LiveScience.com has an article on the use of memory, it talks about how we focus our limited resources on remembering key parts of a scene, and put less memory into less important parts. They illustrate the concept with a photo of a baton hand-off in a relay race, only spotlighting the faces and the connecting hands and identifying race numbers of the team you are interested in, together with some random striking item like another runner’s pink hair. The rest of the picture is pixelated with very low definition.
There is another point – which the article doesn’t mention – in that the highlighted pieces only make sense in the context of the complete picture. It would not be useful if there was NO memory allocated to the ‘irrelevant’ bits – the complete picture is essential for understanding what is actually happening. You need to see the highlighted information (the baton being handed-off) in the context of the race.
Information is only useful when contextualized.