To June Okumura:

Yes, I agree it is difficult to tease out the difference leadership image makes to voters from all the other variables. But first Ben Nyblade and I in an article in 2004 in the BRITISH JOURNAL OF POLITICAL SCIENCE (and then updated more recently by Robert Pekkanen and me in our book THE RISE AND FALL OF JAPAN'S LDP) have a chart that does show that voters are perceiving the PM differently than the party since Naksone.