Behavioral consequences of unilateral inferior colliculus lesions in the rat

This study was carried out to determine the behavioral sensitivity to sound of rats with unilateral lesions of inferior colliculus (IC) located ipsilateral or contralateral to the projection pathway from one ear. Absolute thresholds for the detection of a broad-band noise burst were compared for rats with a profound conductive hearing loss in one ear and a lesion placed either ipsilateral or contralateral to the normally functioning ear. The rats were trained to make withdrawal responses to avoid a shock when they detected the presence of a noise burst. Sound pressure level was systematically lowered to obtain psychophysical curves from which absolute thresholds could be determined. Complete lesions of the contralateral IC resulted in substantial elevations in absolute threshold relative to normal whereas equivalent lesions of the ipsilateral IC produced relatively little elevation. In neither case did unilateral destruction of the IC produce a total inability to respond to sound. Contralateral IC lesions that included the dorsal nucleus of the lateral lemniscus (DNLL) produced a significantly greater elevation in behavioral thresholds than complete lesions limited to the IC. The results indicate a predominance of the contralateral over the ipsilateral pathway to IC for maintaining normal thresholds. They also indicate that other pathways that bypass the IC are likely involved in detecting the presence of a sound.