FREQUENCY TUNING AND SPONTANEOUS ACTIVITY IN THE AUDITORY-NERVE AND COCHLEAR NUCLEUS MAGNOCELLULARIS OF THE BARN OWL TYTO-ALBA

Single-unit recordings were obtained from the brain stem of the bam ow l at the lever of entrance of the auditory nerve. Auditory nerve and n ucleus magnocellularis units were distinguished by physiological crite ria, with the use of the response latency to clicks, the spontaneous d ischarge rate, and the pattern of characteristic frequencies encounter ed dong an electrode track. The response latency to click stimulation decreased in a logarithmic fashion with increasing characteristic freq uency for both auditory nerve and nucleus magnocellularis units. The a verage difference between these populations was 0.4-0.55 ms. The most sensitive thresholds were similar to 0 dB SPL and varied little betwee n 0.5 and 9 kHz. Frequency-threshold curves showed the simple V shape that is typical for birds, with no indication of a low-frequency tail. Frequency selectivity increased in a gradual, power-law fashion with increasing characteristic frequency. There was no reflection of the un usual and greatly expanded mapping of higher frequencies on the basila r papilla of the owl. This observation is contrary to the equal-distan ce hypothesis that relates frequency selectivity to the spatial repres entation in the cochlea. On the basis of spontaneous rates and/or sens itivity there was no evidence for distinct subpopulations of auditory nerve fibers, such as the well-known type I afferent response classes in mammals. On the whole, barn owl auditory nerve physiology conformed entirely to the typical patterns seen in other bird species. The only exception was a remarkably small spread of thresholds at any one freq uency, this being only 10-15 dB in individual owls. Average spontaneou s rate was 72.2 spikes/s in the auditory nerve and 219.4 spikes/s for nucleus magnocellularis. This large difference, together with the know n properties of endbulb-of-Held synapses, suggests a convergence of si milar to 2-4 auditory nerve fibers onto one nucleus magnocellularis ne uron. Some auditory nerve fibers as well as nucleus magnocellularis un its showed a quasiperiodic spontaneous discharge with preferred interv als in the time-interval histogram. This phenomenon was observed at fr equencies as high as 4.7 kHz.