  To explore resonance phenomena in the nonlinear region, we show by
experimental measurements and theoretical analyses that resonance happens in
modulation instability (MI) from non-instantaneous nonlinearities in
photorefractive crystals. With a temporally periodic modulation in the external
bias voltage, corresponding to a modulation in the nonlinear strength, an
enhancement in the visibility of MI at resonant frequency is reported through
spontaneous optical pattern formations. Modeled by such temporally periodic
nonlinear driving force to the system, theoretical curves obtained from a
nonlinear non-instantaneous Schr\"{o}dinger equation give good agreement to
experimental data. As MI is a universal signature of symmetry-breaking
phenomena, our observation on the resonance in MI may provide a control on
chaotic, solitary, and turbulence waves.
