Aromatherapy could be used to tackle a range of psychological illnesses, scientists have claimed.
Research by Christine Broughan, a psychologist at Coventry University, has found that personal associations with aromas could have a strong influence on their effects.
These can compound or eliminate the physical impact caused by the chemical components of the oils themselves.
"While we recognise that the odours have a physiological property, much of their effect seems to be down to personal preference and conditioning," Dr Broughan said. "Our research points to something within aromatherapy that could be really useful for treating a wide range of psychological disorders."
In tests in which volunteers were exposed to different odours in a controlled environment, Dr Broughan found evidence that the chemical constituents of one essential oil commonly used in aromatherapy - lavender - had a sedative effect while another - peppermint - acted as a stimulant.
Reaction times in a simple mental task were lengthened when subjects smelled lavender oil or a chemically matched synthetic oil. Other experiments found that this effect was heightened in volunteers who perceived the odour to be particularly pleasant.
Dr Broughan noted differences in responses between individuals. This suggested that indirect psychological influences played a significant role in a therapy's efficacy.
Dr Broughan said that a patient with depression might be treated by associating a certain odour with happy events using psychological conditioning. Such a link is hard to break once established - many people still gag when they smell an alcoholic drink that gave them a bad experience years ago.
A small bottle of a scent associated with happiness might be carried around and used when the patient needed help, Dr Broughan suggested. She said the approach was less likely to interfere with the individual's work as the olfactory system acts independently of the other senses.
The findings are published in the International Journal of Aromatherapy .