Week 12: Speech and Language

Speech and Language Neurophysiological Overview

Language Dysfunction: Broca's Aphasia

  • 1839: Louis Leborgne lost the ability to speak.

  • Studied by French surgeon Paul Broca.

  • Good language comprehension but inability to produce speech.

  • Developed paralysis of right-side limbs.

  • 1861 autopsy revealed a lesion over the perisylvian region.

  • Broca termed this condition "aphemia."

Language: Localized and Modular

  • Language is a complex, multidimensional, modular system.

  • Functionality is localized within the brain.

  • Speech and language are separate but related cognitive systems.

  • Individual components can be affected while others remain functional, including:

    • Speech production

    • Speech perception

    • Speech comprehension

    • Lexical access

    • Grammatical processing

    • Reading

Normal Speech

  • Normal speech occurs at a rate of < 200 words per minute.

  • English can be accurately perceived at rates up to 400 words per minute, which is approximately 30 sounds per second (Liberman, 1967).

  • The perceptual limit of the human ear for non-speech sounds is around 15 sounds per second (Miller, 1948).

  • Speech is considered a highly efficient human code; no artificial code is more efficient.

Speech Production: Coarticulated and Efficient

  • Speech sounds overlap in time.

  • Coarticulation is pervasive in speech.

  • Production and perception have evolved together; consonants and vowels overlap, yet we perceive them effortlessly.

  • The system works because it is discrete and categorical.

Three Elements of Speech Production:

  1. Initiation:

    • Generation of energy source.

    • Movement of air.

  2. Noise Source:

    • Phonation: Vibration of vocal folds.

    • Turbulence: Noise created by constricted airstream.

    • Transient burst: Release of cavity.

  3. Sound Shaping:

    • Modification of vocal tract changes the acoustics of the sound source.

    • Articulation.

Initiation in Speech Production

  • All sounds generated in the vocal tract involve displacement of air.

  • Initiation: Mechanism by which air is caused to flow within the tract.

    • Ingressive (into tract).

    • Egressive (out of tract).

  • Three sets of organs responsible for initiation:

    • Diaphragm + lungs (pulmonic).

    • Larynx + glottis (glottalic).

    • Tongue (lingual).

Speech Production: Source & Filter

  • Vocal folds in action create a glottal noise source (phonation).

  • Adult male: ~120 Hz.

  • Adult female: ~210 Hz.

  • Broadband signal rich in harmonics.

  • Capable of exciting the vocal tract throughout the full range of speech frequencies.

Source-Filter Model for Vowel Production

  • Glottal sound source + filter 1 + filter 2 + filter 3.

Consonant Production: International Phonetic Alphabet

  • Involves place and manner of articulation.

  • Various places of articulation: Bilabial, Labiodental, Dental, Alveolar, Postalveolar, Retroflex, Palatal, Velar, Uvular, Pharyngeal, Glottal.

  • Manner of articulation includes plosive, nasal, trill, fricative, lateral fricative, approximant, lateral approximant.

Speech Dysfunction: Broca's Area

  • Leborgne's case: Inability to speak with good language comprehension and right-side paralysis due to damage in the left anterior hemisphere.

  • Speech production is independent from comprehension.

  • First evidence for localization of language function to a specific region of the cerebrum.

  • Ventroposterior region of frontal lobe.

  • Broca's area is important for speech production, supported by fMRI studies.

Semantic Dysfunction: Wernicke's Area

  • Carl Wernicke (1875): Damage to the left posterior superior temporal gyrus leads to reflexive but meaningless speech.

  • Some aphasics hear normally but have great difficulty understanding speech.

  • May speak fluently, but without meaningful content.

  • Other aphasics comprehend language but lack the ability to produce syntactically complete utterances.

  • Deficit associated with damage to the posterior & inferior regions (Broca’s area) of the left frontal lobe.

  • Speech perception is independent from production.

Language Dysfunction: Distinct Functional Components

  • Broca's Area:

    • Located in the frontal lobe.

    • Motor speech area.

    • Helps in movements required to produce speech.

    • A patient can understand the speech of others but cannot produce speech.

  • Wernicke's Area:

    • Located in the temporal lobe.

    • Sensory area.

    • Helps in understanding speech and using the correct words to express thoughts.

    • A patient can produce speech but cannot understand the speech of others.

Aphasia: Neurogenic Language Disorder (Brain Injury)

  • Ability to move speech articulators can be compromised without affecting the ability to use language.

  • Damage to auditory pathways can impede the ability to hear without interfering with language functions.

