A former White House aide charged with obstructing a leak investigation, I. Lewis Libby, suffered a legal setback yesterday as a federal judge refused to allow defense lawyers to call an expert witness to explain how memory failure could explain inaccurate testimony by Mr. Libby or others.

Judge Reggie Walton concluded that jurors were well-equipped to consider memory issues without needing advice on the subject from an academic expert.

“On a daily basis the average juror is personally faced with innumerable questions of memory and cognition, as everyone in their daily lives is called upon to store, encode, and retrieve information he or she has been subjected to,” the judge wrote in his 27-page ruling. “Although the average juror may not understand the scientific basis and labels attached to causes for memory errors, jurors inevitably encounter the frailties of memory as a commonplace matter of course.”

Judge Walton bluntly labeled the proposed expert testimony a “waste of time.”

Mr. Libby, who served as chief of staff to Vice President Cheney and resigned after being indicted last fall, faces five felony charges that he obstructed justice and lied in the probe of the disclosure in the press of the identity of a veteran CIA employee, Valerie Plame. The Justice Department appointed a special prosecutor, Patrick Fitzgerald of Chicago, to examine whether administration officials deliberately leaked Ms. Plame’s name to retaliate for Iraqrelated criticism leveled by her husband. Mr. Fitzgerald has indicated that he does not plan to charge anyone for causing the leak.

In the ruling yesterday, Judge Walton made clear that he was impressed by the way Mr. Fitzgerald dissected a criminology professor the defense offered as a witness at a hearing last week on the memory issue. According to press accounts, Elizabeth Loftus of the University of California at Irvine reversed herself repeatedly and backpedaled from some of her own research under withering questioning by the Chicago prosecutor.

“The defendant has an arsenal of litigation tools at his disposal to challenge the recollection of the government witnesses. And the utility of one of these tools was vividly illustrated by government counsel’s skillful cross-examination of Dr. Loftus, which demonstrated the fundamental impermanence of both short-term and long-term memory,” Judge Walton wryly wrote.

Ms. Loftus was never proposed as a witness for the trial. Instead, the defense sought to use her testimony to bolster an effort to get a University of California psychology professor, Robert Bjork, onto the witness stand.

An attorney for Mr. Libby, William Jeffress, did not return a call yesterday afternoon seeking comment for this article.

Mr. Libby, who has maintained his innocence, is accused of lying to investigators and the grand jury about his conversations with three reporters. He testified that he learned about Ms. Plame’s CIA affiliation from journalists. They said he imparted the information to them. Prosecutors contend that Mr. Libby learned from government officials that Ms. Plame’s affiliation was classified. The prosecution claims the senior White House aide deliberately attempted to stymie the leak probe by jumbling the sequence of events.

Judge Walton took particular aim at one of Ms. Loftus’s studies, which he noted was instigated by public defenders in Washington. “The authors stretch the results and reach conclusions that are neither supported by the data or even the questions asked of prospective jurors,” the judge wrote.

Judge Walton said much legal-related memory research has related to mistaken eyewitness identification, often the context of violent crimes that he said had no relation to Mr. Libby’s case. “This court, therefore, has difficulty concluding that the studies provided by the defendant are applicable in any meaningful way to the case at hand,” the judge wrote.

The fight over the memory expert was evocative in some respects of the old aphorism that a liberal is a conservative who’s been charged with a crime. In this instance, Mr. Libby’s defense sought to turn to a highly educated college professor to help jurors inform their decision, while the government said it preferred to rely simply on the common sense of the average citizens in the jury box.