It’s a Thursday night at O’Brien’s in Allston. A small television above the bar shows sports. Another TV in a corner across the room beams promises of Keno windfall. On the wall, Elvis Presley shakes hands with a bemused Richard Nixon. The lighting is diffuse and sickly green; with a smoking ban weeks away, a substantial gray-yellow haze wafts wall-to-wall, half obfuscating a dingy stage that recedes into a corner, cloaked in shadow. On it prowls a heavy-metal band called Primordial Dwarf, whose singer howls with guttural relish into a microphone held like a poleax. Meanwhile, two dozen or so people loll around the place, tilting back Budweiser longnecks and pulling lazily on Winston Lights.

As the grinding din sputters into its death throes, a woman wearing a heavy anorak walks through the club’s front door and makes her way gingerly but deliberately through the sparse crowd, as if pushing against the music’s oppressive volume. She’s pretty, on the still-uphill side of middle age, with raven-dark hair and delicate features. With her decent demeanor and plain-Jane jacket, she seems a little out of place in this murky dive. When she meets a kid with glasses and a thick ponytail, gently touching his elbow as she speaks into his lowered ear, she looks like a mother on a moral mission to retrieve her errant son from this den of deafening diablerie.

But Nancy Mroczek, PhD is not an overprotective parent. And the young man with whom she’s speaking is not her son at all — he’s her bass player. Nancy Mroczek, PhD is in a band. Usually her band is called Nancy Mroczek PhD, but sometimes it’s called Nancy S Mroczek PhD, and for a very short time it was called Dr. Nancy and the Plugheaded Monkeys.

And yes, Nancy Mroczek, PhD really is a PhD. She’s a licensed psychologist. She has a doctorate from the University of Minnesota. She specializes in behavior, behavior medicine, and neuropsychology. She also hosts a weekly TV show, Toward a Quality of Life, on Boston Neighborhood Network’s channel 23, where she holds forth on everything from the Back Bay and Bill Clinton to bad karma. She moonlights as a commodities trader and is registered as a certified trading adviser with the Commodity Futures Trading Commission. Sometimes she’s a filmmaker.

She also rocks. That’s not something that psychologists-cum-commodities-traders who are "in the fifth decade of life" often do. But as Mroczek sheds her bulky coat to reveal a sleek black jacket and a pair of yellow form-fitting pseudo-suede pants, it’s clear she means to prove it.

On stage, her bands warm up. There’s the bass player, Daniel Karz-Wagman, 20, his thumb thumping out percolating funk lines. Drummer Joe Caputo, 24, does some fills. James Curl, 21, bends over his guitar, chopping fuzzy power chords and fingering spidery, scratchy riffs. All three are students at Berklee College of Music.

"It’s gonna be short and sweet tonight," Mroczek says as the band kicks hard into her song "Rock This Joint," a pogo-ing rave-up that recalls the bracing melodic clangor of early-’80s Los Angeles punkers X. In fact, she sounds a lot like that band’s frontwoman, Exene Cervenka, as she sings:

spread your wings

you can do a million things

take it slow

let it roll

what’s ahead you want to see

past is past

not for now

live in possibility

The audience looks immediately intrigued by the sight of this sharp-dressed lady barking out the staccato syllables of a punchy pop-punk anthem about the potential for realizing unrealized potential. Just a moment ago they were being manhandled by a mediocre metal band. But this! This is something ... different.

A gaggle of girls sits with eyes glued and mouths agape. A blonde hangs alone with a beer at the bar, shaking her hair as she loses herself in a frenetic, seated dance. Others in the audience stand and sway stiffly to the savage beat. The Nancy Mroczek PhD band is kicking up a hell of a racket, loud and fast and catchy as a cold.

And Nancy Mroczek, PhD the singer is getting right down to it. As she warbles, she gyrates and wiggles. Then she marches in place and makes a little salute-type motion from her forehead. She strikes a contrapposto stance and wobbles back and forth. She leans dramatically to listen to her band, a hand cupped to her ear. She dances in circles reminiscent of a narcotized tribal chieftain’s rain dance. She hops up and down frenetically. She does something resembling the chicken dance. Then she indulges in a little gentle head-banging.

