Tonotopic organization and functional characterization of the auditory thalamus in a songbird, the European starling

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Abstract
1. The diencephalic auditory nucleus of the European starling, the nucleus ovoidalis, shows rostrocaudal and dorsoventral diameters of 500–800 μm and a mediolateral diameter of 800–1000 μm. This small and sharply delimited nucleus is composed of densely packed neurons (Fig. 1). 2. Its tonotopic organization consists of evenly spaced isofrequency contours, with best frequencies decreasing ventrally (Fig. 2). The frequency range was found to be 150 Hz to 7030 Hz (Fig. 4b). 3. Apart from tonotopic organization, other characteristics of single units demonstrate the uniformity of the neuronal population. Units have high spontaneous activities (mean 61 pps; Fig. 4a), and show mainly stimulus correlated tonic discharge patterns. In most cases, excitatory frequency bands are enclosed by inhibitory frequency bands (Fig. 3). 4. Single units were tested, applying various stimulus classes differing in time structure (BPN, sine, FM up, FM down, SFM, SAM) but sharing a common frequency band. All neurons tested responded to all classes. Evaluation of stimulus class preference, however, revealed that BPN and SFM caused the strongest responses, whereas FM and SAM were less effective (Fig. 5). 5. Comparison of the single unit responses in the ovoid nucleus with those known for avian auditory forebrain and midbrain centres strongly suggests a relay function for the diencephalic nucleus.