4.4 Neural basis of language

The brain's language processing network is a complex system involving multiple regions. Broca's area in the left frontal lobe handles speech production, while Wernicke's area in the left temporal lobe manages comprehension. Other areas contribute to reading, writing, and phonological processing.

Language functions are typically lateralized to the left hemisphere, but the right hemisphere plays a role in prosody and figurative language. Brain damage can lead to various language disorders, including aphasia and dyslexia. Neuroimaging techniques like fMRI and PET help researchers study language processing in the brain.

Brain Regions and Language Processing

Brain regions for language processing

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• Broca's area
  • Located in left frontal lobe, typically inferior frontal gyrus
  • Involved in speech production and articulation
  • Damage leads to Broca's aphasia, difficulty producing fluent speech (agrammatism, telegraphic speech)
• Wernicke's area
  • Located in left temporal lobe, typically superior temporal gyrus
  • Involved in language comprehension and processing meaning
  • Damage leads to Wernicke's aphasia, fluent but meaningless speech (neologisms, word salad)
• Other brain regions involved in language processing
  • Angular gyrus: Involved in reading and writing (dyslexia, agraphia)
  • Supramarginal gyrus: Involved in phonological processing (rhyming, syllabification)
  • Arcuate fasciculus: White matter tract connecting Broca's and Wernicke's areas (conduction aphasia)

Lateralization of language functions

• Left hemisphere dominance for language
  • Most language functions lateralized to left hemisphere in majority of individuals
  • Evidence from studies of brain damage (aphasia) and neuroimaging techniques (fMRI, PET)
• Right hemisphere contributions to language
  • Prosody and emotional aspects of language (intonation, sarcasm)
  • Figurative language (metaphors, idioms)
  • Processing complex narrative structures (story comprehension)
• Individual differences in lateralization
  • Some left-handed individuals may have more bilateral or right-hemisphere language representation
  • Plasticity and reorganization of language functions in cases of early brain damage (hemispherectomy)

Language Disorders and Neuroimaging

Brain damage effects on language

• Aphasia
  1. Broca's aphasia: Non-fluent speech, difficulty with grammar and word finding, relatively preserved comprehension
  2. Wernicke's aphasia: Fluent but meaningless speech, poor comprehension, difficulty with repetition
  3. Global aphasia: Severe impairment in both production and comprehension of language
• Dyslexia
  • Specific learning disorder affecting reading abilities
  • Difficulties with phonological processing, word decoding, and spelling (phoneme-grapheme correspondence)
  • Associated with atypical activation patterns in language-related brain regions (temporo-parietal cortex)
• Other language disorders
  • Specific Language Impairment (SLI): Difficulty with language acquisition and use in absence of other cognitive or sensory impairments (developmental disorder)
  • Agraphia: Impairment in writing abilities, often associated with damage to angular gyrus (alexia)

Neuroimaging in language research

• Functional Magnetic Resonance Imaging (fMRI)
  • Measures changes in blood oxygenation levels as proxy for neural activity (BOLD signal)
  • Allows localization of language-related brain activation during various tasks (word generation, sentence comprehension)
• Positron Emission Tomography (PET)
  • Uses radioactive tracers to measure glucose metabolism or blood flow in brain (FDG-PET, O-15 PET)
  • Provides information about regional brain activity during language tasks (word repetition, story listening)
• Electroencephalography (EEG) and Magnetoencephalography (MEG)
  • Measure electrical activity or magnetic fields generated by neural activity (event-related potentials, oscillations)
  • Offer high temporal resolution for studying timing of language processing (N400, P600)
• Diffusion Tensor Imaging (DTI)
  • Type of MRI that maps diffusion of water molecules in brain tissue (fractional anisotropy, mean diffusivity)
  • Allows visualization of white matter tracts involved in language processing (arcuate fasciculus, uncinate fasciculus)