Changes In Odor Concentration Research Articles

Multielectrode recordings of cockroach antennal lobe neurons in response to temporal dynamics of odor concentrations

The initial representation of the instantaneous temporal information about food odor concentration in the primary olfactory center, the antennal lobe, was examined by simultaneously recording the activity of antagonistic ON and OFF neurons with 4-channel tetrodes. During presentation of pulse-like concentration changes, ON neurons encode the rapid concentration increase at pulse onset and the pulse duration, and OFF neurons the rapid concentration decrease at pulse offset and the duration of the pulse interval. A group of ON neurons establish a concentration-invariant representation of odor pulses. The responses of ON and OFF neurons to oscillating changes in odor concentration are determined by the rate of change in dependence on the duration of the oscillation period. By adjusting sensitivity for fluctuating concentrations, these neurons improve the representation of the rate of the changing concentration. In other ON and OFF neurons, the response to changing concentrations is invariant to large variations in the rate of change due to variations in the oscillation period, facilitating odor identification in the antennal-lobe. The independent processing of odor identity and the temporal dynamics of odor concentration may speed up processing time and improve behavioral performance associated with plume tracking, especially when the air is not moving.

Changes in odorants and flavor profile of heat-processed beef flavor during storage.

Flavor quality changes of heat-processed beef flavor (HPBF) upon storage are greatly affected by storage conditions. Flavor characterization of HPBF stored for 168 days resulted in 17 key odor-active compounds having flavor dilution factors log3 FD ≥ 3 and odor activity values >1. Based on the concentration changes of these key odorants, a total of 13 HPBF samples were divided into four groups by hierarchical cluster analysis. Combined with flavor profile analysis, the overall flavor of HPBF gradually changed from a characteristic beef aroma to burnt and fermented soybean paste-like notes. Some of the 17 key odorants, such as 2-methyl-3-furanthiol, 2-furfurylthiol, methyl furfuryl disulfide, and difurfuryl disulfide, contributed to the characteristic beef aroma of the freshly prepared product and in the early stage of storage. On the other hand, the odor-active compounds 2-methylbutanal, dimethyl disulfide, dimethyl trisulfide, and 2-ethyl-3,5-dimethylpyrazine were associated with the later stage of storage showing a positive correlation with the intensity of burnt and fermented soybean paste-like notes. A molecular understanding of flavor profile changes of HPBF during storage will provide the basis for developing characteristic beef aroma notes and maintaining the keeping quality of beef products upon storage. PRACTICAL APPLICATION: The relationship between sensory quality and changes in odorant concentration of heat-processed beef flavor during storage will provide the molecular basis for developing high-quality thermal reaction meat flavorings.

Transduction and Adaptation Mechanisms in the Cilium or Microvilli of Photoreceptors and Olfactory Receptors From Insects to Humans.

Sensing changes in the environment is crucial for survival. Animals from invertebrates to vertebrates use both visual and olfactory stimuli to direct survival behaviors including identification of food sources, finding mates, and predator avoidance. In primary sensory neurons there are signal transduction mechanisms that convert chemical or light signals into an electrical response through ligand binding or photoactivation of a receptor, that can be propagated to the olfactory and visual centers of the brain to create a perception of the odor and visual landscapes surrounding us. The fundamental principles of olfactory and phototransduction pathways within vertebrates are somewhat analogous. Signal transduction in both systems takes place in the ciliary sub-compartments of the sensory cells and relies upon the activation of G protein-coupled receptors (GPCRs) to close cyclic nucleotide-gated (CNG) cation channels in photoreceptors to produce a hyperpolarization of the cell, or in olfactory sensory neurons open CNG channels to produce a depolarization. However, while invertebrate phototransduction also involves GPCRs, invertebrate photoreceptors can be either ciliary and/or microvillar with hyperpolarizing and depolarizing responses to light, respectively. Moreover, olfactory transduction in invertebrates may be a mixture of metabotropic G protein and ionotropic signaling pathways. This review will highlight differences of the visual and olfactory transduction mechanisms between vertebrates and invertebrates, focusing on the implications to the gain of the transduction processes, and how they are modulated to allow detection of small changes in odor concentration and light intensity over a wide range of background stimulus levels.

The Performance of Olfactory Receptor Neurons: The Rate of Concentration Change Indicates Functional Specializations in the Cockroach Peripheral Olfactory System.