  • Damage to specific brain regions can compromise essential language functions while leaving the sensory and motor infrastructure of verbal communication intact.

  • Aphasias diminish the ability to comprehend and/or produce language while sparing the ability to perceive the relevant stimuli or produce intelligible words.

Aphasia: Characteristics of Major Types

  • Broca's Aphasia:

    • Halting speech.

    • Tendency to repeat phrases or words (perseveration).

    • Disordered syntax and grammar.

    • Disordered structure of individual words.

    • Comprehension intact.

  • Wernicke's Aphasia:

    • Fluent speech.

    • Little spontaneous repetition.

    • Syntax and grammar are adequate.

    • Contrived or inappropriate words.

    • Comprehension not intact.

Aphasia: More Complex Than Binary Taxonomy

  • Considerations:

    • Is speech fluent?

    • Can you comprehend spoken messages?

    • Can the person repeat words or phrases?

  • Types of Aphasia:

    • Global, Mixed, Broca's, Transcortical motor, Wernicke's, Transcortical sensory, Conduction, Anomic

Language in the Brain: Localized and Lateralized

  • Language function is localized.

  • Key language processing is lateralized in the left hemisphere.

  • Cortical representation of language is distinct from:

    • Motor planning and control of the vocal tract (speech production).

    • Auditory cortices (speech perception).

    • Visual cortex (reading).

  • Two areas of special importance exist.

  • Neural substrates for language depend on sensory and motor functions.

  • Regions specifically devoted to language transcend more basic cognitive demands.

  • Essential function of cortical language areas: symbolic representation.

Lateralization: Evidence from Split-Brain Studies

  • Ongoing uncertainty about the degree to which language and complex cognitive functions are localized.

  • New evidence from studies of patients with severed corpus callosum and anterior commissure.

  • Treatment for medically intractable epileptic seizures.

  • Established hemispheric lateralization of language.

  • Sperry et al. demonstrated other functional differences between left and right hemispheres.

  • Split-brain patients:

    • Can name objects held in the right hand.

    • Cannot name objects held in the left hand.

  • The right hemisphere:

    • Can process left haptic information.

    • Cannot communicate with language areas.

  • Using right hemisphere: no verbal account or indirect description is possible.

  • The left hemisphere responds to written commands.

  • The right hemisphere typically responds only to nonverbal stimuli.

  • Distinctions reflect broader hemispheric differences.

  • The left hemisphere in most humans is specialized for verbal and symbolic processing important in communication.

  • The right hemisphere is specialized for visuospatial and emotional processing.

Lateralization: Right Hemisphere Functions

  • Organization of semantic information throughout the cortex.

  • Some language deficits arise from right hemisphere damage.

  • The right hemisphere is important for prosody.

  • Complex interactions exist between levels of language structure, distribution of processing, and representation.

Localization: Individual Variation

  • Individual speaker differences exist in the precise locations of language areas.

  • Bilinguals vary further in the distribution of representation and localization of information between languages.

Handedness: Relationship to Lateralization

  • Approximately 90% of the population is right-handed.

  • Animal paw preferences exist, but a strong human right preference is noted.

  • Does handedness relate to other lateralized functions?

  • No evidence for a direct relationship between language and handedness.

  • 97% of humans, including the majority of left-handers, have verbal language functions represented in the left hemisphere.

  • Handedness, like language, exemplifies the advantage of having a specialized function in one hemisphere or the other to maximize 'wiring efficiency."

Brain Asymmetries: Anatomical Basis

  • Geschwind (1960s): The superior aspect of the temporal lobe differs.

  • Planum temporale is larger on the left side in approximately 2/3 of humans.

  • A similar difference is observed in great apes but not in other primates.

  • The planum temporale is near (but not congruent with) language regions.

  • Anatomical differences between hemispheres of the brain are unlikely to correlate with language lateralization or other lateralized functionality.

  • No clear structural correlates of functional differences have been identified.

Genetic Influences: Influence on Language Development?

  • Genes play some role in all phenotypic features.

  • Language/reading problems can run in families.

  • Rare inherited disorder: FOXP2 (chromosome 7).

  • Affected individuals are disfluent with atypical morphology.

  • Other cognitive functions are also affected.

  • The degree of language specificity is unclear.

Signed Languages: Same Properties, Different Modality

  • The cortical organization of language: Language regions are broadly organized for processing symbols, independent of communication/modality.

  • Signed languages are characterized by the same design features as spoken languages.

  • Auslan, ASL, and BSL also function as discrete combinatorial systems.

  • Hierarchically structured: lexicon, phonology, morphology, syntax, pragmatics.

