C.1 Return to the puzzle

I started, thirty thousand words ago, with a bistable galloping triplet. Two tones — high A, low B — in an A-B-A-rest pattern, repeating. At small frequency differences and slow rates you heard a single galloping rhythm. At large differences and fast rates you heard two separate streams. The acoustic signal arriving at your eardrum was identical in the two cases (or differed only by parameters that smoothly interpolated between them). Yet your percept changed qualitatively.

We can now describe what happened. The cochlea, at every frequency, was responding to its incoming pressure wave with a traveling wave that peaked at the corresponding characteristic place. The auditory nerve carried that information faithfully — the same spike trains, more or less, regardless of how you perceived the sequence. The brainstem and IC computed the same spatial and spectral features. A1 represented the same tonotopic pattern.

What changed was cortical organization. At small frequency differences and slow rates, the cortex grouped the two tones into a single perceptual stream — they were near enough in frequency, slow enough in time, that grouping by frequency proximity and temporal coherence dominated. At large differences and fast rates, those cues weakened and the cortex segregated the tones into two streams. In the bistable region, the cortex could not decide, and the percept oscillated between the two organizations.

You felt your auditory experience flip. The flip is the cortex committing to one interpretation over another — a commitment made under uncertainty, by an inferential system, on the basis of cues that are insufficient to determine a unique grouping. The puzzle of movement 1 was not a puzzle about a strange acoustic phenomenon. It was a puzzle about what hearing is.