1.2 What just happened

The pressure variations in air, the cochlear traveling waves, the auditory-nerve firing patterns — all of these are essentially identical regardless of which of the two interpretations you experienced. The cochlea, in particular, does not care about grouping. It receives the same sequence of tones either way. Yet at one set of parameters your brain organizes those tones into one rhythm; at another, into two; in between, the percept becomes unstable.

This is auditory streaming, one of the most studied phenomena in the cognitive science of hearing. It is a clear demonstration that what you hear is not just what enters your ears. The same input produces qualitatively different percepts, depending on factors — frequency separation, rate, attention, prior context — that the cochlea has no access to.

The brain is doing something that the ear does not. It is organizing the sound into objects, into streams, into sources. The ear gives a representation of what frequencies were present, when, at what intensity. The brain decides what counts as the same thing repeating, what counts as two different things at once, what was signal and what was background. None of that is in the cochlear output. All of it is in the listener.

This is the thesis of the entire essay. Hearing is what the brain does with what the ear sends.