  Results on transverse mass spectra of neutral pions measured at central
rapidity are presented for impact parameter selected 158$\cdot A$ GeV Pb + Pb,
and Pb + Nb collisions. The distributions cover the range $0.5
\mathrm{GeV}/c^{2} \le m_{T} - m_{0} \le 4 \mathrm{GeV}/c^{2}$. The change of
the spectral shape with centrality is studied in detail. In going from
peripheral to medium central collisions there is a nuclear enhancement
increasing with transverse mass similar to the well known Cronin effect, while
for very central collisions this enhancement appears to be weaker than
expected.
