In simplest terms a dual-choice olfactometer is a piece of experimental equipment which allows the testing of the apparent preference of one odour against another, as shown by the organism “choosing” which smell to approach. This choice is assessed by observation of the behaviour of the organism within a choice arena or chamber.

It therefore needs to be an enclosed space where organisms can be presented with a choice between two odours. These odours may be two different added odours (variables), or there may only be one odour added which is tested alongside a control where no odour variable is offered. By carefully designing a strategy of testing the variables against a control, and also against each other, it is possible to develop a data set which indicates which variables are most preferred by the organism.

Dual-choice olfactometers are designed to minimise the possibility of other factors influencing the observed behaviour. If behaviour is being influenced by other factors then little confidence can be placed in the experimental outcome. For example, if the organism can see the source of the odour it may prefer the sight of the source, so this would no longer be a test of olfactory driven behaviour. Similarly, if the air flow through the variable delivery system is unbalanced then the organism may prefer the higher or lower air flow and make its choice based upon this mechanism. Special attention is therefore needed to “design out” these potential confounding variables, and “dry-runs” where no variables are loaded will help to spot any bias in the system which can be remediated.

So far I have only referred to “the organism”. This is because we can, in principle, make this type of choice arena for any organism that has a sense of smell and can display a behavioural response which indicates preference, where one exists. Humans regularly take part in odour preference studies, particularly in areas such as perfume development, although we are able to communicate preference in more ‘intelligent’ ways than just pointing at or moving towards the favoured smell.

Adult female mosquito

Obviously, for this project the organisms being studied will be adult mosquitoes, which have a history of being involved in olfactometer studies. In the next blog, I will look at some of the designs that have been used previously, and discuss the results of a design workshop that took place a few weeks ago.

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