In this paper, we introduce a new model of human sentence processing. The psychological issues addressed include the use of lexical information, specifically subcategorization information, during the initial stage of syntactic structure assembly, the issue of linear parsing, i.e. immediate attach?ment of words to the sentence structure, within a head-driven grammar framewoik, and the resolution of attachment ambi?guities. W e will demonsu^te that the variety of psycholinguis?tic phenomena can be accounted for by the assumption of principled behavior of linguistic signs, which are implemented in an object-oriented fashion. The model provides a serial implementation of Parametrized Head Attachment.