Module 10: Human Communication

Language and the Brain

Lateralisation of Language Function

• In 90% of people language is lateralised to the left - the neural circuitry required for language production and comprehension appears to be dominant for the left half of the brain compared to the right half of the brain.

• The right-hemisphere is responsible for spatial navigation, processing of emotional tone, processing of metaphors, and Prosody

  • Prosody is the use of changes in intonation and emphasis to convey meaning in speech besides the meaning specified by particular word

Split-Brain Operations: includes cutting the corpus callosum, completely separating two hemispheres.

• People with such condition cannot vocalise what they have seen in their left visual field, but they can draw that thing with their left hand.

Neural Basis of Language Production and Comprehension

Because language production and comprehension often requires sensory perceptions and memories, on both hemispheres, it involves :

• Occipital lobes

• Temporal lobes

• Parietal lobes

• Some parts of the prefrontal cortex

Specific Language Prod and Comp areas:

• Broca’s area - speech production

• Wernicke’s area - speech recognition - dictionary for auditory entry of words

• Primary auditory cortex

• Posterior language area - interface between Wernicke’s area and perceptions and memories, processes meaning of words

The Bilingual Brain

• Bilingual people with lesions to the brain can sometimes have aphasia for one language and not another

• As studies show, bilingual brains have separate regions for different languages, but they have some overlapping.

• Density of grey matter in left parietal cortex is higher for bilingual people, and increases the earlier the second language acquisition has happened

Voice Recognition

• Spoken language tells us a lot about speaker’s identity

• Phonoagnosia: difficulty determining identity based on voice, with otherwise intact language comprehension and production

  • Results from lesions to the right hemisphere, specifically, parietal lobe, and anterior superior temporal cortex

Aphasia and Neural Basis of Language Production and Comprehension

Broca’s Aphasia

• Characterised by slow, laborious, non-fluent speech, where words are mispronounced, but meaningful

• Difficulty using function words, tend to use content words

• Better language comprehension then production, but comprehension is still affected

Broca’s aphasia occurs after damage to Broca’s area, surrounding areas in the left frontal lobe, or some underlying subcortical white matter that transmits information across the brain (sometimes also basal ganglia)

• Therefore, seems to be linked to motor memories (how to move muscles to say different words)

• Broca’s area and surrounding regions contain the “programs” for how to articulate words - has direct connections to areas of the motor cortex involved in speech

Three main symptoms:

1. Agrammatism: problems understanding grammatical devices (problems in comprehension - issues with passive sentences)

2. Anomia: difficulty finding words

3. Articulation difficulties: incorrect order of the sounds, but patients know, and try to correct themselves

Pure Word Deafness

Pure word deafness is the ability to hear, to speak, and usually read and write, without being able to recognise, and thus comprehend, spoken words. Patients can read lips, recognise emotional intonation, and non-speech sounds

• Is caused by damage to the superior temporal lobe (Wernicke’s area)

Studies have found that unintelligible speech sounds and intelligible speech are processed in different areas of the anterior superior temporal gyrus

Transcortical Sensory Aphasia

Patients cannot understand spoken words, but can repeat them; no ability to produce meaningful speech.

• Is caused by damage to Posterior Language Area, which sends information between the auditory representation of words (Wernicke’s area) and the meaning of these words, which are memories held in other parts of the cerebral cortex

Wernicke’s Aphasia

Involves difficulties in spoken word comprehension and recognition, as well as difficulties converting thoughts into words.

• Is caused by damage to both the Wernicke’s area, and posterior language area

• Poor language recognition and comprehension, but produce fluent meaningless speech.

• Exhibit prosody, and emotional intonation, follow social conventions, sentences appear to have grammatical structure

• Individual is unaware of the language difficulties

Anomic Aphasia

Individuals have normal grammar and language comprehension, but have difficulty finding words.

• For nouns - left temporal and parietal lobes, without affecting Wernicke’s

• For verbs - frontal lobes, near Broca’s area

Conduction Aphasia

Occurs due to damage to the direct connection between Wernicke’s and Broca’s - Arcuate Fasciculus (AF)

Characterised by inability to repeat spoken words, but no difficulty in producing or comprehending speech otherwise

Sometimes individuals will say a semantically similar word instead of repeating

Thus, two pathways between Wernicke’s and Broca’s:

Direct: via AF to convey sounds, we use it fr unfamiliar words - damaged in conduction aphasia

Indirect: via posterior language area, transmits information based on semantics, not on sounds they make - intact in conduction aphasia

Sign Language

Some research suggests that sign language preceded spoken language. For instance, mirror neurons are active when we see or perform particular grasping, holding, or manipulating movements. Some mirror neurons are found in Broca’s area and likely help us mimic others’ hand movements, as Broca’s area is active when we observe and imitate finger movements.

