Former White House aide I. Lewis "Scooter" Libby will not be allowed to use a memory expert at his perjury and obstruction trial, a federal judge ruled Thursday, blocking a key tactic in Libby's defense strategy.

Libby, who is accused of lying to investigators in the CIA leak case, wanted an expert to testify that memory is unreliable, especially during times of stress. Libby says he had national security issues on his mind and any misstatements he made about the leak of CIA officer Valerie Plame's name were mistakes, not lies.

U.S. District Judge Reggie B. Walton said allowing a memory expert would be a waste of time and would only confuse the jury. Walton said jurors, like everyone else, understand that memory sometimes falters and can judge for themselves whether witnesses are reliable.

The issue provided the case's first courtroom drama last week when Special Prosecutor Patrick Fitzgerald spent hours questioning the research of memory expert Elizabeth Loftus. Fitzgerald picked apart the psychologist's testimony until she acknowledged errors and misstatements in her findings.

The trial is set to begin in January. Libby's lawyers have said the former aide to Vice President Dick Cheney will testify.

Plame's CIA status was leaked in 2003 as her husband, former ambassador Joseph Wilson, criticized the Bush administration's prewar intelligence on Iraq. Plame believes she was outed as retribution, a claim Fitzgerald spent three years investigating. Nobody has been charged with violating a law by discussing Plame.