The relationship between a smell and memory is stored in the orbitofrontal cortex, a brain region near the eye sockets. Researchers from the University of Amsterdam (UvA) have discovered an important cog in the brain mechanism that deals with this storage. With this, they can explain how the brain learns to remember the value and emotional consequence of sensory stimuli.
The smell of a banana can evoke different reactions. Does the smell indicate that the banana will taste good? Or does it actually indicate that the banana is nutritious and, therefore, valuable? The association between a smell and its value is important when making the decision whether or not to eat the banana.
The researchers found that in response to two different smells, cells in the orbitofrontal cortex become more active for one of the smells, depending on the coupling of that smell with a reward. When the NMDA receptor - a protein molecule important for the chemical signal transmission between brain cells - was blocked, the cells were unable to make a distinction between the smells.
With this discovery, an important molecular switch for the development of preferences in response to environmental stimuli has been revealed. The results of the study were published in the scientific journal Neuron on 21 November 2012.
PhD students Marijn Wingerden, Martin Vinck and colleagues working in the research group headed by Cyriel Pennartz at the UvA’s Swammerdam Institute for Life Sciences showed that rats distinguish between two different smells. The animal’s movements in reaction to one smell (e.g. lemon) were rewarded (e.g. sugar with water), while the same movements in reaction to another smell (rosemary) led to an unpleasant outcome (e.g. quinine solution). Meanwhile, the researchers measured the electrical activity of numerous individual cells in the orbitofrontal cortex. A number of the cells were found to respond to certain smells.
Each day of the experiment, new smells were emitted. In this way, the animals were entasked with finding out which of the two smells was 'positive' (receiving an reward), and which smell was 'negative' (leading to an unpleasant outcome). This experiment revealed how cells in the orbitofrontal cortex displayed increasing differences in terms of electrical response to the smells, running parallel with the learning behaviour.
The second step was to study whether a pharmacological substance had an effect on the electrical activity. This substance acts on a receptor which is important for the chemical signal transmission between brain cells. The substance appeared to block the activity of the orbitofrontal neurons without affecting the ‘normal’ properties of the cells.
Finally, the researchers discovered that blocking the NMDA receptor also affected the rhythmic synchronisation of the orbitofrontal neurons. This finding is very interesting for psychiatric disorders involving the orbitofrontal cortex, such as depression and obsessive-compulsive disorders.