Disconnection syndromes sharath

An overview of white matter tracts and various dysconnection syndromes

1. DISCONNECTION SYNDROMES Dr Sharath Chandra p Neurology PG

2. OVERVIEW Definition Anatomy of white matter fiber tracts Disconnection syndromes ---Language network disorders ---Praxis network disorders ---Visual network disorders --- C allosal disconnection syndromes

3. DEFINITION Disconnection syndromes —disorders in which a cluster of clinical deficits occurs because of the inability of one portion of the hemisphere to communicate normally with another portion due to a focal process disrupting pathways connecting one brain area with another. or D isconnection syndromes --- higher function deficits resulting from white matter lesions or lesions of the association cortices , the latter acting as relay stations between primary motor, sensory and limbic areas .

4. ANATOMY OF WHITE MATTER FIBER TRACTS CLASSIFICATION - Theodore Meynert was the first to classify white matter fibres into three groups. Group I -- projection fibres , the ascending or descending pathways arising and terminating in the cortex, Group II -- commissural fibres which connect the cortex of both hemispheres, Group III-- association fibres which connect cortical regions within a hemisphere.

5. The association and commissural fibers connect one area of the cortex with another. Association and commissural fibers come primarily from the supragranular cortex (layer 1-layer 3). Projection fibers connect the cortex with lower centers . Projection fibers arise primarily from the infragranular cortex (layer 5 and 6) and go to lower centers of the nervous system.

6. Association fibers

7. Short association fibers loop from one gyrus to an adjacent gyrus , running in the depths of a sulcus in the most superficial layer of the cortical white matter - referred to as arcuate fibers or U-fibers . The long association fibers run deeper into the white matter than the short association fibers . Major long association fiber bundles are — Sup. and inf. longitudinal fasciculi, Sup.and inf. occipitofrontal fasciculi , U ncinate fasciculus and C ingulum .

8. ASSOCIATION FIBERS

9. Arcuate fasciculus: The arcuate/superior longitudinal fasciculus connects perisylvian frontal, parietal and temporal cortices Fibers of arcuate fasciculus provide communication between posterior receptive (Wernicke's) & anterior motor ( Broca's ) speech centers.

10. I nferior longitudinal ( occipitotemporal ) fasciculus is a thin layer of fibers near the geniculocalcarine tract and it connects the occipital and temporal lobes.

11. S uperior occipitofrontal (subcallosal) fasciculus is a compact bundle that lies deep in hemisphere just below the corpus callosum. It connects posterior portions of the hemisphere with the frontal lobe. I nferior occipitofrontal fasciculus runs near the temporal lobe. The inferior fronto -occipital fasciculus connects the orbital and lateral frontal cortices to occipital cortex

12. C ingulum : I s a white matter tract that runs deep to the cortex of the cingulate gyrus . It is a part of the limbic system and interconnects the cingulate gyrus, the parahippocampal gyrus and the septal area.

13. The uncinate fasciculus connects orbitofrontal to anterior and medial temporal lobes.

14. Lesions involving these long association bundles are responsible for cortical disconnection syndromes —clinical deficit occurs because of the inability of one portion of the hemisphere to communicate normally with another portion.

15. COMMISSURAL FIBERS

16. They connect an area of one hemisphere with the corresponding mirror-image area of other hemisphere . The primary brain commissures are: -- The corpus callosum -- The anterior commissure and -- The hippocampal commissure .

17. C orpus callosum: L argest of the commissural systems. C onnects the neocortical areas of the two hemispheres. B ody/trunk , the major portion; the anterior G enu , which tapers into the R ostrum and a thickened posterior termination, the S plenium . Fibers connecting the anterior portions of the frontal lobes, including the speech areas, course through the anterior third . The body carries fibers from the posterior portions of the frontal lobes and the parietal lobes.

18. S The splenium contains fibers from the temporal and occipital lobes . Fibers that sweep around the anterior portion of the interhemispheric fissure, forming the genu, are referred to as the forceps minor (forceps frontalis ). Fibers that sweep around posteriorly, forming the splenium , are referred to as the forceps major (forceps occipitalis ). These areas communicate by the transcallosal connections of their respective association cortices.

