8Premotor TheoryPremotor Theoryshifting attention is nothing more than preparing an eye movement that will not be executedPerry and Zeki found Right SMG activation (+visual areas/FEF etc) when making eye saccades and for covert attention shifts (equal for left v right shiftsFits parietal patient data
9rTMS over parietal cortex induces extinction in normals +rTMS over parietal cortex induces extinction in normalsrTMS over occipital abolishes target detectionPascual-Leone et al., (1994). Induction of visual extinction by rapid-rate transcranial magnetic stimulation of parietal lobe.Neurology Dec;44(12):2419.
10It has been suggested that extinction results from an inability to disengage from the ipsilesional stimulusweakened or delayed afferent inputs to the affected hemispherecompetition or capacity-limited processesBaylis et al - maximal extinction at simultaneous presentationNot a) as ext. reduced when ipsi item leadsNot b) as b) predicts maximal ext. when contra item leads ipsiNeuropsychologia 40 (2002) 1027–1034Visual extinction with double simultaneous stimulation:what is simultaneous?Baylis, Simon, Baylis & RordenAlso ipsilesional stimuli seem to have a temporal advantage over contra
15Copying Drawing from memory Reading Patient RBCopying Drawing from memory ReadingGrappleApple(omission)FractionTraction(substitution)AcupuncturePicture(both)
1612BeforeAfter34Sketches drawn by the artist Tom Greenshields before and after his stokeAnton Raderscheidt Self-portrait painted during recovery from a right hemisphere stroke which resulted in left hemispatial neglect
18How does neglect affect reaching? Curved hand paths in neglect (Goodale et al, 1990).N+controlDistortion of visual space orgeneral distortion of space?
19Heilman: Attention - Intention model Neglect may occur as consequence of failure to ATTEND or INTEND towards contralateral stimuliProcessing of spatial information is divided into left and right hemispheresRH has special role in space-related behaviour - RH does left and right. LH only does rightNeglect is attributed to hypoarousal of damaged hemisphere
20Kinsbourne: Vectorial model Space-related behaviour is directionalEach hemisphere is reponsible for directing attention in the horizontal plane contraversivelyThe LH is dominant and must be inhibited by the RHDamage to the RH lessens this inhibitionResulting in a pathological rightward attentional biase.g. Ladavas
21Different models explain different aspects of the syndrome. Bisiach: Representational modelSpace is topographically represented across the two hemispheresDifferent models explain different aspects of the syndrome.How the syndrome is defined determines which model appears to be the most attractive.Damage to one hemisphere destroys the representational analogue of the contralesional real world
26Curved hand paths in neglect Jackson et al., (2000)
2730025507510012515017520022525027530-30-20-101020VV/BaseVV/AHa)VP/BaseVP/AHb)Jackson & Newport. (2001). Prism adaptation produces neglect-like patterns of hand path curvature in healthy adults. Neuropsychologia –814
28Balint’s Syndrome (1909)A cluster of co-occuring visuomotor and visuospatial disturbancesBálint's syndrome results from bilateral damage to the posterior parietal lobes (usually either progressive cerebrovascular complications or lateral gunshot wounds) but lesions often also take in parts of occipital and temporal cortex as well as white matter damageThe triad of disorders associated with Bálint’s syndrome are:Simultanagnosia - inability to see visual field as a wholeOcular apraxia - deficit of visual scanningOptic ataxia - inability to reach accurately under visual guidance
31Description of a Balint’s patient (JJ): a 65 year old man with a history of recurrent cerebral posterior haemorrhages affecting the occipital, parietal and temporal cortices of both cerebral hemispheres .He frequently bumps into objects, unsure of where they are or not noticing them at all. He says, “I can see them but it is as if I can’t”. He complains of difficulty in finding objects and people around him. He has difficulty in performing everyday tasks. For example, he may misreach when trying to cut bread. Or he may unsuccessfully spend over an hour trying to wire an electric plug because he has difficulty in placing his fingers in the right place. When pouring tea, he may miss the cup entirely. He often fails to eat all the food on his plate because he does not see it, or knocks food off the plate with cutlery.
