APRAXIA, AGNOSIAS, AND HIGHER VISUAL FUNCTION ABNORMALITIES

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1 APRAXIA, AGNOSIAS, AND HIGHER VISUAL FUNCTION ABNORMALITIES PERCEPTION Correspondene to: Dr John D W Greene, Southern General Hospital, Institute of Neurologial Sienes, 1345 Govan Road, Glasgow, G51 4TF, UK; john. greene@sgh.sot.nhs.uk C JDWGreene J Neurol Neurosurg Psyhiatry 2005; 76(Suppl V):v25 v34. doi: /jnnp ognitive neurology deals mainly with disorders of memory (for example, is the patient s poor memory due to early dementia or to anxiety/depression?) or language (as in stroke). It should be remembered, however, that other areas of ognition may be seletively impaired. This review will over disorders of pereption and of higher order motor output, both in terms of pathologial loss and pathologial gain of funtion. AND ITS DISORDERS A patient must be onsious in order to pereive the world around them. An exploration of onsiousness is outside the remit of this artile though reently reviewed by others. 1 2 The patient must also have the apaity to attend seletively in order to fous on one part of the sensorium. Pereptual proessing is then neessary to identify what is being pereived through the various sensory modalities (namely vision, hearing, touh, smell, taste), thus allowing aess to semanti knowledge and through this understanding of the environment. Initially, pereptual information is basi and modality speifi, but as it is proessed by higher order entres, meaning is asribed to perepts, and information beomes multi-modal (fig 1). Ultimately, semanti knowledge is aessed using the various sensory streams. For example, if standing in the path of an onoming train, basi pereption will involve visual information, hearing the train oming, and feeling vibration from the ground. These separate streams then ome together, aessing relevant semanti knowledge and thus allowing the individual to understand what is taking plae. In disussing pereption, I shall fous mainly on vision and hearing, as the other three forms of pereption are of lesser linial importane. Pereption is not a passive proess, but is modulated by attention. There is feedbak from higher order entres down to primary sensory ortex. Similarly, attention influenes what is pereived. Attention itself has many levels or subomponents, inluding seletive, divided, sustained attention, et. Of relevane to pereption is seletive attention, a proess by whih the individual fouses speifially on partiular areas of sensory experiene, rather than simply passively absorbing all suh experiene. Patients with subortial dementia an lose this ability to attend seletively, resulting in inreased distratibility due to inability to ignore bakground extraneous stimuli. To some extent, we see what we expet to see. For example, if waiting for someone at a rowded venue, we may have several false positive reognitions of strangers beause we are primed to expet to see the friend. VISION Normal visual proessing Visual proessing involves signal relay from the retina via the lateral geniulate nuleus to the striate ortex (area V1). In the visual ortex itself, there is initially strong retinotopi loalisation, suh that striate oipital lesions ause defiits restrited to segments of the visual field. Extrastriate visual ortex is organised more by proess than by visual field loation that is, different areas of extrastriate ortex are involved in olour, motion pereption, et. Extrastriate defiits therefore ause defiits in aspets of vision, suh as pereption of motion, affeting the entire visual field. Broadly speaking, extrastriate regions group into two streams: the ventral oipitotemporal what stream is involved in objet reognition, while the oipitoparietal where stream is involved in spatial proessing. 3 The ventral stream runs below the alarine fissure into the medial temporal lobe, while the dorsal stream extends superolaterally from the striate ortex into oipitoparietal and temporoparieto-oipital areas. Defiits of visual proessing, and how to test for them Defiits in visual attention an result in neglet, whereby attention is no longer direted either to self that is, body part or to external objets suh as food on a plate. v25

2 v26 Neglet is almost always for the left hemifield (figs 2 and 3). This is explained by the left hemisphere monitoring the right hemispae, while the right hemisphere monitors both hemispaes. A left hemisphere lesion will still allow the right hemisphere to survey the entire visual field, hene neglet does not our. By ontrast, with a right hemisphere lesion, the left hemisphere monitors only the right hemispae; therefore beause of the lak of monitoring the left hemispae, left sided neglet ours. Sensory inattention and neglet may be assessed linially by the examiner moving one, other, or both fingers in right and left hemifields and asking the patient whih finger has moved. In visual extintion, the patient will note either finger moving separately, but will only detet the finger in their right hemifield