Localized distribution of olivocochlear fibers

One of the problems the auditory system has to deal with is that of preserving the identity of a signal against a distracting background, a role in which the centrifugal pathway has been implicated. There are two ways in which this might act: (a) by adjusting the general threshold level of the hair cells to be below saturation level, (b) by inhibition at the boundary edges of the active-fiber array. The hypotheses were tested anatomically by studying the organization of the olivocochlear bundle using retrograde axonal transport. True Blue, (TB) a fluorescent dye was introduced into discrete regions of the guinea pig cochlea. When TB contacted the whole cochlea labeling appeared throughout the whole ipsilateral lateral superior olive (LSO), together with some lesser labeling in the contralateral periolivary nuclei and trapezoid body. When TB was applied only to the apical turn labeling was confined to the dorsolateral region of the LSO, while basal turn treatment resulted in labeling restricted to the medial region. This cochleotopic organization corresponds to the tonotopic arrangement of the afferent cells (cat) and suggests that the OCB acts by a localized rather than a diffuse mechanism.