only transiently activated at the onset of odorant delivery. In addition, type III neurons displayed transient excitatory responses both at the onset and termination of odorant presentation. Interestingly, type IV neurons were broadly inhibited by most of the odorants. Taken together, aPC GABAergic neurons adopt different strategies to affect the cortical circuitry. Our results will allow for better understanding of the role of cortical GABAergic interneurons in sensory information processing.

Machine-Learning-Based Olfactometer: Prediction of Odor Perception from Physicochemical Features of Odorant Molecules

Gas chromatography/olfactometry (GC/O) has been used in various fields as a valuable method to identify odor-active components from a complex mixture. Since human assessors are employed as detectors to obtain the olfactory perception of separated odorants, the GC/O technique is limited by its subjectivity, variability, and high cost of the trained panelists. Here, we present a proof-of-concept model by which odor information can be obtained by machine-learning-based prediction from molecular parameters (MPs) of odorant molecules. The odor prediction models were established using a database of flavors and fragrances including 1026 odorants and corresponding verbal odor descriptors (ODs). Physicochemical parameters of the odorant molecules were acquired by use of molecular calculation software (DRAGON). Ten representative ODs were selected to build the prediction models based on their high frequency of occurrence in the database. The features of the MPs were extracted via either unsupervised (principal component analysis) or supervised (Boruta, BR) approaches and then used as input to calibrate machine-learning models. Predictions were performed by various machine-learning approaches such as support vector machine (SVM), random forest, and extreme learning machine. All models were optimized via parameter tuning and their prediction accuracies were compared. A SVM model combined with feature extraction by BR-C (confirmed only) was found to afford the best results with an accuracy of 97.08%. Validation of the models was verified by using the GC/O data of an apple sample for comparison between the predicted and measured results. The prediction models can be used as an auxiliary tool in the existing GC/O by suggesting possible OD candidates to the panelists and thus helping to give more objective and correct judgment. In addition, a machine-based GC/O in which the panelist is no longer needed might be expected after further development of the proposed odor prediction technique.

The olfactory bulb volume in patients with idiopathic Parkinson's disease

BACKGROUND AND PURPOSE

This study addresses the question of whether the neuropathological findings on the olfactory bulb (OB) in idiopathic Parkinson's disease (IPD) correspond to a detectable change in volume of the OB. Additionally, the relationship between OB volume and residual olfactory function, clinical disease characteristics and age are investigated.

METHODS

Fifty-two IPD patients were investigated and compared to 31 healthy age-matched controls. All participants were scanned using a 3 T magnetic resonance imaging MRI scanner including a T2 DRIVE sequence in coronal slices through the OB. The OB volumes were measured via manual segmentation of the OB. Olfactory testing was carried out using the Sniffin' Sticks test battery.

RESULTS

The OB volume in the IPD group was 42.1 mm³ (SD ± 11.6) for the right and 41.5 mm³ (SD ± 11.7) for the left OB and showed no difference from the controls. Additionally, there were no significant correlations between OB volume and disease characteristics such as disease duration or Unified Parkinson's Disease Rating Scale motor score. Likewise, patients' residual smell function did not correlate with their OB volume. In contrast, controls indicated a correlation between smell function and OB volume.

CONCLUSION

The study shows that high resolution MRI does not show a detectable volume loss of the OB in PD patients. It is concluded that OB measurement using in vivo high resolution MRI at 3 T is not helpful to identify IPD.

Looking for the roots of cortical sensory computation in three-layered cortices

Despite considerable effort over a century and the benefit of remarkable technical advances in the past few decades, we are still far from understanding mammalian cerebral neocortex. With its six layers, modular architecture, canonical circuits, innumerable cell types, and computational complexity, isocortex remains a challenging mystery. In this review, we argue that identifying the structural and functional similarities between mammalian piriform cortex and reptilian dorsal cortex could help reveal common organizational and computational principles and by extension, some of the most primordial computations carried out in cortical networks.

