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Language pathways in the brain: Did you know there are two auditory pathways?
In human language processing, language communication and understanding play a vital role. Psycholinguistic research shows that language processing is a uniquely human ability, with humans far superior in the grammatical understanding and systematicity of language compared to other closely related primates. Since the 20th century, the mainstream model of language processing has been based on the Wernicke-Lichtheim-Geschwind model, which was mainly derived from case studies of brain-injured patients. With the advancement of intracranial electrophysiological recording technology and the emergence of non-invasive technologies such as fMRI, PET, MEG and EEG, scientists have gradually discovered that there are dual auditory pathways in the brain and proposed a two-pathway model.
According to this model, there are two different pathways between the brain's auditory cortex and frontal lobes, each playing a different role in language processing.
Specifically, the auditory ventral stream (AVS) is responsible for sound recognition, so it is called the "what pathway" of hearing. The auditory dorsal stream (ADS) is responsible for the localization of sounds, so it is called the "where pathway" of hearing. This pathway in humans (especially in the left hemisphere) is also responsible for speech production, repetition, lip-reading, and phonemic working and long-term memory.
According to the "where to what" model of language evolution, the versatility of ADS can be traced to different stages of language evolution. The differentiation of these two pathways first occurs in the auditory nerve. The anterior branch enters the anterior auditory nucleus of the brainstem to form the ventral auditory pathway; while the posterior branch enters the dorsal and ventrodorsal auditory nuclei to form the dorsal auditory pathway. These pathways are not only important in spoken communication but are also relevant to the processing of signed language and written content.
Early neurolinguistics model
In the 20th century, neurolinguistic research mainly relied on the Wernicke-Lichheim-Gerschwind model. This model is primarily based on the study of symptoms of language-related disorders in brain-injured patients. According to this model, language is received through a specialized word-receptive center (Wernicke's area), located at the left temporoparietal junction. This area then passes information to the area responsible for language production (Broca's area), which is located in the left inferior frontal gyrus. Since this model considers almost all language input to pass through Wernicke's area and all language output to pass through Broca's area, this makes it difficult to clearly identify the basic properties of these two areas.
Because the understanding and expression of language do not entirely depend on these two regions, this makes it very difficult to identify their homologous regions in other primates.
However, with the advent of fMRI and its application in lesion mapping, this model was shown to be based on incorrect correlations between symptoms and lesions. The refutation of this seminal and dominant model thus paves the way for new models of language processing in the brain.
Current neurolinguistic models
Anatomy
Over the past two decades, significant advances have been made in our understanding of the neural processing of sound in primates. By recording neural activity in the auditory cortex of monkeys in the early stage, and subsequent studies using histological staining and fMRI scanning, we identified three auditory areas in the primary auditory cortex and nine surrounding combined auditory areas. Anatomic tracing and lesion studies further demonstrate dissociation between anterior and posterior auditory areas.
Recent evidence showing homology in auditory areas between humans and monkeys also supports the validity of the dual-channel model of human language processing.
Auditory ventral pathway
The auditory ventral pathway (AVS) connects the auditory cortex to the middle temporal gyrus and temporal pole, which in turn connects to the inferior frontal gyrus. This pathway is responsible for sound recognition and is therefore called the "what pathway" of hearing. The functions of this pathway include sound recognition, sentence understanding, etc.
Dorsal auditory pathway
In contrast, the auditory dorsal pathway (ADS) connects the auditory cortex to the parietal lobe, and ultimately to the inferior frontal gyrus. In humans and non-human primates, the ADS is mainly responsible for sound localization, so it is called the "where pathway" of hearing. In addition, this pathway (especially on the left side) is responsible for speech production, speech repetition, lip reading, and working and long-term memory for phonemes.
With the deepening understanding of these pathways, the scientific community has a new understanding of the mode of language processing. The operation of the two is not just a linear relationship, but a complex interaction. Such research opens up a deeper exploration of the evolution of human language.
Have you ever wondered how future research might further reveal the mysteries of language processing in the brain?
