The accretion-ejection mechanism acting in Active Galactic Nuclei
(AGN) is one of the astrophysical main issues. The X-ray emission in
AGN, associated with the accretion flow, is strongly coupled with the
radio emission, associated with a jet. Strong X-ray versus radio
correlations are found even in radio-quiet AGN, where jets are absent
or very weak. To explore the origin of radio emission in radio-quiet
AGN, we have surveyed the sub-parsec scale radio cores of a complete
sample of well known local AGN, mapping the faintest and least
luminous nuclei at a level of sub-mJy flux densities. I will present
the wide variety of sub-parsec cores and structures observed, not
always easy to interpret within a common physical scenario. Finally, I
will discuss the possible reasons why, at milli-arcsecond resolution,
the radio emission does not correlate with the X-ray emission anymore,
at odds with the low efficiency accreting model predictions.