The two main accused in the Nithari serial killings Mohinder Singh Pandher and Surendra Kohli are undergoing Narco Analysis tests in Gandhinagar in Gujarat.

What exactly are these tests and how do they help investigators? And more importantly are the results of these tests admissible in court?

The Narco Analysis Test:
A person is able to lie by using his imagination. In the Narco Analysis Test, the subject's imagination is neutralised by making him semi-conscious. In this state, it becomes difficult for him to lie and his answers would be restricted to facts he is already aware of.

Experts inject the subject with Sodium Pentothal or Sodium Amytal. The dose is dependent on the person's sex, age, health and physical condition. A wrong dose can result in a person going into a coma, or even death.

The subject is not in a position to speak up on his own but can answer specific but simple questions. The answers are believed to be spontaneous as a semi-conscious person is unable to manipulate the answers.

P300 Test:
When the brain recognises a person or a sound, it generates a particular type of electric wave, which is called a P300.

Sensors are attached to the head of a person undergoing a P300 test and the subject is seated before a computer monitor. 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 some connection with the stimulus, in this case pictures or sounds.

The subject is not asked any question.

These test are used by investigators to cross check their findings, determine if a suspect is telling the truth or make him reveal facts pertaining to a case.

Are these tests admissible in court
Lawyers are divided on whether the results of Narco Analysis and P300 tests are admissible as evidence in courts.

"Confessions made by a semi-conscious person is not admissible in court. A Narco Analysis Test report has some validity but is not totally admissible in court, which considers the circumstances under which it was obtained and assess its admissibility," advocate P R Vakil  told rediff.com.

"Under certain circumstance, a person may hold a certain belief. By repeatedly thinking about an issue in a particular way, he begins to believe that what he is thinking is right. But it need not necessarily be the truth," Vakil explained.

"Results of such tests can be used to get admissible evidence, can be collaborated with other evidence or to support other evidence. But if the result of this test is not admitted in a court, it cannot be used to support any other evidence obtained the course of routine investigation."

Criminal lawyer Majeed Memon said, "If the courts give permission to conduct these tests, then only it can decide the admissibility of the test results and other related evidence. Such reports can be used as evidence or to support other evidence."

Another criminal lawyer Sham Keswani has a different view. "Such tests don't have any legal validity. They can only assist the police investigation.

"But, in case a person is not affected by the chemical, he might take some wrong names (to mislead investigators). The results of such tests can be used to support other evidence," he said.