15. Language

Language & Lateralization

Properties of Language and Language Acquisition

Neurobiology of Language

Language

Definition and Universal Qualities

Acquisition

Definition of Language

Human Communication Through Language: Language is defined as a rule-bound arrangement of symbols (including spoken, written, signed forms) utilized to convey a vast range of thoughts, actions, concepts, etc. Language is exceptional as it is:

  • Dynamic: Its meanings and usage evolve over time.

  • Arbitrary: There is no inherent connection between a word and the meaning it communicates. For example, the word "dog" has no intrinsic tie to the animal itself.

  • Grammatical: Language follows a series of rules that govern structure and usage, facilitating coherent communication.

Communication vs Language

Distinction between Communication and Language

  • Communication: Occurs when a signal successfully conveys information. For instance, nonhuman animals exhibit communication through vocalizations, but they lack the structural complexity of true language.

  • Language: Must possess specific features, unlike communication seen in nonhuman primates, which relies mainly on species-typical vocalizations triggered by specific stimuli in brain regions such as subcortical, limbic, and brain stem.

Vocalizations in Nonhuman Primates

  • Vocalizations are directly tied to particular events (e.g., attack, feeding), indicating a non-arbitrary nature. A bonobo named Kanzi has demonstrated the ability to use symbols to convey messages, but no evidence supports complex linguistic communication among nonhuman species in natural settings.

The Deep Structure of All Languages

Components of Language

  1. Phonemes: The smallest units of sound in speech.

  2. Morphemes: Basic units of meaning formed from phonemes. Some languages primarily use morphemes as words, while others include prefixes and suffixes.

  3. Grammar: The set of rules that determine how words are combined to form sentences.

  4. Words: Distinct, meaningful units of language that consist of one or more morphemes. There is no consensus on a universal definition of what constitutes a "word" across languages.

Language Acquisition

Neurological Observations

  • Neurologists Penfield and Roberts noted that early-life brain injuries impact language less severely than similar injuries in adults, suggesting developmental differences.

  • Second language acquisition post-10 years of age rarely reaches native-like fluency, indicating a critical period for language development.

  • Puberty is believed to mark the end of this critical period, which constrains language learning and development.

  • The infant's capacity for language learning through observation contrasts sharply with the adult's inability to learn a new language through mere exposure.

Vocabulary Development

  • The average English speaker boasts a vocabulary ranging from 20,000 to 35,000 words.

  • According to B.F. Skinner, language acquisition is explained through reinforcement: caregivers respond to vocalizations, providing positive feedback that encourages repetition of certain sounds and words.

  • Although grammar can be complex, the majority of language users rely on it intuitively.

  • Approximately 40-50% of the global population speaks more than one language.

Theoretical Perspectives on Language Acquisition

  • Noam Chomsky posits the existence of an innate grammar system underlying many languages, suggesting that humans are predisposed to recognize grammatical structures and facilitate language acquisition right from birth.

FOXP2 Gene

  • A rare mutation in the FOXP2 gene caused developmental verbal dyspraxia in several generations of the KE family in England. This condition results in speech deficits attributed to difficulties in executing the precise motor commands required for language production (e.g., muscle control over lips and tongue).

  • These issues are not caused by paralysis or general muscle weakness, presenting only within speech-related functions.

  • Regardless of whether grammar is innate, humans possess genetic and neurological adaptations for language, such as superior muscular control over vocal cords and enhanced respiratory regulation compared to other primates.

Summary of Language Properties

  • Language represents a specialized communication form characterized by dynamic, arbitrary symbols bound by structured rules. All languages encompass phonemes, morphemes, words, and syntax.

  • The ability to acquire language largely diminishes after the critical preadolescent development window.

Lateralization and the Neurobiology of Language

Lateralization

Split-Brain Patients

Broca's and Wernicke's Areas

Lateralization of Language

  • Typically, the left hemisphere of the brain is where language functions are localized. Consequently, the right ear exhibits heightened sensitivity to speech sounds.

  • The enlarged planum temporale, located on the superior surface of the temporal lobe and critical for language processing, is generally larger in the left hemisphere. This phenomenon has been observed even in infants prior to any language acquisition.

  • The Wada Test: An anesthetic is administered to one of the carotid arteries to determine language lateralization. A loss of language post-anesthesia in the left hemisphere confirms its role in language processing.

