When did "text" come to be defined as something other than words?

I think the answer to this question may be in the OED, but I don't have access to the service. I am discussing "texts" using definitions (from dictionary.com) like this:

text: any theme or topic; subject.

and this

text: anything considered to be a subject for analysis by or as if by methods of literary criticism.

Within the context of cultural anthropology I'm having a discussion with my students about how the definition of a text has expanded over the years to include not just texts comprising words but also visuals (e.g., images in advertisements).

I've come across the latter usage of text in certain educational books:

"Like written texts, visual texts have been carefully constructed by their composers to shape meaning, and to affect and influence the viewer."

Or

"This resource covers how to write a rhetorical analysis essay of primarily visual texts with a focus on demonstrating the author’s understanding of the rhetorical situation and design principles."

An ngram search for "visual texts" doesn't have many results before the 1960s, and some of the results refer to visual texts in single or double quotes to highlight the non-standard usage.

"visual texts"

or

visual "texts"

My question is:

• Since dictionaries are formally acknowledging that a text need not be comprised of words only, when did the concept of a 'visual text' become a subject of analysis in its own right?" (e.g., the MacIntosh logo of an apple; see http://creativebits.org/interview/interview_rob_janoff_designer_apple_logo)

'Text' is commonly used to describe things other than words in fields such as the history of art, literary theory and so on.

According to Wikipedia (http://en.wikipedia.org/wiki/Text_(literary_theory))

In literary theory, a text is any object that can be "read," whether this object is a work of literature, a street sign, an arrangement of buildings on a city block, or styles of clothing. It is a coherent set of signs that transmits some kind of informative message. This set of symbols is considered in terms of the informative message's content, rather than in terms of its physical form or the medium in which it is represented.

I suspect it's only been commonly used this way since the 1960s. Wikipedia (http://en.wikipedia.org/wiki/Critical_theory#Language_and_communication) states:

From the 1960s and 1970s onward, language, symbolism, text, and meaning came to be seen as the theoretical foundation for the humanities, through the influence of Ludwig Wittgenstein, Ferdinand de Saussure, George Herbert Mead, Noam Chomsky, Hans-Georg Gadamer, Roland Barthes, Jacques Derrida and other thinkers in linguistic and analytic philosophy, structural linguistics, symbolic interactionism, hermeneutics, semiology, linguistically oriented psychoanalysis (Jacques Lacan, Alfred Lorenzer), and deconstruction

It is not only common to refer to any object of interpretation as a text at the collegiate level, it is written into the very course catalog descriptions. For example, one of the aesthetics courses I took in grad school was called "Reading Texts: Developing Cultural Fluency" and the main text for that course was Performance Studies: The Interpretation of Aesthetic Texts. Certainly this usage dates from at least Hans-Georg Gadamer's Truth and Method (1975) and is inferred by Roland Barthes in "The Photographic Message," no. 1, Communications (Paris, 1961). While Barthes wrote that "(t)he photographic image (...) is a message without a code," he did speak of the reading the photographic image as parallel with the reading of its caption and title.

So, if I were you, I would likely argue that the 1961 article represents the archetypal--if not originating--use of "text" to refer to non-written objects of interpretation.