Nancy Pontika has published "Why Principal Investigators Funded by the U.S. National Institutes of Health Publish in the Public Library of Science Journals" in Information Research.
Here's an excerpt:
The Institutes-funded investigators submitted to the Public Library of Science journals because they favour the high impact factor, fast publication speed, fair peer-review system and the articles/ immediate open access availability.
Conclusions. The requirements of the National Institutes' public access policy do not influence the investigators' decision to submit to one of the Public Library of Science journals and do not increase their familiarity with open access publishing options.