Graham Bell

Odour mixture suppression: evidence for a peripheral mechanism in human and rat

Rarely do we encounter a single odorant in our environment. Perception of odours, therefore, usually depends on the reception and neural processing of many components. However, little is known about how and where odour mixtures are processed. Evidence is presented here that suppression of one odour by another, a common result of mixing odours, is primarily a peripheral event. Having demonstrated with human subjects that perception of one or both odorants in two-component mixtures is dependent on the polarity and perceived intensity of the odorants, the same mixtures were presented to rats that had been injected with a metabolic marker, [3H]2-deoxyglucose (2-DG). By measuring the metabolic activity in the glomeruli of the rat olfactory bulb, where the axons of the receptor cells terminate, it was found that in a mixture where humans had perceived only one odour, there is a dramatic reduction in metabolic activity of glomeruli specific to the suppressed odour. In mixtures where both odorants were perceived, metabolic activity characteristic of both components was observed. These findings indicate that similar mechanisms underlie the perception of odour mixtures in the two species. Since metabolic activity revealed by 2-DG in glomeruli occurs predominantly in presynaptic receptor axons, the reduced activity seen after stimulation with odour mixtures indicates that a mechanism for mixture suppression begins at the receptor cells. Therefore, the ability of one odorant to suppress another in a mixture is probably determined by their relative chemical polarities, which effects access to and competition for membrane receptor sites in the olfactory epithelium.

Cross-cultural determinants of food acceptability: Recent research on sensory perceptions and preferences

Abstract Studies of cross-cultural chemosensory perceptions and preferences are examined from the point of view of their ability to explain differences in food selection in different cultures. It is unclear from the limited literature whether psychophysical judgements of taste or of other sensory qualities of foods differ cross-culturally. However, preference for these same qualities appears to be dependent on the context in which they are experienced, and thus cultural preference differences are evident, most probably as a function of the different dietary experiences of different cultures.

Detection of propionic acid vapor by rats with lesions of olfactory bulb areas associated with high 2-DG uptake

This study tested the functional significance of recent 2-deoxyglucose (2-DG) results demonstrating a discrete focus of activity in the olfactory bulb of rats exposed to the vapor of propionic acid. Rats with lesions that destroyed this area of the olfactory bulb performed as well as did sham operated and lesioned controls in detection of amyl acetate, butanol, geraniol, propionic acid, and on a test for propionic acid threshold. Our results demonstrate that an area of the olfactory bulb containing a major focus of metabolic activity induced by exposure to an odor can be removed without producing a deficit in the detection of that odor.

Detection and discrimination of propionic acid after removal of its 2-DG identified major focus in the olfactory bulb: a psychophysical analysis.

Prior 2-deoxyglucose and c-fos studies have demonstrated increased metabolic activity in a rostral dorsomedial area of the olfactory bulb in response to the vapor of propionic acid. We used psychophysical tests to assess the effect of removing this area of the bulb on odor sensitivity and discrimination. Normal rats, those with lesions of the rostral dorsomedial bulb or with control lesions of the lateral olfactory bulb were tested for propionic acid absolute detection and intensity difference thresholds and ability to discriminate propionic acid from other odors. There were no differences among groups for absolute or intensity difference threshold or on simple 2-odor discrimination tests but both groups with bulbar lesions made more errors than controls on a relatively difficult odor-mixture task. The results demonstrate that removal of an area of the bulb identified as responsive to propionic acid is essentially without effect on sensitivity to that odor or ability to discriminate it from other odors.

Cross-cultural comparisons of Japanese and Australian responses to manipulations of sourness, saltiness and bitterness in foods

Panels of Japanese and Australian consumers evaluated four foods in which tastant levels had been manipulated to produce four samples of each food. The foods (and tastants) were: orange juice (citric acid); grapefruit juice (caffeine); salad dressing (citric acid); and cornflakes (sodium chloride). The panels gave ratings of intensity, liking, and just right for the manipulated taste, and overall liking for all samples. In addition, a variety of other sensory attributes were rated. There were no cross-cultural differences in the perception of the manipulated taste intensity in any of the foods. For hedonic ratings, the patterns of response to increasing taste intensities varied according to food and culture. However, there was good cross-cultural agreement regarding the optimal level of tastant for each food. Multiple regressions revealed a common core of attributes that explained the variance of overall liking, although, for each food, there were differences between the Japanese and Australians in which additional attributes contributed to the best fit models.

