He was given outline drawings with foreground and background assigned. He did not report an outline with foreground background assignment reversed on its second appearance was an outline drawing he had had previously. Evidently, there are similar effects on recognition produced by figure ground reversal in vision and touch. Patterns made of lines can convey abstract relationships, and show expressive postures. Rather, evolution developed a capacity for pictures, and only a few decades of thousands of years ago was this evolutionary spinoff put to use by pictures.
Pictures use elements such as pairs of contours of lines, finding abstract relations, or axes.
Similarly, in music, pairs of notes can be perceived as having particular relations e.g. a pair of notes separated by an octave are perceived as related. Deaf subjects, including congenitally deaf subjects, found pairs of tangible notes separated by an octave easier to assess than notes separated by an interval that was not a simple arithmetic multiple of an octave. Some referents for drawings are matters of physical relief, and so are easy to assess as correctly or incorrectly depicted. These can only be assessed as matters of agreement. The referents can be pain or numbness for example.