Slow and continuous changes in odor concentration were used as a possible easy method for measuring the effect of the instantaneous concentration and the rate of concentration change on the activity of the olfactory receptor neurons (ORNs) of basiconic sensilla on the cockroach antennae. During oscillating concentration changes, impulse frequency increased with rising instantaneous concentration and this increase was stronger the faster concentration rose through the higher concentration values. The effect of the concentration rate on the ORNs responses to the instantaneous concentration was invariant to the duration of the oscillation period: shallow concentration waves provided by long periods elicited the same response to the instantaneous concentration as steep concentration waves at brief periods. Thus, the double dependence remained unchanged when the range of concentration rates varied. This distinguishes the ORNs of basiconic sensilla from those of trichoid sensilla (Tichy and Hellwig, 2018) which adjust their gain of response according to the duration of the oscillating period. The precision of the ORNs to discriminate increments of slowly rising odor concentration was studied by applying gradual ramp-like concentration changes at different rates. While the ORNs of the trichoid sensilla perform better the slower the concentration rate, those of the basiconic sensilla show no preference for a specific rate of concentration increase. This suggests that the two types of sensilla have different functions. The ORNs of the trichoid sensilla may predominately analyze temporal features of the odor signal and the ORNs of the basiconic sensilla may be involved in extracting information on the identity of the odor source instead of mediating the spatial-temporal concentration pattern in an odor plume.

Developing and testing of an air dilution flow olfactometer with known rates of concentration change

BackgroundConcentration is a variable aspect of an odor signal and determines the operation range of olfactory receptor neurons (ORNs). A concentration increase is perceived as an odor stimulus. The role that the rate of concentration increase plays thereby has been studied with electrophysiological techniques in ORNs of the cockroach. A key prerequisite for these studies was the development of an air dilution flow olfactometer that allowed testing the same change in concentration at various rates. New methodThe rate of concentration change was controlled and varied by changing the mixing ratio of odor-saturated and clean air by means of proportional valves. Their input voltages were phase shifted by 180° to hold the mixed air at a particular constant volume flow rate. ResultsUsing this stimulation technique, we identified, in a morphologically distinct sensillum on the cockroach’s antenna, antagonistically responding ON and OFF ORNs which display a high sensitivity for slow changes in odor concentration. Comparison with existing methodsThe olfactometer is unique because it enables delivering slowly oscillating concentration changes. By varying the oscillation period, the individual effects of the instantaneous odor concentration and its rate of change on the ORNs’ responses can be determined. ConclusionsThe olfactometer provides a new experimental approach in the study of odor coding and opens the door for improved comparative studies on olfactory systems. It would be important to gain insight into the ORNs’ ability to detect the rate of concentration change in other insects that use odors for orientation in different contexts.

Odor Concentration Change Coding in the Olfactory Bulb.

Dynamical changes in the environment strongly impact our perception. Likewise, sensory systems preferentially represent stimulus changes, enhancing temporal contrast. In olfaction, odor concentration changes across consecutive inhalations (ΔCt) can guide odor source localization, yet the neural representation of ΔCt has not been studied in vertebrates. We have found that, in the mouse olfactory bulb, a subset of mitral/tufted (M/T) cells represents ΔCt, enhancing the contrast between different concentrations. These concentration change responses are direction selective: they respond either to increments or decrements of concentration, reminiscent of ON and OFF selectivity in the retina. This contrast enhancement scales with the magnitude, but not the duration of the concentration step. Further, ΔCt can be read out from the total spike count per sniff, unlike odor identity and intensity, which are represented by fast temporal spike patterns. Our results demonstrate that a subset of M/T cells represents ΔCt, providing a signal that may instruct navigational decisions in downstream olfactory circuits.

Implications for human odor sensing revealed from the statistics of odorant-receptor interactions.

Binding of odorants to olfactory receptors (ORs) elicits downstream chemical and neural signals, which are further processed to odor perception in the brain. Recently, Mainland and colleagues have measured more than 500 pairs of odorant-OR interaction by a high-throughput screening assay method, opening a new avenue to understanding the principles of human odor coding. Here, using a recently developed minimal model for OR activation kinetics, we characterize the statistics of OR activation by odorants in terms of three empirical parameters: the half-maximum effective concentration EC50, the efficacy, and the basal activity. While the data size of odorants is still limited, the statistics offer meaningful information on the breadth and optimality of the tuning of human ORs to odorants, and allow us to relate the three parameters with the microscopic rate constants and binding affinities that define the OR activation kinetics. Despite the stochastic nature of the response expected at individual OR-odorant level, we assess that the confluence of signals in a neuron released from the multitude of ORs is effectively free of noise and deterministic with respect to changes in odorant concentration. Thus, setting a threshold to the fraction of activated OR copy number for neural spiking binarizes the electrophysiological signal of olfactory sensory neuron, thereby making an information theoretic approach a viable tool in studying the principles of odor perception.

