If you happened to have been born between about 1978 and 1981, there's a fair chance you count yourself an obsessive of the Southern California rock band Weezer. The affection would not make sense to those even just a bit older or younger, who might regard Weezer's guitar pop as clever and pleasing but also somewhat too shallow to have much lasting significance. Those of a certain age, though, experienced the group's 1994 eponymous debut release, known to fans as the Blue Album, as a thing of precise and overflowing emotion -- 10 tracks that functioned like keys to secret locks in the teenage brain, opening up all the awkwardness and anxiousness of those melodramatic high school years.

We all have music like this, music that burns into the soul when we're young and remains essential for the rest of time. For me it was the Blue Album and anything the Smashing Pumpkins did up until about 1998. For you it's something else, but it's surely something -- there's a tape or record or CD that once knocked you out with a force that, cheesy as it is to remember, felt like true love. Put on one of those songs now and, if it's been a long time, the effect is like an old movie; the scenes play back for you in entire exhilarating reels. What's happening when music captures you in this way deserves some scrutiny. You may feel like the songs are grabbing your heart, but what's actually going on is in your head. There, says Daniel J. Levitin in his new book "This Is Your Brain on Music," an "exquisite orchestration of brain regions" are engaged in a "precision choreography of neurochemical uptake and release." Why human beings make and enjoy music is, in Levitin's telling, a delicious story of evolution, anatomy, perception and computation -- a story that's all the more thrilling when you consider its result, the joy of living in a world filled with music.

Levitin is a neuroscientist and a former record producer. He is one of those people -- think of a Nick Hornby character -- for whom music has always been a source of infinite aesthetic and emotional pleasure. He is also one of those people lucky enough to have turned his abiding interest into worthwhile work. Levitin's primary scientific pursuit concerns how music operates on the human brain, though it might be more fitting to say that he uses music to study how everything works in the human brain. By looking at how our brains process music -- at how we turn collections of sounds into patterns that we think of as songs, how we remember and categorize those patterns, and how we feel them as intense emotion -- Levitin and other scientists have uncovered important neural processes that had previously eluded researchers. The brain systems they discovered explain why music -- whether in high school or in life beyond -- can touch you so deeply: Our brains seem to have evolved to maximize musical ability. Indeed, Levitin argues, music has been essential to our very success as a species.

Levitin's book has an unfortunate PSA- related title, but otherwise "This Is Your Brain on Music" is delightful. Levitin explains the intricacies of two difficult subjects -- neuroscience and music theory -- without ever losing the reader; it helps that he's got what Stevie Wonder refers to in a blurb as "an encyclopedic knowledge of popular music," which allows him to reference a wide range of traditions, from classical to the blues to jazz, rock, country and modern pop, in order to convey certain complex ideas.

Levitin also has a knack for the choice analogy. Here is how, for example, he describes why it's remarkable that our brains can extract audio information from the chaotic collection of air molecules bouncing against our eardrums: "Imagine that you stretch a pillowcase tightly across the opening of a bucket, and different people throw ping-pong balls at it from different distances," Levitin writes. "Each person can throw as many ping-pong balls as he likes, and as often as he likes. Your job is to figure out -- just by looking at how the pillowcase moves up and down -- how many people there are, who they are, and whether they are walking toward you, walking away from you, or are standing still. This is analogous to what the auditory system has to contend with in making identifications of auditory objects in the world, using only the movement of the eardrum as a guide."

Levitin's point is that when we're listening to music, our brains are engaged in an enormously complex computational task -- so complex that no man-made computers have yet been able to do anything nearly as sophisticated with sound. Another insight is that much of what we think of as the sounds of the world actually occur inside our heads, not outside. The air molecules that strike our eardrums, for instance, carry no inherent "pitch." Instead, the molecules oscillate at a specific rate, and our brains measure the rate, and then construct an internal representation -- a high or low pitch -- based on that frequency. (In the same way, light waves carry no color -- our eyes and brains construct color by measuring the frequency of the waves.) In other words, sound is essentially a psychological phenomenon. If a tree falls in the forest and no one is there to hear it, does it make a sound? "Simply no," Levitin points out. "A suitable measuring device can register the frequency made by the tree falling, but truly it is not pitch unless and until it is heard."

Your brain doesn't just come up with an internal representation of sound, it also derives meaning -- in particular, pleasure -- from sound. But how it does so surprises even neuroscientists. In his lab, Levitin has found that when people listen to a song they like -- as opposed to something that they don't like, or simply noise -- one area of the brain that's activated is the cerebellum. This seemed odd: The cerebellum is, evolutionarily, one of the oldest parts of the brain, what some people call the reptilian brain; its main purpose is to coordinate the movement and timing of our bodies, and not, scientists believed, anything more sophisticated, such as the experience of emotions. But if the cerebellum wasn't involved in emotion, why was it being activated only when people listened to something that they liked -- an emotional choice -- rather than just anything at all?

To answer the question, Levitin and his colleagues used an advanced technique known as "functional and effective connectivity analysis" to follow how music moves through the brain. What they discovered was startling. Contrary to long-held assumptions, the cerebellum did turn out to play a role in some emotions -- particularly the way we derive pleasure from the rhythm, or groove, of a piece of music. When we listen to a song, our ears send signals not only to the auditory cortex, the region of the brain that processes the sound, but also straight to the cerebellum. When a song begins, Levitin says, the cerebellum, which keeps time in the brain, "synchronizes" itself to the beat. Part of the pleasure we find in music is the result of something like a guessing game that the brain then plays with itself as the beat continues. The cerebellum attempts to predict where beats will occur. Music sounds exciting when our brains guess the right beat, but a song becomes really interesting when it violates the expectation in some surprising way -- what Levitin calls "a sort of musical joke that we're all in on." Music, Levitin writes, "breathes, speeds up, and slows down just as the real world does, and our cerebellum finds pleasure in adjusting itself to stay synchronized."

