Cerebral Dominance and Language Subhadeep Dutta Gupta

Imagine being unable to say, "I am hungry," "I am in pain," "thank you," or "I love

you.“ Being trapped inside your body, a body that doesn't respond to

commands. Surrounded by people, yet utterly alone. Wishing you could reach out, to

connect, to comfort, to participate. For 13 long years, that was my reality.

Most of us never think twice about talking, about communicating. I've thought a lot

about it. I've had a lot of time to think. --- Martin Pistorius

Introduction:

• Language is communication through words or symbols for words. Language

processing is a unique trait of human species

• By 6 years of age, children understand about 13,000 words, and by the end of

high school, about 60,000 words

• It is estimated that 90 % of right – handed persons and 70% of the left – handed

persons have a left-hemisphere language specialization

Components of language:

Phonology: sounds that compose language and the rules that govern their combination.

Syntax : rules for combining words into phrases and sentences and determining relations among words.

Prosody: contribute to such linguistic functions as intonation, tone, stress, and rhythm (emotional valence).

"Babies and children are geniuses until age 7 and then there's a systematic decline."

• Hemispheric asymmetries are present in most vertebrates, including fish,

amphibians, reptiles, birds and mammals

• Recently, lateralization has also been shown in invertebrates, e.g. fruit flies,

honeybees or octopuses

Concept of Hemispherical lateralization:

A structural asymmetry in the Drosophila brain

-- Nature 2004;427(6975):605–6.

• Functional cerebral asymmetries (FCAs) might have arisen to avoid processing

delays deriving from slow interhemispheric transfer, or to prevent

interhemispheric conflicts or functional incompatibility

• Another long standing hypothesis to explain FCAs is by saving neural capacity

due to a reduction of redundant processes

• While a specific neural circuit in one hemisphere is processing a specific task,

the homologous area in the opposite hemisphere can perform different or

complementary processes, allowing a more efficient use of cortical capacity

Concept of Hemispherical lateralization:

-- Behavioural Brain Research 187 (2008) 297–303

Concept of Hemispherical lateralization:

-- Cognitive Neuroscience; 3rd Edition

Franz Joseph Gall, Austrian physician and anatomist :

The cortex, forms the basis of mental function

Each mental faculty has its own seat, a circumscribed area of cortex

Theory of localization of function

Jean-Baptiste Bouillaud:

In majority of the speech disorder cases, the

lesion was localized in the anterior part of the brain

The beginning:

1. Pathological cases

The beginning:

A, B : Leborgne’s Brain C, D: Lelong’s Brain

-- Brain; (2007) Vol 130 (5) p1432-1441

Pierre Paul Broca (1824-1880)

Famous case of ‘Monsieur Tan’

Damage/lesion at Left Inferior Frontal Gyrus: nonfluent, sparse, dysprosodic, and agrammatical speech

Broca’s area ; Broadmann Area 44,45

Broca’s limitation: Thinking about language as a unitary function localized in a single cortical

region

One step further….• Karl Wernicke distinguished between patients who had lost the ability to

comprehend language and those who could no longer produce language

• Lesion of the posterior and superior temporal lobe on the left hemisphere

• such aphasic patients do not understand language

• retain the ability to produce utterances with reasonable grammatical and

emotional content

2. Experimental Techniques

Montreal Procedure:• Penfield and Jasper applied very small electrical charges directly to the cortical

surface in epileptic patients.

Wilder Penfield and Herbert Jasper

Diagram from Penfield’s original study

illustrating sites in the left hemisphere at which

electrical stimulation interfered with speech

Sperry’s Split Brain Experiment

-- Neuroscience; 3rd Edition

Wada technique

-- Cognitive Neuroscience; 3rd Edition

3. Brain regions associated with language

Broca’s Area: • The IFG is often activated bilaterally but shows left-hemispheric dominance

during tasks requiring naming, judgments of phonology, semantics, and

syntax

• Broca’s region is also activated during acquisition of grammatical rules,

discrimination of speech sounds, production of words, estimation of time

intervals, and reproduction of rhythms

• Thus Broca’s region seems to be involved in both perception and production

of speech

-- Physiology (2005) 20: 60–69

Broca’s Area and the Mirror Neuron System

• Broca’s area : recognition of hand and mouth actions, with recent evidence

suggesting a key role in recognizing actions as part of the “mirror” or

“observation–execution matching”

• Rostral part of the monkey ventral premotor cortex (area F5) is the monkey

homolog of Broca's area in the human brain

-- Trends Neurosci. 1998 May;21(5):188-94.

