The Role of the Thalamus in Declarative and Procedural Linguistic Memory Processes.

Bruce Crosson

doi:10.3389/fpsyg.2021.682199

Abstract

Typically, thalamic aphasias appear to be primarily lexical-semantic disorders representing difficulty using stored declarative memories for semantic information to access lexical word forms. Yet, there also is reason to believe that the thalamus might play a role in linguistic procedural memory. For more than two decades, we have known that basal ganglia dysfunction is associated with difficulties in procedural learning, and specific thalamic nuclei are the final waypoint back to the cortex in cortico-basal ganglia-cortical loops. Recent analyses of the role of the thalamus in lexical-semantic processes and of the role of the basal ganglia in linguistic processes suggest that thalamic participation is not simply a matter of declarative vs. procedural memory, but a matter of how the thalamus participates in lexical-semantic processes and in linguistic procedural memory, as well as the interaction of these processes. One role for the thalamus in accessing lexical forms for semantic concepts relates to the stabilization of a very complex semantic-lexical interface with thousands of representations on both sides of the interface. Further, the possibility is discussed that the thalamus, through its participation in basal ganglia loops, participates in two linguistic procedural memory processes: syntactic/grammatical procedures and procedures for finding words to represent semantic concepts, with the latter interacting intricately with declarative memories. These concepts are discussed in detail along with complexities that can be addressed by future research.

Highlights

AND FOUNDATIONAL CONCEPTSThe thalamus is a group of nuclei deep within each cerebral hemisphere, abutting the third ventricle at its medial aspect
The fact that some conclusions are more tentative in nature indicates areas in which future research is necessary to validate and refine constituent concepts or perhaps to replace such concepts with conclusions better grounded in results from that future research
One purpose of this section is to take a step toward such future research by clarifying what issues need to be resolved to understand the role of the thalamus in declarative vs. procedural linguistic processing

Summary

AND FOUNDATIONAL CONCEPTS

The thalamus is a group of nuclei deep within each cerebral hemisphere, abutting the third ventricle at its medial aspect. Systematic work from Murray Sherman’s laboratory in the last two decades has convincingly indicated that the higher order thalamic relays are capable of transferring information from one cortical region to another (see Sherman and Guillery, 2006; Usrey and Sherman, 2019 for reviews), and detailed models of how basal ganglia loops support motor functions (e.g., Nambu et al, 2000; Nambu, 2003) were applied explain how lesion and functional imaging studies indicate some role for the basal ganglia in language (e.g., Crosson et al, 2007), with thalamic nuclear regions acting as the gateway from the basal ganglia to the cortex. Literature on thalamic aphasia, functional imaging of the thalamus during language tasks, and linguistic effects of thalamic stimulation are reviewed in the sections on linguistic declarative and procedural memory In his 1949 book The Concept of Mind, the philosopher Gilbert Ryle described two forms of memory: “knowing that” (declarative memory) and “knowing how” (procedural memory; Ryle, 1949). We will discuss the possibility that word-finding involves the use of both semantic-lexical associations and learned procedures that enhance the accuracy and efficiency of pairing semantic concepts with a word to represent them

Thalamic Connectivity in Declarative and Procedural Memory

LINGUISTIC PROCEDURAL MEMORY AND THE THALAMUS

Is the Thalamus Involved in Syntax and Grammar?

Is There a Procedural Memory Component to Word Finding?

CONCLUSION AND SYNTHESIS