"Rock and ROll!" someone shouts as the song ends abruptly.

"Yeah, every now and then you gotta rock, right?" she shoots back.

"Who’s that sexy woman?" someone screams.

"I love you!" howls another.

But there’s one in every crowd.

"Hey!" shouts some wag. "The vice-principal is on stage!"

APPARENTLY, SOME folks have a hard time appreciating the idea of a middle-aged professional psychologist cutting loose with some righteous noise in a smoky dive bar. But that kind of limited and constricting outlook on life is precisely the sort of thing Nancy Mroczek, PhD, in her professional capacity, could help remedy.

Because, as the name of her TV show suggests, the thrust of Mroczek’s psychology work deals with helping patients tackle the "problems of everyday living." Her Web site (mroczek.8k.com) advertises her expertise in leading patients along the path of self-discovery, and helping them make changes based on "incorporating behavioral, existential, dynamic, and non-directive (non-judgmental) ways of proceeding."

She’s covered a lot of ground over a 27-year career. "I’ve done research, I’ve even had published research," Mroczek says. "I’m also a specialist in neuropsychological testing, and there have been periods when I’ve done a lot of that. I’ve also done a lot of behavior medicine for people that have medical conditions and need to learn certain things about how to live, but also investigate things about themselves that contribute to the problems that they’re having."

Knowing her background, then, an affirming song about self-actualization like "Rock This Joint" might make that much more sense. But when asked where her singing intersects with her psychology, she responds without a beat.

"It doesn’t," she says. "No. No, it doesn’t. Persons who are in the field generally wouldn’t be doing this kind of music. They might be sitting at Boston Symphony. I just don’t mix the two. And that’s been unfortunate. That’s really unfortunate."

The simple fact is that Mroczek has a need to write and perform music. For 25 years, she’s been penning tunes about the state of the world and of the people who inhabit it. But for most of that time, they were only so many sheets of paper.

"Songs were coming into my head and feeling like being expressed," she says, "but then I stopped. Because there were so many, and I wasn’t actuating them. So what was the point?"

That all changed in 1998, when Mroczek decided to take the proverbial bull by the horns. She lived and worked blocks away from one of the best music schools in the country. Why not take advantage of its talent pool?

"I knew some people at Berklee and one thing led to another," she recalls. "Sometimes I put up ads, and sometimes I knew people, and it just became one hook-up after another."

Five years on, she estimates more than a dozen guys have passed through the ranks.

Karz-Wagman, the linchpin of the band’s current line-up, confesses to having felt some trepidation when he responded to a flier on a Berklee bulletin board and heard the voice on the other end of the line.

"At first I kind of didn’t really understand," he says in a languid drawl. "I called up the number and I was like, wait ... that sounded like an older lady. And she’s like, ‘Yeah, I got this group.’ And I was like, oh, well, maybe she coordinates some people or something. And she was like ‘I’m the singer.’ And I was like, whuuut?!"

His misgivings soon proved unfounded.

"She starts playing me these recordings and stuff, and it was like, just so different, y’know? It was just like a total ... rock ... spectacle."

"It was pretty crazy," concurs Curl, who pulls double duty in a punk band called Lost on Main St. "I didn’t know what to expect at first. I was kind of weirded out. But she’s very approachable, and she made us all feel really comfortable. I didn’t feel like I was just there as a studio musician when I started playing."

"She’s not from a musical background, which makes being in her band a good experience for me," says Caputo. "She conveys her ideas about the music in a different way. She doesn’t have standard musical knowledge. She tries to get across her ideas in ... movements and descriptive words."

"She’s more concerned with the lyrics and not as concerned with the melody, which I really like," adds Curl. "I like her lyrics a lot. A lot of people don’t really concentrate much on lyrics anymore."