Orienting behavior

Interaction with the external world

Broca’s Area: Cerebral asymmetry• Humans have been shown to display left hemisphere dominance of neuropil

space and pyramidal neuron dendritic branching in Broca’s region (Schenker et

al., 2007)

• The cytoarchitectonic areas that comprise Broca’s region (BA44 and BA45) have

a greater volume in the left hemisphere as compared with the right in humans

(Amunts et al., 1999)

-- Physiology, 2005 (20); 60-69

Broca’s Aphasia:

• Q) What have you been doing in the hospital?

“ Yes, sure. Me go, er, uh, P.T. non o’ cot, speech….two

times….read….wr….ripe, er, rike, er, write…..practice….get-ting

better.”

Meaning:

“ I go to P.T at one o’ clock to practice speaking reading, and

writing and I’m getting better.”

-- Case study of David Ford as reported by psychologist Howard Gardner

• The hallmark of Broca’s aphasia is a telegraphic style of speech, in

which mainly content words (nouns, verbs, and adjectives carrying

content specific to the sentence) are used

• Q) Were you in the coast guard?

“No, er, yes, yes... ship... Massachu... chusetts... Coastguard...

years.” He raised his hands twice, indicating the number

“nineteen.”

-- Case study of David Ford as reported by psychologist Howard Gardner

• The Planum temporale, is a sheet-like, triangular structure, which lies on the

superior surface of the temporal lobe within the Sylvian fissure

• Lesion of PT number of associated auditory discrimination and speech

comprehension deficits (Caplan and Markis, 1995)

Wernicke’s Area:

-- J Neurophysiol 2009;101:2725-2732

Wernicke’s Area: Cerebral asymmetry

-- Brain Research Reviews 29 (1999) 26–49

Wernicke’s Aphasia:

• Speak in long sentences that have no meaning, add unnecessary words, and

even create made-up words

• "You know that smoodle pinkered and that I want to

get him round and take care of him like you want

before."

• Great difficulty understanding speech, and they are often unaware of their

mistakes

Arcuate Fasciculus

• The arcuate fasciculus is a white-matter fibre tract that links lateral temporal

cortex with frontal cortex via a dorsal projection that arches around the Sylvian

fissure

• On average, the human arcuate fasciculus is larger in the left than the right

cerebral hemisphere

• The cortical terminations of humans differed from chimpanzees and macaques,

with humans having much stronger terminations posteriorly, in the MTG and

ITG, as well as anteriorly, in pars opercularis (BA 44), pars triangularis (BA 45),

pars orbitalis (BA 47) and surrounding regions

• This raises the possibility that the expanded pathway in humans supports the

transmission of word-meaning information stored in the MTG and angular gyrus

to pars triangularis and orbitalis for both sentence comprehension and

sentence construction during spontaneous speech-- Nature Neuroscience (2008) 11, 426 – 428

-- Cortex. 2008 Sep; 44(8): 953–961

Arcuate Fasciculus: Cerebral asymmetry

Language - related regions of the left hemisphere mapped by positron emission tomography

(PET) in normal human subjects -- Neuroscience (D.Purves), 4th Edn

Distribution of 730 activation peaks from 129 language studies: top panel – activation peaks for phonological (blue), semantic (red), and syntactic (green) tasks; bottom panel – results of a cluster analysis.

-- Neuroimage (2006) 30(4).1414–32.

A developmental Magnetoencephalography study

-- Annu. Rev. Neurosci. (2008);31:511-534.

