According to court records, the experts would have testified that the fMRI a procedure similar to an MRI that measures brain activity by detecting changes in blood flow showed that Valdes has abnormality in his brain activation patterns, and that his "free will and ability to control and inhibit his behavior" are affected by "neuropathological dysfunction stemming from long-term use/abuse of opioid drugs."
Defense attorneys said they would use testimony from the experts as building blocks to establish a diminished mental capacity defense. But prosecutors argued that the testimony should be inadmissible because it does not show that Valdes suffers from a mental illness that prevents him from being responsible for the murders.
DeCaria wrote in his ruling that he was concerned with the fMRI protocol, saying that it has not been studied and reviewed "by an independent third party," and that it is not typically used in a criminal case. DeCaria noted that fMRIs are generally used to detect "mild concussive injuries," not for mapping the effects of chronic drug use on the brain.
DeCaria also wrote that the fMRI results did not show Valdes' mental state at the time of the alleged crimes a requirement to support a diminished mental state defense.
Valdes' seven-week trial is scheduled to start on Jan. 20, 2015. The trial was supposed to begin last month, but one of Valdes' public defenders, Gary Barr, asked to be taken off the case in April, citing health issues.
Valdes was in court Tuesday, where another attorney, Haylee Mills, was assigned to the case. Valdes will be back in court again on Aug. 19 for a status conference.
Weber County prosecutors are seeking the death penalty for Valdes.
On Nov. 25, 2009, Valdes allegedly fatally stabbed Roddy during an argument about stolen prescription drugs. Police believe Valdes then beat Jeffries, Roddy's mother, before leaving her unconscious with a plastic trash bag around her head, and then stuffing her and Roddy into a closet.
Miranda Statler, Valdes' girlfriend, was charged with moving the bodies, stealing Matthew Roddy's car and lying to police. She pleaded guilty in 2010 to obstructing justice and was given a one-to-15 year prison sentence. According to the Utah State Prison officials, Statler, 31, was released from prison Tuesday and is on parole.