Chapter 4 — Classical Connectionist Models

The Seven Classical Aphasia Syndromes

Two rules organize the whole table:
① Repetition tells you centre-vs-disconnection: impaired = on the perisylvian arc, intact = off it.
② Fluency tells you anterior-vs-posterior: non-fluent = anterior (M side), fluent = posterior (A side).

#	Syndrome	Lesion	Fluency	Comprehension	Repetition	Hallmark
1	Broca's	M (centre)	Non-fluent	Intact	Impaired	Halting, effortful speech with articulatory difficulty; comprehension spared.
2	Wernicke's	A (centre)	Fluent	Impaired	Impaired	Fluent paraphasic / jargon speech; severe comprehension deficit.
3	Conduction	A–M tract	Fluent	Intact	Impaired ★	Wernicke's 1874 prediction (Lichtheim later specified the impaired repetition). Hallmark: impaired repetition with otherwise spared language.
4	Transcortical motor	B–M pathway	Non-fluent	Intact	INTACT ✓	Cannot initiate speech, but can repeat (arc intact).
5	Subcortical motor (pure word dumbness / dysarthria)	M → musculature	Non-fluent	Intact	Execution-only deficit	Articulation problem only — NO paraphasias. Central language intact.
6	Transcortical sensory	B–A pathway	Fluent	Impaired	INTACT ✓	Fluent but empty speech; repetition spared.
7	Subcortical sensory (pure word deafness)	Periphery → A	Fluent	Speech-only deficit	Cannot hear input	Cannot understand spoken speech; reading & writing intact.

★ Rows highlighted in red sit on the perisylvian arc (sites 1, 2, 3) — they share impaired repetition. Conduction aphasia is the diagnostic giveaway: fluent + comprehending + cannot repeat → lesion of the A–M tract.

Key Concepts & Dates

Broca (1861)	Localized articulate language to posterior left third frontal convolution. Established aphasia study + cerebral dominance.
Wernicke (1874)	Founded connectionism: centres + information flow. Predicted conduction aphasia.
Lichtheim (1884/85)	Complete 7-syndrome taxonomy. Added concept centre B and writing (E) / reading (O) centres.
Henry Head (1926)	Famously called later undisciplined connectionist work "chaos".
Benson & Geschwind (1971)	Modern revival — adopted Lichtheim's taxonomy + added 3 syndromes. Still clinically dominant.
The Centre Triad	A centre = (i) a brain region + (ii) a psycholinguistic function + (iii) a linguistic representation. Memory hook: "site + skill + store." Shows up everywhere.
Two justifications	Wernicke demanded the model fit BOTH: physiology — Meynert's reflexes, language as a "complex modulated reflex" (sensory → motor); and psychology — ontogeny: children learn speech by imitation, needing the same auditory→motor transfer.
Qualitative-difference principle (Lichtheim)	When a centre receives inputs from 2+ others, losing any ONE input gives a qualitatively different deficit — not merely "more or less" of the same impairment.
The five "on-line" tasks	Speaking · understanding speech · reading · writing · repetition. Testing a patient across these five lets you localize the lesion — the model's clinical engine.
"Data rich and theory poor"	Description of 1861–1874: many case reports + autopsies, no unifying framework. Wernicke supplied the theory.
Localizationism vs Holism	Connectionist (localizationist): language sits in discrete centres in specific gyri. Holist: brain does NOT use dedicated sub-component centres. Still unresolved.
Faculty model	Each centre = a whole on-line task (speaking, understanding, reading, writing). No fine-grained analysis within a task — a key LIMITATION.
The "uniquely inviolate" B	Lichtheim said no aphasia results from a lesion OF B — only of its connections. Critique target: suggests B is part-psychology, part-anatomy.

Critique Cheat-Sheet (likely exam question)

Strength	Weakness
Imposed order on a "data rich, theory poor" field.	Incomplete specification — direction of information flow along lines is unclear, even self-contradictory.
Predictive — generated conduction aphasia, later confirmed.	Hybrid model — part psychology, part neuroanatomy. B is admittedly diffuse and uniquely inviolate; and Wernicke even drew language in the right hemisphere in his schematic — a tell that the anatomy is loose.
Clinically useful — assess on-line tasks → localize lesion + infer pathology.	Vague case data — Lichtheim's seven cases are thin; predictions underdetermined by evidence.
Disciplined — Wernicke required consistency with psychology AND physiology + "simple" centres.	Later connectionists (Kussmaul, Grashey, Moeli) multiplied centres without justification — Head (1926) called it "chaos".
Defined a level of description (the on-line tasks) that shaped all later work.	Faculty-level only — no fine-grained analysis of representations or processing within a task.

Reading & Writing — Lichtheim's extension (Fig 4-4)

Lichtheim grew the diagram to cover written language by adding two more centres:

O	Centre for the visual form of words — reading.
E	Centre for the motor sequences of writing.

★ The non-obvious twist (exam-bait): writing is NOT driven by B + O alone. Centre E also needs input from M (Broca's oral-motor word store). Lichtheim's evidence was clinical — agraphia routinely co-occurs with Broca's aphasia, pointing to an M→E link. He even made it falsifiable: can a conduction aphasic (A–M tract cut) still write? Three possible outcomes would each reveal a different writing pathway — he lacked the data to decide, so it stayed a logical exercise.

Key Terms (free marks)

Connectionism	Building complex language functions by connecting simple, localized components.
Centre	Brain region + psycholinguistic function + stored linguistic representation — "site + skill + store."
Connection / pathway	A fibre tract carrying information between centres; holds NO stored representation.
Disconnection syndrome	A deficit from severing a pathway while the centres stay intact (e.g. conduction, transcortical).
Paraphasia	A wrong-word / wrong-sound error of selection: phonemic (by sound) or semantic (by meaning).
Neologism	An invented non-word, not derivable by normal word-formation rules.
Jargon	Fluent, well-intoned speech that conveys no information.
Paragraphia	Paraphasia in writing — the written counterpart of paraphasia.