  After nearly sixty years since its introduction, the phenomenological
bell-shape Perey-Buck spatial nonlocality in the optical model potential for
nucleon-nucleus scattering has remained unaccounted for from a microscopic
standpoint. In this article we provide a quantitative account for such
nonlocality considering fully nonlocal optical potentials in momentum space.
The framework is based on a momentum-space in-medium folding model, where
infinite nuclear matter $g$ matrices in Brueckner-Hartree-Fock approximation
are folded to the target one-body mixed density. The study is based on chiral
next-to-next-to-next-to-leading order (N3LO) as well as Argonne $v_{18}$
nucleon-nucleon bare interaction models. Applications focus on $^{40}$Ca($p,p$)
scattering at beam energies in the range 11-200 MeV, resulting in the
identification of a separable structure of the momentum-space optical potential
of a form we coin as $JvH$, with a nonlocality form factor as one of its terms.
The resulting nonlocaliy form factor features a bell-shape with nonlocality
range $\beta$ between 0.86 and 0.89 fm, for both proton and neutron beams at
energies below 65 MeV. An analytic toy model is introduced to elucidate the
underlying mechanism for the nonlocality in the optical model, providing an
estimate of its range based on the Fermi motion of the target nucleons and the
long-range part of the $NN$ interaction.