Interneurons, tau and amyloid-β in the piriform cortex in Alzheimer's disease

Impaired olfaction has been described as an early symptom of Alzheimer's disease. Neuroanatomical changes underlying this deficit in the olfactory system are largely unknown. Interestingly, neuropathology begins in the transentorhinal cortex and extends to the neighboring limbic system and basal telencephalic structures that mediate olfactory processing, including the anterior olfactory nucleus and olfactory bulb. The human piriform cortex has been described as a crucial area in odor quality coding; disruption of this region mediates early olfactory deficits in Alzheimer's disease. Most neuropathological investigations have focused on the entorhinal cortex and hippocampus, whereas the piriform cortex has largely been neglected. This work aims to characterize the expression of the neuropathological amyloid-β peptide, tau protein and interneuron population markers (calretinin, parvalbumin and somatostatin) in the piriform cortex of ten Alzheimer-diagnosed (80.4 ± 8.3 years old) and five control (69.6 ± 11.1) cases. Here, we examined the distribution of different interneuronal markers as well as co-localization of interneurons and pathological markers. Results indicated preferential vulnerability of somatostatin- (p = 0.0001 < α = 0.05) and calretinin-positive (p = 0.013 < α = 0.05) cells that colocalized with amyloid-β peptide, while the prevalence of parvalbumin-positive cells was increased (p = 0.045 < α = 0.05) in the Alzheimer's cases. These data may help to reveal the neural basis of olfactory deficits linked to Alzheimer's disease as well as to characterize neuronal populations preferentially vulnerable to neuropathology in regions critically involved in early stages of the disease.

Categorical dimensions of human odor descriptor space revealed by non-negative matrix factorization

In contrast to most other sensory modalities, the basic perceptual dimensions of olfaction remain unclear. Here, we use non-negative matrix factorization (NMF) – a dimensionality reduction technique – to uncover structure in a panel of odor profiles, with each odor defined as a point in multi-dimensional descriptor space. The properties of NMF are favorable for the analysis of such lexical and perceptual data, and lead to a high-dimensional account of odor space. We further provide evidence that odor dimensions apply categorically. That is, odor space is not occupied homogenously, but rather in a discrete and intrinsically clustered manner. We discuss the potential implications of these results for the neural coding of odors, as well as for developing classifiers on larger datasets that may be useful for predicting perceptual qualities from chemical structures.

Flexible cognitive resources: competitive content maps for attention and memory

The brain has finite processing resources so that, as tasks become harder, performance degrades. Where do the limits on these resources come from? We focus on a variety of capacity-limited buffers related to attention, recognition, and memory that we claim have a two-dimensional 'map' architecture, where individual items compete for cortical real estate. This competitive format leads to capacity limits that are flexible, set by the nature of the content and their locations within an anatomically delimited space. We contrast this format with the standard 'slot' architecture and its fixed capacity. Using visual spatial attention and visual short-term memory as case studies, we suggest that competitive maps are a concrete and plausible architecture that limits cognitive capacity across many domains.

From chemotaxis to the cognitive map: the function of olfaction

A paradox of vertebrate brain evolution is the unexplained variability in the size of the olfactory bulb (OB), in contrast to other brain regions, which scale predictably with brain size. Such variability appears to be the result of selection for olfactory function, yet there is no obvious concordance that would predict the causal relationship between OB size and behavior. This discordance may derive from assuming the primary function of olfaction is odorant discrimination and acuity. If instead the primary function of olfaction is navigation, i.e., predicting odorant distributions in time and space, variability in absolute OB size could be ascribed and explained by variability in navigational demand. This olfactory spatial hypothesis offers a single functional explanation to account for patterns of olfactory system scaling in vertebrates, the primacy of olfaction in spatial navigation, even in visual specialists, and proposes an evolutionary scenario to account for the convergence in olfactory structure and function across protostomes and deuterostomes. In addition, the unique percepts of olfaction may organize odorant information in a parallel map structure. This could have served as a scaffold for the evolution of the parallel map structure of the mammalian hippocampus, and possibly the arthropod mushroom body, and offers an explanation for similar flexible spatial navigation strategies in arthropods and vertebrates.