FIGURE 9.

Physical prerequisites shaped distinct binaural processing and coding strategies in mammals and birds. Head size (abscissa) and hearing range (ordinate) during the time of the middle-ear development define the binaural cue (gray scale: white = ILD; gray = ITD) that is most easily exploitable for horizontal sound localization by mammals (blue) and archosaurs/birds (brown). The binaural cue in turn shaped the emergence of distinct neuronal mechanisms that are optimized for the processing and encoding of the particular cue (boxes on right-hand side of panel). Early mammals (Morganucodon, dark blue) were very small and had high-frequency hearing. Therefore, they used ILDs as original binaural cue. Subsequent evolutionary changes in head size and/or hearing range (e.g., cats or humans, light blue) allowed the use of ITDs. However, the neuronal mechanisms (precise temporal integration of excitatory and inhibitory inputs) and coding principles (population code) remained similar to early, high-frequency hearing mammals (dark and light blue boxes on right-hand side of panel). Early archosaurs (brown) were very large and had low-frequency hearing. Therefore, they used ITDs as original binaural cue. Subsequent evolutionary changes in head size and/or hearing range in birds (e.g., chicks or barn owls, brown) allowed the continued use of ITDs. Thus, the neuronal mechanisms (delay-lines) and coding principles (labeled line) remained the same as in early archosaurs (brown box on right-hand side of panel).