Coding for auditory space in the superior colliculus of the rat

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Abstract
Although the rat is often used to determine behavioural sound-localization capabilities or neuronal computation of binaural information, the representation of auditory space in the rat brain has not been investigated so far. We obtained extracellular recordings from auditory neurons in the superior colliculus of anaesthetized rats and examined them for spatial tuning characteristics and topographical order. Many neurons (73%) showed significant tuning, with a single peak in the azimuth response profiles based on spike rates and response latencies. Best azimuth values from neurons in one SC were generally tuned to contralateral and rarely to frontal or ipsilateral directions. Tuning width was mostly broad; at supra-threshold sound pressure levels (35 dB SPL), 55% of the units had a tuning width of > 120° in contralateral space. Additionally, tuning width increased with stimulation intensity. A significant but considerably scattered topographical order of best azimuth directions was observed in the deep layers of the superior colliculus with frontal directions being represented closer to the rostral pole. Tuned auditory units in the intermediate layers of the superior colliculus, however, showed no systematic spatial arrangement. This pattern was confirmed by analysing best azimuth directions from simultaneously recorded units. Our results indicate that the rat superior colliculus contains a representation of auditory space which is similar to that described for other small mammals.