Split-Brain Patients

  • The surgical severance of the corpus callosum in patients with severe epilepsy leads to split-brain syndrome. Individuals with this condition cannot verbally report words presented in the left visual field (processed by the right hemisphere). However, rudimentary language abilities exist in the right hemisphere, allowing for the correct selection of objects based on verbal cues from the left visual field.

Neuromyth: Lateralization of Human Traits

  • Although brain function is often lateralized, especially for language (predominantly in the left hemisphere), the association of personal traits (like logic or creativity) with hemispheric dominance lacks scientific support.

  • A significant fMRI study by Nielson et al. (2013) revealed that while brain activity can indicate hemispheric lateralization, no consistent patterns of hemispheric dominance could be established.

Broca’s Area

  • Location: Broca's Area is situated on the left inferior frontal gyrus.

  • Aphasia: Broca's aphasia, resulting from damage to this area, is characterized by challenges in language expression, while comprehension remains relatively intact (referred to as "nonfluent aphasia").

  • Patients often recognize their struggles with language and exhibit various symptoms, including:

    • Difficulty producing coherent speech at a motor level.

    • Agrammatism: struggles with grammatical speech production.

    • Anomia: difficulty finding the appropriate word.

  • Paul Broca's landmark work in the 19th century with a patient who could only say "tan" but understood language led to the post-mortem discovery of damage in what would be named Broca’s Area.

Wernicke’s Area

  • Location: Found in the left superior temporal lobe.

  • Aphasia: Damage to Wernicke's Area results in Wernicke's aphasia, which impairs language comprehension (also known as "fluent aphasia").

  • Symptoms include:

    • Fluid but nonsensical speech; patients can produce grammatical sentences but often lose meaning.

    • A lack of awareness about their communication deficits.

    • Impaired ability to process spoken language (termed "word deafness") and written language (termed "word blindness"). Patients with Broca’s aphasia typically do not face these issues.

Recovery of Function in Language

  • Post-injury or stroke recovery is possible for individuals with damage to language-critical areas, but it generally favors nonfluent aphasia recovery over fluent. Global aphasia, characterized by a total loss of language, exhibits a poor recovery prediction with minimal healing.

  • Effective speech therapy interventions promoting functional recovery and compensatory strategies particularly benefit nonfluent aphasia patients.

Interaction Between Broca’s and Wernicke’s Areas

  • Communication between Broca’s and Wernicke’s Areas is facilitated through the arcuate fasciculus, a tract of axons that connects these language centers. This anatomical connection supports a model of language processing where comprehension (Wernicke’s) and production (Broca’s) are distinctly managed but interact closely.

  • Transcranial magnetic stimulation studies in healthy subjects indicate complex interactions between these regions, such as a delay in reaction times for synonym assessments when Broca’s area is temporarily inactivated (Gough et al., 2005).

The Language Network

  • Functional Magnetic Resonance Imaging (fMRI) studies demonstrate that language areas in the frontal and temporal lobes are activated concurrently during linguistic tasks, confirming the collaborative nature of Broca’s and Wernicke’s areas within the broader language network.

  • This activation occurs across various language input modalities (e.g., spoken, written, signed) and engages the network at different levels—from phonemic to sentence processing. The degree of activation might vary based on the complexity of language tasks.

Neurons Across the Language Network

  • Research shows that individual neurons within the language network respond to different linguistic dimensions, such as distinguishing between sentences and words, or words and non-words. Neuron distribution reflects consistent responsiveness to multiple contexts but lacks significant differentiation across areas (like Wernicke’s versus Broca’s).

The Language Network and Compensatory Plasticity

  • Despite the classical model suggested by aphasia studies, the language network remains highly interdependent. The question arises: why does damage to Broca’s or Wernicke’s not always result in global aphasia?

  • The concept of compensatory plasticity supports this—following the injury of one brain area, other regions can adapt to fulfill lost functionalities. While this may lead to total recovery in some instances, deficits often persist in others (Hartwigsen & Saur, 2017).

Summary of Lateralization and Language Neurobiology

  • Language primarily lateralizes to the left hemisphere of the brain.

  • Broca’s Area is crucial for language output, while Wernicke’s Area specializes in language comprehension.

  • Damage to these areas results in distinct nonfluent and fluent aphasias, respectively.