Music and the Human Brain
Historically, much of the knowledge of specialized functions of the brain was deduced from the failure of the functions following a stroke or an accident. Recently magnetic resonance imaging (MRI) has shed a lot of additional light on how the brain operates. Nerve impulses from the cochlea arrive at the brain and are first processed to extract specific categories of information. Additional levels of processing eventually result in our emotional reaction to music.
Levitin lists eight "dimensions" of music, meaning attributes that can be individually varied without affecting the other dimensions. Among the most important dimensions are: perceived pitch; rhythm; timbre; melody; and reverberation. It seems that the brain contains specialized modules for extracting each of these attributes. The evidence for this is that brain damage can cause a loss of one of the functions without affecting the others. (There is similar evidence in the case of speech, where it is possible to lose an ability to speak verbs, or to speak nouns, for example). The brain appears to be a collection of highly specialized modules that are seamlessly integrated by an hierarchy of higher order modules.
Consider the perception of timbre, the attributes that distinguish a saxophone from a trumpet. When both are playing the same note A4 each instrument creates a fundamental tone at 440 Hz, and the same spectrum of harmonics at frequencies of 880, 1320, 1760 Hz and so on. The relative amplitudes of the harmonics, and their variations over time, are what give the instrument its characteristic sound. Nerves from the regions of the cochlea excited by these frequencies send signals to the brain, and presumably a brain module recognizes the different pitches, similar to the way we recognize different colors - although at this level the recognition is probably not conscious. Now what? How on earth does the brain disentangle the overlapping saxophone and trumpet harmonic series, and re-assemble them, so we hear two distinct instruments, each playing a single note, rather than a mishmash of pitches? Levitin suggests that a difference of a few milliseconds in time between the arrival of the two harmonic series is the basis for this amazing feat. Directional clues might be used as well. I also suspect that our memory of what each of these instruments sounds like when played solo helps in this process, . The brain module that does this will even fix the bass response of a inferior stereo system. If an instrument plays a note that produces tones at 39, 78, 117, 156 Hz and so on, but your stereo system can't produce a 39 Hz tone, your brain will fill in the gap and you will hear a 39 Hz pitch! This phenomenon is called "restoration of the missing fundamental." All of this processing occurs automatically, largely in parallel, and without any conscious effort.
Different pitches heard at the same time are processed by a module that extracts harmony. Different pitches heard at different times are processed by another module that extracts melody. Again we know this because Sacks describes patients who have lost one capability without affecting the other. Sacks describes a gifted musician who had a stroke. Suddenly he was unable to recognize a tune as simple as "happy birthday." Yet his perception of pitch and rhythm was intact, and he could read music and hum a melody. So the problem was specifically an inability of auditory processing of a sequence of pitches.
Emotional response to music is arguably the highest response level - although the most primitive areas of the brain appear to be heavily involved. It also apparently arises from a specialized part of the brain. Sacks gives examples of people who have the full tool kit for music processing, including a strong emotional response, and then suffer an accident. They retain the ability to perceive all of the structural characteristics of music, but completely lose the emotional response. He quotes one patient: "[music] had always been the primary unfailing source that nourished my spirit. Now it just didn't mean anything."
There are indeed a lot of complex cognitive processes happening inside our brain. There are 'modules' that allows us to perceive/interpret various aspects of the sound stimuli - but I have not come across any evidence that our brain is able to 'remember' raw, uninterpreted sound.
This means that when we hear that something is different, for example saying that a speaker has a congested mid range - what we are saying is that we have to adjust the internal circuits to interpreted the new sound signature. Research has shown that the human brain is extremely adaptable. Even with a pretty bad sound system it will sound pretty good within 20 minutes!
Sound comparison is not what it seems on the surface!