Brian M:
Is it possible to access the immaterial jhanas with open eyes?
Yes, it is possible, although it is typically more difficult as compared to closed-eye. That's also generally the case with all of the jhanas except 1 and 2 for reasons that I'm too lazy to write out.
Brian M:
If so, how does the experience differ from closed-eye access?
They can seem like completely different manifestations depending on how strong your concentration is and what you're focusing on during its' manifestation. If you think about it, the visual field has a high impact on one's experience (hence why the eyes are often closed or half-closed during meditation), and so the entire flavor of experience is usually quite altered when compared to closed-eye. If you are able to access them with eyes closed, then try imagining the visuals you get that way but think of them as if they were morphing the open-eye visual field in the same way that it's morphing with eyes closed. In example, if your eyes are closed and it's morphing like a 2D blob of some sort, then the visual field will probably be shifting around and morphing in the same way if you were open-eyed. You can try accessing a jhana then gradually opening your eyes, and perhaps get an idea of what a stable one would look like. Note that the particularities of these manifestations are probably quite idiosyncratic, but one thing that is typically present is a shifting or waving or morphing or expansion or contraction of the visual field in some form or fashion.
Brian M:
Does entry into an immaterial jhana manifest as a change in visual experience?
The entry itself may be sudden or gradual. Which one it is probably depends on a lot of things, but likely depends on your current strength of concentration, the approach you're taking to enter the jhana, and your familiarity with the jhana being left and/or entered into. This is the case with visual manifestations with both open and closed eye practice.
Regards,
Trent
