Every time <Ctrl> <space> is pressed the image-sequence advances by one.
Consider a folder with the following images :-
Note that 'img_0011_l.jpg' is not part of an image-pair.
You load 'img_0010_l.jpg' and 'img_0010_r.jpg' as an Independent(Left/Right) file.
If you press the 'space' key, the image-sequence advances by two images to display 'img_0011_l.jpg' and 'img_0012_l.jpg'.
The right-hand side is displaying the left-image of an image-pair.
If you now press the 'Ctrl Space' key, the image-sequence only advances by one so that the image-pair 'img_0012_l.jpg' and
'img_0012_r.jpg' is displayed.