Establishing action-perception circuits as a neural basis for meaning-carrying linguistic symbols – the role of frontal speech motor areas and fronto-temporal connectivity

This dissertation investigates how brain language models can explain the neural representation of phonological word forms and their function in speech comprehension. Action-perception theory of language provides a possible explanation based on Hebbian learning principles. In this perspective, word forms are thought to be represented by cell assemblies, or action-perception circuits (APCs), which are distributed across frontal and temporal cortices, as well as inferior parietal areas. These areas are referred to collectively as the perisylvian language areas. While it is generally acknowledged that the perisylvian language areas function as the core regions for language, considerable debate and open questions still exist regarding the precise role of the different perisylvian regions, and in particular concerning the role of long-distance connectivity within the perisylvian areas along the dorsal stream by way of the arcuate fasciculus (AF). Strong AF connectivity is present only in humans, but not in non-human primates, and therefore has been proposed as the specific substrate underlying human word learning abilities. However, a neurobiological explanation why AF connectivity is so important remains to be found. A neuroanatomically grounded and neurophysiologically plausible computational model simulating the emergence of phonological action-perception circuits was used to investigate this. Results suggest that the AF is critical for reverberating activity across the perisylvian cortex, which in turn functions as the substrate for verbal working memory. A further question is what the function of this auditory-motor link is for speech comprehension. One perspective claims that speech comprehension relies on a single lexical interface in temporal cortex, and that any influence of motor systems on speech processing is restricted to ‘artificial’ phonological tasks or particularly noisy listening conditions. In contrast, action-perception theory of language predicts that as a result of Hebbian-based distributed APCs, fronto-parietal systems, in particular motor systems, can take an additional causal role in speech comprehension. In line with this, there is compelling evidence for a contribution of fronto-parietal systems to speech comprehension, even in noise-free and passive tasks, as demonstrated by an extensive review of functional magnetic resonance imaging (fMRI) studies, especially recent ones employing multivariate pattern analysis (MVPA), as well as transcranial magnetic stimulation (TMS) studies. Specifically, the question whether motor systems causally contribute not only to phonological tasks using isolated, meaningless syllables, but also to comprehension of meaningful words, is addressed in a TMS study where phonemes are placed in the context of whole words, hence allowing them to serve their ‘normal’ function as meaning-discriminating units. In summary, the results of this dissertation demonstrate that the neural representation of word forms can be explained in terms of APCs distributed across perisylvian…