Content

• The Language System

• History of Language Localization in the Brain

• Neuroscience of Language Today

  • When Does Processing Take Place?
  • When and How is Language Processed?

• Language Disorders

• Neural Plasticity

  • In cases of Damage to Language Areas
  • Individual Differences in the Healthy Brain

• Current Language Models

• Neuroprosthetics for Language

• Learning objectives

  • You are familiar with the pioneers of neural language research and their key findings
  • You understand how language is studied neuroscientifically today and why historical language models have only been partially confirmed
  • You know the causes, key symptoms, and syndromes of aphasia
  • You are aware of current language models and how they differ from one another
  • You can explain how neuroprosthetics for language can "decode" thoughts

The Language System

• Language != Speech/Speaking
• Language includes several modalities
• A system of rules that we use, which determines what we say, how we understand language, and how we communicate.

What makes Human Language Unique?

• Human language can express unbounded means by a finite number of elements (Recursion: Hauser, Chomsky & Fitch, 2002, Science)
  • dog bites cat != cat bites dog
• Primates have a different understanding of rule violations.

• It depends on the rule/type of grammar:

History of Language Localization in the Brain

• What happens in the case of a brain injury?
• Paul Broca:
  • The patient could only utter the syllable "tan-tan".
  • Leborgne ("Monsieur Tan") visited the clinic due to an infection in his right leg.
  • After Leborgne's death, Broca discovered a lesion in the left inferior frontal lobe.
  • Broca reported seven additional cases (including Mr. Lelong).
  • The lesion led to an impairment in speech articulation.

• Carl Wernicke:
  • The first to think of "theoretically different centers"
  • Wanted to anchor the functional architecture for speech in the neural architecture.
  • Acoustic nerve (a-a_1), sensory speech center (a_1), Broca's area (b), motor speech tract (b-b_1)
  • Anticipated 5 syndrome types based on different brain lesions:
    1. Disruption of the acoustic nerve a–a_1 => deafness (no aphasia)
    2. a_1 projection site of the acoustic nerve (“sensory speech center”) => the patient cannot understand spoken words although he is not deaf (later Wernicke’s region)
    3. Connection a_1–b. Patient can fully understand speech and can speak (conduction aphasia – Leitungsaphasie)
    4. Disruption of speech movement center b. This leads to muteness, with a few simple words spared => Broca’s aphasia
    5. Disruption of tract b–b1. This should cause the same kind of motor aphasia (paralysis of the speech muscles)

• The historical language model of Wernicke-Lichtheim:

• Forgotten Pioneers: Marc and Gustave Dax:
  • Marc Dax (1770 - 1837) - Physician from Montpellier
    • 1836: claimed that spoken language is localized in the left hemisphere, based on the observation that: loss of speech occurred only with right-sided hemiplegia and never with left-sided hemiplegia (i.e., paralysis of the arm and leg on one side of the body often a consequence of strokes).
  • Gustave Dax (1815 - 1874) - Son of Marc Dax, collected additional data, which he published in 1865
    • 87 cases: right-sided hemiplegia + loss of speech
    • 53 cases: left-sided hemiplegia without loss of speech
    • Only 6 exceptional cases
  • 1865: Broca built upon Dax's findings on the left hemisphere.

Neuroscience of Language Today

• Which brain regions are active?

• Structure and Function

• The language modality determines which brain areas are involved in processing:

• Electric brain activity:
  • When does processing take place?
  • Our brain is always active
  • Neurons generate electrical potentials during signal transmission
  • Large assemblies of synchronously firing neurons generate measurable electrical activity on the scalp

• From the acoustic speech signal to linguistic processing (Neural Speech Tracking)
  • The acoustic speech signal cannot simply be divided into individual words (continuous sound stream)
  • Our brain tracks the amplitude of speech - it synchronizes its activity with its rhythm

• There exist different levels of linguistic processing:

Language Disorders

• Aphasia
  • (Greek aphasía) = speechlessness
  • Acquired language disorder
  • Any type of brain damage can cause aphasia
  • It affects several language modalities
  • Not a purely motor speech disorder (dysarthria), which affects the control and exectution of speech movements
  • Causes and Prevalence:
    • Etiology
      • Stroke (80%)
      • Traumatic brain injury
      • Tumor
      • Enzephalitis
      • Dementia
    • Incidence
      • After 30% of all strokes
      • In Switzerland approx. 3500 new cases per year
    • Prevalence
      • Approx. 5000 people in Switzerland are affected by aphasia as a result of a stroke
  • Symptoms
    • Lexical level (semantics)
      • Semantic Paraphasia (Speech Error)
        • Semantic - related to the meaning of the word
        • An unintended word is used: which can be semantically related to the intended word or not.
        • E.g.: mother (instead of wife)
      • Semantic Neologism
        • New words are created
        • These words are not part of the standard vocabulary
        • E.g.: John wants to by a new vacuum mouth.
      • Words finding difficulties
        • E.g.: John wants to buy a ...
    • Speech sound level (phonology)
      • Phonematic Paraphasia
        • Phonematic: concerning sound formation
        • Phonetic modification of a word by substituting, omitting, rearranging, or adding individual sounds (neologism through the meaningless stringing together of sounds)
        • E.g.: "papple" for apple or "lelephone" for telephone
    • Grammatical level (syntax)
      • Agrammatism and Telegram Style
        • Short sentences with simple syntactic structure
        • Few verbs
        • Omission of function words and infection forms
        • E.g.: John ... vacuum cleaner ...
      • Paragrammatism
        • Excessive sentence structures
        • Sentences are complex and characterized by incorrect duplication of sentence elements
        • E.g.: Peter does, he wants, he wants, not necessarily ... but maybe ...

• Weak correlation between lesion and aphasia syndrome
  • Discrepancy between theory and practice

Neural Plasticity in Cases of Damage to Language Areas

• Delayed but normal language development in children with left-hemispheric brain injuries.
• Basser et al. (1962) and Lenneberg (1967) suggested that language functions, in the case of early damage to the left hemisphere, are taken over by the right hemisphere (Lenneberg hypothesis)
• Neural plasticity after perinatal stroke (Newport et al., 2022)

• Individual differences in the healthy brain

• A comparison of different languages: the language network across 45 different languages

Current Language Models

• Dual-stream Model:

• Open question: Are sounds (phonology), meaning (semantics) and sentence structure (syntax) processed separately or integrated?
  • Meta-analyses show a strong overlap in processing at different levels.

• Processing of linguistic levels in shared or separate networks?
  • Parallel networks for semantics, phonology and syntax - Hagoort (2014)
  • The dorsal processing stream is crucial for syntactic processing - Friederici (2018)
  • The same networks contribute to the processing of semantics, phonology and syntax - Fedorenko et al. (2024)

Neuroprosthesis for Language

How far are we really from reading thoughts from brain signals?

• Electrocorticography (ECoG)

• Brain-Computer Interface (BCI) Restoration of Arm and Voice (BRAVO) Clinical Trial
  • Person with severe paralysis of the limbs and vocal tract
  • Unable to speak
  • BCI decodes 78 words per minute (approx. average speaking rate is 150 words per minute)

• Are we mind-reading?
  • No, not really
  • Motor brain signals were decoded (i.e., those nerve impulses that activate the muscles)
  • "Inner speech" is much harder to decode because the articulatory representation (how and which muscles are activated for speaking) is weaker

Take-Home

• Broca (speech production), Wernicke (language comprehension), and the Dax family (lateralization) laid the foundations for the study of the neurobiology of language.
• Thanks to modern methods like fMRI & EEG, we can now investigate where, when, and how language is processed in the brain.
• Historical language models could only be partially confirmed.
• Aphasias are acquired language disorders due to brain damage, with symptoms at the semantic, phonological, and semantic levels.
• The dual-stream model distinguishes between ventral (comprehension) and dorsal (production) processing pathways.
• It remains unclear whether phonology (sounds), semantics (meaning), and syntax (sentence structure) are processed separately or integrated.
• Neuroprosthetics decode motor speech signals, not thoughts - decoding inner speech remains a challenge.