Garrett’s Sentence Processing Model (1988)

Core idea: sentence production (and understanding) follow an inherent, stage-based recipe; errors can be meaning-related or form-related depending on the stage.
Overall structure: one semantic level (message) plus 4 post-semantic levels (functional, positional, phonetic, motor).

Message Level (semantic): constructs the content and abstract semantic specification; assigns initial thematic roles (agent, theme) and maps to a potential syntactic frame.
Functional Level: semantic info + syntactic lemmas; determines argument structure and word-class sub-categorisation; links lemmas to syntactic functions; identifies thematic roles based on the verb.
Positional Level: post-lexical; retrieves specific phonological word forms (lemmas) and creates a structural sentence frame; establishes word order and planning frames; inserts phonological forms into the frame.
Phonetic and Articulatory Levels: phonetic encoding and actual articulation; motor planning and execution.

Generates the message or conceptual form of the sentence; nonlinguistic content that will be expressed in language.
Forms abstract semantic specifications and assigns syntactic functions (e.g., agent vs. theme).
Example concept: contest between agent performing an action on a theme (verb determines roles).

Sub-processes: semantic info (concepts, meanings) and syntactic info (lemmas, argument structure, sub-categorisation frames).
Ends with an unordered set of lemmas linked to syntactic functions via the chosen verb’s argument structure.
Example: The man (agent) gives the money (theme) to the boy (recipient/patient).
Verb-driven: thematic roles are dictated by the verb (e.g., agent for kick, theme/recipient for given action).

Retrieval of phonological word forms for chosen lemmas; creation of a sentence frame (structure) and selection of planning frames.
Active sentence frame: det N – aux V – det N (with appropriate slots); Passive frame: det N – aux V – by det N.
Insertion of phonologically specified items into the sentence frame according to grammatical roles (subject, verb, object).
Example framing: "The girl is kicking the boy" with proper phonological/grammatical placement.

Phonetic level: detailed phonetic encoding of forms.
Motor level: initiation of actual muscular activity; coordination with articulatory system.

Agrammatism: disruption in processing of grammatical morphology; common in Broca’s aphasia.
Content words processed earlier at the Functional Level; function words retrieved later at the Positional Level; phonological specification for function words is late.
Symptoms: omissions of morphological markers, simplified sentence structures, reduced verb production.
Paragrammatism (in fluent aphasias): substitutions of grammatical morphology (e.g., "I’m very want it").
Classic example: passive or active ordering subtle errors (e.g., "the boy is pushed by the girl").

A model of sentence comprehension mirrors the production model in reverse; interpretation involves decoding phonology, mapping to syntactic roles, and retrieving word meanings.

Auditory input → Phonetic level (decode phonemes) → Positional level (identify subject, verb, object) → Functional level (identify thematic roles and retrieve meanings) → Message level (conceptual interpretation of the event).
Demonstrates how semantic content and syntax are integrated across levels to produce a coherent sentence.

Methods: natural language sampling (conversation, narrative, picture description).
Formal analyses: evaluate word class proportions, sentence length, speech rate, predicate-argument structure, and embedding.
Standard tools: VAST, NAVS, SALT-derived measures, etc.

Use the breakdown level to tailor therapy:
- Verb retrieval and positional-structure therapy targets for positional level failures.
- Mapping therapy for thematic roles targets at the functional level.
- Use sentence production or complementation tasks to strengthen functional-to-positional mapping.