when presented simultaneously. This is due to the ipsilesional stimulus extinguishing the ontralesional stimulus from awareness. Similarly line bisetion tasks an be employed in the lini (fig 4), as an letter anellation tasks in a visual array. Drawing a lok fae is another useful test (fig 5). Anosognosia refers to a patient s inability to reognise onsiously the presene of somati dysfuntion indiative of a disease proess. Patients with left hemiplegia aused by stroke may be entirely unaware of their defiit. Anosognosia tends to our predominantly, though not exlusively, with non-dominant parietal lesions and is regarded more a disorder of attention than of pereption. Moving next to visual pereption itself, learly retinal, opti nerve pathology or any lesion of the system from the retina via the lateral geniulate nuleus to the primary visual ortex Figure 1 The semanti system illustrating the sensory modalities whih an aess semanti knowledge. will affet vision. While we are onerned with higher order visual defiits, suh onfounding onditions must be exluded as part of the general assessment and examination. Lesions to the ventral or dorsal streams of visual proessing an give rise to different linial defiits. Ventral stream lesions (fig 6) may produe defets suh as objet agnosia, prosopagnosia, alexia and ahromatopsia, while dorsal stream defets (fig 7) inlude akinetopsia and Balint syndrome. These will be disussed further below. Disorders of ventral stream Agnosias The term agnosia is used to desribe a modality speifi inability to aess semanti knowledge of an objet or other stimulus that annot be attributed to impairment of basi pereptual proesses that is, it relates to a normal perept stripped of its meaning. Agnosias may apply to any sensory modality, but here we shall deal firstly with visual agnosias. Visual agnosis annot reognise by sight objets that they would have previously known. They an neither produe unique semanti identifying information nor name the perept. Visual agnosia is sometimes further subdivided into appereptive visual agnosia, where a defiit in high level pereption is impliated (usually widespread, bilateral oipitotemporal infartion), and assoiative visual agnosia, where high level pereption is preserved but the perept is unable to ativate semanti identifying information (usually anterior left temporal lobe). The term visual agnosia is best restrited to the inability to aess semanti information

3 Figure 2 Left hemisphere damage results in no visual field defiit, while right hemisphere damage results in neglet to the left. through the visual modality only, with retained semanti knowledge aessible through other modalities suh as auditory. So alled assoiative visual agnosia often involves a more general loss of semanti knowledge, with no ability to aess this via any sensory modality. Figure 4 Neglet patient does not see left half of line, so when asked to put a ross halfway along the line, he puts it halfway along the seen line that is, three quarters of the way along the line. A generalised visual agnosia may our in a diffuse hypoxi insult, suh as in arbon monoxide poisoning. However, more seletive agnosias may also our. Indeed, defiits may seletively impair ability to identify words or faes (that is, dyslexia and prosopagnosia, respetively), and an our with foal temporal lesions (fig 8). Testing for visual agnosia Bedside ognitive tests inlude objet naming and ability to provide semanti information about unnamed items. Visuopereptual funtion an be tested by asking the patient to draw the objet or opy a drawing. The patient an be asked to desribe what is seen, and mime its use. If holding the objet allows the patient to identify the objet where vision has not, this would be in keeping with a visual agnosia rather than a loss of semanti knowledge. (In the latter, identifying information will not be able to be provided regardless of modality of sensory input.) Bedside testing may be usefully amplified by a formal neuropsyhologial evaluation. Visuo-pereptual funtion may be tested by unusual views tests, overlapping line drawings, partially degraded or fragmented images, judgement of line orientation, fae analysis, and mathing from different angles as well as the Visual Objet and Spae Pereption battery. v27 J Neurol Neurosurg Psyhiatry: first published as /jnnp on 16 November Downloaded from Figure 3 Right gyriform enhanement of right parietal infart. Figure 5 Clokfae illustrating neglet. Patient is not able to attend to left hemispae, and puts numbers all on right half of lokfae. on 1 November 2018 by guest. Proteted by

