help - isearch highlits my buffer into a sort of Christmas tree

[nospam55]

Hi

when I say e.g. 

  C-s a

my new emacs highlights every visible occurrence of 'a' , and not only
the next (found) one ; the result is reminiscent of a beautiful Christmas tree 
with dozens of little red lamps which however in the emacs situation is in my 
feeling
a bit confusing. I would like to disable this behavior.

How to get highlighted only the found occurrence ?

  Thank you :-)