those

In the context below, what pronoun "those" stands for?
It refers to "shifts in pitch" or "melodies and/or tunes"

Neuroscientists subjected volunteers with amusi--a 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 thenanother. The volunteers were unable to discern adifference between the tones. But the volunteers were able to track timed sequences of musical tones and perceive slight changes in timing.

thanks for your reply in advance.

best regards,