19. The anterior commissure connects the olfactory bulbs, amygdala, and basal forebrain regions of the two sides . It lies in the lamina terminalis . Fornix splits around the anterior commissure into pre- and postcommissural parts. The hippocampal commissure runs between the two crura of the fornix, beneath the body of the corpus callosum and connects the hippocampal formations .

20. PROJECTION FIBERS

21. The projection fibers connect the cerebral cortex to other regions of CNS below it. Efferent projection fibers/ corticofugal /descending fibers arise from infragranular cortex(primarily layer 5) and descend to the basal ganglia, thalamus, reticular formation, brainstem motor nuclei and spinal cord . Afferent projection fibers/ corticopetal /ascending fibers ascend from thalamus and striatum , and project to the cortex and terminate in the supragranular cortex.

22. CORONA RADIATA: A mass of white matter composed of the projection fibers, which converge from the cerebral cortex to the internal capsule and fan out from the internal capsule towards the cortex. INTERNAL CAPSULE: Transmits the projection fibers like corticospinal , corticonuclear and corticopontine fibers and also the thalamic radiations . FORNIX: Is composed of projection fibers, which take origin from the hippocampus. The fibers in the fornix are connected to the neurons of the mamillary body.

23. CORTICOPETAL FIBERS: Ascending/ sensory fibers. Predominantly thalamocortical fibers . Anterior thalamic radiation– from the medial and anterior nuclei of the thalamus to the frontal lobe , thro’ the anterior limb of Int. capsule. Superior thalamic radiation— from the ventral posterior nuclei of the thalamus to the somatosensory area in the postcentral gyrus , thro’ the genu and posterior limb of Int.capsule . Posterior thalamic radiation— from the thalamus to the occipital lobe , thro’ the retro- lentiform part of the Int.capsule . Includes the optic radiation, from the LGB to the visual cortex . Inferior thalamic radiation– from the thalamus to the temporal lobe , thro’ the sub- lentiform part of the Int.capsule . Includes the auditory radiation from the MGB to the auditory cortex.

24. CORTICOFUGAL FIBERS: Corticopontine fibers--- originate from all four lobes of cortex. Frontopontine fibers – thro’ ant.limb , genu , post.limb of int.capsule . Parietopontine fibers – thro’ retro and sublentiform parts of int. capsule. Temporopontine fibers – thro’ sublentiform part of int.capsule . Occipitopontine fibers – thro’ retrolentiform part of int.capsule . Pyramidal fibers--- Corticonuclear fibers – for motor cranial nerve nuclei thro’ genu of int.capsule . Corticospinal fibers – thro’ post.limb of int.capsule .

25. Corticothalamic fibers--- from various parts of the cerebral cortex to the thalamus forming part of the thalamic radiations. Extrapyramidal fibers--- Corticostriate fibers – from all parts of cerebral cortex to the caudate nucleus and putamen . Corticorubral fibers – from the motor areas of the frontal lobe to the red nucleus. Corticoreticular fibers – from the motor cortex and the parietal lobes to the reticular nuclei.

26. Specialized cortical streams that process specific attributes of sensory information in Rt. And Lt. hemispheres: Occipital & inf.med . Temporal lobes – recognition of visual stimuli ( who it is; what it is) . Lat .&sup. Temporal lobes — Auditory recognition( who;what ). Parietal lobes – spatial perception & action onto stimuli ( where it is; how to use it). Frontal lobes – organizing & monitoring information and actions ( when; why) . Limbic structures (hippocampus )– episodic memories (which ).

27. Classic disconnection syndromes

28. DISCONNECTION SYNDROMES Karl Wernicke (1848–1904) is considered the father of disconnection theory . Wernicke considered the brain as a mosaic-like arrangement of areas containing ‘memory images’ related to motor acts and sensory experiences.

29. These memory image areas were localized in primary sensory and motor areas according to the following principle: The acoustic images find their abode within the cortical terminals of the acoustic nerve ; the visual images , within the cortical endings of the optic nerve ; and the olfactory images in that of the olfactory nerve ; and so on. Likewise the motor images or movement located is in the cortical sites of the motor nerve origins. Here higher functions arise through associative connections and disorders of higher function from their breakdown .

