FAILURES OF OBJECT RECOGNITION. Dr. Walter S. Marcantoni

Transcription

1 FAILURES OF OBJECT RECOGNITION Dr. Walter S. Marcantoni VISUAL AGNOSIA -damage to the extrastriate visual regions (occipital, parietal and temporal lobes) disrupts recognition of complex visual stimuli -in visual agnosia, the object cannot be visually recognized, but can be recognized through other modalities **areas 20, 21, and 38 are often referred to as area TE 1

2 AGNOSIA: DISORDER OF OBJECT RECOGNITION OR MEMORY? SUBTYPES OF AGNOSIA visual agnosia apperceptive agnosia associative agnosia associative color agnosia integrative agnosia category-specific agnosia 2

3 APPERCEPTIVE VISUAL AGNOSIA -apperceptive visual agnosia describes failures in object recognition linked to problems in perceptual processing -the patient is unable to recognize, copy or match simple shapes -basic visual functions such as acuity, color, motion are preserved -can sometimes name parts of the object, but is unable to develop a percept of the object (synthesize the sensory information) -results from gross bilateral damage to the extrastriate visual regions APPERCEPTIVE VISUAL AGNOSIA: RECOGNTION 3

4 APPERCEPTIVE VISUAL AGNOSIA: COPY APPERCEPTIVE VISUAL AGNOSIA: MATCHING 4

5 APPERCEPTIVE VISUAL AGNOSIA: TESTS OF PERCEPTUAL ACUITY Incomplete Letters Task Golin Picture Task Warrington (1985) : -patients with agnosia following right hemisphere lesions have more difficulty than patients with left hemisphere lesions in the Incomplete Letters Task, despite the more severe language problems for the left hemisphere group -patients with agnosia following right hemisphere lesions required more complete drawings in order to correctly identify the objects APPERCEPTIVE VISUAL AGNOSIA: DEFICIT IN PERCEPTUAL CATEGORIZATION According to Warrington: -the presence of apperceptive visual agnosia in patients with right hemisphere lesions is not one of contour discrimination (they can make fine discrimination between shapes of simple geometric figures) -the critical problem is that of perceptual categorization (the patient s ability to achieve object consistency is disrupted) -Unusual Views Test and Shadows Test requires subjects to categorize information according to their perceptual qualities 5

6 ASSOCIATIVE VISUAL AGNOSIA -associative visual agnosia patients derive normal visual representation but cannot use this information to recognize things -patient is unable to recognize the object despite an apparent perception -can copy a drawing rather accurately (indicating a coherent percept), match it to another object, use the object correctly, but cannot identify it -the problem seems to be situated at the level of association between the visual representation of the object and its semantic links (stored information about the object, category function) -bilateral lesion of the anterior inferior temporal lobe ASSOCIATIVE VISUAL AGNOSIA 6

7 ASSOCIATIVE VISUAL AGNOSIA: COPY ASSOCIATIVE VISUAL AGNOSIA: SEGMENT COMPLEX DRAWINGS INTO THEIR PARTS 7

8 ASSOCIATIVE VISUAL AGNOSIA: COPY FROM MEMORY -can copy an anchor, can define it, but cannot draw it from memory demonstrating a deficit in accessing a mental image of the object -failure of object recognition is a memory defect that effects not only past knowledge about the object, but also acquisition of new knowledge ASSOCIATIVE VISUAL AGNOSIA: DEFICIT IN SEMANTIC CATEGORIZATION -Matching By Function Test requires subjects to categorize stimuli on the basis of their semantic properties (i.e. how they are used). Warrington (1982) -patients with posterior lesion in either the right or left hemisphere showed impairment on this task, but the deficits occur for different reasons -patients with right-sided lesions cannot do the task because they fail to recognize many objects, especially those depicted in an unconventional manner - patients with left-sided lesions can frequently recognize objects in isolation, but they cannot make the functional connection between the two visual percepts 8

9 WARRINGTON S TWO-STAGE MODEL OF OBJECT RECOGNITION -the first stage of object categorization is perceptual, the process required to overcome perceptual variability in the stimulus (right hemisphere) -the second stage involves semantic categorization in which perceptual representation is linked to semantic knowledge DIFFICULTIES WITH WARRINGTON S TWO-STAGE MODEL OF OBJECT RECOGNITION 1) neuropathological findings have not always proved a correspondence between associative agnosia and left hemisphere lesions. More typically these patients have bilateral lesions that affect occipitotemporal regions 2) unilateral right hemisphere lesions restricted to the occipitotemporal region can produce an agnosia more similar to associative subtype than the apperceptive subtype 9

10 ASSOCIATIVE AGNOSIA SUBTYPE: INTEGRATIVE AGNOSIA Humphreys and Riddoch (1994) -patient H.J.A. -no problem with shape-matching tasks; no trouble copying objects; successful in matching photographs of objects from unusual views -recognized objects in isolation, but had great difficulty when the contours of 2 objects overlapped -integrative agnosia deficits in the recognition of objects due to the failure to group and integrate the component parts into a coherent whole; patients with this deficit can faithfully reproduce drawings of objects, however their percept is of isolated, unconnected parts or contours INTEGRATIVE AGNOSIA: LINE DRAWING -patient C.K. 10

