Rehan M. Khan

Predicting Odor Pleasantness from Odorant Structure: Pleasantness as a Reflection of the Physical World

Although it is agreed that physicochemical features of molecules determine their perceived odor, the rules governing this relationship remain unknown. A significant obstacle to such understanding is the high dimensionality of features describing both percepts and molecules. We applied a statistical method to reduce dimensionality in both odor percepts and physicochemical descriptors for a large set of molecules. We found that the primary axis of perception was odor pleasantness, and critically, that the primary axis of physicochemical properties reflected the primary axis of olfactory perception. This allowed us to predict the pleasantness of novel molecules by their physicochemical properties alone. Olfactory perception is strongly shaped by experience and learning. However, our findings suggest that olfactory pleasantness is also partially innate, corresponding to a natural axis of maximal discriminability among biologically relevant molecules.

Mechanisms of scent-tracking in humans.

Whether mammalian scent-tracking is aided by inter-nostril comparisons is unknown. We assessed this in humans and found that (i) humans can scent-track, (ii) they improve with practice, (iii) the human nostrils sample spatially distinct regions separated by ∼3.5 cm and, critically, (iv) scent-tracking is aided by inter-nostril comparisons. These findings reveal fundamental mechanisms of scent-tracking and suggest that the poor reputation of human olfaction may reflect, in part, behavioral demands rather than ultimate abilities.

Attentional modulation in human primary olfactory cortex

Central to the concept of attention is the fact that identical stimuli can be processed in different ways. In olfaction, attention may designate the identical flow of air through the nose as either respiration or olfactory exploration. Here we have used functional magnetic resonance imaging (fMRI) to probe this attentional mechanism in primary olfactory cortex (POC). We report a dissociation in POC that revealed attention-dependent and attention-independent subregions. Whereas a temporal subregion comprising temporal piriform cortex (PirT) responded equally across conditions, a frontal subregion comprising frontal piriform cortex (PirF) and the olfactory tubercle responded preferentially to attended sniffs as opposed to unattended sniffs. In addition, a task-specific anticipatory response occurred in the attention-dependent region only. This dissociation was consistent across two experimental designs: one focusing on sniffs of clean air, the other focusing on odor-laden sniffs. Our findings highlight the role of attention at the earliest cortical levels of olfactory processing.

A metric for odorant comparison.

In studies of vision and audition, stimuli can be systematically varied by wavelength and frequency, respectively, but there is no equivalent metric for olfaction. Restricted odorant-feature metrics such as number of carbons and functional group do not account for response patterns to odorants varying along other structural dimensions. We generated a multidimensional odor metric, in which each odorant molecule was represented as a vector of 1,664 molecular descriptor values. Revisiting many studies, we found that this metric and a second optimized metric were always better at accounting for neural responses than the specific metric used in each study. These metrics were applicable across studies that differed in the animals studied, the type of olfactory neurons tested, the odorants applied and the recording methods used. We use this new metric to recommend sets of odorants that span the physicochemical space for use in olfaction experiments.

Smelling a Single Component of Male Sweat Alters Levels of Cortisol in Women

Rodents use chemosignals to alter endocrine balance in conspecifics. Although responses to human sweat suggest a similar mechanism in humans, no particular component of human sweat capable of altering endocrine balance in conspecifics has yet been isolated and identified. Here, we measured salivary levels of the hormone cortisol in women after smelling pure androstadienone (4,16-androstadien-3-one), a molecule present in the sweat of men that has been suggested as a chemosignal in humans. We found that merely smelling androstadienone maintained significantly higher levels of the hormone cortisol in women. These results suggest that, like rodents, humans can influence the hormonal balance of conspecifics through chemosignals. Critically, this study identified a single component of sweat, androstadienone, as capable of exerting such influence. This result points to a potential role for synthetic human chemosignals in clinical applications.

Olfactomotor activity during imagery mimics that during perception

Neural representations created in the absence of external sensory stimuli are referred to as imagery, and such representations may be augmented by reenactment of sensorimotor processes. We measured nasal airflow in human subjects while they imagined sights, sounds and smells, and only during olfactory imagery did subjects spontaneously enact the motor component of olfaction—that is, they sniffed. Moreover, as in perception, imagery of pleasant odors involved larger sniffs than imagery of unpleasant odors, suggesting that the act of sniffing has a functional role in creating of olfactory percepts.

Brain Mechanisms for Extracting Spatial Information from Smell

Forty years ago, von Békésy demonstrated that the spatial source of an odorant is determined by comparing input across nostrils, but it is unknown how this comparison is effected in the brain. To address this, we delivered odorants to the left or right of the nose, and contrasted olfactory left versus right localization with olfactory identification during brain imaging. We found nostril-specific responses in primary olfactory cortex that were predictive of the accuracy of left versus right localization, thus providing a neural substrate for the behavior described by von Békésy. Additionally, left versus right localization preferentially engaged a portion of the superior temporal gyrus previously implicated in visual and auditory localization, suggesting that localization information extracted from smell was then processed in a convergent brain system for spatial representation of multisensory inputs.

Olfaction: The world smells different to each nostril

The flow of air is greater into one nostril than into the other because there is a slight turbinate swelling in one. The nostril that takes in more air switches from the left to the right one and back again every few hours, but the effect of this switching on the sense of smell has been unclear. Here we show that this difference in airflow between the nostrils causes each nostril to be optimally sensitized to different odorants, so that each nostril conveys a slightly different olfactory image to the brain.

The world smells different to each nostril.

The flow of air is greater into one nostril than into the other because there is a slight turbinate swelling in one. The nostril that takes in more air switches from the left to the right one and back again every few hours, but the effect of this switching on the sense of smell has been unclear. Here we show that this difference in airflow between the nostrils causes each nostril to be optimally sensitized to different odorants, so that each nostril conveys a slightly different olfactory image to the brain.

Sniffing human sex-steroid derived compounds modulates mood, memory and autonomic nervous system function in specific behavioral contexts

We asked whether the effects of exposure to two human sex-steroid derived compounds were context dependent. The effects of sniffing 4,16-androstadien-3-one (AND) and 1,3,5(10),16-estratetraen-3-ol (EST) on mood, memory, and autonomic nervous system responses were explored in 72 participants. Subjects were tested with AND, EST, or a Control compound within four mood contexts: neutral, sexually aroused, sad and happy. These moods were successfully induced using selected film segments (P < 0.0001). During the neutral context, none of the compounds affected mood or autonomic nervous system function. However, compound effects were significantly increased within arousing contexts. During the sexually arousing context, both compounds increased sexual arousal (P < 0.029). During the sad context, AND maintained positive mood in women (P< 0.050) and increased negative mood in men (P < 0.031). Memory for events during the sad context was impaired by AND in women (P < 0.047) but not in men. Finally, effects of AND on physiology were observed during the sexually arousing context whereby AND increased skin temperature in both sexes (P < 0.022) but reduced abdominal respiration rate in men only (P < 0.034). These results suggest that sex-steroidal compounds modulate mood, memory and autonomic nervous system responses and increase their significance within specific behavioral contexts. These findings lend support to a specific role for these compounds in chemical communication between humans.