Auditory cortex neurons sensitive to correlates of auditory motion: underlying mechanisms.

Neuronal response properties such as phasic vs. tonic, onset vs. offset, monotonicity vs. non-monotonicity, and E/E vs. E/I, can be shown to act synergistically suggesting underlying mechanisms for selectivity to binaural intensity correlates of auditory sound source motion. Both identical (diotic), and oppositely directly dichotic AM ramps were used as stimuli in the lightly anesthetized cat, simulating motion in four canonical directions in 3-dimensional space. Motion in either azimuthal direction evokes selective activity in cells which respond best to the onset of monaural sound in one ear and show a decreased response to binaural stimulation (E/I or I/E). In some cells specificity is increased by "off" components in the non-dominant ear. Although these cells fire only at the onset of stationary sound, they fire throughout oppositely directed AM ramps. Motion toward or away from the head evokes responses from EE cells; strong binaural facilitation increases selectivity for motion in depth. The sharpness of direction of tuning was related to the degree of binaural facilitation in E/E cells. Selectivity for sound moving away from the head is correlated with "off" responses, while "on" responses correlate with preference for motion toward the head. Most units showed a monotonic rate function as AM ramp excursion and rate was increased. One third were selective for slower rates of intensity change and may therefore encode slower rates of stimulus motion, as well as direction of movement. The findings suggest that neural processing of auditory motion involves neural mechanisms distinct from those involved in processing stationary sound location and that these mechanisms arise from interactions between the more traditionally studied response properties of auditory cortex neurons.