30. CONDUCTION APHASIA Motor component of language ( the images of speech movements ) is localized in frontal region ( Broca’s area ) and the sensory component of language ( auditory images of words ) is localized in the posterior part of the superior temporal gyrus ( Wernicke’s area ). Lesions of the Broca and Wernicke centres led to pure motor aphasia and pure sensory aphasia . Wernicke hypothesized that lesions of the association tracts connecting them lead to conduction aphasia - It consists of repetition deficit and paraphasic speech (the use of incorrect words or phonemes while speaking) with impaired writing but with intact comprehension and fluency .

31. Reason for classifying this aphasia as a disconnection syndrome-- Wernicke and Broca areas are spared and involved is the connection between them— the arcuate fasciculus. Lesion is located in the cortex and subcortical white matter in the upper bank of the left sylvian fissure involving the supramarginal gyrus . In most of the reported cases the left auditory complex, insula and supramarginal gyrus were involved.

32. APRAXIA Liepmann recognized 3 anatomic loci for apraxia . Callosal apraxia--- a lesion of ant. corpus callosum disconnected the right hemisphere from the left leading to unilateral left-hand apraxia . Sympathetic apraxia --- a lesion of the left motor area caused a bilateral apraxia , masked on the right by the paresis caused by the lesion. Parietal apraxia -- lesions in the left supramarginal gyrus of the parietal lobe disconnected the left-hand area from visual, somatosensory and auditory input, leading to bilateral apraxia .

33. Several sites of damage ( corpus callosum, left motor association cortex and underlying white matter, and the white matter deep to the supramarginal gyrus ) could produce apraxia by disconnecting pathways joining selected sensory, motor and language regions . Spontaneous movements are normal (e.g. using a spoon while eating) - - when asked to perform or copy gestures with his hand (e.g. point to your nose) or manipulate imaginary objects (e.g. show how you use a harmonica) he does in an absurd fashion.

34. ALEXIA WITHOUT AGRAPHIA Also called Pure Word Blindness/ visual verbal agnosia / PURE ALEXIA . Inability to read . Written language comprehension is lost but writing ability is intact . Literate person loses the ability to read aloud, to understand written script, and to name colors, i.e.to match a seen color to its spoken name, visual verbal color anomia . Understanding spoken language, repetition, writing spontaneously and to dictation and conversation are all intact.

35. Letter -by-letter reading can be seen but not to join them together ( asyllabia ). The most striking feature of this syndrome is the retained capacity to write fluently- but the patient cannot read what he has written ( alexia without agraphia ). Dejerine proposed that a ‘ visual verbal centre ’,(Visual memory centre for words) whose function is the storage of visual images of words, is located in the left angular gyrus .

36. For Wernicke and his school, disconnection and its syndromes had implied a white matter lesion to the association tracts connecting two areas . For Geschwind , disconnection syndromes went beyond this to imply a lesion of association cortex itself , particularly that in the parietal lobe . Even a pure lesion of association cortex could cause a disconnection syndrome , little distinction being made between such lesions and those restricted to white matter tracts .

37. GESCHWIND’S DISCONNECTION SYNDROMES

38. SENSORY—LIMBIC DISCONNECTIONS Limbic structures are important for learning and emotional response. Disconnection of a specific sense modality from limbic structures would result in the failure of a stimulus to evoke memories. Disconnection of somatosensory cortex from the limbic lobe resulted in pain asymbolia (no response to pain in the presence of normal tactile discriminatory function). Disconnection of auditory cortex from the limbic lobe resulted in verbal learning impairment .

39. SENSORY– WERNICKE’S AREA DISCONNECTIONS Geschwind distinguished four syndromes. Tactile aphasia -- the inability to name a held object in the presence of preserved speech and naming in other sensory modalities .( tactile anomia ). Pure word deafness - - an inability to understand spoken words in the presence of preserved hearing. Pure alexia Modality -specific agnosias – inability to recognize objects in the presence of intact elementary sensation.

40. SENSORY– MOTOR DISCONNECTIONS In the left hemisphere, disconnections of the hand motor cortex from posterior sensory areas or Broca’s area from Wernicke’s area resulted in apraxia and conduction aphasia respectively .

