Lavi Secundo

A Mechanistic Link between Olfaction and Autism Spectrum Disorder

Summary Internal action models (IAMs) are brain templates for sensory-motor coordination underlying diverse behaviors [1]. An emerging theory suggests that impaired IAMs are a common theme in autism spectrum disorder (ASD) [2–4]. However, whether impaired IAMs occur across sensory systems and how they relate to the major phenotype of ASD, namely impaired social communication [5], remains unclear. Olfaction relies on an IAM known as the sniff response, where sniff magnitude is automatically modulated to account for odor valence [6–12]. To test the failed IAM theory in olfaction, we precisely measured the non-verbal non-task-dependent sniff response concurrent with pleasant and unpleasant odors in 36 children—18 with ASD and 18 matched typically developing (TD) controls. We found that whereas TD children generated a typical adult-like sniff response within 305 ms of odor onset, ASD children had a profoundly altered sniff response, sniffing equally regardless of odor valance. This difference persisted despite equal reported odor perception and allowed for 81% correct ASD classification based on the sniff response alone (binomial, p < 0.001). Moreover, increasingly aberrant sniffing was associated with increasingly severe ASD (r = −0.75, p < 0.001), specifically with social (r = −0.72, p < 0.001), but not motor (r < −0.38, p > 0.18), impairment. These results uncover a novel ASD marker implying a mechanistic link between the underpinnings of olfaction and ASD and directly linking an impaired IAM with impaired social abilities.

The perceptual logic of smell.

Mammals have ∼1000 different olfactory receptor subtypes, each responding to a number of different odorants, and each odorant activating a number of different receptor subtypes. These molecular and anatomical underpinnings of olfaction imply a perceptual structure of very high dimensionality that relies on combinatorial coding. In contrast to this expectation, the study of olfactory perception reveals a structure of much lower dimensionality. Moreover, a low-dimensionality approach to olfaction enabled derivation of perception-based structural metrics for smell. These metrics provided meaningful predictions of odorant-induced neural activity and perception from odorant structure alone. Based on this low functional dimensionality, we speculate that olfaction likely does not functionally rely on 1000 different receptor subtypes, and their persistence in evolution may imply that they have additional roles in non-olfactory functions such as in guidance of embryogenesis and development.

Individual olfactory perception reveals meaningful nonolfactory genetic information

Significance Cyrano de Bergerac observed that “a large nose is the mark of a witty, courteous, affable, generous and liberal man.” Here we report that individual noses, not how they look but rather how they function, indeed say a lot about a person. Each person expresses a nearly unique set of different olfactory receptor genes, and therefore may have unique olfactory perception. We developed a highly sensitive perceptual test we call the “olfactory fingerprint” that captures this variability. Individual olfactory fingerprints are therefore mirrors of individual olfactory genomes. We demonstrate that such fingerprints predict genetic features linked to the olfactory system, such as aspects of immune regulation. Thus, a precise measure of olfactory perception reveals meaningful nonolfactory genetic information. Each person expresses a potentially unique subset of ∼400 different olfactory receptor subtypes. Given that the receptors we express partially determine the odors we smell, it follows that each person may have a unique nose; to capture this, we devised a sensitive test of olfactory perception we termed the “olfactory fingerprint.” Olfactory fingerprints relied on matrices of perceived odorant similarity derived from descriptors applied to the odorants. We initially fingerprinted 89 individuals using 28 odors and 54 descriptors. We found that each person had a unique olfactory fingerprint (P < 10−10), which was odor specific but descriptor independent. We could identify individuals from this pool using randomly selected sets of 7 odors and 11 descriptors alone. Extrapolating from this data, we determined that using 34 odors and 35 descriptors we could individually identify each of the 7 billion people on earth. Olfactory perception, however, fluctuates over time, calling into question our proposed perceptual readout of presumably stable genetic makeup. To test whether fingerprints remain informative despite this temporal fluctuation, building on the linkage between olfactory receptors and HLA, we hypothesized that olfactory perception may relate to HLA. We obtained olfactory fingerprints and HLA typing for 130 individuals, and found that olfactory fingerprint matching using only four odorants was significantly related to HLA matching (P < 10−4), such that olfactory fingerprints can save 32% of HLA tests in a population screen (P < 10−6). In conclusion, a precise measure of olfactory perception reveals meaningful nonolfactory genetic information.