  • Visual modality rather than acoustic/auditory.

  • Bellugi et al.: Cortical localization of language abilities in deaf individuals with brain lesions.

  • No verbal language, signing whole life in deaf communities, right-handed.

  • Patients with left-hemisphere lesions to language areas: measurable deficits in sign production and comprehension.

  • Lesions in the same areas on the right: no signing aphasia, but right hemisphere abilities are impaired: visuospatial & emotional processing.

  • Language instinct emerges universally in early infancy.

  • Babbling prefigures mature language.

  • Babbling in hearing infants: predictable patterns of sound production → adult phonologies evidenced in the environment.

  • An innate capacity for language imitation is a key part of the acquisition.

  • Hearing offspring of deaf, signing parents babble with hands.

  • Gestures approximate mature signs of parent languages.

  • The amount of manual babbling increases with age towards accurate, meaningful signs.

  • Key strategies for language acquisition are independent of modality.

First Language Acquisition: Critical Period?

  • Language fluency requires linguistic experience relatively early in life.

  • Neural circuitry is especially susceptible to modification during early development.

  • Malleability diminishes with maturation.

  • Critical period: A window for extensive neural modification supporting a behavior.

  • Learning an L2 before age 7 results in native-like adult performance.

  • The requirement for experience during the critical period is revealed in children who become deaf at different ages.

  • Effects on language skills are more marked when the onset of deafness occurs early in life.

  • Younger children who have acquired some speech but then lose their hearing suffer a substantial decline in spoken language.

  • Brain activation while doing language-based tasks reveals neural regions pertinent to diminished language-learning skills in adults.

Reading: Relationship to Language

  • The interpretation of visual symbols is closely related to language.

  • Not a fundamental component of language, but predicated on language.

  • A wide range of reading skills, dependent on other factors.

  • Dyslexia may involve:

    • Poor reading skills.

    • Difficulty processing speech sounds.

    • Difficulty translating visual to verbal information.

    • Difficulty writing.

    • Not related to general intelligence.

  • Common: ~ 5 to 15% of children, more often male.

  • Runs in families: genetic basis?

Dyslexia: Insights into Language

  • fMRI: Specific left brain areas are activated during reading.

  • Some areas are also activated by spoken language.

  • Visual Word Form Area (VWFA): left occipito-temporal sulcus.

  • Selectively activated by written characters.

  • Not activated by spoken words or low-level visual stimuli.

  • The organization of VWFA depends on experience.

  • Activation levels in children predict word–phoneme decoding abilities.

  • Dyslexics:

    • Weaker BOLD signal in VWFA.

    • Underdeveloped cortex + white matter tracts.

  • Why a functionally specific brain region for reading?

  • The VWFA could not have evolved to support reading.

  • A brain region with specific processing characteristics.

  • Dehaene & Cohen: Brain circuits 'recycle' information for new, culturally specified functions.

  • Alphabets may exploit efficient use of receptive fields.

Animal Communication: Expressions of Language?

  • Many animal (+ plant, fungi) communication systems are characterized by great complexity.

  • Cultural transmission of whale song.

  • Complex encoding in bee dance.

  • The sophistication of signaling and ecological implementation exceeds that observed in human language & speech in some dimensions.

  • Is human language fundamentally different?

  • Are differences categorical or gradient?

Animal Communication: Biological Foundations

  • Different species exploit different modalities in communication systems:

    • Chemical, olfactory, tactile, seismic, visual, acoustic.

  • The medium will shape/limit properties of the system.

  • Does not directly inform key issues of language.

  • Human signed languages use the visual modality.

  • All human communication is typically multimodal.

Animal Communication: Evolutionary Function

  • Animal communication systems evolved to:

    • Facilitate mating.

    • Repel competitors or enemies.

    • Signal aggression or submission.

    • Warn of predators.

    • Communicate about the environment/food.

  • Human language can facilitate these functions.

  • Language is more than communication.

  • Other species communication is characterized by:

    • Instinctive behaviors.

    • Learnt behaviors.

Human Language: Design Features (Hockett, 1960)

  • Arbitrariness: Most linguistic signs are not iconic.

  • Displacement: Discuss dislocated events and places.

  • Productivity: Individual speaker, new expression; limitless possibility of expression.

  • Reflexivity: Metalanguage of language.

  • Duality: Sounds combine to make words; words combine to make sentences.

  • Cultural transmission: Animal communications are largely instinctive, but a person’s specific language is learned rather than hard-coded.

  • Discreteness: A finite set of elements combine to create an infinite set of messages.