Sound localization in chinchillas, III: Effect of pinna removal.

The ability of chinchillas to make left/right, front/back, and vertical locus discriminations was determined before and after surgical removal of the pinnae. The animals were tested behaviorally using a conditioned avoidance procedure. In the left/right localization tests, removal of both pinnae had no effect on localization acuity for broadband noise but did result in a small decrement in performance when localizing low-pass filtered noise. In the front/back localization tests, removal of a single pinna resulted in a small but consistent decrement in performance when the sound sources were located in the hemifield on the same side as the intact pinna, and a greater decrement when the sound sources were located in the hemifield on the side of the missing pinna; removal of both pinnae resulted in the largest decrement in performance. Finally, vertical localization acuity and performance when localizing low-pass filtered noise were greatly impaired following removal of both pinnae. These results demonstrate the importance of the pinnae in performing front/back and vertical localization tasks in which binaural cues are not available.