PS3019 Cognitive and Clinical Neuropsychology

Transcription

1 PS3019 Cognitive and Clinical Neuropsychology Lectures 7 & 8 Dissociation between perception and action in brain damaged and healthy individuals Essential Reading - Goodale & Milner, Sight Unseen, Chapters 1, 2, 3, 6, 7 - Milner and Goodale, The Visual Brain in Action, Chapters 4, 5, 6, 7 Additional Reading -Kolb & Wishaw, pp Norman, J. (in press). Two visual systems and two theories of perception. Available at: Case D.F. Carbon monoxide (CO) poisoning - Anoxia CO replaces oxygen on hemoglobin - Brain cells die in absence of oxygen DF s symptoms - unable to identify shape of objects - objects recognised by colour - ability to identify shades of the same colour and texture - unable to identify boundaries of overlapping objects of with similar colour or texture Deficits not related to loss of knowledge about objects - unable to copy line drawings of objects - able to produce line drawings of objects from memory D.F. apperceptive agnosia ( Visual Form Agnosia in more precise modern terminology) (Greenberg, 1969) as revealed also in Efron s test (1969). Now standard test for VFA. DF s Correct discriminations out of 20 D.F. shows deficits in shape discrimination Preserved ability to grasp size in D.F. Methodology: hand reaching precisely measured using infrared-emitting markers, tracked by infrared-sensitive cameras During reaching movement finger and thumb open proportionally to object size D.F. and controls tested using three-dimensional Efron s blocks (Goodale et al., 1991) Similar reaching patterns in D.F. and controls

2 Preserved ability to grasp shape in D.F. Pebble like objects (Blake s shapes) used to test robotic grasping abilities Curvature of different parts of the object to be taken into account Ability to negotiate obstacles Unfamiliar environment with obstacles of different height As in controls, D.F. was able to step over the obstacles leaving Just enough clearance to ensure that the foot does not touch the obstacle D.F. grasping not very different from controls Matching and posting task Goodale, et al., (1991) Matching turn card to match the orientation of the slot Posting reach out and post the card into the slot How do we know that D.F. symptoms are not due to general poor vision that makes it impossible for her to process detailed aspects of shape but leaves enough visual processing ability to carry out generalised actions towards objects? Double dissociations can be used to rule out the possibility that deficits in a particular task are just related to task difficulty Need for evidence from another syndrome presenting opposite patterns of impaired and spared functions D.F. undistinguishable from controls in the posting task only Optic ataxia as part of Balint s syndrome Balint (1909) patient with bilateral damage to parietal (and dorsal occipital) cortex Symptoms can vary according to the specific damage suffered by different similar patients e.g. in addition to reaching problems inability to shift gaze inability to focus attention on more than one part of a visual scene Inability to reach for visible objects with right hand deficit could not be purely perceptual since patient could reach for objects using left hand Ability to reach for specific body parts on request the deficit cannot be purely motor as reaching can be achieved using tactile or proprioceptive information Holmes (1918) considered the deficit as mainly spatial i.e. inability to use egocentric co-ordinates to locate objects Therefore, specific problems in translating vision into action

3 More recent studies clarify some confusion Perenin et al. (1983; 1988) extensive testing of patients with optic ataxia, filmed when performing different tasks e.g. patient with tumour in right parietal lobe, reaching with left hand Jeannerod et al. (1994) Healthy subjects early hand opening during reaching gradual hand closure when approaching target Optic ataxia patients hand widely open throughout movement Control Patient s hand-orientation error Patient s localisation error Jakobson et al. (1991); Goodale et al. (1993) patient V.K. bilateral parietal damage recovering from Balint s syndrome - longer grasping latency - abnormal succession of grip adjustments controls patient V.K. Maximum grip opening in V.K and healthy control B.S. when reaching for objects of different size Patient R.V. (Goodale & Milner 2004) Bilateral damage to parietal lobe Generally able to copy line drawings Generally able to discriminate Blake s shapes Patients can show correct size of objects with thumb and index if required and can discriminate objects shape and orientation. i.e. disorder specifically related to reaching. Unable to pick them up efficiently Experimental support for the What versus How systems Functions of vision (Milner and Goodale, 1995) Different computational demands for perception and visually guided action Dorsal stream (reflexive perception) sensory-motor mechanisms mainly egocentric requires information about size, location and motion of target objects absolute metrics of sizes and distances Ventral stream (cognitive perception) internal representation of the external world mainly allocentric relative metrics Timing requirements of the two visual systems - Organisms do not stay still - In primates complex movements of body, head and eyes produce changing patterns on the retina - Object recognition requires a stable representation of objects irrespective of these changes (e.g. requires shape and size constancy) - The visual control of action towards objects requires the computation of the exact relationship of our body (e.g. our hand) and the object at a particular point in time (e.g. not in what relationship they were 1 sec before) - If the two systems are independent, different predictions can be made concerning action guided by current visual input and by object representation (e.g. in delayed or pantomimed grasping)

