Always on my mind
Our brains recognise octaves as special
‘Happy birthday’ is a well-known tune, written surprisingly recently (technically, it is still in copyright). As with all songs, if its notes are all raised by an octave (or multiple octaves) it remains instantly recognisable. A much smaller shift in frequency, if it does not match an octave, has a much more dramatic impact on melody and makes the tune harder to spot.
Remarkably, our brains have an innate ability to spot the fact that notes an octave apart are the same. This capacity is even present in unborn infants, whose heart rate changes when they experience novel sounds. An octave shift, though, has a relatively small effect on heart rate.
Perhaps even more remarkably, other primates share this ability. Rhesus monkeys trained to distinguish ‘same’ from ‘different’ can spot the similarity between different versions of ‘Happy birthday’ (and other simple songs), but only when they are played an octave apart.