Neuroscientists subjected volunteers with amusia -- difficulty telling different melodies apart and remembering simple tunes -- to shifts in pitch comparable to those that occur when someone plays one piano key and then another  The volunteers were unable to discern a difference between the tones
But the volunteers were able to track timed sequences of musical tones and perceive slight changes in timing
People who are unable to tell pitches apart in isolation are able to do so in the context of a melody by relying upon timing.