4 Healthy participants required to reach for objects (e.g. Efron blocks) in real time or to pantomime the reaching movement after a delay Goodale et al. (1994). If on-line reaching movements rely on dorsal functions and delayed reaching on ventral functions, then we should expect that the latter should be selectively impaired by damage of the ventral stream (e.g. in patients with agnosia such as D.F.) A) Control B) D.F Reaches to remembered objects are different from reaches to present objects but there is little difference in reaches made after a 2sec or 30sec delay Nevertheless, D.F. is able to pantomime typical actions to common objects. Her deficit is not in the ability to pantomime per se. In contrast with D.F. patients with optic ataxia (damage of dorsal stream), tested by Milner & Goodale in collaboration with Jeannerod, Michel, Rossetti & Dijkerman, show a deficit in pointing to a present light or grasp a present object, but their pantomimed movements are not very different from those of healthy controls. What can be concluded from these experiments on delayed responding is that the dorsal stream works in real time and stores the coordinates required for action only for a very short period of time. By contrast, the ventral stream operates on much longer time scales. Evidence for the use of different frames of reference in the two systems Frame of reference for scene analysis allocentric relational properties of objects recording of absolute metrics of complex scene computationally demanding allows representation of causal relationship between objects that can be used by inferential and planning processes Frame of reference for action egocentric absolute properties of objects information only temporarily available allows fast accurate movements of the body in relation to objects It should be possible to verify this distinction experimentally Virtual Reality assessments of size contrast effects (Hu & Goodale, 2000) Apparatus size and location of virtual object can be accurately manipulated robot arm places real object accordingly to ensure realistic grasp Grasping the Ebbinghaus illusion (Haffenden & Goodale, 1998) Manipulations of the size of the surrounding circles can make the internal circles perceptually identical or perceptually different Physically identical Perceptually different Two virtual objects presented simultaneously to induce contrast effects Tasks: indicate manually the size of target object grasp target object (with or without delay) Results manual report of object size affected by contrast delayed grasping affected by contrast immediate grasping unaffected by contrast Real grasping affected by actual object size in perceptually identical trials. Results replicated with other types of illusions Physically different Perceptually identical

5 Wong and Mack (1981) Frame moves creating illusion of target displacement - saccades to perceived targets directed to retinal location - saccades to remembered targets directed to perceived location What depth cues are used by the two systems? Ventral stream pictorial cues (dependent on object relations and context) Dorsal stream stereopsis and convergence (dependent on relation of object and observer, not sensitive to context) How do the dorsal and ventral system communicate? Dorsal and ventral stream used different frames of references the ventral stream has the appropriate frames of reference for identifying the object the dorsal stream has the relevant frame of reference for picking it up The retinal image is the starting point of processing for both systems There is evidence for backwards connections from higher to lower visual areas The ventral stream could refer back to the retinal image to indicate the target object to the dorsal stream Lateral Intraparietal Area (LIP) seems active when the focus of attention changes. It could act as the attentional spotlight that indicates on the retinal image which object the dorsal stream has to reach for. Contributions to action from the Ventral Stream Ventral stream and grip force For known objects grip force is affected by top down processed based on the properties of those objects (e.g. phone book or box of crackers) For unknown objects grip force is affected by size alone Visual illusions using pictorial cues affect ventral but not dorsal processing Jackson (2000) Ponzo illusion affects grip force Grip force determined by ventral stream Goodale and Milner, 2004 D.F. unable to adjust grip force on the basis of size of objects Semantics of action Reaching determined by knowledge of how to use a tool even if the action required is not the most direct and efficient Concurrent memory tasks (word recall) impair this component of action D.F. uses most direct action without taking into account semantics of action It seems to be a ventral function Ventral stream and consciousness in animals Computationally, it would make sense to be aware of the output of the ventral stream processing only while the dorsal stream takes care of the details of action control e.g. locomotion among tree branches playing tennis Ventral streams contribution to action Identify goal of action Identify function of objects Planning movements Control of grip force How can we address experimentally what is consciously experienced by animals so that we can relate it with cell activity within the inferior temporal cortex? - binocular rivalry studies

6 Binocular rivalry When presented with incompatible images to the two eyes we typically perceive (are aware of) one or the other and only rarely a combination of the two. The two alternative images take turns in consciousness Logothetis (1998) - monkeys trained to press different levers for different stimuli e.g. face or sunburst pattern - condition of binocular rivalry with the two stimuli presented simultaneously - recordings from inferior temporal neurons responding to either faces or sunburst patterns - correlation between lever response and appropriate cell firing e.g. face leaver pressed correlates with activation of face cells but not sunburst cells and vice-versa - V1 neurons do not show the same pattern. They show the same pattern of activation irrespectively of what monkeys consciously perceive. Ventral activity and consciousness in humans Kanwisher et al., (1997) -fmri Fusiform Face Area (FFA) - responding to faces Parahippocampal Place Area (PPA) - responding to landmarks, houses and scenes Andrews et al. (2002) - ambiguous face-vase picture - participant to press keys according to perception - activity in FFA for perception of faces - activity Lateral Occipital (responding to objects) for perception of vase - verbal report of what participants see under binocular rivalry -face and house - activity of FFA and PPA correlated with reported perception Ventral stream vs.dorsal stream and traditional vs. ecological approaches to visual perception See Norman, J. (2002) for a detailed discussion e.g. Lee & Lishman (1975) swinging room as an example of the ecological approach vision to guide action unconscious egocentric Summary The what and how dichotomy Clinical evidence - Case D.F. - Agnosia - Optic ataxia Experimental evidence - Requirements of the two systems different time scales different frames of reference - Experimental evidence from healthy participants (and D.F.) - Characterisation of the two systems (what and how too simplistic?) - Ventral system and consciousness - Dorsal-ventral = ecological-traditional?