4 v28 Figure 6 Ventral stream lesion: omputed tomographi (CT) san showing left posterior erebral artery infart. Alexia Language is a speialised symboli representation of the world around us, allowing us to ommuniate our inner thoughts with others. With regard to pereption, language may be aessed through the visual or auditory modality, namely reading or hearing speeh respetively. Reading is a very omplex ativity. It requires the eye to fixate on the written word, pereption, eye movements, and entral language to allow understanding of the written word. Defiits in any of these areas an impair the ability to read that is, alexia. Alexia may be peripheral (where there is diffiulty transmitting the visual perept to the intat language entres) or entral (due to an impaired language Figure 8 Axial magneti resonane image (MRI) showing bilateral anterior temporal lobe atrophy in semanti dementia. system) (table 1). Examples of peripheral alexia inlude impaired visual auity as a result of oular problems, or a visual field defet, even if this does not involve entral fixation. Disorders of visual attention suh as visual neglet may also impair reading ativity. For example, neglet dyslexia results in the patient being unable to read the left hand side of words for example, for SISTER, the patient will only pereive -TER. On the motor side, impaired ability to oordinate eye movements suh as oular motor apraxia or saadi intrusions may also impair reading ability. Alexia without agraphia represents a high level visual defiit resulting in inability to read, and is an example of a disonnetion syndrome. Here there is an inability to omprehend written material. The patient an write what he has seen, but is then unable to read bak what he has written. It is a sort of ategory-speifi form of visual agnosia for words, in some ways similar to prosopagnosia. The defiit Figure 7 Dorsal stream lesion: CT san showing left parietal meningioma. Figure 9 Figure illustrating the disonnetion syndrome of alexia without agraphia. Seen written material annot aess Wernike s area, resulting in alexia. The fibres from the oipital ortex to the motor ortex are, however, spared, so the patient may write what he has seen.

5 Table 1 Types of dyslexia is aused by the inability of pereptual information from primary visual ortex to get to language areas that is, they are disonneted. The patient an reognise words spelled aloud, showing that this is an aess problem rather than a primary language defiit. Preserved writing illustrates that pathways from primary visual ortex to pre-motor and motor ortex involved in ontrol of writing movements are preserved. It is best understood by referene to fig 9. The syndrome is often aompanied by a right homonymous hemianopia, olour anomia, or ahromatopsia, and ours with lesions affeting the left oipital lobe and the posterior fibres of the orpus allosum. This syndrome is rare and more often overlooked. Disorders of fae pereption Prosopagnosia refers to a patient s inability to reognise a person simply by studying their fae. One other means of Site of pathology Peripheral dyslexia Alexia without agraphia Disonnetion syndrome Left medial oipital lobe Neglet dyslexia Diffiulty with left side of words or page Right hemisphere Central dyslexia Surfae dyslexia Loss of semanti knowledge Left temporal lobe Diffiulty with irregular words Deep dyslexia Only able to read via meaning, not by letter-by-letter. Unable to read non-words (e.g. CHOG, LAVE) Left hemisphere reognition ome into play (for example, if the person has a harateristi voie or gait, et), this allows aess to unique semanti identifying information that is, there is no loss of knowledge of the person. Covert reognition of apparently unreognised faes is supported by studies of galvani skin responses, these being greater when viewing faes that were previously known, implying that fae identifiation may our at unonsious levels. Prosopagnosia is best understood by means of employing urrent models of fae reognition (fig 10). It an our with ventral oipitotemporal pathology, espeially right sided. 4 Other fae reognition disorders Several other onditions previously thought to be of psyhiatri origin are now onsidered organi, with ognitive neuropsyhology now providing plausible explanations for their phenomenology. 5 Figure 10 A model of fae proessing. Prosopagnosia results in damage to fae reognition units. v29 J Neurol Neurosurg Psyhiatry: first published as /jnnp on 16 November Downloaded from on 1 November 2018 by guest. Proteted by

6 v30 Capgras syndrome is a form of delusional misidentifiation in whih the patient believes that familiar people have been replaed by impostors. Attempts to explain this ondition involve theories of fae reognition. Normally, it is laimed that when viewing a fae, the onsious stream will result in reognition, while the unonsious stream may invoke feelings of empathy if it is a liked person. Normally, there is no onflit between these parallel streams, and reognition ours. It is laimed that in Capgras, there may be damage to streams of unonsious proessing. This an result in a dissoiation between the onsious stream identifying a person as, say, wife, and the lak of unonsious empathi feelings. The lak of onordane between these two streams (that is, looks like wife but no feelings of empathy) may therefore be resolved ognitively by the ontention that the loved one has been replaed by an impostor (explaining