41. DISCONNECTION BETWEEN THE HEMISPHERES Leads to Callosal disconnection syndromes .

42. LANGUAGE NETWORK DISORDERS THE PARALLEL PERISYLVIAN LANGUAGE NETWORK : Three projection zones corresponding to Broca’s territory, Geschwind’s territory (angular and supramarginal gyri) and Wernicke’s territory . D irect pathway connects Broca’s and Wernicke’s territory directly and corresponds to the arcuate fasciculus (red) . Direct pathway being involved in phonologically based language functions such as repetition.

43. Indirect pathway between Broca’s and Wernicke’s territory passes through the inferior parietal lobe . C onsists of a posterior segment connecting temporal and parietal cortex (yellow) and an anterior segment connecting parietal and frontal cortex(green) . Involved in semantically based language functions, such as auditory comprehension (posterior segment) and vocalization of semantic content ( anterior segment).

44. Effects of l esions in the inferior parietal lobe : Purely sub cortical lesion , affecting the long segment(arcuate fasciculus) only -- classical conduction aphasia with a repetition deficit in the presence of normal auditory comprehension and verbal fluency. S ub cortical lesion affecting both direct and indirect pathways – a global aphasia despite intact cortex .

45. Cortical lesion encroaching on Geschwind’s territory produce : (i) Only the anterior portions of Geschwind’s territory ( the cortical end station of the anterior segment), a retrorolandic lesion --- non - fluent aphasia with spared repetition and comprehension. All of Geschwind’s territory (the cortical end stations of both anterior and posterior segments) -- will be one mixed transcortical aphasia with normal repetition but both reduced verbal fluency and comprehension. (iii) Lesion extending into the deep white matter--- a global aphasia with impaired repetition, fluency and comprehension .

46. Hyperfunction : I n the indirect pathway--- Semantically based symptoms , eg .,Auditory hallucinations in schizophrenics. In the direct pathway--- Disorders of excessive repetition , eg ., the echolalia of autism.

47. The angular gyrus was described as the region which turns written language into spoken language .(angular gyrus is the visual centre for words ), Storing the memory of the ‘rules of translation’ from written to spoken language. If the angular gyrus is deprived of visual input, reading is impossible. Production of pure alexia seemed to require separation of both right and left hemisphere visual regions from the left angular gyrus . Splenial damage prevented visual information processed by the intact right visual cortex from reaching intact language centers in the left hemisphere.

48. PURE WORD DEAFNESS Impairment of auditory comprehension and repetition and an inability to write to dictation. Spontaneous writing and the ability to comprehend written language are preserved. Patients with pure word deafness may declare that they cannot hear . Audiometric testing and auditory evoked potentials --no hearing defect. Nonverbal sounds such as a doorbell , can be heard without difficulty . Lesions are bilateral, in the middle third of the superior temporal gyri , that interrupt the connections between the primary auditory cortex in the transverse gyri of Heschl and the association areas of the superoposterior cortex of the temporal lobe .

49. PURE WORD MUTISM (APHEMIA) In localized injury of the dominant frontal lobe , the patient loses all capacity to speak while retaining perfectly the ability to write , to understand spoken words, to read silently with comprehension, to repeat spoken words. Right facial and brachial paresis may be associated. Lesion was confined to the cortex and subjacent white matter of the lowermost part of the precentral gyrus .

50. ANOMIC (AMNESIC, NOMINAL) APHASIA There are pauses in speech, groping for words, circumlocution and substitution of another word that is intended to convey the meaning. Perseveration is prominent. The patient may fail to name a shown object . When shown a series of common objects, the patient uses or demonstrate the use , instead of giving their names. Normal fluency of spontaneous speech and preserved comprehension and repetition.

51. BILATERAL TACTILE APHASIA Objects seen and verbally described could be named, but not those felt with either hand . Recall of the names of letters, digits and other printed verbal material is almost invariably preserved and immediate repetition of a spoken name is intact . Caused by a left parietooccipital lesion . The patient's understanding of what is heard or read is normal. Deficit is principally one of naming – as indicated by patient's correct use of the object and by an ability to point to the correct object on hearing or seeing the name to choose the correct name from a list.