11 INTEGRATIVE AGNOSIA: DEFICIT IN GROUPING AND INTEGRATING FEATURES -patients with integrative agnosia have difficulty grouping common elements together ASSOCIATIVE AGNOSIA SUBTYPE: CATEGORY-SPECIFIC AGNOSIA Warrington and Shallice (1984) -patient J.B.R. -had severe associative agnosia that was disproportionately worse for living objects than inanimate ones -category specific agnosia recognition impairments that are restricted to certain classes of objects 11

12 CATEGORY-SPECIFIC AGNOSIA: SELECTIVE LOSS OF KNOWLEDGE ABOUT VISUAL PROPERTIES -according to Warrington semantic knowledge is structured: -brain injuries that produce agnosia in humans do not completely destroy the connections to semantic knowledge; because damage is not total it seems reasonable that circumscribed lesions might destroy tissue that is devoted to processing similar types of information TESTS USED TO DEMONSTRATE CATEGORY-SPECIFIC AGNOSIA 12

13 SPARED ABILITY TO RECOGNIZE NONLIVING OBJECTS: THEORY I -according to Damasio (1990) the spared ability to recognize common objects has been attributed to the fact that our visual knowledge is supplemented by kinesthetic codes developed through our interactions with these objects SPARED ABILITY TO RECOGNIZE NONLIVING OBJECTS: THEORY II -according to Gaffan and Heywood (1993) living things are inherently more difficult to discriminate than are nonliving things because members of categories such as animals and fruits share more salient and distinctive features than do categories such as tools 13

14 PROSOPAGNOSIA -prosopagnosia is a failure to recognize familiar faces -the subjects sees the eyes, the ears, a nose and mouth, but cannot recognize the particular configuration of these features that identifies an individual face -they still remember the individual and can recognize them by their voice, posture, even their clothes -usually due to lesions of the fusiform gyrus (BA 37) WHY ARE FACES SPECIAL? -for primates (including humans) faces are very important for social communication -for example, recognizing one s mother, a close friend or family member, inferring an individual s emotional state -this importance has led to the idea that a special pathway exists in the visual system for the analysis of faces -3 criteria can be useful in considering whether face and other forms of perception utilize distinct processing system: 1) do the processes involve physically distinct mechanism 2) are the systems functionally independent 3) can each operate without the other 14

15 NEURAL MECHANISMS OF FACE PERCEPTION: CLINICAL RESULTS NEURAL MECHANISMS OF FACE PERCEPTION: PHYSIOLOGICAL RESULTS -studies with monkeys reveal that neurons in the fusiform gyrus respond preferentially to faces 15

16 NEURAL MECHANISMS OF FACE PERCEPTION: IMAGING RESULTS -studies with humans reveal greater activation in the fusiform gyrus when viewing faces NEURAL MECHANISMS OF FACE PERCEPTION: PSYCHOLOGICAL EVIDENCE -asymmetry in facial processing -subjects are asked which of the 2 pictures, B or C, most closely resembles picture A -control subjects chose picture C significantly more often than picture B -picture C corresponds to the part of picture A that falls in the subject s left visual field -the women pictured chose B, the view she is accustomed to seeing in the mirror 16

17 DISSOCIATION OF FACE AND OBJECT PERCEPTION: CLINICAL RESULTS I -patient C.K. showed difficulty in recognizing objects but not faces DISSOCIATION OF FACE AND OBJECT PERCEPTION CLINICAL RESULTS II -some researchers have argued that prosopagnosics suffer from deficits in within-category discrimination (i.e. problems recognizing objects in particular categories where certain subtle differences distinguish the individual items such as cars and buildings) as opposed to between category discriminations -the argument is not valid as there exists clinical evidence to the contrary 17

18 DISSOCIATION OF FACE AND OBJECT PERCEPTION PSYCHOLOGICAL RESULTS I -inverting any object makes it harder to recognize, this effect is disproportionate for faces -we are particularly sensitive to the configuration of upright faces (Thatcher Illusion) -inverted image is processed by the same cortical region as the other stimuli -upright picture is processed by a face perception system DISSOCIATION OF FACE AND OBJECT PERCEPTION PSYCHOLOGICAL RESULTS II -experimental results support the notion that face perception requires a representation that is simply a concatenation of individual parts 18

19 TWO SYSTEMS FOR OBJECT RECOGNITION CLINICAL RESULTS I -recognition can be based on two forms of analysis: holistic processing and part based (analytic) processing -the contribution of these 2 systems varies for different classes of stimuli -analysis by parts is essential for reading and is central for recognizing objects -a unique aspect of face recognition is its dependence on holistic analysis; this process also contributes to object recognition TWO SYSTEMS FOR OBJECT RECOGNITION CLINICAL RESULTS II 19

20 ASSOCIATIVE COLOR AGNOSIA -in associative color agnosia a patient cannot match a particular color with a particular object, even though color perception is intact -the patient cannot: -choose an object with a given color amid a series of objects -indicate a color on verbal command -name the color of an object -evoke the color of a common object (tomato, orange) -lesions cause a disconnection between the areas responsible for color perception (V4) and semantic association regions 20