A Neurolinguistic Investigation of Symbolic Representation: Cognitive Mechanisms and Impairments

Abstract

Humans engage with a variety of symbols in daily life. Perhaps the most common symbol form is language where we represent myriad concepts, ideas, and notions through strings of sounds and letters. Human communication is also facilitated through other non-verbal symbol modalities ranging from body language/gestures (e.g., crossed arms, thumbs up) to objects (clothing as an indication of social status), and signs (red octagon means stop). Child language research has explored symbolic cognition and its emergence in development, but fundamental questions remain regarding symbolic thinking in adulthood and the impact a symbolic deficit can have on communication. A novel, neurologically-constrained model of symbolic representation is proposed and investigated which aims to addresses open challenges in exploring symbolic cognition. In Chapter 1, SymCog, a novel toolkit to evaluate symbolic representation is developed and normed in a series of three studies. This toolkit and its task, the Symbol Identification Task, are premised upon matching animated video depictions of abstract concepts to their corresponding verbal and non-verbal symbols. Results showed individuals can consistently match different symbol modalities (words, images) to shared concept animations. Chapter 2 assesses performance in the Symbol Identification Task in neurotypical adults receiving inhibitory brain stimulation [i.e., transcranial direct current stimulation (tDCS) of the temporoparietal junction]. Chapter 3 evaluates two cases of persons with aphasia (PWA) in the same task. The goal of these studies was to isolate the neurological-constraints of the model and identify how symbol processing can be impaired. Findings showed reduced performance in the inhibitory stimulation group; highlighting the role of the temporoparietal junction in symbol processing. Results from PWA revealed one case had poor performance across both word and image symbol modalities. Chapter 4 further investigates neurological constraints of the model in an EEG/ERP study. This study assessed N400 deflections during a modified Symbol Identification Task, where frontal-central electrodes were most active. Taken together, these chapter findings suggested two cortical regions potentially support symbolic cognition, including the temporoparietal junction and left central frontal cortex. The results also suggest that impairments in one symbol modality (e.g., words in aphasia) might also present in other modalities for shared concepts. These studies offer insights regarding the cognitive mechanisms involved in symbolic cognition and how they can be impaired. With the use of contemporary tools, such as the SymCog toolkit, future research can better recognize the human capacity to understand and use symbols.