Effect of Cutting the Commissure of Probst on Sound Localization and Auditory Orientation Response in the Albino Rat

Abstract

Albino rats were tested for their ability to localize sounds in space and for their reflex head orientation to auditory stimulation before and after a micro knife transection of the commissure of Probst. The animals were trained to approach a sound source on a two choice test apparatus in order to obtain a water reward. The minimum audible angles were then determined for midline (left vs. right) sound localization using the method of descending limits to define the thresholds for auditory spatial acuity. The commissure of Probst was then transected through its midline decussation with a micro knife inserted into the brain according to stereotaxic coordinates. Following a postoperative recovery period of at least one week, the rats were retrained and again tested for sound localization. After completing the sound localization tests, the animals were tested for reflex head orientation with a single burst of 70dB (SPL) broad-band noise. In most cases, the lesions were within 200-300 microns of the midline, and damage at locations lateral to the knifecut was very slight. The condition of the damaged fibers was confirmed by both anterograde and retrograde tract tracing methods. After a complete transection of the commissure of Probst, the number of cell bodies in the dorsal nucleus of the lateral lemniscus (DNLL) was greatly reduced. Most of the contralaterally projecting neurons in the DNLL underwent retrograde degeneration. Complete transection of the commissure of Probst resulted in elevated behavioral thresholds for midline sound localization. The minimum audible angles were elevated, and reflex head movements towards the sound source were abolished in most animals with extensive lesions. These results suggest that the Commissure of Probst fibers, including the crossed projections from the DNLL, play a role in auditory spatial perception, especially in the ability to discriminate small auditory angles. These projections also appear to play an important role in the reflex pathway for auditory head orientation.