  We have measured activation gaps for odd-integer quantum Hall states in a
unidirectional lateral superlattice (ULSL) -- a two-dimensional electron gas
(2DEG) subjected to a unidirectional periodic modulation of the electrostatic
potential. By comparing the activation gaps with those simultaneously measured
in the adjacent section of the same 2DEG sample without modulation, we find
that the gaps are reduced in the ULSL by an amount corresponding to the width
acquired by the Landau levels through the introduction of the modulation. The
decrement of the activation gap varies with the magnetic field following the
variation of the Landau bandwidth due to the commensurability effect. Notably,
the decrement vanishes at the flat band conditions.
