The superior paraolivary nucleus (SPON) is a prominent nucleus of the superior olivary complex. In rats, this nucleus is composed of a morphologically homogeneous population of GABAergic neurons that receive excitatory input from the contralateral cochlear nucleus and inhibitory input from the ipsilateral medial nucleus of the trapezoid body. SPON neurons provide a dense projection to the ipsilateral inferior colliculus and are thereby capable of exerting profound modulatory influence on collicular neurons. Despite recent interest in the structural and connectional features of SPON, little is presently known concerning the physiological response properties of this cell group or its functional role in auditory processing. We utilized extracellular, in vivo recording methods to study responses of SPON neurons to broad band noise, pure tone, and amplitude-modulated pure tone stimuli. Localization of recording sites within the SPON provides evidence for a medial (high frequency) to lateral (low frequency) tonotopic representation of frequencies within the nucleus. Best frequencies of SPON neurons spanned the audible range of the rat and receptive fields were narrow with V-shaped regions near threshold. Nearly all SPON neurons responded at the offset of broad band noise and pure tone stimuli. The vast majority of SPON neurons displayed very low rates of spontaneous activity and only responded to stimuli presented to the contralateral ear, although a small population showed binaural facilitation. Most SPON neurons also generated spike activity that was synchronized to sinusoidally amplitude-modulated tones. Taken together, these data suggest that SPON neurons may serve to encode temporal features of complex sounds, such as those contained in species-specific vocalizations.
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