Odor stimulus

Masking. Masking can be defined as the reduction of olfactory perception of a defined odor stimulus by means of presentation of another odorous substance without the physical removal or chemical alteration of the defined stimulus from the environment. Masking is therefore hyperadditive it raises the total odor level, possibly creating an overpowering sensation, and maybe defined as a reodorant, rather than a deodorant. Its end result can be explained by the simple equation of 1 + 1 = >2 (Fig. 2a). 

The denomination of odors was schematically related to two separate domains, both related to the memory stimulus of an event concomitant with the perception of the odor. One domain was based on an actual reference point that contains the odor vectors the other was associated with an odor stimulus based on imagination, ie, what image is evoked by the stimulus. With such a system, the final descriptive terminology used would more often than not be expressed in esoteric language, causing confusion and even communication breakdown. The work of Jaubert (1) was the origin of a more standardized descriptive system in the field of aroma description. 

Different odor substances stimulate different patterns of ORCs in the olfactory epithelium, owing to the different sensitivity spectra of the ORCs (28). The pattern of activity in the epithelium evoked by a particular odor substance constitutes the first molecular image of that stimulus, which represents the determinants of the stimulating molecules (13). Thus, although olfaction is not a spatial sensory modality, in contrast, for example, to vision and somatosensation, the initial representation of an odor stimulus in the olfactory pathway does have spatial structure. 

Our neurophysiological studies have focused on three important properties of the sex-pheromonal signal its quality (chemical composition of the blend), quantity (concentrations of components), and intermit-tency [owing to the fact that the pheromone in the plume downwind from the source exists in filaments and blobs of odor-bearing air interspersed with clean air (47, 48)]. Each of these properties of the pheromonal message is important, as the male moth gives his characteristic behavioral responses only when the necessary and sufficient pheromone components A and B are present in the blend (44), when the concentrations and blend proportions of the components fall within acceptable ranges (49), and when the pheromone blend stimulates his antennae intermittently (39, 50). In our studies, we examine how each of these important aspects of the odor stimulus affects the activity of neurons at various levels in the olfactory pathway. 

To be ready for the next odor stimulus, jS-adrenoceptor kinase (Bark) 2 inactivates a receptor only a tenth of a second after the first stimulation occurred. This kinase phosphorylates the activated receptor, which allows another protein, B-arrestin, to bind to the receptor and inactivate it. This is a specific example of a group of enzymes that deactivate hormone or neuroreceptors (Dawson et al., 1993). 

The best investigated odor-taste interactions occur in conditioned flavor aversions. Tastes that precede a delayed food-related illness are often avoided after only one experience. Odors are not avoided under similar conditions. However, if taste and odor are presented together before the malaise, animals will avoid odor when encountered later by itself. Taste affects odor, but not vice versa. If only the taste intensity is increased, both taste and odor aversion increase. Conversely, if only the odor stimulus is increased, only the odor aversion increases (Garcia etal, 1986). 

PLS is best described in matrix notation where the matrix X represents the calibration matrix (the training set, here physicochemical parameters) and Y represents the test matrix (the validation set, here the coordinates of the odor stimulus space). If there are n stimuli, p physicochemical parameters, and m dimensions of the stimulus space, the equations in Figure 6a apply. The C matrix is an m x p coefficient matrix to be determined and the residuals not explained by the model are contained in E. The X matrix is decomposed as shown in Figure 6b into two small matrices, an n x a matrix T and an a x p matrix B where a << n and a << p. F is the error matrix. The computation of T is such that it both models X and correlates with T and is accomplished with a weight matrix W and a set of latent variables U for Y with a corresponding loading matrix B. ... 

