Responsiveness of units in the auditory neostriatum of the guinea fowl (Numida meleagris) to species-specific calls and synthetic stimuli

Abstract

1. The tonotopically organized auditory neostriatum (Field L1,L2,L3) was examined in 7 animals for single units which respond to Iambus-like calls and other calls (responsive units), or which clearly distinguish Iambus-like calls of the guinea fowl from other calls of the species (selective units). A high proportion of responsive (168 units: 46%) and of selective units (58 units: 16%) was localized in an area with best frequencies of units between 1 and 2 kHz. Only selective units are described in this paper. 2. Stimulation with 16 natural variations of the Iambus call showed that the 58 selective units formed a population of comparable neurons, in that 80% responded to more than 9 Iambus variations. High response coincidences of neurons to particular Iambus variations indicated that the neuronal preference relied on characteristic lines in the Iambus spectrum and not on accidental properties (Figs. 1–6). 3. In most Iambi the energy is concentrated in spectral lines between 1 and 2.3 kHz. This band is called the doininant spectral band. The highest response score for Iambus variations contained those units which responded to tones of 1.8 and 1 kHz within the dominant band (Figs. 8, 11). One kHz is the fundamental frequency of the harmonic Iambus spectrum, and 1.8 kHz comes close to the formant, i.e. the energy maximum of an average Iambus spectrum. Selective units rarely responded to 1.5 kHz, and Iambus variations with a strong 1.5 kHz component least frequently elicited responses from units (Figs. 8, 11). 4. Half of the selective units showed multiple response maxima when stimulated with tone bursts. One or several maxima were between 1 and 2 kHz. Most other units showed suppression of activity between 1 and 2 kHz or no tone responses (Figs. 9–11). The absence of a tone response was no indication of a higher call selectivity of units. 5. Pulse trains of different periodicity (P) generate harmonic spectra with varying fundamental frequencies (Figs. 13, 14). These stimuli frequently yielded neuronal response maxima at 1 kHz fundamental frequency (the fundamental of Iambus-like calls) as well as maxima at 1.8 kHz and below 700 Hz. Such maxima were also observed in units with no pure tone response or with no tone response in this band (Figs. 9–11). 6. Pulse trains which are amplitude modulated (PAM) generate harmonic spectra with sidebands (Fig. 12). A majority of selective units gave prominent responses to PAM with a combination of 900 Hz to 1.2 kHz fundamental frequency and sidebands of 50 Hz to several hundred Hz distance from the harmonics (Figs. 9, 10). This corresponds to the characteristic spectral lines of Iambus calls (Figs. 13, 14). 7. In other units the spectral envelope of PAM stimuli had to be adjusted to the formant of Iambi by bandpass filtering the signal in order to obtain prominent responses (Fig. 11). These stimuli (Figs. 13, 14) mimic focal properties, i.e. the complete spectral construction of Iambi. 8. One of the most important findings appears to be that selective units were far more specialized to distinguish Iambi from other wide-band calls with spectral overlap in the dominant spectral band than they were to distinguish Iambi from their simple or complex spectral components.