Harmonic fusion and pitch affinity: Is there a direct link?

Why is the perceptual octave stretched? An account based on mismatched time constants within the auditory brainstem

This paper suggests an explanation for listeners' greater tolerance to positive than negative mistuning of the higher tone within an octave pair. It hypothesizes a neural circuit tuned to cancel the lower tone that also cancels the higher tone if that tone is in tune. Imperfect cancellation is the cue to mistuning of the octave. The circuit involves two neural pathways, one delayed with respect to the other, that feed a coincidence-sensitive neuron via excitatory and inhibitory synapses. A mismatch between the time constants of these two synapses results in an asymmetry in sensitivity to mismatch. Specifically, if the time constant of the delayed pathway is greater than that of the direct pathway, there is a greater tolerance to positive mistuning than to negative mistuning. The model is directly applicable to the harmonic octave (concurrent tones) but extending it to the melodic octave (successive tones) requires additional assumptions that are discussed. The paper reviews evidence from auditory psychophysics and physiology in favor-or against-this explanation.

The perception of octave pitch affinity and harmonic fusion have a common origin

Musicians say that the pitches of tones with a frequency ratio of 2:1 (one octave) have a distinctive affinity, even if the tones do not have common spectral components. It has been suggested, however, that this affinity judgment has no biological basis and originates instead from an acculturation process ‒ the learning of musical rules unrelated to auditory physiology. We measured, in young amateur musicians, the perceptual detectability of octave mistunings for tones presented alternately (melodic condition) or simultaneously (harmonic condition). In the melodic condition, mistuning was detectable only by means of explicit pitch comparisons. In the harmonic condition, listeners could use a different and more efficient perceptual cue: in the absence of mistuning, the tones fused into a single sound percept; mistunings decreased fusion. Performance was globally better in the harmonic condition, in line with the hypothesis that listeners used a fusion cue in this condition; this hypothesis was also supported by results showing that an illusory simultaneity of the tones was much less advantageous than a real simultaneity. In the two conditions, mistuning detection was generally better for octave compressions than for octave stretchings. This asymmetry varied across listeners, but crucially the listener-specific asymmetries observed in the two conditions were highly correlated. Thus, the perception of the melodic octave appeared to be closely linked to the phenomenon of harmonic fusion. As harmonic fusion is thought to be determined by biological factors rather than factors related to musical culture or training, we argue that octave pitch affinity also has, at least in part, a biological basis.