The response properties of auditory neurones in the midbrain of the domestic fowl (Callus gallus) to monaural and binaural stimuli

Abstract

Extracellular recordings were made from auditory units in the midbrain (NMLD) of the domestic fowl. Tonal stimuli delivered monaurally to either ear revealed that contralateral excitation predominated and was coupled with ipsilateral inhibition (EI, 46%), excitation (EE, 21%) or no effect (EO, 17%). Conventional V-shaped tuning curves were readily obtained from contralateral excitatory thresholds, however complex effects were noted such as, inhibitory sidebands and double sensitivity peaks. The distribution of best frequency (BF) thresholds ranged from 60 Hz-5.0 kHz, with the most sensitive region between 1.0–2.0 kHz. Analysis of cell types with BF revealed wide distributions for EI and EE cells, however EO cells had a distinct bias for BF's above 1.0 kHz. Binaural stimuli were used to study unit sensitivity to experimentally produced differences in interaural intensity and time (phase). EI cells generally showed good sensitivity for interaural intensity differences up to 20 dB, across a wide range of BF's. EE cells which demonstrated interaural time sensitivity did so over several milliseconds (maximum physical delay 100 μs). These effects appeared to be related to the period of the stimulus (below 800 Hz) and reflected a tendency for unit responses to be phase-locked. Considering the limits of head size and high frequency sensitivity in the domestic fowl, the present neural data suggests that binaural intensity differences may be the only viable cue during sound localization.