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In recent times, a number of high profile suspects and criminals -- from extradited gangster Abu Salem to stamp scamster Abdul Karim Telgi -- have taken the test in Bangalore.

But why Bangalore? And what exactly is the brain-mapping test? A quick guide:

What is the brain-mapping test?

It is a test that maps the brain to reveal 'guilty knowledge.'

The brain-mapping test is done to interpret the behaviour of the suspect and corroborate the investigating officers' observation and the suspect's statements.

During the tests, forensic experts apply unique technologies to find out if a suspect's brain recognises things from the crime scene that an innocent suspect would have no knowledge of.

In a nutshell, experts say the brain fingerprinting test -- as the brain-mapping test is also called -- matches information stored in the brain with information from the crime scene.

Studies have shown that an innocent suspect's brain would not have stored or recorded certain information, which an actual perpetrator's brain would have stored.

What is the process during the test?

During the test, the accused is first interrogated to find out whether s/he is concealing any information.

Then sensors are attached to the subject's head and the person is made to sit before a computer monitor. S/he is then shown certain images or made to hear certain sounds.

The sensors monitor electrical activity in the brain and register P300 waves, which are generated only if the subject has connection with the stimulus -- picture or sound.

The subject is not asked any questions.

Who invented this test?

American neurologist Dr Lawrence A Farwell. An expert in brain wave science, he called his technique 'brain-wave fingerprinting' or 'brain mapping test.'

Dr Farwell found that a MERMER (Memory and Encoding Related Multifaceted Electro Encephalographic Response) is initiated in the accused when the brain recognises information pertaining to the crime.

Why are suspects in India taken to Bangalore for brain mapping?

India's only forensic science laboratory that conducts brain-mapping or brain-wave fingerprinting test is located in Bangalore.

What techniques does the laboratory use on suspects?

According to Dr S Malini, assistant director at the laboratory, they use an equipment called the 'neuroscan' and obtain an EEG (Electro Encephalogram) and ERP (Event Related Potential), both of which are studied to analyse the case.

"There is an in-built programme that automatically removes responses obtained due to 'tensions' unrelated to the crime," she says.

How accurate is the brain-mapping test?

The accuracy rate is 99.99 per cent, says Dr Malini. In fact, brain mapping is one of the most effective ingredients of forensic sciences these days. In the US, it is said the Federal Bureau of Investigation uses brain-mapping tests to convict criminals.

Any examples to show that brain-mapping tests are effective?

According to Dr Malini, Abdul Karim Telgi, the kingpin in the multi-crore fake stamp paper scam case, tested positive in the P-300 brain-mapping test performed on him.

Telgi underwent the test last year to ascertain whether he made payments to former Maharashtra deputy chief minister Chhagan Bhujbal, allegedly to seek favours.

The report on the Telgi brain mapping test said: 'The major findings reported by the brain mapping tests are indicative of the possession of knowledge about the activities by Karim Telgi; brain activation during preparation processing, while evoking primary encoding indicates active participation of Karim Telgi in all these activities.'