March 16, 2009 -- Feb. 18, 1981 was the day Bob Petrella met his friend Susan Angelo.
"It was a Wednesday," he said.
He remembers. She doesn't.
In 1976, Petrella met his friend Tom Challis.
"I remember the first time we really talked, though, was New Year's Day 1978 when we watched Oakland and Denver," Petrella said. "It was over at Patty's house."
For the average person, the ability to recall the odd date -- a birthday, anniversary or reunion -- might not be that impressive. But Petrella has a different gift. He remembers almost everything.
Impressive Memory Surprises Friends
Petrella, 58, is only the fourth person in the United States discovered to have what has been described as a super-autobiographical memory. Give him a date or event and he's likely to remember it. Sometimes he remembers more about his friends' lives than they remember about their own.
"He called me, I swear, six years after I had met him," said Angelo. "I got a telephone message from him one day. He said happy anniversary, we met six years ago today."
"This guy's from another planet," said Challis. "Like '74 ... when Nixon was impeached, I think about ... graduating from high school. This guy, he remembers [that] it hailed one day in July or something that same year."
"It's Thursday when [Nixon] announced it," Patrella said.
"See what I'm saying," said Challis.
Petrella lives a fairly normal life, working in Los Angeles as a producer for the Tennis Channel on cable television. Some people who have known him for years are still stunned by his memory.
The day Princess Diana was killed?
"That was a Saturday and it was Aug. 30, 1997," he said. "I just remember sitting at home and turning [on] CNN or whatever. It was all over the news ... and then the next day the Steelers lost to Dallas, 37-7, which was Aug. 31."
March 30, 1981?
"Reagan was shot, and that night Indiana beat North Carolina for the NCAA championship. Isaiah Thomas played for Indiana and James Worthy and Sam Parkins played for North Carolina. It was a year before Michael Jordan joined the team."
Aug. 15, 1969?
"That was Woodstock. That was a Friday," he said.
Memory Potential Is Infinite
Petrella's memory is a mixture of personal experiences and verifiable public events. He leans toward sports, but historical dates are embedded in his brain as well.
It's a mystery how he does it.
"We don't know how it works, and we would like to know how it works," said Dr. James McGaugh, the founding director of the Center for the Neurobiology of Learning and Memory at the University of California, Irvine.
Petrella and the three others with super memory have been the subjects of study at the center.
"These are not learning machines. These are not people with so-called photographic memory," said McGaugh. "These are people who learn certain things about their lives and don't forget."
The doctors have screened about 2,000 people with 60 questions that only those with super memory have a chance of answering.
"I always explain to people [that it's] like I'm walking around with a video camera on my shoulder and every day is a videotape," Price told ABC News last year. "So if you throw a date out at me, it's as if I pulled the videotape out, put it in the VCR and just watched the day as it happened, like from my point of view."
Doctors are just beginning to observe, let alone understand, this phenomenon.
"First, three of our four top subjects are left-handed, and our fourth has strong tendencies to be left-handed [but] uses a right hand in writing," said McGaugh. "They all have slightly obsessive tendencies. They save a lot of things. They keep a lot of things. Salvation Army will never get rich off these people because they keep it, and so they covet collections the way they covet their memories. We find that interesting."
Also, it turns out certain parts of the super-autobiographical memory subjects' brains are larger than they are in people who don't have super-autobiographical memory. Also, their potential to store information appears infinite.
"There's no capacity limitation on what we can learn," said McGaugh, "no limit to our capacity to learn."
Memory: 'It's Like a Hard Drive'
The remembering also is completely unintentional.
Petrella remembers schoolboy sports events in Pennsylvania and that he got glasses in 1956. He remembers all but two of his birthdays since he turned 5. He recalls where he was and what he did with high school buddies. Grainy images of the 1970s are vivid pictures in his head.
"I remember all my ATM codes," he said. "I remember people's numbers. [I] lost my cell phone Sept. 24, 2006. A lot of people, if they lost their cell phone, they would panic because they have all these numbers. I didn't have any numbers in my cell phone because I know everybody's numbers up here [in my head]."
"It's like a hard drive," he said. "You want to throw some of these dates in the trash and put more, maybe some creative things on -- because they are some inane things, a lot of things. Sometimes that's a bad thing, because when I am going through ... a bad situation or a bad circumstance. And then I go back and I go, 'boy, this is how I felt on, say, May 3, 1986."
At the same time, Petrella has the kind of forgetfulness anyone can have. He'll walk into another room and forget why he entered. Or he'll forget that he left his car in a tow zone.
Petrella is most astounding at sports events. If shown a freeze frame from a game 35 years ago, he'll recall details of the sporting event.
And he knows just about everything when it comes to his beloved Pittsburgh Steelers.
"Oh, that was my most memorable Super Bowl," he said when asked about the Steelers' third Super Bowl win. "Jan. 21, 1979, 35-31, over Dallas. Terry Bradshaw was the most valuable player. Threw for 318 yards and four touchdowns."