Computational modeling and experimental validation of odor detection behaviors of classically conditioned parasitic wasp, Microplitis croceipes.

A prototype chemical sensor named Wasp hound® that utilizes five classically conditioned parasitoid wasps, Microplitis croceipes (Cresson) (Hymenoptera: Braconidae), to detect volatile odors was successfully implemented in a previous study. To improve the odor-detecting ability of Wasp Hound®, searching behaviors of an individual wasp in a confined area are studied and modeled through stochastic differential equations in this paper. The wasps are conditioned to 20 mg of coffee when associated with food and subsequently, tested to 5, 10, 20, and 40 mg of coffee. A stochastic model is developed and validated based on three positive behavioral responses (walking, rotation around odor source, and self-rotation) from conditioned wasps at four different test dosages. The model is capable to reproducing the behaviors of conditioned wasps, and can be used to improve the ability of Wasp Hound® to assess changes in odor concentration. The model simulation results show the behaviors of conditioned wasps are significantly different when tested at different coffee dosages. We conjecture that the searching behaviors of conditioned wasps are based on the temporal and spatial neuron activity of olfactory receptor neurons and glomeruli, which are strongly correlated to the training dosages. The overall results demonstrate the utility of mathematical models for interpreting experimental observations, gaining novel insights into the dynamic behavior of classically conditioned wasps, as well as broadening the practical uses of Wasp Hound.

Range and Absolute Odour Intensity Affect Olfactory Generalisation in the Honeybee

The human olfactory psychophysical literature is rich with anecdotal reports of variation in the perceived quality between weak and strong concentrations of the same odour (Wilson and Stevenson 2006). Psychophysical experiments using animals have also found similar effects of concentration on odour quality. The proboscis extension reflex (PER) is an appetitive Pavlovian conditioning assay that has been used to investigate olfactory stimuli in the honeybee. In this series of experiments I aim to identify the sensitivity of honeybees to changes in odorant concentration across the range of a honeybee’s sensorium (0.01–100 Pa). I compared generalisation to test odours that differed in molecular identity, odour concentration or both and found that large changes in odorant concentration (1,000 fold change) can produce greater shifts in perceptual similarity than a change in the odorant’s molecular structure. Our findings suggest a failure in concentration invariance when identical odours differ greatly in concentration. I also found poorer olfactory sensitivity (between identical odours of differing concentration) and acuity (between novel odours of identical concentration) at low odour concentrations (0.01 Pa).

Frequency Transitions in Odor-Evoked Neural Oscillations

In many species, sensory stimuli elicit the oscillatory synchronization of groups of neurons. What determines the properties of these oscillations? In the olfactory system of the moth, we found that odors elicited oscillatory synchronization through a neural mechanism like that described in locust and Drosophila. During responses to long odor pulses, oscillations suddenly slowed as net olfactory receptor neuron (ORN) output decreased; thus, stimulus intensity appeared to determine oscillation frequency. However, changing the concentration of the odor had little effect upon oscillatory frequency. Our recordings in vivo and computational models based on these results suggested that the main effect of increasing odor concentration was to recruit additional, less well-tuned ORNs whose firing rates were tightly constrained by adaptation and saturation. Thus, in the periphery, concentration is encoded mainly by the size of the responsive ORN population, and oscillation frequency is set by the adaptation and saturation of this response.

Navigational Strategies Used by Insects to Find Distant, Wind-Borne Sources of Odor

Insects locate many resources important to survival by tracking along wind-borne odor plumes to their source. It is well known that plumes are patchy distributions of high concentration packets of odor interspersed with clean air, not smooth Gaussian distributions of odor intensity. This realization has been crucial to our understanding of plume-tracking behavior, because insect locomotory movements and sensory processing typically take place in the range of tens to hundreds of milliseconds, permitting them to respond to the rapid changes in odor concentration they experience in plumes. Because odor plumes are not comprised of smooth concentration gradients, they cannot provide the directional information necessary to allow plume-tracking insects to steer toward the source. Many experiments have shown that, in the species examined, successful source location requires two sensory inputs: the presence of the attractive odor and the detection of the direction of the wind bearing that odor. All plume-tracking insects use the wind direction as the primary directional cue that enables them to steer their movements toward the odor source. Experimental manipulations of the presence and absence of the odor, and the presence, absence, or direction of the wind during plume tracking, have begun to resolve the relationship between these two sensory inputs and how they shape the maneuvers we observe. Experiments, especially those undertaken in the natural wind and odor environments of the organisms in question and those directed at understanding the neural processing that underlie plume tracking, promise to enhance our understanding of this remarkable behavior.