the dissonane). The lak of onordane between onsious and unonsious streams has been asribed to a disonnetion between the ognitive and emotional streams, anatomially represented by fusiform gyrus of the right temporal lobe and the limbi system, respetively. Fregoli syndrome is the linial mirror image of Capgras in that the patient will ause strangers of being a familiar person in disguise. Intermetamorphosis is a ondition where the patient, on looking at a real fae, has a subjetive experiene of the fae morphing from one known fae into another, and is thought to represent inappropriate ativation of fae reognition units. Disorders of olour Foal impairments of the olour system mirror defiits to the semanti system in general. Ahromatopsia refers to a loss of the ability to pereive olours. Patients usually desribe this as like wathing blak-and-white television. It results from lesions affeting the medial oipito-temporal region, in partiular fusiform gyri. Colour agnosia is not a pereptual defiit (as demonstrated by intat mathing tasks) but reflets a loss of semanti knowledge of olour. It is really a ategory speifi semanti memory impairment for olour, and the term olour agnosia is unfortunate. In olour anomia, there is both preserved pereption and semanti knowledge regarding olour, but simply a defiit in olour naming. These defiits may be tested by assessing olour disrimination, olour knowledge, and olour naming. Defiits of dorsal stream Disorders of motion pereption Seletive impairment of motion pereption is rare, and tends to be assoiated with damage to areas of extrastriate visual ortex analogous to V5 that is, bilateral lesions of the lateral oipitotemporal area. Patients may have no impression of motion in depth or of rapid motion. Fast targets appear to jump rather than move. Partiular diffiulties are enountered judging the speed and diretion of ars. Disorders of spatial pereption Balint syndrome is a disorder of spatial pereption omprising three aspets: simultanagnosia (the inability to omprehend a omplex sene in its entirety that is, only one omponent of the sene is pereived at a time), opti ataxia (inability to reah by hand for targets presented visually), and oular motor apraxia (inability to diret gaze to a visual target). Patients omplain of visual diffiulties, and may appear funtionally blind. They exhibit a urious searhing head thrust, by whih they aim to searh their environment item by item (sometimes unfortunately but aurately ompared to the head thrusts of a hen searhing for food). Pathology usually is bilateral superior parieto-oipital. It may be erebrovasular due to watershed infartion, or be neurodegenerative as in the posterior ortial atrophy variant of Alzheimer s disease. Dressing apraxia and onstrutional apraxia Confusingly, these are not apraxias as suh, but are rather visuospatial defiits resulting in diffiulty dressing and drawing. Dressing apraxia may be tested by asking the patient to don a jaket whih has had the sleeves deliberately turned inside out. Bedside tests of onstrutional apraxia inlude drawing overlapping pentagons, the Nekar ube, or a lok fae. Left hemisphere damage results in simplified drawings, while right sided pathology leads to an explosion of the onstituent parts of the drawing. Other higher order defiits of vision Topographagnosia Getting lost in familiar surroundings an be due to defiits in either ventral or dorsal visual assoiation orties. Anton s syndrome This is a form of anosognosia restrited to vision, in whih the patient denies there is any visual disturbane despite being funtionally blind. It is assoiated with pathology affeting the primary visual ortex. Blindsight The existene of this ondition is disputed, but it is laimed that in the ontext of blindness aused by primary visual ortex damage, residual unonsious visual funtion may our, subserved by subortial strutures suh as the lateral geniulate nuleus. Visual gain Pathologial gain of visual funtion an result in positive phenomena that is, halluinations. 6 Visual halluinations are a strong pointer to organi disease. Common auses inlude the aute onfusional state and Lewy body disease. Charles Bonnet syndrome This omprises positive visual phenomena ourring in areas of visual field defiit, whether this be total or partial. It often arises in the elderly as a result of oular pathology for example, age related maular degeneration. The images tend to be omplex (for example. animals, people) and insight is usually retained. Pedunular halluinosis Following midbrain stroke, vivid halluinations may arise, whih tend to our in the evenings, and have a tendeny to disappear over weeks. HEARING Drawing analogies with vision, normal hearing requires intat end organs whih feed information to the primary auditory ortex. Again, information is initially strongly tonotopially represented, but loses this as it is proessed in the assoiation ortex, whih is speialised for types of

