Final answer:
Difficulty in localizing sound from directly overhead is due to the absence of horizontal cues such as interaural time differences and interaural intensity differences. The pinnae use monaural cues to aid in vertical localization, but these are less effective compared to binaural cues for horizontal localization.
Step-by-step explanation:
When a sound comes from directly overhead, it is difficult to localize because there are few or no cues for horizontal sound localization. We normally rely on differences in the sounds that reach each ear to locate origins of sounds in the horizontal plane, such as the interaural time differences and interaural intensity differences. However, sounds from directly above do not provide these horizontal cues, making localization challenging. Instead, our ears use monaural cues from the pinna (the outer part of the ear) to help distinguish sounds coming from above or below.
The pinnae are shaped to interact with sound waves differently based on the sound's source direction. For vertical sound localization, these monaural cues play a crucial role as the binaural cues are not effective for sounds originating from directly above or below.
In summary, the human auditory system is well-equipped for horizontal sound localization through processing interaural time differences and interaural intensity differences in the auditory brain stem, but it has less ability to localize sounds coming from a vertical direction due to the lack of horizontal cues.