Human Olfaction

Olfaction, or sense of smell, is one of the many sensory/perceptual systems that humans use to process external stimuli. Human olfaction is relatively understudied. More literature has focused on olfaction in animals for which olfaction plays a larger role in critical functioning and survival. This bibliography provides key resources for information about human olfaction and then outlines twelve general topics that are of considerable value to understanding this primary sensory system. It begins by providing some general resources that outline the functions of olfaction in humans. Next, it explores the anatomy and physiology of the human olfactory system, recognizing that our understanding has been largely guided by studying animals. It addresses several important theories of human olfaction, including ones that have attempted to understand the initial receptor-level processing of stimuli and ones that have been developed to explain the whole system. After considering some historical issues in studying olfaction, it outlines the various ways that human olfaction is studied. It considers some issues in olfaction that are related to cognitive processes, such as language and memory. A critical topic in the chemical senses of smell and taste that is somewhat less relevant in other sensory systems is that of hedonics (pleasure or liking). This bibliography explores sex differences, which, though small, seem to be more prevalent than in other sensory systems. In addition, this bibliography explores the effect of age on olfaction, both in terms of the development of olfaction in children and the decline of olfaction in aging. The entry ends with clinical implications, primarily of smell loss. In light of the Covid-19 pandemic, which brought newfound appreciation for the importance of sense of smell in human experience, a summary of the effect of Covid-19 on human sense of smell is included. In this bibliography pioneering studies are emphasized, although reviews are included in cases where the literature is too extensive to be summed up by single studies.

Sex Differences in Odor Hedonic Perception: An Overview

Odor hedonic evaluation (pleasant/unpleasant) is considered as the first and one of the most prominent dimension in odor perception. While sex differences in human olfaction have been extensively explored, gender effect in hedonic perception appears to be less considered. However, a number of studies have included comparisons between men and women, using different types of measurements (psychophysical, psychophysiological,…). This overview presents experimental works with non-specific and body odors separately presented as well as experimental studies comparing healthy participants vs patients with psychiatric disorders. Contrary to sensitivity, identification or discrimination, the overall literature tends to prove that no so clear differences occur in odor hedonic judgment between men and women. On the whole, gender effect appears more marked for body than non-specific odors and is almost never reported in psychiatric diseases. These findings are discussed in relation to the processes classically implied in pleasantness rating and emotional processes.

State of the art olfactometers. Different types

The complexity of human olfaction is very high and the importance of being able to measure it directly, objectively and qualitatively has led experts to search for mechanisms that can be applied. Human beings use this sense, which is one of the oldest, to recognize danger and distinguish between pleasant and unpleasant odors. Smells are mixtures of molecules that, at different concentrations in the inhaled air, stimulate the olfactory area and are recognized at the brain level. Therefore, there is a coding and decoding system. Human olfactometer techniques use equipment designed to be able to measure its intensity and quality of volatile substances. If we are able to measure this sense, we will be able to know its variations and be able to make clinical diagnoses in normal and pathological conditions and diagnose the losses that occur in certain infectious, degenerative diseases, traumatic processes and other variants. For many years, systems have been developed that can measure subjective olfaction in humans, as well as objective forms, but it is also true that there is no equipment available that is fast, simple handling and that can be applied in daily clinical services. Aim of the Study Present the recent achievements in olfactometer technology; Elaborate the scientific articles about olfactometry published mainly in the last 10 years; To gather the information published in the last years in relation to the usefulness, existence in the market and purposes of equipment that can measure the odors, what we will call the Smell-o-meter or olfactometer for human use. Material and Methods: In the first part of this research we will gather most of the information existing so far in international bibliography, as well as the achievements and utilities obtained to date. Following, we will analyze all the new concepts related to smell-o-meters devices that exist on the market and assess the possibility, based on what has been done so far, to seek new practical systems for application in the medical field.

Human olfaction: A view from the top

Although our understanding of human olfactory perception has increased dramatically, it remains less well understood how olfaction interacts with cognitive processes. In this overview, we review the claim that olfaction evolved to predominantly embody “novelty detection”, a framework that leaves little room for cognitive representations and processes. We challenge this framework by pointing to several perceived limitations. Instead, we argue that human olfaction is best understood in terms of its reliance on top-down processes; visual or verbal contexts may generate predictions of odor qualities, and a fundamental role of olfaction is to evaluate such predictions. We hypothesize that olfaction is to a large extent dependent on “top-down” cognitive processes. We discuss how this framework differs from other contemporary approaches to human olfactory perception. We conclude that in contrast to other frameworks that characterize olfaction primarily in terms of stimulus-driven perception, human olfactory processing is best understood by invoking a “view from the top”.

The Structural Properties of Odorants Modulate Their Association to Human Odorant Binding Protein

The binding of known odorant molecules to the human odorant-binding protein (hOBP) was evaluated in silico. Docking experiments elucidate the preferable binding site and binding affinity of odorant molecules to hOBP. The physicochemical properties molecular weight (MW), vapor pressure (Vp), hydrophobicity level (logP), number of double bonds (NºDB), degree of unsaturation (DoU) and the chemical classification, were selected for the study of odorant modulation. Here, these properties were analyzed concerning 30 pleasant and 30 unpleasant odorants, chosen to represent a wide variety of compounds and to determine their influence on the binding energy to hOBP. Our findings indicate that MW, logP and Vp are the most important odorant variables, directly correlated to odorant-binding energies (ΔGbinding) towards hOBP. Understanding how the odorants behave when complexed with the OBP in human olfaction opens new possibilities for the development of future biotechnological applications, including sensory devices, medical diagnosis, among others.

Accounting For Subjectivity In Experimental Research On Human Olfaction

Although olfaction is a modality with great inter-individual perceptual disparities, its subjective dimension has been let aside in modern research, in line with the overall neglect of consciousness in experimental psychology. However, following the renewed interest for the neural bases of consciousness, some methodological leads have been proposed to include subjectivity in experimental protocols. Here, we argue that adapting such methods to the field of olfaction will allow to rigorously acquire subjective reports, and we present several ways to do so. This will improve the understanding of diversity in odor perception and its underlying neural mechanisms.

A System-Wide Understanding of the Human Olfactory Percept Chemical Space

The fundamental units of olfactory perception are discrete 3D structures of volatile chemicals that each interact with specific subsets of a very large family of hundreds of odorant receptor proteins, in turn activating complex neural circuitry and posing a challenge to understand. We have applied computational approaches to analyze olfactory perceptual space from the perspective of odorant chemical features. We identify physicochemical features associated with ~150 different perceptual descriptors and develop machine-learning models. Validation of predictions shows a high success rate for test set chemicals within a study, as well as across studies more than 30 years apart in time. Due to the high success rates, we are able to map ~150 percepts onto a chemical space of nearly 0.5 million compounds, predicting numerous percept–structure combinations. The chemical structure-to-percept prediction provides a system-level view of human olfaction and opens the door for comprehensive computational discovery of fragrances and flavors.