Responses of inferior colliculus neurons in young adult C57BL/6J mice as a function of stimulus azimuth
McFadden, Sandra L.
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In order to examine responses of inferior colliculus (IC) neurons as a function of sound source location in the azimuthal plane, extracellular recordings were made from 97 neurons in 2-month-old C57BL/6J mice anesthetized with sodium pentobarbital and chlorprothixene. Tone bursts (200 ms duration, 5 ms rise/fall) were presented at seven fixed angles in the free field. Best frequencies (BF) of the neurons ranged from 6.3 kHz to 50 kHz, with the greatest proportion in the 16-20 kHz range. Most neurons (92/97) were presented with BF stimuli at two or more intensities; 60 units were also tested with stimulus frequencies below BF, and 59 units were tested with frequencies above BF. Only six neurons (6.5%) tested with BF stimuli responded as though they were binaurally excited, with discharge rates varying little as a function of stimulus azimuth. Most units (93.5%) had discharge rates that varied markedly with speaker location at one or more stimulus intensities. Maximal excitation nearly always occurred when the speaker was positioned in the quadrant contralateral to the recording site. Some neurons behaved as though they were monaurally excited; in these cases, response variations could be attributed to the head shadow effect (attenuation of the signal at the distal ear by the presence of the animal's head in the sound field). In other cases, response variations were greater than could be accounted for by the head shadow effect alone, suggesting binaural interactions. Responses of neurons depended on stimulus frequency and intensity as well as on azimuth angle. Although similar types of units (e.g., omnidirectional, hemifield) appear to be found in the mouse as in other animals (e.g., cat), proportions and functions may differ between species. Neurons in the mouse IC appear to be well suited for encoding laterality of a sound source, rather than precise azimuth angle.