Image Gallery Attachment Pages: Next/Previous Navigation Reversed

I’ve set up attachment pages (full screen enlargements) for my (default) image gallery, running the 2012 theme in WordPress 4.0. Having clicked on a thumbnail in my (default) image gallery and gone to the relevant attachment page, it is then possible to navigate directly from one attachment page to another using the Next/Previous commands which appear thereon. Logically, these should scroll Left – Right/Right – Left, respectively but their functions are reversed at present. Is there some code and/or a plugin which will sort them out, permanently?