Behavioral neurology in language and aphasia: from basic studies to clinical applications

With increasing aging population, cognitive deteriorations due to neuro- degenerative diseases or stroke are so commonly observed that it is thought to be inevitable with aging. For dementia and stroke, a communication disorder due to language deterioration is one of the main problems. Behavioral neurology aims to clarify the relationship between brain function and behavior; language deterioration is one of the main targets, and its clinical applications are really useful for making better understanding of patients. Language is sequences of sound or characters that carry meanings for communication. From the evolutionary perspective, "language" can be thought about by considering birds and dogs: the basal ganglia and anterior cingulate, and the thalamus and cerebral cortex are thought to provide the neurobiological background, respectively. Humans can use language. The language area is a newly developed brain area in evolution and is mainly localized in the left cerebral hemisphere. Semantic memory has also developed in humans. There are two routes, the superficial and deep routes, with the latter associated with meaning, and three brain areas are involved: the peri-Sylvian area, per-peri-Sylvian area, and right hemisphere. Using these principles, language symptoms of dementia with progressive non-fluent aphasia (PA), semantic dementia (SD), Alzheimer's disease (AD), and vascular dementia (VaD) can be understood. Namely, the symptoms of PA is understood by the dysfunction of peri-Sylvian language area, those of SD and AD by that of peri-peri-Sylvian language area, and those of some VaD cases and AD cases by that of right hemisphere.