"The first drug that has any efficacy, even very small, will get used a lot, " says Wade Smith, a neurologist and neurovascular stroke specialist at the University of California, San Francisco.

FORBES: Brain Repair

On 27 April, Coats and Wade told Parker and Smith they could leave the flat.

BBC: 'Unimaginable' suffering and death of Lynda Spence

Mr Parker, Philip Wade, Colin Coats and Paul Smith deny murdering Ms Spence.

BBC: Lynda Spence trial: Flooring 'replaced in torture flat'

David Parker, 37, Paul Smith, 46, Philip Wade, 41, and Colin Coats, 41, deny the charges against them.

BBC: Lynda Spence

Mr Coats, 42, Mr Parker, 38, Mr Smith, 47, and Mr Wade, 42, deny abducting, torturing and murdering Ms Spence at a flat at Meadowfoot Road, West Kilbride, between 14 and 28 April 2011.

BBC: Lynda Spence trial: Missing woman was 'sent text threat'

Smith was cross-examined by Mr Wade's defence lawyer, Gary Allan QC, on Wednesday.

BBC: Lynda Spence trial: Ex-accused 'did not see murder'

Although Parker and Smith eventually gave evidence against Coats and Wade they did not intervene to save Lynda, claiming they were too scared to do anything.

BBC: 'Unimaginable' suffering and death of Lynda Spence

Mr Parker, Mr Smith, Colin Coats, 42, and Philip Wade, 42, deny murder.

BBC: Lynda Spence

Parker claimed that he and his friend, Smith, were recruited by Mr Coats and Mr Wade to guard the financial adviser, who remains missing, at his flat in West Kilbride.

BBC: Lynda Spence

Parker said that Mr Wade and Mr Coats both arrived the next day and told him and Smith they could go.

BBC: Lynda Spence

Smith and Parker eventually admitted their part in the plot while Wade confessed to a friend that he had helped dispose of a body.

BBC: 'Unimaginable' suffering and death of Lynda Spence