  Generalized nucleon polarizabilities for virtual photons can be defined in
terms of electroproduction cross sections as function of the 4-momentum
transfer $Q^2$. In particular, the sum of the generalized electric and magnetic
polarizabilities $\Sigma=\alpha+\beta$ and the spin polarizability $\gamma$ can
be expressed by virtual photon absorption cross sections integrated over the
excitation energy. These quantities have been calculated within the framework
of the recently developed unitary isobar model for pion photo- and
electroproduction on the proton, which describes the available experimental
data up to an excitation energy of about 1 GeV. Our results have been compared
to the predictions of chiral perturbation theory.
