On the mechanism of major-minor perception
1) Humans, like all other mammals that have been tested so far (apes, cats, and mice), have a pitch mechanism that extracts a unitary percept from a series of harmonics, e.g., a pitch corresponding to 300 Hz is extracted from a series of 900 Hz, 1200 Hz, and 1500 Hz.
2) The evolutionary background is that all these animals produce sounds with their breathing apparatus. The latter contains air tubes like in a flute or in a bassoon and therefore emits sound containing harmonics.
3) The unitary percept of pitch supports identification and localization of sound sources in natural noisy environments and can therefore be considered as highly conserved in evolution.
4) Physiological data of the past 20 years indicate that pitch extraction is accomplished in the auditory midbrain (inferior colliculus, ICC) by a spectro-temporal mechanism. Apparently, low frequency neurons (e.g., for 300 Hz) receive input from units for the harmonics (e.g., 900 Hz, 1200 Hz, and 1500 Hz) and detect the presence of a harmonic series by coincidence detection. Much of the research was done by Gerald Langner and colleagues.
5) A side effect of the pitch mechanism has been highly exploited in music, particularly in the harmony traditions of European music. If several voices, or instruments, sound simultaneously (or in close temporal proximity) the pitch mechanism extracts a sub-pitch (or bass pitch, or root pitch) from the total sound. An example would be the sub-pitch corresponding to 100 Hz that is extracted, if there are single pitches of 300 Hz, 400 Hz, and 500 Hz. This is only possible in some chords, like in major chords. It is not possible in minor chords. This is the physical and physiological difference between major and minor. The phenomenon is well described in music theory and also in psychoacoustics. It is not yet well known, however, that the decisive physiology most likely takes place in the auditory midbrain, and not in the auditory cortex.
6) The mode-specific output of the midbrain has at least two effects at the cortical level. It adds a unitary bass-pitch signal in the case of major, and it adds a characteristic spectral difference, perceived as timbre difference, between major and minor.
7) The ICC is situated adjacent to the periaqueductal gray (PAG) and also in very close proximity to other midbrain centers that are known to be part of the mammalian reward systems. Exceptional temporal order, as is found in major chords, may have a direct or indirect influence on these reward nuclei, which could be one of the reasons why music can cause pleasure. This would also explain why major is regarded as more joyful than minor, despite the fact that both are equally consonant.
The assumed mechanism can be tested with persons that are deaf to dissonance but still can hear the joyful-sad difference between major and minor. Such restricted musical abilities can appear in persons with partial amusia.
The subject is asked to label major and minor chords, presented randomly, as "joyful" or "sad". In one half of all chords the bass bandwidth where the sub-pitch would appear is masked by a comfortable broadband noise. Hypothesis: the subject will only note the correct "joyful-sad" difference for the chords that are presented without low-side masking.
A major chord (element A) and its sub-pitch (element B) are presented in the form of a unitary sequence with alternating elements, ABABAB. In one half of the test sequences, the sub-pitch is detuned by one semitone. Hypothesis: the subject will hear the difference between correct and detuned sub-pitches, despite the inability to perform normal consonance tasks.