Coding of narrow-band and wide-band vocalizations in the auditory midbrain nucleus (MLD) of the Guinea Fowl (Numida meleagris)

Abstract

1. Neurones in the auditory midbrain nucleus (MLD) of Guinea Fowl were examined for specific encoding of vocalizations. Single units were recorded in chronic awake preparations of birds. The animals were presented with various artificial stimuli and tape recordings of conspecific calls. 2. Several Guinea Fowl vocalizations are characterized by multiple-line spectra or by inhomogeneous noise components (Fig. 4). Such broad energy distributions were called ‘frequency complexes’. Neuronal preference particularly of calls which contained frequency complexes was examined by three methods: (a) Use of natural variations of calls; (b) selective filtering of calls; (c) technical synthesis of complex components of calls. 3. Of the neurones in the material, 60% showed responses to complex stimuli not simply predictable from pure tone responses (complex neurones). None of the neurons gave exclusive responses to a particular call but some preferred certain calls. Several types of complex neurones were distinguished which might be considered suitable for the detection of frequency complexes in calls. 4. Units with broad inhibitory bands in response to pure tones were excited by harmonic (pulse) spectra or other frequency combinations which fell into the inhibitory band (Figs. 4–6). Similarly units with weak but wide-band excitation by pure tones often responded to multiple-line spectra (Fig. 6). 5. Neurones with preference for several tone frequencies favoured spectra which overlapped those optimal frequencies (Fig. 3). 6. FM sensitivity was seen in combination with preference for frequency complexes (Fig. 8).