Relation of binaural interaction and spectro-temporal characteristics in the auditory midbrain of the grassfrog

The relation between binaural interaction type and spectro-temporal characteristics was studied for single units in the auditory midbrain of the grassfrog. Tonal and continuous wideband noise ensembles have been used as stimuli. Spectro-temporal sensitivities were determined for ipsi-, contra- and bilateral stimulus presentation by a closed sound system. Binaural interaction was classified in monaural EO (one ear excitatory), binaural EE (both ears excitatory) and EI (one ear excitatory, the other inhibitory) and purely inhibitory categories. Binaural interaction appeared to be rather invariant to alterations in stimulus intensity and type. A very clear correlation was observed between best frequency and binaural interaction type: EE units are predominantly of high best frequency, whereas EI units are predominantly of low best frequency. The correlation with latency was less significant: EE units tended to have somewhat shorter latencies that EI units. EO units take an intermediate position. Comparisons of ipsi-, contra- and bilateral spectro-temporal sensitivities, revealed differences in best frequency, latency and temporal discharge pattern. In some units a complex interplay of excitatory and inhibitory monaural influences was demonstrated. A number of units was recorded, which were characterized by multiple activation or suppression areas. The majority of these units exhibited frequency-dependent binaural interaction types. In some units it was noticed that binaural interaction type can be dependent on state of adaptation. A comparison of binaural interaction types of neighbouring units provided only weak evidence for a binaural organization in the anuran auditory midbrain, since simultaneously recorded pairs shared the same binaural interaction type only slightly more than expected by mere chance (chi 2-test, P less than 0.10).