When the brainstem tracks a tone, the two ears add up and do not interact

The frequency-following response is the brainstem’s habit of firing in time with the waveform of a low tone, visible on the scalp as a train of small peaks a few milliseconds after the sound. Gerken and colleagues recorded it in five normal-hearing listeners using brief 500 Hz tone bursts, and asked a narrow question: when you present the tone to both ears instead of one, what does the response do?

It adds. The binaural response could be read as the straightforward sum of the two monaural responses. The authors concluded that there are two independent populations of neurons, one driven by each ear, each capable of generating the response on its own, and that the peaks come from the collective activity of phase-locked single cells rather than from a microphonic effect of the cochlea. The sharpness of the individual waves led them to argue that the generators must be spatially compact.

This is a small, technical, fifty-year-old paper, and it earns its place in the library for one reason: it marks where binaural interaction is not. The frequency-following response is the measure most often invoked when people say that audio “entrains” the brainstem, and at this level, in this study, the two ears were not interacting at all. Whatever combines the signals from the two ears to produce the sensation of a binaural beat is happening somewhere else, in structures like the ones Kuwada and colleagues recorded from four years later. Two phase-locked responses sitting side by side is not the same thing as a brain following a beat.

Limitations. Five listeners, one carrier frequency, one stimulus type, no behavioural measure and no claim about anything a person would notice. The conclusion about two independent populations rests on the responses summing cleanly, which is suggestive rather than decisive. We grade it weak on the rubric because that is what a five-person mechanism study scores, and we would not cite it for anything beyond the specific, still-useful point that it makes.