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Response Properties of Single Neurons in the Zebra Finch Auditory Midbrain: Response Patterns, Frequency Coding, Intensity Coding, and Spike Latencies

¹ Department of Psychology, University of California, Berkeley, California 94720; ² Department of Psychology, University of Washington, Seattle, Washington 98195

Submitted 2 July 2003; accepted in final form 23 September 2003

The avian mesencephalicus lateralis, dorsalis (MLd) is the auditory midbrain nucleus in which multiple parallel inputs from lower brain stem converge and through which most auditory information passes to reach the forebrain. Auditory processing in the MLd has not been investigated in songbirds. We studied the tuning properties of single MLd neurons in adult male zebra finches. Pure tones were used to examine tonotopy, temporal response patterns, frequency coding, intensity coding, spike latencies, and duration tuning. Most neurons had no spontaneous activity. The tonotopy of MLd is like that of other birds and mammals; characteristic frequencies (CFs) increase in a dorsal to ventral direction. Four major response patterns were found: 1) onset (49% of cells); 2) primary-like (20%); 3) sustained (19%); and 4) primary-like with notch (12%). CFs ranged between 0.9 and 6.1 kHz, matching the zebra finch hearing range and the power spectrum of song. Tuning curves were generally V-shaped, but complex curves, with multiple peaks or noncontiguous excitatory regions, were observed in 22% of cells. Rate-level functions indicated that 51% of nononset cells showed monotonic relationships between spike rate and sound level. Other cells showed low saturation or nonmonotonic responses. Spike latencies ranged from 4 to 40 ms, measured at CF. Spike latencies generally decreased with increasing sound pressure level (SPL), although paradoxical latency shifts were observed in 16% of units. For onset cells, changes in SPL produced smaller latency changes than for cells showing other response types. Results suggest that auditory midbrain neurons may be particularly suited for processing temporally complex signals with a high degree of precision.

This article has been cited by other articles:

				S. M. N. Woolley, P. R. Gill, T. Fremouw, and F. E. Theunissen Functional Groups in the Avian Auditory System J. Neurosci., March 4, 2009; 29(9): 2780 - 2793. [Abstract] [Full Text] [PDF]

				S. M. N. Woolley, P. R. Gill, and F. E. Theunissen Stimulus-Dependent Auditory Tuning Results in Synchronous Population Coding of Vocalizations in the Songbird Midbrain J. Neurosci., March 1, 2006; 26(9): 2499 - 2512. [Abstract] [Full Text] [PDF]

				S. M. N. Woolley and J. H. Casseday Processing of Modulated Sounds in the Zebra Finch Auditory Midbrain: Responses to Noise, Frequency Sweeps, and Sinusoidal Amplitude Modulations J Neurophysiol, August 1, 2005; 94(2): 1143 - 1157. [Abstract] [Full Text] [PDF]