HNA II — Speech Functions & Hemispheric Asymmetry

Complete Viva Notes (Guyton & Hall + Ganong/Sembulingam)

1. What is Speech as a Function of HNA?

• Comprehension – understanding what you hear or read
• Expression – forming and producing meaningful words

2. Cortical Speech Centers (Broca & Wernicke Areas)

Guyton & Hall — Broca area (word formation, frontal lobe) and Wernicke area (language comprehension, temporal lobe)

3. Function of Broca's Area

• Located in the inferior frontal gyrus, just in front of the primary motor cortex
• Receives input from Wernicke's area via the arcuate fasciculus
• Converts language concepts into detailed, coordinated motor programs for vocalization — it drives the muscles of the lips, tongue, pharynx, and larynx
• Projects via a speech articulation area in the insula → to the motor cortex → to produce actual speech sounds
• Think: "Broca = speaks / produces speech"

"Broca area processes the information received from Wernicke area into a detailed and coordinated pattern for vocalization and then projects the pattern via a speech articulation area in the insula to the motor cortex." — Ganong's Medical Physiology

4. Function of Wernicke's Area

• Located in the posterior part of the superior temporal gyrus, just behind the primary auditory cortex
• Understands both auditory and visual language — it is the "comprehension centre"
• It links sensory input (sound of words, visual images of letters) to meaning
• Most intellectual functions are language-based, so Wernicke's area is one of the most important areas in the whole brain for intellect
• Think: "Wernicke = understands / comprehends language"

"The major area for language comprehension, called the Wernicke area, lies behind the primary auditory cortex in the posterior part of the superior temporal gyrus... It is one of the most important regions of the entire brain for higher intellectual function." — Guyton & Hall

5. How is Speech Produced and Perceived? (The Wernicke–Geschwind Model)

Ganong — Language pathway: Wernicke → Arcuate fasciculus → Broca → Motor cortex → Speech

Pathway for HEARING and REPEATING a word:

1. Sound → Primary auditory cortex (Heschl's gyrus, temporal lobe)
2. → Wernicke's area (word is recognized and understood)
3. → Via arcuate fasciculus (white matter bundle) → Broca's area
4. Broca's area forms motor program → via insula → primary motor cortex
5. Motor cortex → muscles of mouth, tongue, larynx → spoken word

Pathway for READING and SPEAKING a word:

1. Visual input → Primary visual cortex → Higher visual areas
2. → Angular gyrus (converts visual word into auditory equivalent)
3. → Wernicke's area (comprehension)
4. → Arcuate fasciculus → Broca's area → Motor cortex → Speech

"A major share of our sensory experience is converted into its language equivalent before being stored in the memory areas." — Guyton & Hall

6. What is Hemispheric Dominance?

• One hemisphere is more specialized for certain functions than the other — this is called cerebral dominance or lateralization
• For language and speech: Left hemisphere is dominant in ~95% of right-handers and ~70% of left-handers
• For most other "dominant" functions like: analytical thinking, mathematics, logic, reading, writing → LEFT hemisphere
• The "dominant" hemisphere is determined mostly by genetic factors + early developmental experience

7. Role of the LEFT Hemisphere (Dominant Hemisphere)

• Speech and language (Broca's + Wernicke's areas)
• Reading and writing
• Mathematical calculations
• Logical and analytical reasoning
• Verbal memory — stores information in language form
• Sequential processing of information

"When the Wernicke area in the dominant hemisphere of an adult is destroyed, the person loses almost all intellectual functions associated with language or verbal symbolism, such as the ability to read, perform mathematical operations, and even the ability to think through logical problems." — Guyton & Hall

8. Role of the RIGHT Hemisphere (Non-dominant Hemisphere)

• Music and prosody (tone and rhythm of speech)
• Non-verbal visual experiences — recognizing faces, spatial patterns
• Spatial relationships — understanding body position in surroundings
• Body language and emotional tone (intonation/emotion in voice)
• Artistic and creative abilities
• Holistic (gestalt) processing

"The so-called nondominant hemisphere might actually be dominant for some other types of intelligence." — Guyton & Hall

Note: Both hemispheres communicate via the corpus callosum.

9. Types of Aphasia

10. Clinical Manifestations of Speech Disorders

Broca's Aphasia:

• Speech is slow, labored, telegraphic (short phrases, no connecting words)
• Agrammatism — omits grammatical words ("Give water" instead of "Give me some water")
• Patient understands what you say and is frustrated by their difficulty
• Writing is also abnormal

Wernicke's Aphasia:

• Speech is fluent but meaningless — full of invented words (neologisms) and jargon
• Patient cannot understand spoken or written language
• Unaware of their own errors
• May appear confused or even "psychiatric" to untrained observers

Other Points:

• Agraphia (can't write) + Alexia (can't read) → Angular gyrus lesion
• Stuttering → associated with right cerebral dominance and abnormal supplementary motor area activity
• Prosopagnosia → right temporal lobe damage → can't recognize faces

Quick Summary for Viva