52. PRAXIS NETWORK DISORDERS P raxic functions are multiple functions related to different body parts ( eg., the trunk, face or limb) and motor acts ( eg.,reaching / grasping, motor sequencing and posture in relation to the limb) and has distinct cortical loci and connecting circuitry. A tleast three sets of fibre connections underlying praxis can be identified in the human brain.

53. The dorsolateral frontoparietal set connects the dorsal motor and premotor cortex to the superior parietal lobe (green fibres ). The ventrolateral frontoparietal set connects the ventrolateral motor and premotor cortex to the inferior parietal lobe (red fibres ). The medial frontoparietal set connects the medial frontal lobe to the medial parietal lobe (precuneus ) (yellow fibres).

54. VISUAL NETWORK DISORDERS Visual network is the occipitotemporal projection system . It has 2 sets of fibers. Indirect occipitotemporal projection system : a chain of U shaped fibers, connecting visually specialized cortical areas in the human brain. Relates to visual perceptual qualities. Direct occipitotemporal projection system : the inferior longitudinal fascicle, connecting prestriate cortex to medial temporal structures ( the hippocampus,parahippocampal gyrus and amygdala). Relates to emotional qualities and visual memory .

55. A ventral or lateral temporal cortical lesion : specific deficits related to the cortical specializations lost. Lesion extension into medial white matter : Visual hypoemotionality ( a deficit of visually evoked emotions with preserved emotional responses to non-visual stimuli) 0r Visual amnesia ( a deficit of registering novel visual experiences in short term memory with the preserved ability to register non-visual images). Hyperfunction of specialized visual cortical areas : hallucinations of specific visual attributes. Hyperconnectivity within the occipitotemporal pathways: synaesthesia between visual attributes ( eg ., grapheme- colour synaesthesia ) and excessive emotional responses to visual stimuli ( eg ., in phobic disorders).

56. VISUAL AGNOSIA Agnosia is defined as a loss in the ability to identify environmental stimuli , unexplained by damage to low-level sensory systems (vision, hearing, or touch), language disorders, or more general intellectual deterioration. Visual agnosia is a family of disorders, each related to a specific deficit of visual form perception ( eg ., faces or objects). Loses the ability to recognize even common objects presented visually . A lesion which spared visual cortex but involved its white matter outputs would result in visual sensory images being disconnected from other brain areas .

57. Visual processing bifurcates into two independent but interconnected streams: Ventral stream- -- projecting from the primary sensory visual cortex (V1),thro’ ventral occipital and temporal cortices , terminating in the anterior temporal lobe, and subserving visual object identification and memory ( what/who) Dorsal stream- -- projecting from V1 through the dorso -occipital cortex to the posterior parietal cortex, and subserving object-directed action and spatial analysis for the purposes of arm and eye movements as well as selective attention (where/how). Disorders attributed to the ventral visual stream : agnosia for objects, colors, and faces. Disorders attributed to the dorsal stream : visuospatial agnosia .

58. Vision involves many (>30) distinct cortical areas that are likely to entertain many reciprocal/bidirectional interactions within and between these streams. Visual pathways in Left hemisphere are specialized for representing numerous local parts (crucial for word reading). Visual pathways in right hemisphere are specialized for representing global configurations (crucial for face identification).

59. Lesions of ventral stream 1.Visual object agnosia 2.Colour vision abnormalities. 3.Prospognosia 4.Pure alexia(alexia without agraphia )

60. DISORDERS OF VENTRAL STREAM AGNOSIA--- P erceptual disorder where sensations are present but the ability to recognize a stimulus or identify its meaning is lost. Agnosia results when lesions disconnect and isolate visual,auditory and somatosensory inputs from higher level processing.

61. VISUAL AGNOSIA Person cannot recognize and understand an object by sight even though it is previously known to him/her. Apperceptive agnosia - --deficits in ability to consciously perceive attributes of a stimulus. Associative agnosia - --inability to ascribe meaning to what is perceived.

62. APPERCEPTIVE AGNOSIA Intact sensory visual ability. Defect—early stage of visual processing. Prevents a correct percept of stimulus being formed. Patient is unable to assess the structure and special properties of a visual stimulus. Object is not seen in a meaningful a manner.

