Auditory-Evoked Potentials from Cochlea to Cortex as Influenced by Activation of the Efferent OlivoCochlear Bundle

The crossed olivocochlear bundle (OCB) of Rasmussen was stimulated stereotaxically in acute experiments on cats immobilized by Flaxedil and prepared either under pentobarbital, or chloralose, or with a high-spinal section. Middle-ear muscles were cauterized. The efferent effects on sound-evoked potentials were titrated as equivalent dB changes in sound energy by a matching procedure taking into account the intensity function of the responses to sound alone. Maximal inhibition of the N1 auditory-nerve response to click was equivalent to a 25-dB decrease. The potentials evoked in cochlear nucleus, superior olive, inferior colliculus, medial geniculate, and auditory I area of the cerebral cortex when expressed similarly in equivalent dB changes disclosed a decrease proportional to that of N1. The anomalous Ruben-Sekula effect i.e. reduction of cortical auditory potential without concomitant change in N1 during brainstem stimulation was shown not to involve the OCB inhibition but to depend on a cortical refractory state subsequent to spurious stimulation of second-order auditory axons by inadequately placed electrodes. With suitable precautions, pure OCB stimulation was achieved in most of our experiments, and interference from this effect thus excluded. The OCB activation also paradoxically potentiates the cochlear microphonic potential (CM), but the change amounted at most to a +4-equivalent-dB increase in sound energy. This increase in receptor potential, while important for understanding the synaptic mechanisms of the inner ear, is ignored by the central nervous system, since acoustic signals are simultaneously suppressed in the auditory nerve (Fig. 13). Various parameters of OCB effects were analyzed in detail, e.g. voltage, duration, frequency and number of shocks delivered to the bundle, and interval between the conditioning stimulation and the testing sound. More than three shocks at a frequency higher than 50/sec are needed to produce detectable changes in N1 or CM, and 40 shocks at 400/sec will generally produce maximal effects. The dissipation of the changes after OCB stimulation is rather slow and has an exponential time constant of 90 to 180 msec. These and other intrinsic features of olivo-cochlear axons may be reflected in the operational characteristics of the whole centrifugal extrareticular auditory control system (CERACS), for which the OCB is one of the peripheral effector links. © 1962, Acoustical Society of America.SCOPUS: ar.j