As of now, all of the cases of people who are deaf with aphasia for sign have lesions to the left hemisphere.

The same language related areas of the brain activate in deaf and hearing people when deciding of written words rhymed

Stutter

How it works:

• It's linked to impaired timing and coordination in the speech production system, involving the motor cortex, basal ganglia, and auditory areas.

• Brain imaging shows overactivity in motor areas and underactivity in auditory feedback regions during speech in people who stutter.

• There's also disrupted communication between left and right hemispheres, and sometimes delayed auditory feedback.

Why people stutter:

• Genetic factors: It often runs in families; several genes have been linked to it.

• Neurodevelopmental differences: Brain wiring for speech may develop differently.

• Environmental factors: Stress or pressure can worsen stuttering but don’t cause it.

• It usually begins in early childhood, often during language development.

Impacts 1% of the population

Reading and Writing

Pure Alexia

Inability to read with intact ability to write

• Can recognise words when they are spelled out to them - perceptual disorder

• Caused by lesions that prevent visual information from getting to the visual association cortex of the left hemisphere

Neural basis:

• Damage to the left visual cortex (in the occipital lobe) disrupts visual input from the right visual field, which is crucial for reading.

• Visual information from the left visual field goes to the right visual cortex, but in pure alexia, the splenium of the corpus callosum (the back part) is also damaged.

• This damage prevents visual information from the right hemisphere from crossing over to the left hemisphere, where the language areas (like the visual word form area) are located.

• As a result, although vision is intact and writing is possible, the brain can’t link visual input to language, so reading fails.

VWFA: a region in ventral occipitotemporal lobe that is crucial for reading and processing written word

Acquired Dyslexia

Two pathways for reading:

1. Whole word pathway: used when we recognise words as a whole

   • Follows the ventral stream and involves the Visual Word Form Area (VWFA - fusiform gyrus)

2. Phonetic pathway: used when we are sounding out words letter by letter (unfamiliar words)

   • Less certain about neural basis, but may involve the temporoparietal cortex and the inferior frontal cortex (+ Broca’s), and their connection, VWFA also likely involved

Surface Dyslexia

Difficulty using whole-word reading, thus a need to always sound out the words they are reading.

• People with this condition will have difficulty reading the words “pint“ or “yacht“, but no difficulty reading “hand“.

• Can read non-words

• Due to not being able to recognise whole words, have to rely on their pronunciation of them to understand meaning

Phonological Dyslexia

Individual can read using the whole word method, but not the phonological method.

• Pronouncing familiar words is easy, but learning new words is challenging because they cannot sound them out

Visual Word Form Area

Part of the visual association cortex involved in perceiving written words, found in the fusiform cortex of the left hemisphere.

• Children learning to read, after a couple hours of reading lessons, transfer the visual perception of words from both hemispheres, to only the left one.

• VWFA is very precise and can recognise words even if they are very similar

• Experienced readers take the same amount of time to read 3- and 6-words letters

• Suggested that originally this area was responsible for learning configurations of lines to recognise objects

Writing and the Brain

Dysgraphia

A writing disturbance common with dyslexia (spelling issues, motor control, etc.)

• Brain injury related to aphasia can result in similar disturbances in writing

• Organisation of the motor aspects of writing involves the dorsal parietal lobe and the premotor cortex (+ motor cortex) - damage causes dysgraphia

Skills needed for writing:

• Audition

  • Reading and writing depends on earlier learnt skills

• Vision

  • Trying to remember how to spell the word by trying to visualise it

• Memorisation

  • Learning a sequence of letters to write

• Motor memory

  • We have auto memory of writing or typing very familiar words

Phonological Dysgraphia: cannot write words that are unfamiliar to them because cannot sound them out (damage to Broca’s area, ventral precentral gyrus, insula)

Orthographic Dysgraphia: a person can spell regularly spelled words, but not irregularly - damage to VWFA