Piano tuners who use their ears (as opposed to those who rely on electronic devices) focus their hearing on what most listeners do not even notice. The pinnacle of the tuner's art is the 'setting of the scale' accurately.
This 'setting of the scale' is the laying down of the central octave so that each note is in a correct relationship with the others. Starting with only a single tuning fork, tuners learn to balance the complex inter-related pattern of intervals in order to fix the 12 semitones that make up the central octave. Once this stage has been reached, tuning the octaves up and down the keyboard is relatively easy.
To set the scale, the tuner listens, not to the dominant pitch given out by the various notes played, but to the harmonics these vibrations generate. Once you get the hang of it, it is not really so difficult, but to the uninitiated, it is like listening to an unintelligible language.
Any two notes played at the same time create a chorus-like sound. Since the texture of the sound is difficult to describe in words, it is fruitless to attempt a definition beyond saying that in setting the scale, harmony is achieved when the intervals (mainly the 3rds, 4ths & 5ths) are nicely balanced.
The study mentioned above, does not address the detail of what goes on in the tuner's brain, it simply records that something occurs in there that is different from what goes on in a non-tuner's brain.
Interesting to note that it was observed that the difference is more pronounced among the more experienced tuners - further proof that this is a job in which one has ever more to learn and gain from experience!
© Steve Burden