Question

Neuroimaging data and aphasia research strongly suggest that the arcuate fasciculus consists of three pathways; a direct, ‘deep’ one and two indirect, ‘shallow’ ones. Describe what each of the 3 pathways connect to and briefly briefly describe its role in speech perception and production.

Answer 1

The arcuate fasciculus belongs to the core perisylvian circuitry underlying language. However, functional imaging experiments in a language-based neurodegenerative syndrome known as primary progressive aphasia, clinico-anatomical observations in stroke patients, and intraoperative stimulation studies have been expanding the boundaries of this core circuitry. One of the most interesting developments has been the demonstration that areas in the medial, inferior, and anterior temporal cortices, traditionally considered outside the canonical language network, may play crucial roles in semantic processing. The interaction of these additional areas with the canonical perisylvian language network may be mediated by a set of ventral tracts such as the inferior longitudinal fasciculus, the uncinate fasciculus, and the inferior fronto-occipital fasciculus. The inferior longitudinal fasciculus carries visual information from occipital areas to the temporal lobe and it is likely to play an important role in visual object recognition, semantic processing and in linking object representations to their lexical label. The uncinate fasciculus interconnects the anterior temporal lobe to the orbitofrontal area, including the IFG, and may play an important role in lexical retrieval, semantic associations, and aspects of naming that require connections from temporal to frontal components of the language network (e.g. the naming of actions). The inferior fronto-occipital fasciculus is part of the mirror neuron system and arguably the only direct connection between occipital and frontal cortex in the human brain. It is considered part of the mirror neuron system and there is preliminary evidence suggesting that this tract is not present in monkey. The relevance of this fasciculus to language is not fully understood but may involve reading and writing. These ventral pathways are linked to the perisylvian network at least in two different regions, posteriorly, through short U-shaped fibers connecting Wernicke’s territory to ventral temporo-occipital cortex and anteriorly, through intralobar fibers connecting lateral orbitofrontal cortex to Broca’s territory.