[Koha] searching

Hi, I am using koha 3.08 and in OPAC the masthead file contains the drop down box for the searches with option like keyword,title,author,accession number. But while searching with keyword such as "a" fetches results from all the fields like publication,author,title etc. I want the keyword should fetch result only from title,subject or note field not from all the fields. How should i do this. Thanks in advance.