A new method for measuring reaction times for odour detection at iso-intensity: Comparison between an unpleasant and pleasant odour
Introduction
The discrimination between “good” and bad” smells is one of the main functions of the human olfactory system. Bad smells act as a warning signal and require a rapid decision. They may also require a response, for example, avoidance or withdrawal. The same is not true for non-malodours. Malodours therefore have a greater immediate biological significance to our survival than non-malodours. In terms of the evolution of adaptation, it is important that organisms move away from “bad” stimuli and unfavourable situations and move towards “good” stimuli and situations favourable to their well-being [1]. In our recent study on the response to pleasant and unpleasant odours [2] we found that the olfactory system adapted/habituated more rapidly to malodours than to pleasant odours. At first this seemed counterintuitive, but there was a steep inverse relationship between adaptation and concentration of malodour such that if the system was exposed to a change (increase) in malodour strength, some adaptation was removed and the sensitivity recovered. On the other hand, there was very little change in the degree of adaptation with concentration of pleasant odours. This phenomenon has also been observed in the anterior piriform cortex by functional magnetic resonance imaging where activity was sustained in response to vanillin but decreased steadily over time with the malodour 4-methyl pentanoic acid [3]. The olfactory system therefore achieves a greater sensitivity to changes in malodour concentration than to changes in the concentration of pleasant smells. This has obvious survival advantages. Following from this we hypothesised that malodours would be detected more rapidly than pleasant smells at the same intensity and in this study we use a reaction time odour detection paradigm with intensity matching to test this hypothesis. On the basis of several biophysical characteristics (e.g. dose–response, stimulus frequency, sensitivity) we found that malodours, as a group, behaved similarly [2], so we selected valeric acid as a representative malodour and amyl acetate as a pleasant smell.
Section snippets
Olfactometry
Odour stimulation was achieved using an olfactometer as described previously [4]. Briefly, humidified (70%), warmed air was presented to the nostril at a flow rate of 3 l min− 1. Odorised air could be injected into the flow line (at 1 l min− 1), without altering the pressure or flow rate, by switching between a control odour reservoir (water), the amyl acetate reservoir or the valeric acid reservoir, using teflon-lined solenoid valves (Cole Palmer, Bishops Stortford, UK). A single tube, inserted
Results
The pooled reaction time data for all subjects, male and female, for amyl acetate (pleasant odour) and valeric acid (unpleasant odour) are given in Fig. 1(A) and (B) respectively. The ordinate represents the number of correct odour pulse detections per 200 ms time bin. Odour pulse widths ranged from 35 to 200 ms and are represented by different colours. It can be seen that with increasing pulse duration (increasing stimulus strength) there is a decreasing reaction time. Because the intensity of
Discussion
In a task designed to determine the time taken for the olfactory system to detect an odour, without hedonic judgement, we demonstrate that a malodour, valeric acid, is detected more rapidly than a pleasant odour, amyl acetate, across the range of stimulus strengths in spite of the fact that the olfactory system adapts more rapidly to malodours than pleasant odours [2]. The reaction time at the 50% detection level was 1.74 s for amyl acetate and 1.36 s for valeric acid. Reaction time was found
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