Tonic activity of the crossed olivocochlear bundle in guinea pigs with idiopathic losses in auditory sensitivity

In animals with pre-existing N₁ threshold losses of unknown etiology, transection of the crossed olivocochlear bundle (COCB) at the floor of the fourth ventricle resulted in a marked improvement in thresholds in the region of the loss but not at adjacent frequencies with normal thresholds or increased threshold sensitivities. There was also an increase in N₁ amplitudes. These effects were not obtained if, prior to COCB transection, the COCB were continuously stimulated for l min in these animals, nor were they obtained with COCB transection in animals with normal N₁ thresholds and amplitudes. In the animals with idiopathic N₁ threshold losses, there appeared to be a linear relationship between the amount of threshold sensitivity recovered after COCB transection and the amount of loss existing initially. Parallels between these results and the recently-demonstrated protective COCB effects on temporary threshold shifts in auditory sensitivity are discussed.