7 auditory information for example, speeh, musi, and environmental sounds. Hearing loss Auditory agnosias The inability to understand the meaning of sounds in the ontext of preserved basi auditory pereption is alled auditory agnosia. Stritly speaking, Wernike s aphasia is a form of auditory agnosia for words, though language disorders are outwith the remit of this review. In pratie the term auditory agnosia tends to be applied to non-verbal sounds for example, environmental sounds suh as traffi, airraft noise, et. Most patients with this have bilateral lesions of the auditory ortex. The disorder is normally of aute onset, and initially the patient beomes almost entirely deaf that is, ortially deaf. Normally, however, this improves suh that the patient hears auditory stimuli. Auditory pereption returns to normal (tested linially by the examiner snapping fingers behind patient s head) but the patient remains unable to identify the sound. Although restrited ategory speifi forms may exist, most auditory agnosias refer to a wide group of environmental sounds. Hearing gain Auditory halluinations While hearing voies, espeially if threatening or ausatory, is more likely to be due to psyhiatri illness, organi disease an also result in auditory halluinations. These may be musial in nature, and are assoiated with temporal pathology, often non-dominant. 7 OTHER SENSORY MODALITIES Tatile agnosias have been desribed that is, an inability to aess semanti knowledge of an item by touh, yet with preserved semanti knowledge of the item when aessed through other modalities suh as vision. Extraampine halluinations involve the patient sensing a human presene near them, yet they see no one and there is indeed no one there. While this has been reported in normal individuals under severe stress, suh as Antarti explorers, it is well reognised in Lewy body dementia. 8 THE PARAXIAS Motor funtion Normal motor funtion The formulation of an ation requires the will and intention to do suh a task, and is generated in prefrontal ortex. This signal is then used to ativate left fronto-parietal systems that in turn ativate motor engrams (for example, the pattern and sequene of movements needed to light a math) in premotor ortex. This is then fed down to the primary motor ortex and by the ortiospinal trats to musle, with modulation from erebellum and basal ganglia. LOSS OF MOTOR FUNCTION Apraxia Praxis refers to the ability to perform skilled movements. Dyspraxia refers to diffiulty performing motor ats whih annot be aountable by lak of understanding of the task, weakness, or proprioeptive loss. (Apraxia an even our in the presene of an additional defiit suh as hemiparesis, but to qualify as apraxia, the examiner must be onfident that the apparent diffiulty performing the motor at annot be explained by the hemiparesis alone.) Many patients with Figure 11 MRI showing left parietal arterio-venous malformation. dyspraxia are unaware of their defiits, thus the examiner should speifially enquire from the patient and aregiver as to how the patient manages ativities of daily living suh as brushing teeth, ombing hair, using kithen utensils, et. Dyspraxia is frequently missed by liniians, and may then be deteted by oupational therapists or physiotherapists for example, when seeing patients reovering from stroke. Anatomy and pathology of apraxia Left parietal or frontal premotor lesions are most ommonly assoiated with dyspraxia (fig 11). It is thought that it results from loss of motor engrams or a disonnetion between praxis systems. Orobual apraxia ours with insular and left inferior frontal lesions. The most ommon auses of apraxia are stroke and neurodegenerative disease suh as Alzheimer s or ortiobasal degeneration progressive isolated limb apraxia strongly suggesting the latter. 9 Subdivisions of apraxia Confusingly, traditional subdivisions of apraxia, partiularly ideomotor and ideational, are used in an inonsistent manner. Clinially it is often suffiient simply to desribe whih body parts are affeted (for example, limbs or orobual), and to desribe whih movements are impaired. If a more detailed knowledge of praxis is required, then the following ognitive neuropsyhologial model of praxis and its defiits may be helpful. Praxis is oneptually diffiult, and it may help for the reader to draw analogies between the following aount of praxis, and the more widely understood language system. Praxis requires both a oneptual system of knowledge of tool funtions and ations (for example, the purpose of a srewdriver), and a prodution system inluding sensorimotor ation programmes onerned with the generation and ontrol of movement (that is, the ability to move a limb through spae in the orret diretion and with appropriate speed). 10 v31

