Gaius Plinius Secundus, HN  35.59 (ca. A.D. 65):

Huius est tabula in porticu Pompei quae ante curiam eius fuerat, in qua dubitatur an ascendentem cum clupea pinxerit an descendentem.

There is a picture by this artist [Polygnotos of Thasos] in the portico of Pompeius, which before had been in the Curia of Pompeius, with reference to which, there is some doubt whether the man represented with the shield is in the act of ascending or descending.  (H. Rackham, trans.)