But it's not just the cerebellum that perks up to songs. What's interesting about how our brains respond to music -- rather than, say, language -- is the large number of systems that are activated by the experience. In addition to the cerebellum, music taps into the frontal lobes (a "higher-order" region that processes musical structure), and it also activates the mesolimbic system, which Levitin explains is "involved in arousal, pleasure, the transmission of opiods and the production of dopamine." This is why certain music can feel so pleasurable, producing such deep emotions -- it's simultaneously operating on various parts of our brains, and the response is something on the order of taking a hit of heroin.

Clearly, though, we don't all find pleasure in the same music -- and what determines whether we end up loving Billy Corgan, Billy Idol, Billie Holiday or Billy Shatner is mostly a matter of what we listen to when when we're young. Studies suggest that we start listening to and remembering music in the womb (but playing Mozart to your baby, and indeed playing Mozart to yourself, will not make you smarter -- studies showing that famous effect have largely been debunked). Humans prefer music of their own culture when they're toddlers, but it's in our teens that we choose the specific sort of music that we'll love forever. These years, Levitin explains, are emotional times, "and we tend to remember things that have an emotional component because our amygdala and neurotransmitters act in concert to 'tag' the memories as something important." In addition, our brains are undergoing massive changes up until the teen years -- after that, the brain structure becomes more fixed, and it begins to prune, rather than grow, neural connections. Consequently it's in our teens that we're most receptive to new kinds of music (in much the same way it's easier to learn a new language when you're young than when you're old). After that, you can of course find new stuff to love -- but there is a reason that there's such a thing as "your parent's music," and why, even though I can't get enough Paul Simon, I'm far more emotionally attached to my generation's music.

One more thing on the connection between memory and music bears mentioning, if only for the name: the "earworm." This word, from the German "ohrwurm," describes the annoying feeling of having a song stuck in your head. Alas, Levitin says relatively little research has been done on the phenomenon -- all we really know is that musicians and people with obsessive compulsive disorder are more prone to getting earworms, and that for most people it's small bits of songs, rather than entire songs, that we keep repeating. And if it seems more common that terrible songs get stuck in your head, that might be because bad songs -- and commercial jingles -- are the ones with the simplest phrases. (If you do not want to be infected with an earworm named KFed, do not click here.)

There is compelling support, Levitin says, for the idea that our brains evolved to respond to music in this way; in other words, it is no accident -- and rather it's by evolutionary design -- that we are so good at processing music. One bit of evidence is the ubiquity of music across cultures, and across history. "No known human culture now or anytime in the recorded past lacked music," Levitin notes. More than that, we are amazingly good -- nearly magical, when you consider how much better we are than even supercomputers -- at processing sounds. For instance, your brain can instantly spot a transformation -- that is, another version -- of a song, even if the two are radically different. John Coltrane's "My Favorite Things" varies in tempo, pitch, instrumentation and countless other ways from the Rodgers and Hammerstein, "Sound of Music" version, but you know the two as the same song; the same is true for OutKast's funked-out, hip-hop "My Favorite Things." Computers simply cannot do this; our brains are uniquely efficient at such complex pattern-matching tasks.

But why would humans have evolved to become musical creatures? Among evolutionary biologists, there is great controversy over this question -- and, indeed, over whether musical ability was "selected for," or whether it occurred as an accident of other advances during evolution. But Levitin proposes several reasons why music might have been important to humans over the long sweep of history. Making and listening to music is a social activity, and could thus have improved cohesion among members of the species. "Music may have historically served to promote feelings of group togetherness and synchrony" in ancient societies, Levitin writes. Singing around the campfire, way back in the day, "might have been a way to stay awake, to ward off predators, and to develop social coordination and social cooperation within the group."

Music might also serve as a precursor to more advanced cognitive tasks, especially speech. We know that when kids learn to speak, they don't do so by memorization of every word and phrase -- rather they learn the rules of a language, and then try to apply those rules to new contexts. One way we might learn how to use such rules is through music. "Music for the developing brain is a form of play," Levitin writes, "preparing the child to eventually explore generative language development through babbling, and ultimately more complex linguistic and paralinguistic productions."

Finally there is that most important thing about music: its connection to love, or, more specifically, to arousal and mating. Unlike birds and whales, humans don't produce musical mating calls. But as social animals, humans need strategies to attract potential mates, and music might have been an important part of the process. "As a tool for activation of specific thoughts, music is not as good as language," Levitin writes. But "as a tool for arousing feelings and emotions, music is better than language." If you want your potential mate to remember you, you serenade her, or at least get Peter Gabriel to do it.

This is obvious -- that music elicits emotion better than speech is something we all understand. It's why movies have soundtracks, and it's why couples have favorite songs. "You gotta hear this," Natalie Portman tells Zach Braff in "Garden State," playing him the fine Shins' song "New Slang." "It'll change your life." The scene is more touching than gushy because, of course, it's true: it's not always the Shins, but music does change your life. At the end, they fall in love.