  During microlensing events with a small impact parameter, the amplification
of the source flux is sensitive to the surface brightness distribution of the
source star. Such events provide a means for studying the surface structure of
target stars in the ongoing microlensing surveys, most efficiently for giants
in the Galactic bulge. In this work we demonstrate the sensitivity of
point-mass microlensing to small spots with radii $r_s\lesssim0.2$ source
radii. We compute the amplification deviation from the light curve of a
spotless source and explore its dependence on lensing and spot parameters.
During source-transit events spots can cause deviations larger than 2%, and
thus be in principle detectable. Maximum relative deviation usually occurs when
the lens directly crosses the spot. Its numerical value for a dark spot with
sufficient contrast is found to be roughly equal to the fractional radius of
the spot, i.e., up to 20% in this study. Spots can also be efficiently detected
by the changes in sensitive spectral lines during the event. Notably, the
presence of a spot can mimic the effect of a low-mass companion of the lens in
some events.
