D.G. Laing

The capacity of humans to identify odors in mixtures

One hundred and twenty-three subjects were given the task of identifying the constituents of stimuli consisting of 1-5 odorants. The highest level of identification occurred with single odors and few subjects correctly identified the constituents of mixtures. Since the stimuli were common, dissimilar odors, the results suggest that the capacity of humans to process information about odors perceived simultaneously may be limited, or that odors in mixtures blend to form a new odor with few of the characteristics of the constituent odors.

Natural Sniffing Gives Optimum Odour Perception for Humans

The characteristics of human sniffing episodes during odour perception have been described in an earlier paper, where it has been suggested that the techniques used by individuals may be close to those providing optimum perception. To investigate this suggestion, threshold and intensity tests with butanol, cyclohexanone, and pentyl acetate have been carried out on twenty-one subjects. Olfactory responses obtained by using natural sniffing techniques were compared with those where the number of sniffs, interval between sniffs, and size of sniffs were varied. The results indicate that it is very difficult to improve on the efficiency of sniffing techniques of individuals and that a single natural sniff provides as much information about the presence and intensity of an odour as do seven or more sniffs. A single natural sniff and the first sniff of a a natural sniffing episode were shown to have similar characteristics and most significantly both were unaffected by changes in the concentration and type of odour. Overall, the results indicate that humans achieve optimum odour perception during threshold and intensity measures with their natural multiple-sniff technique or with a single sniff. For the ‘average’ human this occurs with a sniff of inhalation rate 30 1 min−1, volume 200 cm3, and duration 0.4 s. The use of several sniffs in a sniffing episode appears to be a confirmatory action rather than a necessary one, except for the perception of odour mixtures where several sniffs are likely to be needed to aid discrimination of the components. Data from the present and earlier study provide the information necessary for the development of a standard olfactometer and standard procedures for measuring the olfactory responses of humans.

Quality and intensity of binary odor mixtures

The perceived intensity and quality of binary mixtures consisting of the common but dissimilar odorants, benzaldehyde (almond), eugenol (cloves), propionic acid (vinegar) and (-)-carvone (spearmint), were determined over a wide range of environmentally relevant concentrations. Consistent data were obtained from each of the odor pairs. This showed that the intensity of odorants not their quality determined the contribution of each to the quality of a mixture. Except for the pair carvone-propionic acid, both odorants were perceived only when the difference between their intensities was small. The results confirmed an earlier finding that the total intensity of a mixture is less than the sum of the intensities of the two components, and also showed that it is never less than the intensity of the weaker component. Seven major characteristics of binary odor mixtures are defined.

Discrimination of odors from stressed rats by non-stressed rats.

Abstract Comparison of earlier reports of rat stress odors is complicated by the many differences in experimental parameters and responses measured. To evaluate whether these stress odors provide a special signal, rats were subjected to different levels of stressful foot-shock in one half of a simple two-compartment test box whilst the other half was clean and unoccupied. The results show that whilst test subjects preferred the half containing odors from non-stressed rats, this preference was decreased by the presence of stress odors to an extent concordant with the level of stressor applied to the odor donors. There were no differences in plasma corticosterone among the odor donors indicating that this hormone is probably not the source of stress odors. Plasma corticosterone levels of the subjects were similar to each other and to the odor donors. Compared to odors from non-stressed rats, stress odors increased the activity of the subjects. The evidence strongly suggests a special signalling function for stress odors although responses to this signal are not stereotyped.