Improvement of Robustness of Odor Classification in Dynamically Changing Concentration Against Environmental Change

In this paper, we propose a method for improving the robustness of odor classification against humidity change when the odor concentration changes dynamically. We apply a short-time Fourier transform (STFT) to sensor responses to obtain the frequency characteristics, and then employ a stepwise discriminant analysis to select the frequency components effective for the odor classification. We improve the classification performance by selecting the components robust against humidity change and combining them with humidity data. Using a learning vector quantization (LVQ) method, we successfully achieved high classification rate even if the odor concentration changed dynamically and irregularly at different humidity levels whereas the classification rate was insufficient in the case of using only magnitudes of sensor responses.

Adaptive regulation of sparseness by feedforward inhibition

In the mushroom body of insects, odors are represented by very few spikes in a small number of neurons, a highly efficient strategy known as sparse coding. Physiological studies of these neurons have shown that sparseness is maintained across thousand-fold changes in odor concentration. Using a realistic computational model, we propose that sparseness in the olfactory system is regulated by adaptive feedforward inhibition. When odor concentration changes, feedforward inhibition modulates the duration of the temporal window over which the mushroom body neurons may integrate excitatory presynaptic input. This simple adaptive mechanism could maintain the sparseness of sensory representations across wide ranges of stimulus conditions.

Odor-modulated orientation in walking male cockroachesPeriplaneta americana, and the effects of odor plumes of different structure

The location of distant resources using odor information usually also requires information on the flow of air (or water) through the environment together with the expression of internally programmed steering responses. The orientation responses of virgin male Periplaneta americana L. to wind and the female sex-pheromone component (-)-periplanone-B were video-recorded in a laboratory wind tunnel and quantified. P. americana males showed no preferred walking orientation when released in zero wind in the absence of pheromone. When introduced into 25 cm s(-1) wind in the absence of pheromone, 79% of males walked downwind. Upon introduction to a plume of (-)-periplanone-B in wind, 100% of males walked upwind in the pheromone plume to the source. Males were then challenged with wind-borne plumes of (-)-periplanone-B of four different temporal/spatial structures. In nearly all cases, the only statistically significant changes in behavioral parameters measured from their walking tracks were observed from males tracking the treatment consisting of the most turbulent plume. The plume-tracking performances of males challenged with the other three less turbulent plumes were visually and quantitatively similar, regardless of the width. Males tracking all four plumes showed evidence of turns resulting from multiple mechanisms. Some of the observed maneuvers were temporally regular counterturns, suggesting steering according to an ongoing internal program, while others could have been triggered by encountering the change in odor concentration between pheromone and clean air at the lateral boundary of the plume or chemotactically upon the detection of changes in odor concentration.

A measurement system for odor classification based on the dynamic response of QCM sensors

In this paper, an innovative measurement system for odor classification, based on quartz crystal microbalances (QCMs), is presented. The application proposed in this paper is the detection of typical wine aroma compounds in mixtures containing ethanol. In QCM sensors, the sensitive layer is, e.g., a polymeric layer deposited on a quartz surface. Chemical mixtures are sorbed in the sensitive layer, inducing a change in the polymer mass and, therefore, in the quartz resonance frequency. In this paper, the frequency shift is measured by a dedicated, fully digital front-end hardware implementing a technique that allows reducing the measurement time while maintaining a high-frequency resolution . The developed system allows, therefore, measuring variations of the QCM resonance frequency shifts during chemical transients obtained with abrupt changes in odor concentration. Hence, the reaction kinetics can be exploited to enhance the sensor selectivity. In this paper, some measurements obtained with an array of four sensors with different polymeric sensitive layers are presented. An exponential fitting of the transient responses is used for feature extraction. Finally, to reduce data dimensionality, principal component analysis is used.

Compensatory methods for the odor concentration in an electronic nose system using software and hardware

Compensatory methods for changes in odor concentration of a target sample in an electronic nose system have been investigated. An automatic sampler that automatically adjusts the odor concentration of the sample has been developed. The sampler consists of a mass flow controller, two electromagnetic valves and two stainless steel buffers equipped with two sensors for monitoring the odor concentration. The odor concentration of the sample is kept at a certain level by controlling the valves and the mass flow controller according to feed back signals of the monitoring sensors. Using this sampler, identification of unstable volatile samples was successfully performed, regardless of the state of the samples and the environmental conditions. A software-based compensatory method was also examined.

