Display node title in Colorbox of gallery items

I'm not sure this is an issue of Node Gallery or of Colorbox, so I hope someone can tell me which project is more appropriate.

Situation: I've setup a Node Gallery where the gallery page displays thumbnails of all related gallery items. When you click on a thumbnail, you'll see a colorbox (Colorbox module) that display the original image with a caption. So far so good.

However, the caption currently displays the name of the file, not the title of the Gallery Item node it belongs to. No matter which caption setting I use in the Node Gallery Item Thumbnail display.

Example: I've uploaded files file20140001.jpg file20140010.jpg to Gallery 'Test'. On the 'manage items' tab I've changed the titles to 'Testing 01' until 'Testing 10'.

When I view the 'Test' gallery page I can see all the images with the titles 'Testing 01' etc. When I click on the image a colorbox appears and the caption there is 'file20140001.jpg' etc.

What should I do to get the colorbox to display 'Testing 01' etc. as caption?

I've searched the internet, but all tutorials use the file name as title for the node, so it is not clear what is displayed.

I did some more digging. The 'title text', 'alternative text' and 'content text' refer to the title field, the alt field and the name field resp. of the image that is contained in the Node Gallery Item. So the alternative approach would be to synchronize the title of the Node Gallery Item with the title of the image. Is that possible?

Thanks.

Helma