  We do not yet understand how the vertebrate visual system provides for
recognition of ob- jects. Countless experiments have been performed to examine
the contribution of cues such as color, texture, and shadowing, but the most
important cues are the con- tours of the outer boundary. Most objects that we
can name can be identified as a silhouette, or equally well as a line drawing
of the boundary. This has long been ap- preciated, so it is somewhat surprising
that after more than a century of experimental research, we have not yet
established how our visual system encodes this shape in- formation.
