Cable inspired me, so I did a cursory literature search for scientific publications on HLs. I have a list of about 150 citations. If anyone is interested, e-mail me and I'll send it to you. It's too long to post here. Most of them (almost all) are not available on-line, but if you have access to a good research library, you should be able to get many of them. A large portion are in just a handful of journals.