Washington, June 16 Yale University scientists have found that high levels of brain energy are required to maintain consciousness, paving way for a new approach to understand this mysterious state of being.

In simple terms, consciousness can be defined as the ability to respond meaningfully to external stimuli.

A large number of studies of consciousness have used imaging technology to try to pinpoint areas of brain activity during tasks, such as memorization or problem solving.

However, lead author of the paper, Robert G. Shulman, has said that such an approach has two drawbacks-First, functional magnetic resonance imaging has shown that many areas of the brain, not just one or two, are recruited during tasks such as memory tests and not help in studying the state of being conscious.

Secondly, the amount of energy used in such tasks is minute, about one percent of baseline energy available to the brain.

"Neuroimaging has been looking at the tip of the iceberg. We looked at the rest of the iceberg," said Shulman.

Aiming to find out the purpose behind the other 99 percent of energy consumption, the researchers proposed that it is needed to maintain a person in a state of consciousness.

Heavily anaesthetized people are known to show approximately 50 percent reductions in cerebral energy consumption.

When the paws of lightly anaesthetized rats with rather high baseline energy levels were stroked, fMRI signals were received in the sensory cortex and in many other areas of the brain. In heavily anaesthetized rats, the signal stopped at the sensory cortex.

The researchers observed that both the total energy and the fMRI signals changed when the person or animal lost consciousness.

"What we propose is that a conscious person requires a high level of brain energy," said Shulman.

He said that the finding has implications for the understanding of the connection between the brain and consciousness, saying: "You can think of consciousness not as a property of the brain, but of the person."

According to anaesthesiologists, a person to be in a behavioural state of consciousness when he or she can respond to simple stimuli.

Properties of this state, such as the high energy and the delocalised fMRI signals, allow the person to perform the interconnected activities that make up our everyday lives.

He suggested that these more energetic properties of the brain support human behaviour and should be considered when interpreting the much weaker signals that are typically recorded during fMRI studies.

The study will be published in the online edition of the journal Proceedings of the National Academy of Sciences. (ANI)