Identification of single dissimilar odors is achieved by humans with a single sniff

The duration that a single odor needs to be sniffed for identification was determined for 18 humans. A hot wire anemometer and an oscilloscope were used to monitor the duration, volume and inhalation rate of sniffs. In Experiment 1 subjects used 1, 3 or 5 natural sniffs, or an unlimited number of natural sniffs to sample seven dissimilar single odors of moderate perceived intensity, and demonstrated that each odor could be identified with a single sniff. In Experiment 2 subjects demonstrated that each of the odors could be identified with the shortest sniff (0.42 sec) they could physically achieve. In Experiment 3 tests with two of the odorants at several concentrations showed that sniff duration influences identification over a narrow range of concentrations that is just above the recognition threshold. These results together with earlier data that described the optimum conditions for the detection of an odor and the perception of odor intensity, provide information that is necessary for the development of a standard olfactometer and standard methods for human olfactory measurements.

Odor masking in the rat

A computer controlled behavioral apparatus and olfactometer were used to determine the olfactory threshold of rats for propionic acid and to determine their ability to discriminate propionic acid in binary mixtures that contained the dissimilar odorants, limonene or carvone, or a similar odorant, acetic acid. The concentration of propionic acid that rats required to discriminate this substance from the other odorants was substantially above that of the threshold concentration, the increase being greatest with acetic acid. The increased levels of propionic acid needed for detection of this odorant in mixtures is attributed to masking by the other components. This study represents the first time behavioral methods have been used to demonstrate masking with odor mixtures in a rodent. The results indicate that it should be possible to use behavioral methods to define conditions for odor masking in animals so that physiological studies of the underlying mechanisms can be achieved.

The sources of odors from stressed rats.

Abstract Recent studies [8,9] have shown that odors from stressed Norway rats act as signals to which other rats respond primarily by overall changes in activity and exploration. At present the source of these odors is unknown. In this study odors from urine, feces and the bodies of stressed rats were delivered along a runway in which the subjects had been previously trained to run for a water reward in the presence of odors from non-stressed rats. The results indicate that odors are released from the body surface and in the urine but not the feces of stressed rats.

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.

Characterisation of Human Behaviour during Odour Perception

Factors which govern the optimum perception of odours have not yet been defined. This has hindered the development of standard methods and instruments for measuring olfactory responses. As an initial step towards defining these conditions, inhalation rates and volumes, number of sniffs, and sniff duration were measured for twenty-three humans in odour-threshold and odour-intensity tests with pentyl acetate, 1-butanol, and diethylamine. Measurements were made with the aid of a hot-wire anemometer concealed within the outlet of an air-dilution olfactometer. Individuals varied markedly in their sampling techniques but maintained their characteristic sniffing patterns with different odours and olfactory tasks. Only three parameters consistently varied with odour concentration: total volume of odour sampled, total sampling time, and number of sniffs. Maximum inhalation rate was remarkably stable and was independent of the type, concentration, and pleasantness of odour. Values recorded for sniff volumes and inhalation rates indicate that most olfactometers in use do not accommodate human inhalation requirements during a sniffing episode. The many common characteristics in the varied sampling techniques of different subjects suggest that the techniques are close to those providing optimum odour perception. Whether these are inherited or developed through habit or experience is not known.

An investigation of the mechanisms of odor suppression using physical and dichorhinic mixtures.

Psychophysical procedures were used to investigate the effects observed by humans when two odorants were presented simultaneously through the same nostril (physical mixture) or separate nostrils (dichorhinic mixture). The odor pairs were (+)-limonene and alpha-pinene, (+)-limonene and propionic acid, alpha-pinene and propionic acid. With both types of mixtures subjects indicated the acid had no effect on the perception of the other odorants. In contrast, limonene suppressed perception of the acid and pinene in physical and dichorhinic mixtures, pinene suppressed limonene in both types of mixtures but only suppressed the acid in physical mixtures. The magnitude of suppression with dichorhinic mixtures never exceeded that seen in physical mixtures, whilst suppression with physical mixtures was often significantly greater than in dichorhinic mixtures. The absence of suppression in dichorhinic mixtures in almost one-third of instances when suppression occurred with physical mixtures suggests that different mechanisms are involved in the perception of the two types of mixtures. A hypothesis that accounts for non-reciprocal suppression with both types of mixtures is proposed.