63. They cannot match or copy drawings. Visual acuity is normal.

64. TESTS FOR APPERCEPTIVE AGNOSIA MATCHING

65. DRAWING

66. ASSOCIATIVE VISUAL AGNOSIA Early stages are intact upto object centered sketch and can develop perception of object. Percept is not able to access semantics or knowledge of the object. The object is perceived as an object but it has no meaning. Semantic information is stored in left anterior temporal lobe.

67. Can perceive objects from conventional and unconventional view points. Patients can copy objects and pictures, but do not understand or recognize what they have drawn. They also cannot indicate whether two visually distinct examples of same object(e.g. two types of glasses )have identical function.

68. Tests for associative agnosia 1.Questionnare:- pictures of objects,animals and asked about uses ,name and properties. 2.Sorting of stimuli---according to category.

69. 3.Matching visually dissimilar but having same function

70. 4.Real and unreal objects

71. Optic aphasia: Patients cannot name (nor precisely define) objects presented visually, but can still mime their use and categorize them with semantically related items. These patients show greater ability to use objects of daily life. Strikingly, object naming is intact in other sensory modalities . Pure alexia and color anomia, however, are frequently associated. Optic aphasia is attributed to a disconnection or selective degradation of some modality-specific, visually derived components of semantic knowledge. Left temporooccipital lesions are typically reported, with frequent extension to callosal interhemispheric fibers .

72. Colour vision abnormalities Cerebral achromatopsia : perceive colours as grey shades,b /l or non dominant inf occiptotemporal lobe. Colour agnosia: can see colours and discriminate colours but cannot recognise colour,dominant inf occipito temporal lobe Colour amnesia: can see colours,match colours and name colours but cannot colour pictures, dominant hemisphere Colour anomia: cannot name colours

73. Face agnosia : Face agnosia or prosopagnosia is a selective loss in the ability to recognize faces, including previously known faces (relatives, celebrities) or newly seen faces (physician, hospital staff). Recognition of people using other cues (voice, gait, dressing) and knowledge about their identity or biography is usually intact . T wo clinical forms are distinguished. In apperceptive prosopagnosia , the structural analysis of face features and the ability to generate or retrieve the representation of a specific face identity are impaired. Patients cannot discriminate between two different individuals nor match two different pictures of the same individual , regardless of familiarity.

74. In associative prosopagnosia , a structural representation is properly constructed but representations of known face identities seem damaged or disconnected. Patients can match pairs of identical faces but cannot identify the person , provide any information about him/her, or even discriminate between familiar and unfamiliar faces. New faces cannot be learned . Lesions are usually located in the ventral temporooccipital cerebral cortex, including the fusiform, lingual, and posterior parahippocampal gyri , especially of the right hemisphere.

75. Pure alexia : Reading can be selectively impaired by an inability to recognize written words in the absence of other language disturbance , and in particular without writing disorder ( alexia without agraphia , or pure word blindness). It is a failure to identify the global form of words , while recognition and naming of individual letters is preserved. The patient uses a slow and laborious strategy of reading letter by letter , which is most efficient for short words. Reading errors are made for visually similar single letters (G/C, h/b). Reading is normal for words spelled orally by the examiner, or tactually written on the hand, indicating that internal lexical representations are preserved but not accessed by visual inputs.

76. Reading may be spared for Arabic numbers (1998) , iconic symbols ($, %, ideographic Kanji characters in Japanese), familiar acronyms (USA), or familiar proper names (of people, cities), probably because these stimuli are still recognized as a whole through other perceptual pathways. Can correctly identify a known person’s handwriting. Pure alexia is most often caused by a lesion in the territory of the left PCA, usually involving the left inferomedial temporooccipital region together with the ( ventroposterior ) splenium of the corpus callosum .

77. CALLOSAL DISCONNECTION SYNDROMES Callosal fibers connect homologous areas , allowing interhemispheric transfer of information, as well as integrative and reciprocal inhibitory influences that determine coherent behaviour . Callosal disconnection syndromes result from an interruption of these fibers by damage to the corpus callosum itself or adjacent white matter. The most classical disconnection signs observed in practice follow lesions of the posterior half of the corpus callosum, that is, the body/trunk (vascularized by the ACA) and the splenium (vascularized by the PCA). Effects of more anterior disconnections in the genu (ACA territory) are poorly known and presumably involve higher-level executive functions as well as motor intentional processes.