8 v32 How to examine for dyspraxia It is impossible to draw any meaningful onlusions about the presene or absene of praxis, unless tests for it are interpreted in the ontext of the rest of the neurologial examination hene a full neurologial examination is essential. Given the above onfusion regarding subdivisions of praxis, there is no lear onsensus as to how best to test praxis. If, however, we employ the above lassifiation of the oneptual system for ation (that is, knowledge of ations and tools) and the ation prodution system (that is, the ability to arry out motor programmes), then this lends itself to a logial means of exploring dyspraxia in patients. The following aount of testing praxis is exhaustive and lends itself more to linial researh than linial pratie. It is best read in onjuntion with fig 12. Graham et al 10 assessed the oneptual system for ation by testing naming of ations, naming of tools, speifiation of tool use (for example, What would you use a toothbrush for?, and ation reognition ( Whih is the orret movement for brushing teeth? with the examiner miming both the target ation and distrators). The ation prodution system was assessed by hand and finger position imitation, exeution of familiar ation sequenes (for example, Fold a piee of paper, plae in envelope and seal the envelope ), and exeution of single familiar ations. Exeution of single familiar ations an be further assessed by verbal ommand Show me how you would use a Figure 12 A model of praxis, whih an be aessed through various sensory modalities. toothbrush, imitation of examiner s mime, ability to pantomime usage while looking at the tool Pretend you have this in your hand. Show me how you would use it, and by atual tool use Take this in your hand and show me how you would use it. A simpler means of testing praxis at the bedside is to ask the patient to imitate gestures, or to gesture to ommand. Use of imagined objets should also be tested. The above should be done both for limb movements and for orobual movements. Finally, a sequening task suh as the Luria three-step test or alternating hand movements an be done. Table 2 illustrates a useful series of ommands whih allow praxis to be tested at the bedside. 11 Ideational apraxia Damage to the oneptual system leads to both impaired gesture omprehension and disrimination, as well as a prodution defiit. Patients have diffiulty performing in response to ommand and imitation and do not disriminate well between poorly and well performed ats. Patients may make ontent and tool seletion errors, and exhibit loss of tool objet ation knowledge (for example, using a srewdriver as a hammer). Preserved tool naming shows that this defiit annot be explained on the basis of an objet agnosia. Patients may also show a loss of tool objet assoiation knowledge (for example, when shown a partly driven nail, they may hoose a srewdriver rather than a hammer). This is best termed an ideational apraxia (although others use the

9 Table 2 Shema for examining praxis: Show me how you would If defetive by ommand, test by imitation. Buofaial Blow out a math Lik lips Cough Sip through a straw Limb Gestures Wave goodbye Bekon ome here Salute, like a soldier Hith a lift Objet use Comb hair Brush teeth Use sissors Hammer a nail term oneptual apraxia or even posterior ideomotor apraxia, and reserve the term ideational for another disorder). It is most ommonly seen in Alzheimer s disease Ideomotor apraxia By ontrast, ideomotor dyspraxia relates to an impaired ation prodution system. Although it also results in prodution defiits whether by ommand or imitation, it an be disriminated from ideational dyspraxia in that gesture omprehension and disrimination are preserved. Both spatial and temporal prodution errors may our in ideomotor apraxia. The former inlude postural errors that is, use of body part as tool (for example, using finger as toothbrush; this an our in normal ontrols, but if it persists when the patient has been told not to, it is pathologial). Defiits in spatial orientation (for example, keeping sissors in the sagittal plane) and spatial movement (for example, rotating at the shoulder rather than pronating/ supinating when using a srewdriver) may also our. Timing errors relate to the fluidity of movement (for example, utting bread with a knife should employ regular sinusoidal movements). Lesions affeting the premotor ortex may ause this type of dyspraxia. Some patients, often with bilateral frontal and parietal dysfuntion as in dementia, are unable to arry out a series of ats in the proper sequene (for example, folding paper, putting in envelope, sealing the envelope). Confusingly, this has been termed ideational apraxia, but it is really a failure of exeution of familiar ation sequenes, and hene a measure Figure 13 Sagittal MRI of ortiobasal degeneration illustrating striking frontoparietal atrophy, but with sparing of primary sensorimotor ortex. Figure 14 Coronal MRI of ortiobasal degeneration showing striking gyral atrophy. of the integrity of the ation prodution system, and is therefore a type of ideomotor apraxia. Condution apraxia This results in greater impairment imitating movements than when pantomiming to verbal ommand, and is analogous to ondution aphasia. The loation of the lesion responsible is unknown. Verbal disassoiation apraxia Here, the patient annot gesture normally to ommand but an perform