32Simultanagnosia or spatial disorientation An inability to see the visual field as a wholeExamples: difficulty in copying/drawing/writing because they are unable to see both the end of the pen and what is on the paper at the same time.Unable to describe complex scene (e.g. Boston cookie theft).Spatial disorientation: inability to appreciate the ‘spatial properties’ of objects (e.g. relative distance and size estimates are impaired, as are whole body movements in space).Often seen as part of Balint’s, but also seen independently following bilateral damage to the superior parts of the visual association areas of the occipital lobes (BA 18/19)
37Gaze or ocular apraxiaAn inability to move the eyes voluntarily to points in the visual field.Not due to basic oculomotor deficit.Spontaneous, reflexive movements may be spared.Eye movements towards auditory or somatosensory stimuli spared.Functional gaze restricted to narrow band, usually to the right of the midline.Can appear similar to neglect symptoms.Usually co-occurs with visuospatial deficits
41Optic ataxia - an inability to reach accurately under visual guidance Reaching accurately involves reaching in the right direction, with the correct grip scaling and grip orientation/finger placement.Visual guidance means being able to see both the target and the hand throughout.Milner’s posting task
42In humans optic ataxia is associated with (usually unilateral) damage to intraparietal sulcus/superior parietal lobeOptic Ataxia is not due to basic sensory or motor deficit.How do we know this?Can affect one limb in one or both hemispheres(not purely visual or spatial disorder)Can affect both or one limb in only one hemisphere(not purely motor disorder)Optic ataxia is not accompanied by a deficit of position sense although this is often only assessed by informal ‘bedside’ clinical testing (e.g. limb postion matching).
43Disconnectionist account De Renzi, 1982Misreaching deficitDisconnectionBoth hands to contralesional fieldVisual information from damaged hemisphere to both (contra and ipsi) motor areasContralesional hand to contralesional fieldVisual information from motor area in damaged hemisphere onlyIpsilesional hand to contralesional fieldVisual information from damaged hemisphere to motor area in spared hemisphereBoth hands to both fieldsvisual information from both to both motor areas (i.e. bilateral parieto-occipital junction damage)
44But…there are two broad categories of optic ataxia: foveal and extra-foveal and disconnectionist modeldoes not work for non-foveal optic ataxia.If areas are disconnected then OA’s should be impaired whenReaching to both foveal and non-foveal targetsBuxbaum and Coslett (1997) - OA a spatio-motor transformation failure - a failure to encode the target with respect to the position of the arm in limb-based coord system.OA’s rely upon undamaged oculocentric coord system. Reaches directed towards direction of gaze.Evidence: DP (Buxbaum and Coslett, 1997) and Mrs D (Carey et al., 1997).
45Pointing movements in two optic ataxic patients a) patient with left hemisphere lesion reaching with left hand - note leftward error for both right visual hemifieldsb) patient with right hemisphere lesion reaching with left hand - note rightward error for both left visual hemifieldsH1 - misreaching toward the ipsilesional spaceH2a - misreaching toward fixationH2b - imbalance between foveal and peripheral visionFrom Ratcliff and Davies-Jones, 1972
46Magnetic misreachingCase of Mrs. D (Carey et al., 1997)76 year-old woman with a slowly progressive bilateral parietal lobe degenerationMrs. D could only reach to the point of fixation regardless of where the target was with either hand.A manifestation of parietal lobe dysfunctionBreakdown in sensorimotor transformation
47Magnetic misreaching: Mrs D - Carey et al., 1997 LEFT HANDControl (foveating) ° right of fixation ° right of fixation ° right of fixationRIGHT HANDTargetFixation point
48Bimanual reach-to-grasp task Patient JJ - optic ataxiaBimanual reach-to-grasp taska.b.c.We record movement of the thumb marker using electromagnetic recording device. Movement cut-off defined spatially before contact with object.Jackson et al., (2003). Action binding and the parietal lobes: some new perspectives on optic ataxia
49April 2001: Bimanual trials - Effects of gaze angle Gaze rightGaze leftGaze right - No deviationGaze left - Right hand deviates leftwards
52Anosognosia Landmark cases Anosagnosia is the denial of illness which is often seen in brain-injured patients. Frequently associated with hemineglect.Landmark casesVon Monokow (1885) - Reported a 70 year old patient who had suffered bilateral damage to posterior brain areas and exhibited loss of sight of which the patient was not aware (patient attributed visual deficit to loss of ambient light).Anton (1899) - Reported the case of Ursula Mercz who was shown to suffer from cortical blindness but denied this. (termed Anton's syndrome). Patients pupils respond to light but the patient is unable to demonstrate functional sight. Deny any visual difficulty. Confabulate responses, guess, and make excuses for deficit e.g., "the room lights are too dim" or "I don't have my glasses with me"
53Anosagnosia as a failure of monitoring Heilman's intentional modelMotor'intentional'patients unaware of movement failure because the comparator which contrasts intended and actual movements receives no signal that a movement has been intended.Because patients do not try to move the paralysed limb they never discover that it is paralysed.Can explain denial of impairment, but not cases in which patients apparently experience having made movements when none have actually occurred.activationsystemComparatorMotorsystemor monitorEffectorHeilman et al., 1998.
54Desired state and predicted state match Forward modelFrith, Blakemore and Wolpert. (2000).Desired state and predicted state matchActual state feedback is absent or ignored