Attorneys in the first-degree murder trial of Alexander Pring-Wilson attempted to bolster claims that he acted in self-defense with expert testimony on Friday from doctors who said Pring-Wilson suffered a concussion the night of the incident.

Pring-Wilson, a 26-year-old former Harvard graduate student, is accused of the April 2003 killing of 18-year-old Michael D. Colono, who died after Pring-Wilson stabbed him five times with his four-inch pocketknife. The defense opened their case on Friday following two weeks of jury selection and the prosecution’s case.

The two men became involved in a street fight outside of a Pizza Ring on Western Avenue in Cambridge after Colono, who was waiting outside the restaurant for his pizza, insulted Pring-Wilson for apparently drunken behavior, police say.

Dr. Jeremy Schmahmann, a neurologist who teaches at Harvard Medical School, testified Friday that based on his review of the medical records and evidence, including tapes of phone conversations Pring-Wilson had with the police and friends after the incident, “[Pring-Wilson’s] symptoms are consistent with concussion.”

Schmahmann also said that the defendent’s head injury could have been a consequence of a single blow to the head.

Colono’s cousin, Samuel Rodriguez, who was present the night of the killing, has testified to delivering a single punch to Pring-Wilson’s head.

During Schmahmann’s cross-examination by Assistant District Attorney Adrienne Lynch, the chief prosecutor on the case, he conceded that the CAT scan performed on Pring-Wilson on April 13 showed no evidence of hemorrhage. Such evidence would indicate concussion, though its absence does not preclude it, according to Schmahmann.

Lynch also questioned Schmahmann about the alternate version of events that Pring-Wilson gave to police only minutes after the fight.

In a 911 tape played to the jury last week, a breathless and intoxicated Pring-WIlson can be heard telling police that he had just witnessed a stabbing and that the perpetrators had “fucking run off.”

Lynch attempted to demonstrate that Pring-Wilson’s apparent lie to police was a result of a guilty conscience, not of confusion relating to head injury.

“With regard to someone who has a concussion, it wouldn’t predispose one to tell a lie?” Lynch said.

Schmahmann said that such a question was “beyond his expertise.”

Pring-Wilson attorney Amy Kaufman questioned the defense’s second witness of the day in an attempt to convince the jury that Pring-Wilson’s history of sports-related concussions caused him to be defensive as he entered the fight with Colono.

Dr. David Ross, an emergency room physician who treated Pring-Wilson in Septemer 1998 for a concussion suffered as a result of a rugby game, testified that Pring-Wilson told him then that he had suffered “four or five” previous concussions to the head.

Ross said that he had told Pring-Wilson that another head injury could result in brain damage and had advised him to stop playing sports.