Musicians and non-musicians alike are often fascinated by people who have perfect pitch, often referred to as absolute pitch. It's one of the most mysterious phenomenons in the musical world, and perhaps the cognitive/neurological world as well. Numerous studies continue to try to pinpoint the roots of it, often with surprisingly different conclusions. Perhaps it is one of the only concretely definable aspects of innate musical ability, one that typically makes its presence known early on and remains elusive to many of the most gifted and accomplished musicians on the planet. Many great musicians have not had it, and it's by no means a guarantee of musical talent or ability.
Despite the fact that both my parents are professional musicians with perfect pitch, I don't remember having any knowledge of what it was when I started playing music. Unlike many musician parents, overeager to gauge their children's musical abilities, my mother and father never did any of the tests to determine if I had perfect pitch (or if they did they never told me anything about it). After about a year playing saxophone, a year in which I started to listen to a lot of music as well, I realized that I could identify the notes that I heard other alto players play. Without really having an understanding of transpositions of different instruments, it didn't go any further than that in the initial stages, and it seemed pretty dependent on being able to recognize the timbre of specific notes on the instrument. Not knowing what perfect pitch was, I didn't think anything of it, and assumed that it was something any musician could do. It wasn't until people pointed it out to me that I realized that this ability wasn't something that all musicians had.
Never having not had perfect pitch, it's difficult to describe. The Psychology of Music defines it as "the ability to identify the frequency or musical name of a specific tone, or, conversely, the ability to reproduce a frequency, frequency level, or musical pitch without comparing the tone with any objective reference tone, i.e., without using relative pitch." The closest analogy I can come up with is that it is like the ability to recognize basic colors. Once you learn them, it is basically instant, and done on a level more basic than thought. Nobody ever needs a reference color to identify the color before them. Somehow, you just know what's before you, instantly, without having to consider it in any way. It's not an ability you need to practice once you know your twelve notes, and despite the fact that I've never been away from music for too long, I'm sure it wouldn't escape me if it wasn't used for a while. Some musicians with perfect pitch have it to the degree of being able to identify pitches to exacting measure, and can tell you if pitch is in tune or how out of tune it is, sometimes within a few cents. (A cent is 1/100th of the way from one pitch to the next closest pitch.) I can certainly identify when a note is egregiously far from our tonal system, but can't make such small distinctions of pitch. In fact, it wasn't until I tried to play along with Miles Davis' Kind of Blue, famously sharp in its original version (supposedly the tapes on which it was recorded were running slow that day), that I realized just how out of tune it is. I rarely experience the physical pain that some musicians refer to when music is out of tune, and often struggle not to play sharp, which is a common tendency for saxophonists. I'm more bothered when pitch isn't even from note to note or between register than by a single pitch that's slightly out of tune.
One of the most common questions with perfect pitch is whether it's something that can be learned or acquired. There's a company that persistently runs ads in music magazines and journals that not so slickly claims to teach perfect pitch. However, I'm skeptical, as are most people who have it, but it might just be a matter of definition. I think that with practice somebody can learn good pitch memory, perhaps by identifying specific moods and colors with certain tone centers, and from there develop a relative sense of pitch that allows them to identify notes with some degree of consistency and accuracy. As far as I know, this is what the perfect pitch course accomplishes for some. Some studies have proved that perfect pitch is a genetic trait, and that somebody is much more likely to have it if a sibling has it. One study even identified a relatively enlarged left planum template in the brain in people with perfect pitch. The presence of it in my family supports the genetic theory, although my two siblings don't display it. However, the presence of perfect pitch can only be known once somebody has some degree of musical training, making it difficult to study conclusively.
I won't deny that perfect pitch helped me develop quite quickly as a musician early on. I listened to jazz obsessively from about the age of thirteen to twenty, and could internalize musical vocabulary without having to go through many of the motions that somebody without perfect pitch has to go through. As a jazz musician, it's incredibly helpful to be able to easily identify what is going on around you and thus interact with a group. Professionally speaking, it's great to be able to recognize the key that any song is in without having to ask or make efforts to find it. I've also been able to work over the years as a music transcriber, something that I can do quicker than most musicians without perfect pitch.
However, I think that along with perfect pitch comes some common limitations. Most basic, it isn't something that can ever be turned off by people who have it. Thus, somebody with perfect pitch will never experience music the same way most other musicians and non-musicians experience it. Music always enters the brain as specific information, thus subject to a greater degree of analysis than commonly experienced. No note, or even common noises, can enter the consciousness simply as sound. It's always subject to a degree of categorization that most don't experience.
Also, because I can instantly identify notes, my retention for music is not very good. I can instantly play something back to you, but most likely won't be able to remember it the next day or the next week, because I haven't had to grasp the musical relationships in order to learn it. Because music enters my mind as little bits of information, it took me a long time to learn to identify the larger, mostly systematic categorizations that help musicians
learn and understand material. I tend to be slower at transposing than many musicians, probably because I hear pitches as single events and not as much in terms of their relationships, which also affects my ability to identify chord qualities, where most often I first have to identify all the individual notes before I can identify the quality of the chord. Also, I have a lot of trouble playing instruments not pitched in C or Bb, as I tend to be confused when everything around me doesn't relate to my own instrument in a familiar way. When I play alto saxophone, I can hear the notes on the instrument itself quite clearly but hear everything around me in a different pitch center. (Playing the alto flute, which is in the key of G, is a total mess, despite my love for the instrument!)
All in all, I don't want the long list of gripes above to be any sort of whine about how I feel about perfect pitch. Basically, along with many great things about it, I think there are some drawbacks and limitations to it, and thus it's not something to be quite so idealized or coveted. Other than the various party tricks that somebody with perfect pitch has at his/her disposal, I think good relative pitch can benefit any musician as much, if not more, than perfect pitch. However, it's undoubtedly a mystery that continues to fascinate musicians and non-musicians, and hopefully with more study and advancements in genetics and brain-mapping more can be learned about it.
If anybody wants to read more, particularly on the science of it, the Wikipedia entry on it (found here) is quite good and has a lot of good links.