The conjectured holographic duality between vector models with quartic
interaction and higher-spin field theory in the bulk is reviewed, with emphasis
on some versions and generalisations (higher dimensions, beyond the singlet
sector, etc) which have not been much investigated yet. The strongest form of
the conjecture assumes that it holds for any (not necessarily large) number of
massless scalar fields and for any value of the coupling constant. Since the
quartic interaction is of double-trace type, the exact duality (for any value
of the coupling constant) automatically follows from its validity at the
Gaussian fixed point (for vanishing coupling constant). The validity of the
latter also implies that unbroken higher spin symmetries should prevent quantum
corrections in the bulk.Comment: 9 pages, expanded version of the contribution to the Proceedings of
the XVII European Workshop on String Theory (Padova, 5-9 September 2011); v2:
one reference adde