Odor-induced metabolic activity in the olfactory bulb of rats trained to detect propionic acid vapor

The pattern of glucose metabolism in the glomerular layer of the main olfactory bulb was studied in rats trained to sample brief odor stimuli. After injection with [14C]2-deoxyglucose (2-DG). 5 rats were tested for discrimination of propionic acid from air. Over the 45 min test period rats sampled the stimulus for 0.5-0.8s during each trial and their total exposure to the stimulus was 53-147 s. A discrete focus of increased glucose metabolism was found in the central dorsomedial sector of the glomerular layer in each animal. The position of this focus and the overall pattern of glomerular layer activity was essentially identical to that obtained in 4 control rats which were exposed passively to alternating 5 min periods of the odor and clean air for 45 min. The size of the primary focus was only slightly smaller in the trained rats, despite the large difference in total exposure time. The absence of olfactory adaptation during the behavioral tests and the similar pattern of 2-DG uptake in controls and trained animals indicate that adaptation does not play a significant role in the patterns of glucose metabolism induced in the glomeruli of the olfactory bulb by extended exposure to an odor.

Seasonal total methane depletion in limestone caves

Methane concentration in caves is commonly much lower than the external atmosphere, yet the cave CH4 depletion causal mechanism is contested and dynamic links to external diurnal and seasonal temperature cycles unknown. Here, we report a continuous 3-year record of cave methane and other trace gases in Jenolan Caves, Australia which shows a seasonal cycle of extreme CH4 depletion, from ambient ~1,775 ppb to near zero during summer and to ~800 ppb in winter. Methanotrophic bacteria, some newly-discovered, rapidly consume methane on cave surfaces and in external karst soils with lifetimes in the cave of a few hours. Extreme bacterial selection due to the absence of alternate carbon sources for growth in the cave environment has resulted in an extremely high proportion 2–12% of methanotrophs in the total bacteria present. Unexpected seasonal bias in our cave CH4 depletion record is explained by a three-step process involving methanotrophy in aerobic karst soil above the cave, summer transport of soil-gas into the cave through epikarst, followed by further cave CH4 depletion. Disentangling cause and effect of cave gas variations by tracing sources and sinks has identified seasonal speleothem growth bias, with implied palaeo-climate record bias.

Early-Stage Processing of Odor Mixtures

Little is known about how the components of odor mixtures interact to produce the perceived quality and intensity of the mixture. We have shown recently that neural metabolic responses to odors, measured with the radioactive 2-deoxyglucose (2-DG) technique, can be used to test interrelationships between odor mixture components.’ Metabolic activity was suppressed in reliably identifiable patches of glomeruli in the rat main olfactory bulb, when the animals were exposed to a two-component mixture at concentrations at which human subjects had perceived masking of one of

Neural Network Processing of Responses to Odorants by a Biological Nose and a Sensor Array

A neural network is a processing device, either an algorithm or actual hardware, whose design was motivated by biological neural functions. It can be trained to operate as a classifier. The application of neural networks to the study of olfactory processing in vivo and to identifying and classifying complex chemical mixtures from the outputs of chemical sensor arrays is the subject of this study.

Cross-Cultural Studies of Japanese and Australian Taste Preferences

Despite major differences between the diets of Japan and western countries such as Australia, the extent to which the taste perceptions and preferences of Japanese and Australians differ is uncertain. Collaborative research between the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Sensory Research Centre (Australia) and Chuo University (Japan) has investigated cross-cultural taste perceptions and preferences, using a range of tastes and food types.