  We study low-energy virtual Compton scattering off the proton within the
framework of a nonrelativistic constituent quark model.
  The Compton tensor is divided into two separately gauge-invariant
contributions.
  The first consists of the groundstate propagation in the direct and crossed
channels together with an appropriately chosen term to satisfy gauge
invariance.
  The residual part contains the relevant structure information characterized
by the so-called generalized polarizabilities.
  We discuss two different schemes to obtain the generalized polarizabilities
from the residual term.
  Explicit predictions for the generalized polarizabilities are presented for
the Isgur-Karl model.
  Our results are compared with previous predictions in that model as well as
other approaches.
