"When the breath gets really full and refreshing throughout the body, you can drop the evaluation and simply be one with the breath. This sense of oneness...pervading the entire range of your awareness. You’re at one with whatever you focus on, at one with whatever you do. There’s no separate “you” at all..it’s cetaso ekodibhava, unification of awareness—a factor of concentration, present in every level from the second jhana up through the infinitude of consciousness." (Thanissaro Bhikku, Mindfulness Defined)

I have found the phenomenology of "unification of awareness" to be quite surprising, and will attempt a more thorough description than Thanissaro Bhikkhu's. Normally, it appears as if the mind shifts back-and-forth between sense-experiences and suffering (affect / 'being' / shadow being / craving-clinging-becoming / however you like to conceive of it), and the experience of this rapid shifting has been called a number of things ("vibrations" / "attention wave" / etc.). As it presents for me now, and as I recall it presenting in the past, unification of awareness does away with this perception to a significant degree; the mind clearly focuses on sense-experiences, while suffering appears to float on top of the sense-experience substratum without appearing to divert the mind from sense-experience in so gross a way as before. (The name is quite apt; the mind is unified, rather than appearing to move back-and-forth between these different types of experiences.) Given this, one who cultivates unification of awareness will provide themselves with a strong base for attentiveness to sensuousness...as unification of awareness appears to force one towards the senses.

Unification of awareness is also useful to cultivate as a preliminary tool for discernment and insight (as one need only watch suffering fluctuating over sense-experience, which is simple to do).

It is worth seeing (after cultivating it in context of concentration) whether one can learn to summon this jhana factor up at will, or with minimal concentration; such an ability could be quite valuable. One could walk around like this, eyes open. (My experience says that this is quite possible.)

Though the cultivation of felicity was never a practice I used, unification of awareness seems to share something non-affective in common with the kind of experience one has upon generating felicity, and so it would be good to hear from someone more familiar with felicity whether this is so.

Unification of awareness is similar to an EE in a way, with one large difference being that unification of awareness does not mitigate affective experience. (The impression one will have of unification of awareness may be very different in the case of one who has a great deal of it to deal with, compared with one who only has residue, due to the ways that affect is distorting...one may think they have become one with everything...one may think that their experience has turned into something formless...however, as far as I can see, no matter what impression one gets, the experience is fundamentally the same in both cases, with the difference being solely in terms of what affect makes of it.)

I am very confident that unification of awareness would have significant and valuable effects, with respect to the path, on the functioning of the brain (one can cultivate it and see how sharply one's experience shifts as soon as it arises) but I cannot provide hard evidence for this yet.

If anyone finds this unique jhana factor especially useful or not-useful as a support for some other practice, or has any thoughts on its phenomenology, please feel free to post about it here.

N.B. If one is unfamiliar with it, the only way I know of to cultivate at first it is to have significant concentration and enter the 2nd jhana; to distinguish between experience with and without this factor, one can compare the 1st jhana to the 2nd.

Interesting stuff, I think I may have been doing this already but describing it in a slightly different way. What I do is what I describe as lining everything up; taking the entire sense field as object, still experiencing each sense door but, and this is kind of hard to explain, having the visual, auditory, kinesthetic, gustatory, olfactory and mental objects lined up with what is actually occurring, e.g. if I'm standing in the garden, the eyes see what is there to be seen, the ears hear was is there....etc etc and the mental image is a gestalt of the whole thing as it's happening so that everything is being experienced at once, at the sense doors. When I do this, the sense of internal/external experience evaporates and it just "is" but the way you describe affect "floating on top" also sounds very similar to what happens here.

I don't know if that makes sense, which is why I've never really talked about it before, but the practice of it, particularly when walking around open eyed, consistently leads to very refined EE's and was part of what I was doing before the two hour long PCE happened a few weeks ago. I'll think of a better way to describe it 'cause I know what I'm trying to say, but I'm trying to describe it in a way that can be tested and verified although "lining up the sense doors" seems to convey the idea.

Is this anything like what you're talking about?

P.S. I have a suspicion that practicing the yogic technique of dhyana would be a useful way to develop this ability as that's aimed at the same sort of unification, however in my experience this leads to a jhanic absorption. The practice of mahamudra also seems useful for this purpose.

It may be what I'm talking about, but it's hard to say. It certainly is possible to have unification of awareness outside of deep concentration.

The easiest way to test this out is to flip between the 1st and 2nd jhana with a reasonable amount of concentration, and see if the change involved is the same as what you do outside of jhana.