78. They include: Verbal disconnection disorders Motor disconnection disorders Memory disorders

79. I. VERBAL DISCONNECTION DISORDERS: V erbal disconnection can affect any sensory modality ( vision, audition, etc.) depending on the location of callosal damage. A. L eft visual anomia( hemianomia ) --- d/t lesion in the most posterior and dorsal portion of the splenium . Images seen in the right visual hemifield (left hemisphere ) are normally named or described, while those shown in the left hemifield (right hemisphere) cannot (or are even not reported ). When no verbal response is required, the patient can recognize the object, draw it with the left hand, or perform matching tasks with the left hand (but not with the right hand).

80. B.Left hemialexia : Left alexia is a special case of anomia for words shown in the Left visual field. Callosal lesions are usually located in the more ventral portion of the Splenium . The pure alexia or alexia without agraphia may be considered as a form of left hemialexia due to callous disconnection.

81. Left auditory anomia: Left ear extinction . The right hemisphere can still process auditory stimuli, allowing the patient to associate some words or sounds with a corresponding object (e.g., when projected in the left visual field) . However, the lack of transfer from the right hemisphere to the language system of the left hemisphere prevents any verbal report of these stimuli . Lesions affecting interhemispheric auditory transfer are located in the posterior inferior trunk of the corpus callosum or isthmus.

82. D. Left tactile anomia ( pseudoastereognosia ): The patient can describe and name objects palpated with the right hand (without vision), but not with the left hand. The disturbance cannot be explained by astereognosia because the patient can still point to, match, or even draw the object when using only the left hand. The comparison of objects held in the right and the left hand is impossible and can elicit left tactile extinction . Normallly manipulates and uses the object.

83. Naming of letters written on the palm may also be impaired (unilateral tactile alexia), although the perception of letter shape is preserved . Touched left-hand fingers can be moved but not named. Disruption of the crossed replication of hand or finger posture and crossed pointing of body parts touched by the examiner (e.g., fingers) in both directions (right-to-left and vice versa). A failure to recognize one’s own hand (versus the examiner’s hand) when held by the other hand (with eyes closed or behind one’s back) was termed alien hand sign . All these signs reflect damage to the posterior trunk of the corpus callosum.

84. E. Right olfactory anomia: Because olfactory inputs project ipsilaterally , odors presented to the right nostril (right hemisphere) are not named , while those presented to the left nostril (left hemisphere) are. There is no anosmia . This disorder is associated with complete callosal section after epilepsy surgery, and with lesions of the rostral corpus callosum and anterior commissure.

85. MOTOR DISCONNECTION DISORDERS: Difficulty in coordinating bimanual gestures due to the lack of sensorimotor transfer. D/t lesions in the front half of the corpus callosum, affecting fibers between the two supplementary motor areas and primary motor area . A. Crossed optic ataxia: Disconnection between motor reaching systems in one hemisphere and visuospatial information in the other . When an object is presented in the peripheral visual field of a patient, the patient can get it with the ipsilateral but not the contralateral hand. This crossed difficulty is found symmetrically for both sides.

86. Left unilateral motor apraxia: Callosal apraxia most often manifests as a form of ideomotor Apraxia and affects the left limbs, reflecting a disconnection of the right motor cortex from left hemisphere areas involved in the representation of skillful gestures and limb postures. The disorder is most severe to verbal command, and does not improve to imitation when damage is in the midcallosal trunk. Normal performance on imitation may be seen when damage is more anterior in the genu ( unilateral disassociation apraxia) . Left unilateral ideomotor apraxia is typically seen after ACA infarction .

87. C. Agraphia of the left hand: Writing problems selectively affecting the left hand. Unilateral apraxic agraphia is characterized by badly drawn letters and scribbles, but typing and writing with block letters are preserved, reflecting impaired transfer of graphomotor information. Left agraphia may occur independently of left ideomotor apraxia. Unilateral aphasic agraphia is a verbal-motor disconnection with impaired two-hand writing and typing. Lesions are located in the posterior trunk of the corpus callosum or anterior splenium /isthmus for the apraxic and aphasic forms, respectively.

