Cues for vertical sound localization in the upper median plane: Integration of directional band theory and parametric notch-peak head-related transfer function model

A number of studies have examined cues for the perception of the vertical angle of a sound image. Sound localization tests using narrow-band signals revealed that there exist directional bands that were perceived in specific directions (front, above, and rear), regardless of the direction of the actual sound source. On the other hand, sound localization tests using wide-band signals revealed that spectral notches and peaks above 5 kHz in the head-related transfer function (HRTF) contribute to vertical angle perception. Based on this finding, a parametric notch-peak HRTF model (PNP model), which is reconstructed using the minimum number of notches and peaks required for vertical angle perception, has been proposed. Thus, the directional band theory, which claims that the presence of specific frequency components is important, and the PNP model, which suggests the importance of the absence of specific frequency components (notches), appear to be contradictory. In the present study, the authors first focus on the front, above, and rear directions and attempt to integrate the PNP model and the directional band theory. Then, expanding the scope to the entire upper median plane, a hypothesis regarding the cues for vertical angle perception and the extended PNP model are proposed.