Synthesis of hemispheric itd tuning from the readout of a neural map: Commonalities of proposed coding schemes in birds and mammals

A major cueto infer sound direction isthe difference in arrivaltime ofthe sound atthe left and right ears, called interauraltime difference (ITD). The neural coding of ITD and its similarity across species have been strongly debated. Inthe barn owl, an auditory specialist relying on sound localizationto capture prey, ITDs withinthe physiological range determined bythe head width aretopographically represented at each frequency. The topographic representation suggests that sound direction may be inferred from the location of maximal neural activity within the map. Such topographical representation of ITD, however, is not evident in mammals. Instead, the preferred ITD of neurons in the mammalian brainstem often lies outside the physiological range and depends on the neuron's best frequency. Because of these disparities, it has been assumed that how spatial hearing is achieved in birds and mammals is fundamentally different. However, recent studies reveal ITD responses inthe owl's forebrain and midbrain premotor areathat are consistent with coding schemes proposed in mammals. Particularly, sound location in owls could be decoded from the relative firing rates of two broadly and inversely ITD-tuned channels. This evidence suggests that, at downstream stages, the code for ITD may not be qualitatively different across species. Thus, while experimental evidence continues to support the notion of differences in ITD representation across species and brain regions, the latest results indicate notable commonalities, suggesting that codes driving orienting behavior in mammals and birds may be comparable.