1 Disconnection Syndromes DS Higher function deficits resulted

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Disconnection Syndromes (DS) Higher function deficits resulted from lesions of white matter or the association cortices ‘Functional’ disorders such as schizophrenia, autism, dyslexia Founded on: Anatomy, post-mortem dissections, monkeys. More recently: DTI, Tractography 2

DS before Geschwind ‘The Classical associationist era’ Paradigm based on two central tenets: 1) Localization of function in discrete cortical areas 2) Connections between areas through white matter association pathways 3

DS before Geschwind Franz Joseph Gall (1758 -1828): � White � Grey matter � Phrenology (England) 4

DS before Geschwind Theodor Meynert (1833 -1892): � White matter fibers: 1) Projection fibers 2) Commissural fibers 3) Association fibers 5

DS before Geschwind Karl Wernicke (1848 -1904): � The father of Disconnection theory � ‘fundamental psychic elements’ / ‘memory images’ � “… mediated by means of their manifold connections via the association fibres” (Wernicke, 1885) 6

DS before Geschwind Karl Wernicke (1848 -1904): � Higher functions arise through associative connections, Disorders of higher function from their breakdown � Critisizm: No cortical specialization other than motor/sensory � Theoretical framework explained classical DS 7

Conduction Aphasia Motor component (Broca) Sensory component (Wernicke) Lesions: � Broca Pure motor aphasia � Wernicke � Arcuate Pure sensory aphasia Fasciculus Conduction aphasia 8

Visual Agnosia Lesions: � Cortical (Visual cortex) Apperceptive agnosia � Trans-Cortical (associative fiber connections) Associative agnosia 9

The apraxias Hugo Liepmann (1863 -1925) Higher movement disorders � Spontaneous � Planned movements gestures � � Disconnection of visual, auditory and somatosensory areas from motor area 10

The apraxias Left hemisphere dominant for complex movements control Lesions: � Left parietal lobe bilateral apraxia portion of corpus callosum Unilateral apraxia (left) � Anterior � Left motor area (not shown) bilateral apraxia + right paresis 11

Pure Alexia Jules Déjérine (1849 -1917) Left angular gyrus = visual verbal centre Lesions: � Left angular gyrus Alexia + Agraphia � Left central white matter occipital lesion Pure alexia 12

The fall of the classical era Déjérine model suggested � Higher functions located in cortex � An area specialized for higher visual function outside the visual cortex Wernicke’s opposition to higher functional centers The early 20 th century (holistic, anti-localizationist) 1965 (localization theory, DS) 13

Geschwind’s neo-associationism Flechsig’s rule � Include sensory and motor cortices and interhempispheric connections � Association cortex acted as an obligatory relay station Phylogenetic perspective � Higher-order association area in the parietal lobe � Connections that did not depend on the limbic system 14

Geschwind’s neo-associationism The role of the angular gyrus: � Déjérine visual memories of letters and words � Geschwind forming multimodality associations Geschwind focused on DS caused by lesions of association cortex (parietal lobe) “…A ‘disconnexion lesion’ will be a large lesion either of association cortex or of the white matter leading from association cortex” (Geschwind, 1965) 15

Disconnections between sensory areas and limbic cortex Failure of a stimulus to evoke memories/affective response Disconnections between limbic lobe and � Somatosensory � Auditory � Visual cortex pain asymbolia cortex verbal learning impairment etc. system (indirect connections) no symptoms 16

Disconnections between sensory areas and Wernicke’s area Modality-specific language deficits Disconnections can be direct or indirect (through the angular gyrus) Four syndromes: � Tactile aphasia/Anomia � Pure word deafness � Pure alexia � Modality-specific 17 agnosia

Disconnections between sensory areas and motor cortex Left hemisphere disconnections: motor cortex from posterior sensory areas Apraxia � Hand � Broca’s area from Wernicke’s Conduction aphasia 18

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DS After Geschwind contribution: 1. Association cortex as an obligatory relay 2. Hierarchies of associations within the IPL 3. Importance of IPL in phylogeny and ontogeny of language Two research paths: � Damasio (CT, PET, SPET) � Mesulam (Tracing neural connections, computation theory etc. ) 20

Contemporary neuroanatomical basis of higher brain functions Functional subdivision of the association cortex ‘Extended territories’ composed of specialized cortical subregions serving different but related functions Parallel, bidirectional, distributed processing 21

Contemporary neuroanatomical basis of higher brain functions Two key elements underlie higher function deficits: 1. Loss of specialized cortical function 2. Damage to connecting pathways Recent techniques enable research of disorders caused by hyperconnection and cortical hyperfunction 22

Hodotopic framework of clinicopathological correlations Territories composed of specialized subregions Intra-territorial connections (U-shaped) Inter-territorial connection (Long) 23

Hodotopic framework of clinicopathological correlations Topological mechanism (Topos = place) � Prosopagnosia, face hallucinations Hodological mechanism (Hodos = road / path) � Conduction aphasia, autism 24

Hodotopic framework of clinicopathological correlations 25

Language network disorders Direct (Long segment) Indirect (Anterior, Posterior) Pure hodological mechanism: segment Conduction aphasia � Long, anterior, posterior Global aphasia 26

Language network disorders Direct (Long segment) Indirect (Anterior, Posterior) Pure topological mechanism: � Anterior portions of Geschwind’s Non-fluent aphasia � All of Geschwind’s Mixed trans-cortical aphasia of Geschwind’s + deep white matter Global aphasia 27

Language network disorders Direct (Long segment) Indirect (Anterior, Posterior) Hyperfunction: � Indirect � Direct Semantically based sympthoms Excessive repetition (e. g. echolalia) 28

Praxis network disorders Med. Frontal Med. Parietal (DMPF, yellow) Motor SPL (DLFP, green) Motor IPL (VLFP, red) Lesions: � SPL Depends on which praxic subfunctions are affected + White matter Additional abnormalities 29

Visual network disorders Indirect (U-shaped occipito-temporal, red) Direct (Inferior longitudinal fasciculus, green) Lesions: � Direct � Indirect specific deficits related to the cortical specializations lost + medial white matter Visual hypo-emotionality / Visual amnesia 30

Visual network disorders Direct (Inferior longitudinal fasciculus, green) Indirect (U-shaped occipito-temporal, red) Hyperfunction: � Indirect Visual hallucinations Hyperconnectivity: / Direct Unclear (Synaesthesia? Phobia? ) � Indirect 31