88. Right unilateral constructional apraxia: Difficulties in drawing and constructive abilities when using the right hand. However, when a model is shown to the left hemisphere, both the right and left hands fail in copying. Disconnection of the left motor cortex from right hemisphere visuospatial skills. The lesions are usually located in the posterior corpus callosum , affecting fibers between the two parietal lobes.

89. E. Alien hand syndrome and diagonistic apraxia: Alien hand (AH) motor syndromes are a collection of rare but striking disturbances in the voluntary control of movements , some of which are typically due to lesions in the anterior portion of the corpus callosum (or nearby paracallosal fibers) . At least three major syndromes can be distinguished--- Pure callosal alien hand syndrome Frontal alien hand syndrome Sensory or posterior alien hand syndrome.

90. Pure callosal alien hand syndrome: Damage restricted to the midtrunk of the corpus callosum (especially its ventral part ). A lways involves the left/ nondominant hand (in righthanders ). ( i ) The left hand makes unintended movements during purposeful actions. These movements can be antagonistic to the right hand action ( intermanual conflict , e.g., closing a drawer opened with the right hand), or irrelevant but nonantagonistic (e.g., slapping on the table while drawing with the right), or even simply mirroring the right (e.g., reaching for the same object). (ii) The left hand does not move despite an intention to use it during voluntary actions. This can entail the failure to initiate or interrupt an action (e.g., grasping or releasing an object held in the hand), or the execution of an incorrect movement , inconsistent with verbally stated purposes (e.g., drawing circles instead of a line).

91. Frontal Alien Hand: Occurs after lesions in the medial frontal cortex (vascularized by the ACA) on one or both sides. It is also described as an anarchic hand , and may affect either the dominant or nondominant hand (or sometimes both). U nilateral release of elementary, reflexive, exploratory motor behaviors . ( i ) forced grasping of objects, ( ii) impulsive reaching and groping toward objects in near sight, and ( iii) compulsive manipulation of tools. Grasping correlates with lesions relatively confined to the contralateral Supplementary motor area or midcingulate gyrus . G roping and compulsive manipulation of tools indicate more extensive lesions of the anterior cingulate gyrus adjoining the callosal genu. Paradoxically , unilateral limb motor neglect ( hemiakinesia ) may coexist.

92. Sensory or posterior alien hand syndrome: Unintentional and uncontrollable movements of limbs opposite to a unilateral brain lesion occur in patients who show a complex combination of disturbances in proprioceptive sensation ( sensory ataxia ) and visuomotor coordination ( optic ataxia ). This is also termed opticosensory AH and is often accompanied by spontaneous non- goaloriented motor activities such as levitation, drifting, or repetitive scratching or tapping. Feeling of foreigness of the limb ( asomatognosia ), hemispatial neglect, or anosognosia are frequently associated. L arge posterior hemispheric infarcts or hemorrhages involving either the thalamus ( ventroposterolateral and ventrolateral nuclei) and subthalamic region in the PCA territory, or the superior posterior parietal lobes in the MCA territory .( right > left). A dystonic or myoclonic component is observed after thalamo-subthalamic Injury.

93. III. MEMORY DISORDERS: S electively affect recall but not recognition. The deficits concern both verbal and visual memory and are attributed to impaired retrieval strategy. Due to lesions in the callosal fibers linking the hippocampus and medial temporal lobe of the two hemispheres or to a section of the hippocampal commissure or a section of the anterior commissure connecting the medial temporal lobes of two sides.

94. REFERENCES Snells neuroanatomy Dejong s – The neurological examination Adams and victors : Principles of neurology The rises and falls of disconnection syndromes- Brain (2005), 128, 2224–2239- Marco Catani and Dominic H. Ffytche . Disconnection syndromes: An overview of Geschwind's contributions-Neurology 1993;43;862-John R. Absher and D. Frank Benson . James jose – cognitive neurological examination. Apraxias , agnosias and callosal disconnection syndromes; Caplan and Vangi JN Disconnection in language disorders: Annals of IAN-2005,Vol-8

95. THANK YOU