Odors Elicit Three Different Oscillations in the Turtle Olfactory Bulb

We measured the spatiotemporal aspects of the odor-induced population response in the turtle olfactory bulb using a voltage-sensitive dye, RH414, and a 464-element photodiode array. In contrast with previous studies of population activity using local field potential recordings, we distinguished four signals in the response. The one called DC covered almost the entire area of the olfactory bulb; in addition, three oscillations, named rostral, middle, and caudal according to their locations, occurred over broad regions of the bulb. In a typical odor-induced response, the DC signal appeared almost immediately after the start of the stimulus, followed by the middle oscillation, the rostral oscillation, and last, the caudal oscillation. The initial frequencies of the three oscillations were 14.1, 13.0, and 6.6 Hz, respectively. When the rostral and caudal oscillations occurred together, their frequencies differed by a factor of 1.99 +/- 0.01. The following evidence suggests that the four signals are functionally independent: (1) in different animals some signals could be easily detected whereas others were undetectable; (2) the four signals had different latencies and frequencies; (3) the signals occurred in different locations and propagated in different directions; (4) the signals responded differently to changes in odor concentration; (5) the signals had different shapes; and (6) the rostral and caudal signals added in a simple, linear manner in regions where the location of the two signals overlapped. However, the finding that the frequency of the rostral oscillation is precisely two times that of the caudal oscillation suggests a significant relationship between the two. The location of the caudal oscillation in the bulb changed from cycle to cycle, implying that different groups of neurons are active in different cycles. This result is consistent with the earlier findings in the olfactory system of the locust (). Our results suggest an additional complexity of parallel processing of olfactory input by multiple functional population domains.

Modular representations of odorants in the glomerular layer of the rat olfactory bulb and the effects of stimulus concentration

To study the mechanism whereby odorants are encoded in the nervous system, we studied the glomerular-layer activity patterns in the rat olfactory bulb evoked by closely related odorants from different chemical families. These odorants had a common straight-chain hydrocarbon structure, but differed systematically in their functional groups. Neural activity was mapped across the entire glomerular layer by using the ¿(14)C2-deoxyglucose method. Group responses were averaged and compared by using data matrices. The glomerular activity patterns that resulted from this analysis were comprised of modules. Unique combinations of modules were activated by each odorant, demonstrating what may be part of the neural code for odorants. Most of the modules were clustered together in the bulb, perhaps providing for enhanced contrast between related chemicals by means of lateral inhibition. We also determined whether changes in odorant concentration would affect spatial patterns of glomerular activity. Two odorants, pentanal and 2-hexanone, evoked different patterns at increased concentrations, with additional glomeruli being recruited at a great distance from glomeruli in which activity was evoked at lower concentrations. Humans report that both of these odorants change in perceived odor with increasing concentration. Three other odorants (pentanoic acid, methyl pentanoate, and pentanol) did not recruit new areas of glomerular activation with increasing concentration, and humans do not report a changed odor across concentrations of these odorants. The results suggest that changes in modular glomerular activity patterns could underlie altered odor perception across odorant concentrations, and they provide additional support for a combinatorial, spatially based code in the olfactory system.

Small‐scale fluid dynamics of olfactory antennae

A critical step in the process of olfaction is the arrival at the surface of a chemosensory structure of chemical signals from the environment. Many organisms use appendages bearing arrays of microscopic hairs to pick up chemical signals from the surrounding water or air. In this study a general model of fluid flow between neighboring hairs in an array is used to explore the effects of hair size, spacing, and speed on flow near olfactory hairs. If an array of hairs is moved more rapidly: 1) the volume flow rate through it rises, causing higher molecule encounter rates even though a smaller proportion of the odorants passing through the array have time to diffuse to hair surfaces, and 2) velocity gradients along hair surfaces become steeper, causing greater sensitivity to changes in odorant concentration. The more closely‐spaced the hairs, the less sensitive they are to these effects of changing speed.

Location by Olfaction: A Model and Application to the Mating Problem in the Deep-Sea Hatchetfish Argyropelecus hemigymnus

We develop a model for mate location by olfaction and apply it to reproductive behavior in the deep-sea hetchetfish Argyropelecus hemigymnus. The model includes diffusion theory to model the geometry of the region of detectable pheromone around the female, kinetic theory to medel the motion of the males prior to detection of a pheromone field, and a Monte Carlo search routine to estimate the time to locate the female after detection. The model yields results that are consistent with our present understanding of the biology of A. hemigymnus. The success of olfactory communication systems is strongly dependent on diffusion velocity, which is a function of depth in the ocean. The receiving organism must have low detection thresholds and high dynamic sensitivity to changes in odor concentration to use olfaction effectively for long-range detection and location of advertising females. Olfaction appears to be critical to reproductive success and therefore viability for many dispersed and sparse populations living in deep ocean environments.