Found a few references. In a nutshell, Interaural Level Difference (ILD) and Interaural Time Difference (ITD) are the means for interpreting the location of a sound source. Level difference is mostly used for high frequencies due to wavelength relative to head size, yet the time difference is identical for low frequencies as for high. Time difference allows for low frequency localization. (See the abstract at this link, http://adsabs.harvard.edu/abs/2005ASAJ..117.2391B) The precedence effect also comes into play in a typical domestic room where one is listening relatively close to the source, and level differences are aided by proximity reinforcement. Try listening to sine waves coming from only the left or right channel. You can easily identify which speaker is producing the sound unless the frequency hits a room mode, then ringing may obscure the location cues. More reverberant spaces also make it more difficult to localize, even for high frequencies. Location cues tend to be easily obscured, and room acoustics can affect perception, for better or worse, making it difficult to determine a specific cut-off frequency. It's a convenience to say frequencies under X Hz are nondirectional.
This link suggests learning is involved: http://www.ncbi.nlm.nih.gov/pubmed/7602671
This link goes into greater detail: http://www.aip.org/pt/nov99/locsound.html
One more: http://www.ncbi.nlm.nih.gov/pubmed/1564201
This link suggests learning is involved: http://www.ncbi.nlm.nih.gov/pubmed/7602671
This link goes into greater detail: http://www.aip.org/pt/nov99/locsound.html
One more: http://www.ncbi.nlm.nih.gov/pubmed/1564201