Does Open Access Increase Citations?

PLoS ONE has a high impact factor despite publishing 70% of submitted papers. It is, of course, the best known open access journal. Does open access increase the rate of citation? The naive answer would be that it must obviously do so. Taking a barrier to access away must result in more people reading and citing a paper. On the other hand most research libraries are going to subscribe to the top journals in each field so this might be more important for lower ranked journals than highly ranked journals. Analysis can also be confounded if higher quality articles are submitted to open access journals as is argued for PLoS ONE due to its publication fee. A 2006 article in PLoS Biology compared open access and non-open access articles in the same journal - PNAS - and concluded that the open access articles were cited more. But more recent research across broader samples of journals seems to be mixed. I haven't done any detailed research on this but need to include something on this in a presentation on Friday. Any feedback would be welcome.