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Automatic Frequency-Shift Detection in the Auditory System

by Wolff - published on , updated on

Automatic Frequency-Shift Detection in the Auditory System

Automatic Frequency-Shift Detection in the Auditory System: A Review of Psychophysical Findings Demany L, Semal C. Neuroscience 2018, 389, 30-40.

In this review, Laurent Demany and Catherine Semal (Auditory perception team) summarize research that stemmed from the discovery of a paradoxical auditory phenomenon, initially described by L. Demany and C. Ramos in 2005. The elements of a set of pure tones with arbitrarily selected frequencies are typically not audible individually when the tones have synchronous onsets and offsets. Indeed, when such a "chord" is followed by a single tone which is either identical to an element of the chord or halfway in frequency between two elements, it is very difficult to say if the single tone was present in the chord or absent from it. Nevertheless, if the tone following the chord is either slightly higher or slightly lower in frequency than a randomly selected element of the chord, it appears that the direction of this frequency shift ("up" or "down") can be identified relatively well. Surprisingly, therefore, it is possible to hear accurately a frequency/pitch shift in a tone that was not consciously heard out.

The figure presented here displays the performance of 11 listeners (circles) in the "Present/Absent" task and the "Up/Down" task for chords of 5 tones. Performance is measured using the d’ index from signal detection theory (d’ is 0 when performance is at the chance level). The bold curve shows the prediction of a model based on the assumption that the elements of the chords could be heard out individually but evoked percepts affected by Gaussian internal noise. As can be seen in the figure, the experimental results were dramatically at odds with this model.

The human brain has the task of binding successive sounds produced by the same acoustic source into a coherent perceptual stream, and binding must be selective when several sources are concurrently active. Binding appears to obey a principle of spectral proximity: pure tones close in frequency are more likely to be bound than pure tones with remote frequencies. It has been hypothesized that the binding process is realized by automatic "frequency-shift detectors" (FSDs), comparable to the detectors of spatial motion in the visual system. In 2005, this hypothesis was supported by a psychophysical study showing that human listeners are able to identify the direction of a frequency shift between two successive pure tones while the first of these tones is not audible individually due to informational masking by other tones presented synchronously. A number of variants of this study have been performed since 2005, in order to confirm the existence of FSDs, to characterize their properties, and to clarify as far as possible their neural underpinnings. The results obtained up to now suggest that the working of the FSDs exploits an implicit sensory memory which is powerful with respect to both capacity and retention time. Tones within chords can be perceptually enhanced by small frequency shifts, in a manner suggesting that the FSDs can serve in auditory scene analysis not only as binding tools but also, to a limited extent, as segregation tools.