Gravitoinertial force magnitude and direction influence head-centric auditory localization

We measured the influence of gravitoinertial force (GIF) magnitude and dire ction on head-centric auditory localization to determine whether a true aud iogravic illusion exists. In experiment 1, supine subjects adjusted compute r-generated dichotic stimuli until they heard a fused sound straight ahead in the midsagittal plane of the head under a variety of GIF conditions gene rated in a slow-rotation room. The dichotic stimuli were constructed by con volving broadband noise with head-related transfer function pairs that mode l the acoustic filtering at the listener's ears. These stimuli give rise to the perception of externally localized sounds. When the GIF was increased from 1 to 2 g and rotated 60 degrees rightward relative to the head and bod y, subjects on average set an acoustic stimulus 7.3 degrees right of their head's median plane to hear it as straight ahead. When the GIF was doubled and rotated 60 degrees leftward, subjects set the sound 6.8 degrees leftwar d of baseline Values to hear it as centered. In experiment 2, increasing th e GIF in the median plane of the supine body to 2 g did not influence audit ory localization. In experiment 3, tilts up to 75 degrees of the supine bod y relative to the normal 1 g GIF led to small shifts, 1-2 degrees, of audit ory setting toward the up ear to maintain a head-centered sound localizatio n. These results show that head-centric auditory localization is affected b y azimuthal rotation and increase in magnitude of the GIF and demonstrate t hat an audiogravic illusion exists. Sound localization is shifted in the di rection opposite GIF rotation by an amount related to the magnitude of the GIF and its angular deviation relative to the median plane.