  Isotopic effects in the fragmentation of excited target residues following
collisions of $^{12}$C on $^{112,124}$Sn at incident energies of 300 and 600
MeV per nucleon were studied with the INDRA 4$\pi$ detector. The measured yield
ratios for light particles and fragments with atomic number $Z \leq$ 5 obey the
exponential law of isotopic scaling. The deduced scaling parameters decrease
strongly with increasing centrality to values smaller than 50% of those
obtained for the peripheral event groups. Symmetry term coefficients, deduced
from these data within the statistical description of isotopic scaling, are
near $\gamma =$ 25 MeV for peripheral and $\gamma <$ 15 MeV for central
collisions.
