Spatial sound detection and the role of the inferior colliculus in the Long-Evans rat

The ability of Long-Evans hooded rats (n = 10) to detect sounds presented from sources in the horizontal plane at 0 degrees elevation and the effects of bilateral lesions of the inferior colliculus on these abilities were examined. Rats were trained on a directional detection task which required animals to suppress licking responses in a conditioned avoidance paradigm when 100-ms noise bursts were presented at random from speakers at 45 degrees intervals beginning at azimuth (0 degrees). A task performance rate was determined by reducing the correct lick suppression rate for signal trials by the proportion of incorrect suppression responses on non-signal trials. Higher performance rates were observed for stimuli presented from 0-90 degrees than for stimuli presented in the caudal hemifield prior to surgical procedures. Bilateral lesions restricted to the inferior colliculus reduced detection performance (p < 0.05) and shifted the best performance rates from sounds presented at 0-45 degrees to stimuli emitted from a 90 degrees source (p < 0.05). These results demonstrate that pigmented rats show differential detection levels for noise bursts presented from different locations throughout the horizontal interaural plane, and suggest that the inferior colliculus is involved in this aspect of directional hearing.