What makes humans react quickly to negative smells?

Researchers at Karolinska Institute in Sweden have found that negative smells associated with unpleasantness or unease are processed earlier than positive smells and trigger a physical avoidance response. The cognitive process is not only unconscious but is also extremely rapid, this goes against the conventional wisdom that unpleasant smells associated with danger is a conscious cognitive process.

The olfactory organ takes up about 5% of the human brain and enables us to distinguish between many million different smells. A large proportion of these smells are associated with a threat to our health and survival, such as that of chemicals and rotten food. In humans, the olfactory sense seems particularly important for detecting and reacting to potentially harmful stimuli.

Until recently, it was not known which neural mechanisms are involved in the conversion of an unpleasant smell into avoidance behaviour in humans. The reason: lack of non-invasive methods of measuring signals from the olfactory bulb, the first part of the rhinencephalon with direct connections to the important central parts of the nervous system that helps us detect and remember threatening and dangerous situations and substances, says a Karolinska Institutet press release.

The researchers have for the first time made it possible to measure signals from the human olfactory bulb, which processes smells and in turn can transmits signals to parts of the brain that control movement and avoidance behaviour (Proceedings of the National Academy of Sciences).

Three experiments were carried out in which participants were asked to rate their experience of six different smells, some positive, some negative, while the electrophysiological activity of the olfactory bulb when responding to each of the smells was measured. And they found that the bulb reacts specifically and rapidly to negative smells and sends a direct signal to the motor cortex within about 300 milliseconds.

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