The determination of sound source elevation (i.e., in
the vertical planes) holds special problems for all vertebrates.
For example, sources at all locations on the
median vertical plane have equivalent pathways to
both ears. Animals seem to have solved these ambiguities
using any of three mechanisms: (1) bilaterally
asymmetrical ears (e.g., the barn owl: Konishi, 1973),
(2) movements of the head and ears, and (3) directional
frequency ¢ltering of the outer ears, head, and body
(Wightman and Kistler, 1993). The latter depends
upon capacities for frequency analysis. All vertebrates
studied are able to analyze the frequency spectrum with
varying degrees of acuity (Table 1), and thus may use
monaural spectral cues in vertical localization.