How does the brain sense the activation of a defined subset of olfactory receptor neurons In the mouse, all neurons expressing a given OR extend axons that synapse in two glomeruli in each olfactory bulb (Ressler el al., 1994 Vassar et al., 1994 Mombaerts et al., 1996). Therefore activation of a given complement of ORs in the periphery will be represented in the olfactory bulb by the specific activation of a subset of glomeruli. This spatial map may be interpreted by higher brain centers to yield information about the nature and concentration of the odorous stimulus. 

This exercise, especially Part 3, focuses on observing behavior. In the time available, the observed reactions to the odor stimulus will most likely not be frequent enough to lend themselves to statistical analysis (Data Sheet 9.1). 

In this paper we have proposed a simplified model of the insect AL in order to explore the neural code in olfaction. A possible role of the AL is to transform a multidimensional input vector representing the odorant stimulus into a spatio-temporal code given by a sequence of quasi-synchronized assemblies of PNs, in which each PN is individually phase-locked to the LFP. 

As mentioned above, the concentration of an odor stimulus is a key physical feature of naturally occurring odors. The concentration of an odor may vary widely... 

Fig. 8.6 Generalization from one odor stimulus to another increases as a function of variation in odors. Generalization to low concentration odorants is also affected by the intensity of the conditioning mixture. When honeybees were conditioned with high intensity mixtures (2.0 and 2.1 M) they responded less than honeybees conditioned with low intensity mixtures (0.03 and 0.06 M). Honeybees conditioned with highly variable mixtures (var) responded with a higher probability than honeybees conditioned with no variation present in the mixture (const). Reproduced from Wright and Smith, Ihoceedings of the Royal Society B, 2004, vol. 271, iss. 1535, pp. 147-152, with the permission of the Royal Society of London.

Properly synchronization between odour stimulation and imaging was enabled. Data acquisition was delayed 150ms after odour delivery. It was done so, as a tradeoff between the time of response to an odorant stimulus (described to be around 100ms) and also the number of frames available. The odorant pulses were delivered to the left nasal cavity of the frog, through a valve-operated system, in 60 ms air pulses. 

In olfaction, a transition from sea to land means that molecules need to be detected in gas phase instead of water solution. The odor stimulus also changes from mainly hydrophilic molecules in aqueous solution to mainly hydrophobic in the gaseous phase (discussed in Stensmyr et al. 2005). The olfactory system is also, like the rest of the organism, very prone to desiccation and mechanical abrasion in the terrestrial environment. All these new selection pressures take part in reshaping the sense of smell. Behavioral studies have provided evidence that some land-living crustaceans are very effective in detecting food from a distance and in responding to airborne odors, in short, that they have evolved an excellent sense of distance... 

The discriminatory capacity of the mammalian olfactory system is such that thousands of volatile chemicals are perceived as having distinct odors. It is accepted that the sensation of odor is triggered by highly complex mixtures of volatile molecules, mostly hydrophobic, and usually occurring in trace-level concentrations (ppm or ppb). These volatiles interact with odorant receptors of the olfactive epithelium located in the nasal cavity. Once the receptor is activated, a cascade of events is triggered to transform the chemical-structural information contained in the odorous stimulus into a membrane potential [58,59], which is projected to the olfactory bulb and then transported to higher regions of the brain [60] where the translation occurs. 

A serious drawback of naturalistic observations and simple one- and two-choice tests is that these methods depend on untrained responding. Simply because an animal fails to respond is not evidence that it has failed to detect a stimulus. For this reason, training procedures have been developed. Typically, these methods employ classical or operant conditioning. With the former, the presentation of a stimulus is paired with the delivery of a reinforcer. For example, presentation of an odor is paired with the delivery of electric shock. The shock leads to an unconditioned increase in heart rate or a measurable change in some other physiological response. After several pairings, presentations of odor alone lead to conditional or conditioned increases in heart rate. The concentration of the odor stimulus can then be manipulated, and questions can be asked about minimum detectable concentrations etc. Not infrequently, greater stimulus sensitivities are measured when trained animals are used than when data are collected from untrained animals in simple choice tests. 

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