4. Language and Right Hemisphere

Evidences:

• Some patients with RH brain damage have subtle deficits in comprehending

natural language

• Neuroimaging studies often reveal weak neural activities in homologous regions

of the RH during language tasks

• A growing number of studies report RH greater than LH brain activity while

subjects perform higher-level language tasks, such as comprehending

metaphors, getting jokes, deriving themes, and drawing inferences, generating

the best endings to sentences, mentally repairing grammatical errors, detecting

story inconsistencies, and determining narrative event sequences

Functions of RH in language:The right hemisphere is involved in processing certain aspects of:

• Melody

• Prosody, which is the intonation pattern, or sound envelope, of an utterance

• Timing

• Stress

• Pauses

• Accent

The right hemisphere plays an important role in narrative and inference.

Narrative refers to the ability to construct or understand a story line, whereas

inference refers to the ability to “fill in the blanks” and make assumptions

about material that is not explicitly stated

Wernicke- Lictheim’s House model of language

Wernicke-Geschwind model: Serial fashion for language processing.

7 components:

• Wernicke’s area

• Arcuate fasciculus

• Broca’s area

• Angular gyrus

• Primary auditory Cortex

• Primary visual cortex

• Primary motor cortex

Primary Auditory Cortex: Primary Visual Cortex:Reception of auditory stimulus Reception of visual stimulus Angular Gyrus: Shape of the symbols

Wernicke’s Area: Interpretation of the words and meaning given

Arcuate Fasciculus: Information transmission

Broca’s Area: activates the appropriate response to the word

Primary Motor Cortex: Vocalization

Tripartite organization for language system

1. Language Implementation System: Broca’s area, Wernicke’s area, parts of

insular cortex and Basal Ganglia

-- Decodes incoming verbal information and produces appropriate verbal

responses

2. Language Mediation System: Association cortex in the temporal, parietal and

frontal lobes

3. Language Conceptual System: higher level association cortex areas

-- Damasio and Damasio, 1992

Dual Stream Model of Language Processing

A/c to this model, there are two functionally distinct computational/neural

networks that process language information:

• one that interfaces sensory/phonological networks with conceptual-semantic

systems, and

• one that interfaces sensory/phonological networks with motor-articulatory

systems

-- Nature Neuroscience, 2007 (8) 393-402

Left hemisphere biasness – Why??Incorporation of vocalization:

Vocalization itself, appears to be left-hemispheric even in nonhuman species, from

frogs (Bauer, 1993) to mice (Ehert, 1987) to primates (Hook-Costigan & Rogers,

1998), leading Corballis (2003) to propose that it was the incorporation of

vocalization into the human mirror system that extended the left-hemispheric bias

to the cortical level, giving rise perhaps to nonlinguistic asymmetries such as the

left-hemispheric dominance in manual praxis—and indeed the phenomenon of

right-handedness itself.

4. Language Asymmetry and Genetics

FOXP2• Forkhead box P2 (FOXP2) gene – in the long arm of chromosome 7 (7q31)

• Study of British K.E family: Across 3 generations, 15 out of 37 members suffered from ‘Verbal dyspraxia’ -

orofacial movement disorder

Autosomal dominant mutation

Substituted a histidine for an arginine

at site 553 in the FOXP2 sequence

Inferior frontal Basal Ganglia loop

Inferior frontal – cerebellum loop

-- Nature Reviews Neuroscience (2005) 6; 131-138

Lateralization of gene expression in human language cortex

Data from Pletikos et al, 2014) Data from Hawrylycz et al,2012).

Box plots of STG lateralization t-statistics, for genes within the four most significantly

lateralized gene sets in meta-analysis, and an additional non-lateralized gene set

(metallopeptidase activity) for comparison. -- Cortex (2015) 67; 30–36

Large & diffused semantic fields

Small & focused semantic fields

Multiple distantly related words

Coarse interpretation

Summation of weak activation

Bicêtre Hospital, the place of Leborgne's illness.

-- Credit: National Library of France.

We’ve come a long way from the days of phrenology. And much of it is thanks to the man who couldn’t speak—and the doctor who understood just how meaningful that loss would be for the future of science.