– In the annals of murder trials, few testimonies can rival the impact of slain teenager Trayvon Martin’s mother, Sybrina Fulton: “I heard my son screaming.”

She was referring to the voice on an audio recording of a 911 call that has been at the center of the prosecution’s case against George Zimmerman, accused of fatally shooting Martin during a scuffle.

That is, until late Friday when the defense called Zimmerman’s mother, Gladys, to the stand. She, too, identified the same recorded voice as her son’s. She was certain, she said, “because he’s my son.”

Suddenly, this high-stakes trial has become a test of the credibility of two mothers.

If the voice heard screaming in the background of the recording belonged to the dead teen, as the prosecution claims, then it is less easy to believe that Zimmerman was acting in self-defense, as he claims.

If the voice belongs to Zimmerman, then one might conclude that he felt sufficiently threatened to squeeze the trigger. Zimmerman, 29, was acting as a volunteer neighborhood watchman the night he shot Martin, 17, whom he has described as acting suspiciously.

Into the mix enters a ghost witness that is both more amorphous and exponentially more powerful than any other: Maternal Instinct. Clearly, attorneys for both sides are plowing this fertile field, counting on the maternal synchrony of the six jurors — all of whom are women, five of them mothers.

One of the mothers testifying is obviously wrong, and jurors will have to rely largely on their own instincts to determine who is right. The judge has ruled against allowing voice experts to testify.

Their answer likely will be the most probative piece of evidence in the trial, although there can be no certainty. The truth, to the extent it can be surmised, may well hinge on an assumption that is interesting to consider. It isn’t so much a gut feeling but is primal, deep-brain and fiercer than mere logic.

Does maternal instinct convince these mothers that they are right? Or does maternal instinct compel each to protect her son regardless of what may be true?

Second-guessers have an array of questions to entertain: Would a mother lie about such a thing? Could she? Would she wish another man convicted on the basis of her sense of things? Can she know with certainty that the voice in the background, barely audible, is that of her son and not of the other man?

The last question is most compelling. Can she?

Most mothers know the sound of their own child’s cry from the moment of birth. From personal experience, I can vouch for the strange ability to discern one’s own from all others. If my baby was crying in the hospital nursery, I was halfway down the hall to retrieve him before I realized I’d left the bed.

Does this sort of attunement last through time? Logic suggests that as a child’s dependency decreases, so does the acuteness of a mother’s instinctive responses. But experience tells us that the mother-child bond does not diminish with time. Every dead soldier is still his mother’s baby.

Would a mother recognize her son’s cry for help on a recording? It’s possible to believe so, while also possible not to believe so. Recordings often distort voices; other noises interfere. Further complicating are the unconscious desires of the listener. A mother needs to believe that whatever harm came to her son wasn’t his fault.

In fact, it was when defense attorney Mark O’Mara suggested that Fulton might have hoped to hear her son’s voice that she uttered those five words and said, “I didn’t hope for anything. I just simply listened to the tape.”

Trayvon Martin was undoubtedly a beloved son who didn’t deserve to die. He was unarmed, on his way home from a convenience store. Did he attack George Zimmerman? Was Zimmerman, also a beloved son, so mortally afraid that he had no choice but to shoot Martin in the heart?

No one envies the terrible decision these jurors must make. Matters were made worse Friday when medical examiner Shiping Bao testified that Martin likely lived another one to 10 minutes and opined that he suffered and was in pain.

Maternal instinct may not be a reliable witness, but in the absence of verifiable truth, she/it may prove to be the bullet through the heart of this case.

Kathleen Parker is a columnist for The Washington Post Writers Group. She can be contacted at:

kathleenparker@washpost.com