well both when imitating the examiner, and when using atual tools and objets. This suggests that both the oneptual system for ation and ation prodution system are preserved, but simply annot be aessed by verbal ommand. Callosal apraxia The left frontal and parietal areas are dominant for higher order motor funtion. A lesion affeting the anterior orpus allosum will not impair the left hemisphere s ability to ontrol movement of the right arm and leg. The allosal lesion will, however, impair the left hemisphere s ability to ontrol the right premotor ortex, and an therefore result in left sided apraxia aused by this disonnetion. Gain of motor funtion Toxi gain of motor funtion may our at prefrontal, premotor, and motor levels (an example of the latter being foal motor seizures). The prefrontal ortex may exert an inhibitory effet on the ability of pereptual input to result in motor response that is, it lets us arve an autonomous path in the world rather than be a slave to our environment. Prefrontal pathology an remove this inhibition, leading to a ompulsion to at on the surroundings (that is, environment dependeny syndrome or fored utilisation behaviour). A patient with this ondition, on being taken into a room with hammer, nails, and a piture, will hammer a nail into the wall and hang the piture. If asked why he did that, he will say I assume from the items being there that this is what you wanted me to do. Alien hand syndrome Often assoiated with apraxia, in this disorder the limb may perform movements not under volitional ontrol, as if it has a will of its own. Severe alien hand on one side may result in intermanual rivalry, suh as the anarhi hand trying to v33

10 v34 Figure 15 D2-single photon emission omputed tomography (SPECT) san illustrating redued post-synapti dopamine reeptors as ours in Parkinson-plus syndromes, as in ortiobasal degeneration here. throttle the patient while the other hand under volitional ontrol tries to fend it off, but suh irumstanes are rare and tend to our transiently in the ontext of aute erebrovasular disease. Progressive isolated alien limb is essentially pathognomoni of ortiobasal degeneration (figs 13 15). CONCLUSION To summarise, while memory and language disorders tend to dominate ognitive neurology, it is important to have at least a working knowledge of other ognitive defiits. Disorders of gnosis and praxis may result in either pathologial loss or pathologial gain in funtion. Agnosias may involve any sensory modality, not just vision. Defiits of visual proessing are best divided into disorders of the ventral what stream and of the dorsal where stream. Agnosias an be differentiated from more entral semanti defiits by assessing the same objet using different modalities. If knowledge an be aessed via other modalities, then there is a modality speifi agnosia rather than a multi-modal semanti defiit. Apraxia is often missed in the aute stage, as it an be masked by oinident hemiparesis. It is often first notied by physiotherapists in stroke rehabilitation. Subdivisions of apraxia are onfusing. Perhaps the most logial subdivision is into disorders of either the oneptual system for ation (that is, semanti knowledge of tools and ations), or the ation prodution system (that is, the atual motor programmes needed to arry out tasks). Progressive isolated alien limb is very suggestive of ortiobasal degeneration. REFERENCES 1 Rees G, Kreiman G, Koh C. Neural orrelates of onsiousness in humans. Nature 2002;3: Zeman A. Consiousness. Brain 2001;124: Ungerleider L, Mishkin M. Two ortial visual systems. In: Ingle DJ, Mansfield RJW, Goodale MS, eds. The analysis of visual behaviour. Cambridge, Massahusetts: MIT Press, 1982: Evans JJ, Heggs AJ, Antoun N, et al. Progressive prosopagnosia assoiated with seletive right temporal lobe atrophy. Brain 1995;118: Ellis HD, Young AW. Aounting for delusional misidentifiations. Br J Psyhiatry 1990;157: Ffythe D. Visual halluination and illusion disorders: a linial guide. Advanes in Clinial Neurosiene and Rehabilitation 2004;4(2): Berrios GE. Musial halluinations. Br J Psyh 2005;156: Chan D, Rossor MN. but who is that on the other side of you? Extraampine halluinations revisited. Lanet 2002;360: Spatt J, Bak T, Bozeat S, et al. Apraxia, mehanial problem solving and semanti knowledge: ontributions to objet usage in ortiobasal degeneration. J Neurol 2002;249: Graham NL, Zeman A, Young AW, et al. Dyspraxia in a patient with ortiobasal degeneration: the role of visual and tatile inputs to ation. J Neurol Neurosurg Psyhiatry 1999;67: Hodges JR. Cognitive assessment for liniians. Oxford: Oxford University Press, Lhermitte F. Utilisation behaviour and its relation to lesions of the frontal lobes. Brain 1983;106: Lhermitte F. Human autonomy and the frontal lobes. II: Patient behaviour in omplex and soial situations the environmental dependeny syndrome, Ann Neurol 1986;19:335. J Neurol Neurosurg Psyhiatry: first published as /jnnp on 16 November Downloaded from on 1 November 2018 by guest. Proteted by