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When you are in the supermarket pondering over whether to buy apples or oranges a special group of nerve cells in the brain is at work categorising the fruit according to their value, a study conducted at the Harvard Medical School in Boston showed.

A team led by the medical school's Camillo Padoa-Schioppa identified the nerve cells after conducting experiments with monkeys. They were given two fruit juices of different quantities, Nature magazine reported.

The monkeys were initially given the choice of choosing between one drop of grape juice and one drop of apple juice - an easy decision because monkeys prefer grape.

In the second experiment, two drops of apple juice and one drop of grape juice were available with the monkeys again choosing their favourite.

The monkeys began to waver only when they were given the choice between three drops of apple juice and one drop of grape juice.

With the choice of between four drops of apple juice and one drop of grape juice the monkey always chose the less favourable bigger portion.

The researchers therefore found that the monkeys gave three drops of apple juice the same value as one drop of grape juice.

According to the researchers, certain nerve cells situated in the orbitofrontal cortex showed a higher reaction activity when the monkey was confronted with an object of high value such as three drops of grape juice.

Other research has found that damage to the orbitofrontal cortex, part of the limbic system and situated behind the eye orbits, was related to eating disorders and addictions such as gambling and drug abuse.