Mirror Sniffing: Humans Mimic Olfactory Sampling Behavior

Ample evidence suggests that social chemosignaling plays a significant role in human behavior. Processing of odors and chemosignals depends on sniffing. Given this, we hypothesized that humans may have evolved an automatic mechanism driving sniffs in response to conspecific sniffing. To test this, we measured sniffing behavior of human subjects watching the movie Perfume, which contains many olfactory sniffing events. Despite the total absence of odor, observers sniffed when characters in the movie sniffed. Moreover, this effect was most pronounced in scenes where subjects heard the sniff but did not see the sniffed-at object. We liken this response to the orienting towards conspecific gaze in vision and argue that its robustness further highlights the significance of olfactory information processing in human behavior.

Odors enhance slow-wave activity in non-rapid eye movement sleep

Most forms of suprathreshold sensory stimulation perturb sleep. In contrast, presentation of pure olfactory or mild trigeminal odorants does not lead to behavioral or physiological arousal. In fact, some odors promote objective and subjective measures of sleep quality in humans and rodents. The brain mechanisms underlying these sleep-protective properties of olfaction remain unclear. Slow oscillations in the electroencephalogram (EEG) are a marker of deep sleep, and K complexes (KCs) are an EEG marker of cortical response to sensory interference. We therefore hypothesized that odorants presented during sleep will increase power in slow EEG oscillations. Moreover, given that odorants do not drive sleep interruption, we hypothesized that unlike other sensory stimuli odorants would not drive KCs. To test these hypotheses we used polysomnography to measure sleep in 34 healthy subjects (19 women, 15 men; mean age 26.5 ± 2.5 yr) who were repeatedly presented with odor stimuli via a computer-controlled air-dilution olfactometer over the course of a single night. Each participant was exposed to one of four odorants, lavender oil (n = 13), vetiver oil (n = 5), vanillin (n = 12), or ammonium sulfide (n = 4), for durations of 5, 10, and 20 s every 9-15 min. Consistent with our hypotheses, we found that odor presentation during sleep enhanced the power of delta (0.5-4 Hz) and slow spindle (9-12 Hz) frequencies during non-rapid eye movement sleep. The increase was proportionate to odor duration. In addition, odor presentation did not modulate the occurrence of KCs. These findings imply a sleep-promoting olfactory mechanism that may deepen sleep through driving increased slow-frequency oscillations.

Measuring and Characterizing the Human Nasal Cycle.

Nasal airflow is greater in one nostril than in the other because of transient asymmetric nasal passage obstruction by erectile tissue. The extent of obstruction alternates across nostrils with periodicity referred to as the nasal cycle. The nasal cycle is related to autonomic arousal and is indicative of asymmetry in brain function. Moreover, alterations in nasal cycle periodicity have been linked to various diseases. There is therefore need for a tool allowing continuous accurate measurement and recording of airflow in each nostril separately. Here we provide detailed instructions for constructing such a tool at minimal cost and effort. We demonstrate application of the tool in 33 right-handed healthy subjects, and derive several statistical measures for nasal cycle characterization. Using these measures applied to 24-hour recordings we observed that: 1: subjects spent slightly longer in left over right nostril dominance (left = 2.63 ± 0.89 hours, right = 2.17 ± 0.89 hours, t(32) = 2.07, p < 0.05), 2: cycle duration was shorter in wake than in sleep (wake = 2.02 ± 1.7 hours, sleep = 4.5 ± 1.7 hours, (t(30) = 5.73, p < 0.0001). 3: slower breathing was associated with a more powerful cycle (the extent of difference across nostrils) (r = 0.4, p < 0.0001), and 4: the cycle was influenced by body posture such that lying on one side was associated with greater flow in the contralateral nostril (p < 0.002). Finally, we provide evidence for an airflow cycle in each nostril alone. These results provide characterization of an easily obtained measure that may have diagnostic implications for neurological disease and cognitive state.

