Chapter 13: Aphasia
Chapter 13 Aphasia
Key Focus: Types of Aphasia, Brain-Language Links, Parsing Deficits, & Treatment
Overview
This chapter explores aphasia—an acquired language disorder from brain damage (e.g., stroke)—to understand how the brain organizes language. It covers core topics: the left hemisphere’s role in language, classic aphasia syndromes (Broca’s, Wernicke’s, Conduction), why patients struggle with complex sentences, and evidence-based treatments. Real-world relevance: Explains why someone with Broca’s aphasia can understand simple sentences but not passives, and how therapies like singing (MIT) improve speech fluency.
1. Introduction (pp. 491-492)
1.1 What Is Aphasia?
- Definition: Acquired language disorder from brain damage (stroke, tumor, TBI)—no loss of general intelligence (e.g., can solve math but can’t say “I want water”).
- Core Goal: Use aphasia as a “window” to answer:
- Are specific brain regions for language tasks (speaking vs. understanding)?
- Which regions handle grammar vs. word meaning?
- How do regions communicate for fluent language?
- Do all brains organize language the same way?
1.2 Methods to Study Brain-Language Links
| Method | Focus | Example |
|---|---|---|
| Neuroimaging (Intact Brains) | Measure activity in healthy people. | fMRI shows Broca’s Area activates when planning sentences; ERP N400 spikes for “The cat barked” (semantic oddity). |
| Lesion Studies (Damaged Brains) | Link damage to deficits (tests if a region is necessary for a function). | Damage to Wernicke’s Area → comprehension loss. |
1.3 Left Hemisphere Dominance
Nearly all humans use the left hemisphere for core language functions:
A. The WADA Test (Wada & Rasmussen, 1960)
- Procedure: Anesthetize one hemisphere (left/right) via artery injection; test language (name pictures, repeat phrases).
- Result:
- Left hemisphere anesthetized: ~96% right-handers / ~70% left-handers become mute and can’t understand.
- Right hemisphere anesthetized: Speech/understanding intact (minor prosody/humor deficits).
B. Left Hemisphere Damage = Aphasia
- Severe aphasia (no speech/understanding) almost always follows left hemisphere damage.
- Right hemisphere damage: Subtle deficits (e.g., can’t understand sarcasm) but no core aphasia.
2. Aphasiology: Brain Damage & Language Loss (pp. 492-504)
2.1 Historical Debate: Localization vs. Equipotentiality
| Hypothesis | Core Claim | Evidence For/Against |
|---|---|---|
| Equipotentiality | Language needs “mass action” of the whole brain—no specific regions. | ❌ Disproven: Aubertin’s patient stopped speaking when left frontal lobe was pressed. |
| Localization | Specific regions for specific language functions. | ✅ Broca’s patients (Leborgne/Lelong) had left frontal damage and non-fluent speech. |
2.2 Key Historical Discoveries
A. Paul Broca (1861): Broca’s Area
- Patients: Leborgne (said only “tan”) and Lelong (said 5 words) → both understood language.
- Autopsy: Both had lesions in the left inferior frontal lobe (later named Broca’s Area).
- Conclusion: Broca’s Area is critical for fluent speech production.
B. Carl Wernicke (1870s): Wernicke’s Area
- Patients: Susanne Adam/Rother → fluent but meaningless speech (“feffort,” “stam”) + severe comprehension loss.
- Autopsy: Lesions in left posterior temporal lobe (later named Wernicke’s Area).
- Theory:
- Motor Aphasia (Broca’s): Frontal damage → non-fluent speech, intact comprehension.
- Sensory Aphasia (Wernicke’s): Posterior damage → fluent nonsense, no comprehension.
2.3 Classic WLG Model (Wernicke-Lichtheim-Geschwind)
Integrates Broca’s/Wernicke’s work—3 core left-hemisphere structures:
| Structure | Location | Function |
|---|---|---|
| Wernicke’s Area | Temporal/parietal junction | Decode sound → meaning (comprehension); retrieve sound codes (production). |
| Broca’s Area | Left inferior frontal lobe | Build grammar; plan speech movements. |
| Arcuate Fasciculus | White matter tract (Wernicke’s → Broca’s) | Relay sound/meaning info from Wernicke’s to Broca’s. |
How It Works (Example: Saying “I ate breakfast”)
- Concept (“I ate breakfast”) → Wernicke’s Area.
- Wernicke’s retrieves words (“I,” “ate,” “breakfast”) + sound codes.
- Arcuate fasciculus sends info to Broca’s Area.
- Broca’s organizes grammar + plans speech movements.
- Motor cortex executes speech.
2.4 Three Aphasia Syndromes (WLG Predictions)
| Type | Lesion Location | Key Symptoms | Example Speech |
|---|---|---|---|
| Wernicke’s (Fluent) | Wernicke’s Area | - Fluent but meaningless speech (neologisms: “feffort”). - Severe comprehension loss. - Preserved prosody. | “It had a feffort and the stam of foriment.” |
| Broca’s (Non-Fluent) | Broca’s Area + subcortical structures | - Halting, telegraphic speech (omits “the,” “-ed”). - Intact comprehension for simple sentences. - Apraxia (speech movement difficulty). | “Cinderella … ball … dance … shoe.” |
| Conduction | Arcuate fasciculus | - Intact comprehension + fluent speech. - Severe repetition deficit (can’t repeat phrases). - Can paraphrase (e.g., “pastry cook tired” → “baker felt sleepy”). | “The quick brown… thing… jumps over sleepy dog.” |
2.5 Limitations of the WLG Model
- Lesion Location ≠ Symptom: Broca’s aphasia can occur without Broca’s Area damage (basal ganglia lesions); small Broca’s lesions cause only short-term deficits.
