The combined selectivity for amplitude modulation frequency (AMF) and interaural time difference (ITD) was investigated for single units in the auditory midbrain of the grassfrog. Stimuli were presented by means of a closed sound system. A large number of units was found to be selective for AMF (95%) or ITD (85%) and mostly, these selectivities were intricately coupled. At zero ITD most units showed a band-pass (54%) or bimodal (24%) AMF-rate histogram. At an AMF of 36 Hz, which is equal to the pulse repetition rate of the mating call, 70% of the units possessed an asymmetrical ITD-rate histogram, whereas about 15% showed a symmetrically peaked histogram. With binaural stimulation more units appeared to be selective for AMF (95%) as was the case with monaural stimulation (85%). A large fraction of the units appeared to be most selective for ITD at AMFs of 36 and 72 Hz, whereas units seldomly exhibited ITD selectivity with unmodulated tones. Based upon previous papers (Melssen et al., 1990; Van Stokkum, 1990) a binaural model is proposed to explain these findings. An auditory midbrain neuron is modelled as a third order neuron which receives excitatory input from second order neurons. Furthermore the model neuron receives inputs from the other ear, which may be either excitatory or inhibitory. Spatiotemporal integration of inputs from both ears, followed by action potential generation, produces a combined selectivity for AMF and ITD. In particular the responses of an experimentally observed EI neuron to a set of stimuli are reproduced well by the model.
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