Sound Motion Analysis by Cells in the Central Auditory Pathway 
    Several years ago a graduate student thesis undertaken by Dr. Daryl Doan engineered a virtual reality of moving sounds using mathematical equations to dynamically model intensity and time of arrival of sound at the two ears.  The dynamic expression of these equations resulted in sound motion along the horizontal azimuth.  The motion was either circumferential (around the head) or radial (toward or away from the head). 
    The equations were modified using a spherical model to fit the interaural distance between the left and right tympanic membranes of the rat head.  The sounds were presented to anesthetized animals through ear bars attached to each ear individually.  As the moving sounds were presented to the animal, a microelectrode inserted into the primary auditory cortex recorded the cellular response of neurons.  Dr. Doan was able to demonstrate that about 30% of his cells were clearly responsive to simulated motion in one direction, but not the other. 
    These studies are progressing.  The stimulus will be modified to cover continuous motion across a 90 degree range of circumferential motion and over a radial distance of 1 to 5 meters from the head. 
    We also intended to label cells with dye tracers in order to identify the cortical layers from which they originate.  Forthcoming studies will record from the inferior colliculus to see if the motion patterns noted in the cortex exist in brainstem nuclei.