- Insula Is Critical: All patients with speech apraxia have insula damage (WLG ignores this region).
- VLSM Challenges: Sentence comprehension depends on wide-ranging regions (not just Wernicke’s); lesion size matters more than location.
3. Broca’s/Wernicke’s Aphasia & Parsing Deficits (pp. 504-516)
3.1 Broca’s Aphasia: Hidden Comprehension Problems
Early researchers thought Broca’s patients had intact comprehension—not true: they struggle with syntactically complex sentences.
Reversible vs. Non-Reversible Sentences
| Sentence Type | Example | Patient Performance |
|---|---|---|
| Non-Reversible | “The cheese was eaten by the mouse.” | Good (~85% correct) → semantics clarify roles (cheese = eaten). |
| Reversible | “The girl was kissed by the boy.” | Poor (~50% correct) → need syntax to know who kissed whom. |
3.2 Why Broca’s Patients Struggle with Parsing: 4 Hypotheses
| Hypothesis | Core Claim | Strength/Weakness |
|---|---|---|
| Global Parsing Failure | Can’t build syntactic structures—rely on semantics. | ❌ Weakness: Can judge grammaticality (e.g., reject “Was the girl enjoy the show?”) 85% correct. |
| Trace Deletion | Can’t link “fillers” (displaced words) to “gaps” (original positions) → guess roles. | ✅ Explains 50% correct on reversibles; ❌ Weakness: Struggle with simple actives (“The dancer applauds the actor” = 75% correct). |
| Mapping Hypothesis | Can build structures but can’t assign semantic roles (who did what). | ✅ Explains grammaticality judgment skills; ❌ Weakness: Doesn’t explain why mapping fails. |
| Slowed Syntax | Parse sentences too slow—info decays before linking to meaning. | ✅ Cross-modal priming: Activate fillers 0.5s late (normal = at gap); explains preserved grammar knowledge. |
3.3 Wernicke’s Aphasia: Comprehension Loss from Lexical Access Failure
- Core Deficit: Can’t link speech sounds to word meanings (e.g., hear /kæt/ but can’t retrieve “cat”).
- Key Symptoms:
- Lexical substitutions (e.g., “boat” for “coat”).
- Can’t detect own errors (say “feffort” and not realize it’s meaningless).
- Better reading than listening (visual input lasts longer).
3.4 Broca’s vs. Wernicke’s: Side-by-Side
| Feature | Broca’s Aphasia | Wernicke’s Aphasia |
|---|---|---|
| Speech Fluency | Non-fluent, telegraphic | Fluent (but meaningless) |
| Comprehension | Good (simple sentences) | Severe loss |
| Key Deficit | Grammar/speech planning | Sound-meaning linking |
| Error Type | Omit function words (-ed, the) | Neologisms (feffort) |
4. Treatment & Recovery from Aphasia (pp. 516-520)
4.1 Spontaneous Recovery
- Definition: Improvements without treatment—starts days after damage.
- Key Factors:
- Small lesions → full recovery; large lesions → permanent deficits.
- Most recovery in 3–6 months (minor gains for years).
- Younger/healthier patients recover faster.
- Case Example: Wernicke’s patient Susanne Adam went from no comprehension to understanding most speech in 3 weeks.
4.2 Neural Mechanisms of Recovery
- Restoration: Swollen neurons regain function.
- Reorganization: Right hemisphere homologs (similar regions) take over (e.g., right Broca’s Area plans speech).
- Plasticity: New connections form (e.g., Wernicke’s → Broca’s via right hemisphere).
4.3 Evidence-Based Treatments
| Treatment | Target Deficit | Method | Evidence |
|---|---|---|---|
| Melodic Intonation Therapy (MIT) | Non-fluent speech (Broca’s) | Sing phrases → fade melody to speech (recruits right hemisphere). | PET shows more left Broca’s activation; 30–40% fluency gain. |
| Intensive Semantic Training (IST) | Word finding (Wernicke’s) | Practice word properties (syllables, first sound) without speaking → name pictures. | Naming accuracy up from 30% to 70%; generalizes to untrained words. |
| Phonological Components Analysis (PCA) | Word finding (all aphasias) | Identify 5 phonological features (syllables, rhymes) → name pictures. | 7/10 patients improved; gains last 6 months. |
| tDCS/TMS | All types | Non-invasive brain stimulation (boosts plasticity). | tDCS improves naming by 20–30%; TMS suppresses overactive right hemisphere. |
| Amphetamines | All types | Stimulants boost plasticity; paired with speech therapy. | 50% more fluency gain than therapy + placebo. |
4.4 Treatment Principles
- Early Intervention: Start 2–7 days post-stroke (heightened plasticity).
- Intensity: 3–5 sessions/week (45–60 mins).
- Task Specificity: Match therapy to deficit (MIT for fluency, PCA for word finding).
Quick Review Questions
- What is the key difference between Broca’s and Wernicke’s aphasia?
- Why does the WLG model fail to explain modern aphasia data?
- What is the slowed syntax hypothesis, and how does it explain Broca’s parsing deficits?
- What is spontaneous recovery, and what factors influence it?
- How does Melodic Intonation Therapy (MIT) work to improve speech fluency?