Epibolai really just means “focus.” We understand the eyes can focus on a specific range within the sea of information around us, whereas the ears can focus on a different range within this sea of information, and so for the rest of the senses.
As we know, there is a lot more data out there. From radio waves to radiation, there are many perpetuated disturbances (waves/eidola) all around us that carry information that our external senses do not pick up.
Epicurus understood the internal sense of the mind also focuses on a range of this information, which produces mental images (prolepseis). If we have a thought that is not based on images – it is just "a label on an empty box" (hypolepsis). So, we are talking about a “mental picture.”
Just as when we increase our views of the formerly distant tower, we come to see it’s true nature – as we increase our views of these mental pictures, while guided by the laws of nature, we are more acutely able to focus on and more accurately able see what exists within the universe.
The anticipations are these mental pictures, and mental focus (ἡ ἐπιβολή τῆς διανοίας) is our attention to them.