There is no such thing as the Psycho-Linguistic Theory. Several theories are part of the field of Psycho-Linguistic.
See for example: Model of Text Comprehension
Core Assumptions and Statements
Language is a product of reasoning and therefore accessible to general, rational analysis, i.e. in analogy to other cognitive functions. Cognitive linguistics can be seen as the modern instantiation of this view, regarding language-bound functionality of the brain as incorporated and inextricably linked with other functions of the brain and being a learned ability, biologically / genetically based only on general-purpose "reasoning-mechanisms" of the brain. Applied in communication science this theory a.o. means that a particular use of language in messages has more or less persuasive power depending on a.o. the value system, the effort and the motivation of receivers. (Chomsky, Piaget, Vygotsky).
See for Chomsky: Natural (born) language.
See for Piaget & Vygotsky: theories on learning.