Picture a 3 by 3 grid. Then picture the words "gas", "oil", and "dry" spelled downwards in the columns left to right in that order. Looking at the picture in your mind, read the words across on the grid.

Interestingly, I find the task much easier if I do it the other way: visualizing the words spelled across, and then reading off the words going down the grid.

If mental images consist of replayed saccades, this makes perfect sense. To generate the downward images of words and then read across would reasonably be harder than simply replaying the stored "across" patterns, and then reading them down. IOW, visualization is more like vectors and sprites than it is like pixels -- which reflects how sight itself works.