Perception blindness

I wasn't sure whether it's the onset of old age or whether I'm experiencing early signs of alzheimers disease so I did some research on the net to find out.
I have had issues with short term memory recently so dug a bit deeper and found a site listing the ten signs of early onset of alzheimers. It was a relief to find that I only exhibited two of those signs. I guess when there is a history of the disease in the family it makes you aware of the possibilities of inheritance of that disease.
Still, I have other problems that made me look into something I described as "perception blindness" ; being when I'm looking for something and that something is staring at me in the face and I just don't see it. A very disturbing feeling if you've ever experience it.
What I found is that "perception blindness" is a recognized scientific condition and not just a term I came up with to describe my problem.
On this site is a very amusing exercise by video which point out this problem but unfortunately doesn't offer much explanation to the nature of the problem or whether it's something I should worry about.