From Nose to Brain: Un-Sensed Electrical Currents Applied in the Nose Alter Activity in Deep Brain Structures

Rules linking patterns of olfactory receptor neuron activation in the nose to activity patterns in the brain and ensuing odor perception remain poorly understood. Artificially stimulating olfactory neurons with electrical currents and measuring ensuing perception may uncover these rules. We therefore inserted an electrode into the nose of 50 human volunteers and applied various currents for about an hour in each case. This induced assorted non-olfactory sensations but never once the perception of odor. To validate contact with the olfactory path, we used functional magnetic resonance imaging to measure resting-state brain activity in 18 subjects before and after un-sensed stimulation. We observed stimulation-induced neural decorrelation specifically in primary olfactory cortex, implying contact with the olfactory path. These results suggest that indiscriminate olfactory activation does not equate with odor perception. Moreover, this effort serendipitously uncovered a novel path for minimally invasive brain stimulation through the nose.

Sniffing patterns uncover implicit memory for undetected odors

Consciously undetected events are represented at the sensory-motor level and in the neurons of sensory-motor control, for example, consciously undetected visual targets drive eye movements [1] and neural activity [2]. Olfaction offers an opportunity to investigate processing of undetected stimuli through measurements of the sniff-response: odorant-specific modulations of nasal airflow [3-6]. Here, we report evidence that consciously undetected odorants modulate sniffing in a predicted manner. Moreover, in our study we observed that sniff-modulations recurred at least 10 seconds after the onset of an undetected odor, implying that information which was not consciously perceived was nevertheless maintained in memory, available for future decision making.

A Cross Modal Performance-Based Measure of Sensory Stimuli Intricacy

We define a new measure of sensory stimuli which has the following properties: It is cross modal, performance based, robust, and well defined. We interpret this measure as the intricacy or complexity of the stimuli, yet its validity is independent of its interpretation. We tested the validity and cross modality of our measure using three olfactory and one visual experiment. In order to test the link between our measure and cognitive performance we also conducted an additional visual experiment. We found that our measure is correlated with the results of the well-established Rapid Serial Visual Presentation masking experiment. Specifically, ranking stimuli according to our measure was correlated at r = 0.75 (p < 0.002) with masking effectiveness. Thus, our novel measure of sensory stimuli provides a new quantitative tool for the study of sensory processing.

Smelling Pseudomonas aeruginosa infections using a whole-cell biosensor – An alternative for the gold-standard culturing assay

Improved easy-to-use diagnostic tools for infections are in strong demand worldwide. Yet, despite dramatic advances in diagnostic technologies, the gold-standard remains culturing. Here we offer an alternative tool demonstrating that a bacterial biosensor can efficiently detect Pseudomonas aeruginosa infections in patients suffering from otitis externa. Detection was based on specific binding between the biosensor and 2-aminoacetophenone (2-AA), a volatile produced by P. aeruginosa in high amounts. We collected pus samples from ears of 26 subjects exhibiting symptoms of otitis externa. Detection of P. aeruginosa using the biosensor was compared to detection using gold-standard culturing assay and to gas-chromatograph-mass-spectrometry (GC-MS) analyses of 2-AA. The biosensor strain test matched the culture assay in 24 samples (92%) and the GC-MS analyses in 25 samples (96%). With this result in hand, we designed a device containing a whole-cell luminescent biosensor combined with a photo-multiplier tube. This device allowed detection of 2-AA at levels as low as 2 nmol, on par with detection level of GC-MS. The results of the described study demonstrate that the volatile 2-AA serves as an effective biomarker for P. aeruginosa in ear infections, and that activation of the biosensor strain by 2-AA provides a unique opportunity to design an easy-to-use device that can specifically detect P. aeruginosa infections.