Gravitoinertial Force Magnitude and Direction Influence Head-Centric Auditory Localization

1 Ashton Graybiel Spatial Orientation Laboratory and Volen Center for Complex Systems, Brandeis University, Waltham 02454-9110; 2 Department of Brain and Cognitive Science and 3 Research Laboratory of Electronics, Department of Electrical Engineering, Massachusetts Institute of Technology, Cambridge 02139; and 4 Center for Adaptive Systems, Boston University, Boston, Massachusetts 02215 DiZio, Paul, Richard Held, James R. Lackner, Barbara Shinn-Cunningham, and Nathaniel Durlach. Gravitoinertial Force Magnitude and Direction Influence Head-Centric Auditory Localization. J. Neurophysiol. 85: 2455-2460, 2001. We measured the influence of gravitoinertial force (GIF) magnitude and direction on head-centric auditory localization to determine whether a true audiogravic illusion exists. In experiment 1, supine subjects adjusted computer-generated dichotic stimuli until they heard a fused sound straight ahead in the midsagittal plane of the head under a variety of GIF conditions generated in a slow-rotation room. The dichotic stimuli were constructed by convolving broadband noise with head-related transfer function pairs that model 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° rightward relative to the head and body, subjects on average set an acoustic stimulus 7.3° right of their head's median plane to hear it as straight ahead. When the GIF was doubled and rotated 60° leftward, subjects set the sound 6.8° leftward of baseline values to hear it as centered. In experiment 2, increasing the GIF in the median plane of the supine body to 2 g did not influence auditory localization. In experiment 3, tilts up to 75° of the supine body relative to the normal 1 g GIF led to small shifts, 1-2°, of auditory setting toward the up ear to maintain a head-centered sound localization. These results show that head-centric auditory localization is affected by azimuthal rotation and increase in magnitude of the GIF and demonstrate that an audiogravic illusion exists. Sound localization is shifted in the direction opposite GIF rotation by an amount related to the magnitude of the GIF and its angular deviation relative to the median plane.