  A methodology based upon recurrence quantification analysis is proposed for
the study of orthographic structure of written texts. Five different
orthographic data sets (20th century Italian poems, 20th century American
poems, contemporary Swedish poems with their corresponding Italian
translations, Italian speech samples, and American speech samples) were
subjected to recurrence quantification analysis, a procedure which has been
found to be diagnostically useful in the quantitative assessment of ordered
series in fields such as physics, molecular dynamics, physiology, and general
signal processing. Recurrence quantification was developed from recurrence
plots as applied to the analysis of nonlinear, complex systems in the physical
sciences, and is based on the computation of a distance matrix of the elements
of an ordered series (in this case the letters consituting selected speech and
poetic texts). From a strictly mathematical view, the results show the
possibility of demonstrating invariance between different language exemplars
despite the apparent low-level of coding (orthography). Comparison with the
actual texts confirms the ability of the method to reveal recurrent structures,
and their complexity. Using poems as a reference standard for judging speech
complexity, the technique exhibits language independence, order dependence and
freedom from pure statistical characteristics of studied sequences, as well as
consistency with easily identifiable texts. Such studies may provide
phenomenological markers of hidden structure as coded by the purely
orthographic level.
