  High precision measurements of the fusion excitation functions for the
reactions ^{40}Ca + ^{194}Pt, ^{192}Os clearly demonstrate that projectile
excitation significantly modifies the potential barrier distribution. In sharp
contrast, fusion of ^{16}O + ^{144}Sm appears to show no influence of the
projectile excitation on the shape of the barrier distribution. These
apparently conflicting conclusions are reconciled in this work, using realistic
coupled--channels calculations, which show that high energy states produce an
adiabatic potential renormalisation. This result indicates that adiabatic
effects restrict, in a natural way, the states which influence the shape of a
fusion barrier distribution. The analysis of barrier distributions thus offers
a criterion for the relevance of the `counter term' prescription in the
Caldeira-Leggett approach.
