Agnosias & Semantic Deficits
Raffaella Ida Rumiati
Cognitive Neuroscience Sector
SISSA
Trieste, Italy
INTRODUCTION
• The study of several neuropsychological disorders such as agnosia, optic aphasia, semantic dementia, and category selective deficits has provided us with a valuable insight as to the cerebral organization of meaning
• Moreover, disorders of object perception have offered cues as to the human visual recognition abilities
• In this lecture, I am going to review a number of studies that have challenged our contemporary view on these issues
AGNOSIA
• This is a reduced ability to identify stimuli presented in a given sensory modality as a consequence of brain damage
• Depending on which modality is affected, we talk about visual, auditory or tactile agnosia
VISUAL AGNOSIA • This is the most studied type: easier to detect
• Stimuli misrecognized visually, can be recognized:
– through tactile manipulation
– from verbal description
– based on its characteristic sound or noise
Il caso di Heinrich Lissauer (1890) • He described the case of an 80-year-old patient, GL, who had
been blown against a wooden fence by a storm, knocking his head
• After this accident, he could still see but he could not identify common objects visually presented
• GL had almost normal visual acuity for his age, and he could draw accurate copies of seen objects he could not recognize
• His knowledge of objects was preserved: he would refer to them appropriately in conversation, recognize them when he could touch them or listen to their characteristic sound
• Thus GL suffered from visual associative agnosia
• The post-mortem analysis revealed a lesion in the left temporo-occipital junction
The patient could copy the items he could not recognize
(Rubens & Benson 1971)
COPYING LINE DRAWINGS
• He proposed a model of visual recognition that distinguishes two levels:
– apperceptive: that accomplishes early perceptual
processing of the stimuli
– associative: that provides a meaning to the percept by
linking it to previous experience
• Depending on which of the two levels is impaired as a result of brain damage, we will observe apperceptive or associative agnosia respectively
Lissauer‟s Model
After Lissauer
• Some skeptics (Bay 1952; Bender & Feldman 1972; Farah 1990) have argued that:
– visual agnosia does not exist
– so-called agnosic patients have either an elementary sensory deficit or an intellectual decline
• The original dichotomy proposed by Lissauer was maintained but each level has been further fractionated
Lissauer
• Apperceptive
• Associative
Warrington & co.
• Pseudoagnosia: sensory discrimination, shape detection and discrimination
• Apperceptive: figure-ground, incomplete drawings, perceptual categorization
• Associative
WARRINGTON & COLLEAGUES
Pseudoagnosia: Shape discrimination
Efron test
Pseudoagnosia: Shape detection
Apperceptive Agnosia
Figure-Ground
Ghent overlapping figure test
Apperceptive Agnosia
Incomplete drawings
Gollin‟s test
Perceptual
Categorization:
– Patients with RBD
(parietal lesions)
• spared shape
recognition
• impaired identification
and matching of objects
depicted in unusual
views
• deficit particularly
severe when main axis
is shortened or a critical
feature is occluded.
Matching unusual views
Lissauer
• Apperceptive
• Associative
Humphreys & Co.
• Appreceptive:
• Integrative agnosia:
– inability to group and integrate parts of an object into a
coherent whole
• SDS
• Semantic
System
Warrington & Co.
• Pseudoagnosia
• Apperceptive: figure-ground, completion, perceptual categorization
• Associative
Integrative Agnosia
– Deficit in integrating single features of a stimulus in a coherent fashion
– Failure to extract a figure from the background
– Accurate copy of drawings and objects
– Good identification of elementary shapes
– Good semantic memory (e.g. drawing from memory)
HJA, Humphreys & Riddoch, 1987
HG, Grailet et al., 1990
Humphreys & Co.
• THE STRUCTURAL DESCRIPTION SYSTEM
– contains representations which define geometrical
and volumetric properties of objects
– is for objects what the input phonological lexicon is
for words
• THE SEMANTIC SYSTEM
– stores functional knowledge about objects,
associations between them, the context in which
they can be found as well as the encyclopedic
knowledge about them
DIFFERENT TYPES OF ASSOCIATIVE A.
Object decision (chimeras)
• In analogy with the lexical decision task that assesses the integrity of the phonological input lexicon, the object decision task assesses the integrity of the SDS:
– patients are asked to decide whether a given stimulus exists in their repertoire of visual descriptions.
Head Test
• Matching a given “body” of an animal or object to the correct “head” is also supposed to tap the SDS
Other Tests
• Drawing an object from memory, describing its shape, or evoking its perceptual features may not detect the SDS but it could reflect a possible imagery deficit
How to assess the SDS
– Naming from different modalities (semantic errors)
– Sorting items into categories (living vs non living)
– Semantic matching tasks
• Which 2 items are used together (hammer & nail)
• Which 2 items are found in the same context (P & P)
• Which 2 items share the same function (radio & CD player)
– Questions concerning visual perceptual and functional
associative knowledge (Barbarotto et al. 1996; Silveri &
Gainotti 1988)
– Pantomiming the use of objects
*All these tests can also be administered using verbal stimuli
Testing the Semantic System
Picture-to-Picture Matching
Pyramid & Palm Tree Test
Word-to-Word Matching
Pyramid & Palm Tree Test
pine tree life preserver
tulip palm tree
pyramid
Barbarotto et al. 1996
Questionnaire
HAMMER 1. supraordinate info: is it an object, a vegetable or an
animal?
2. category info: is it a tool, a musical instrument or a gem?
3. subordinate perceptual info: is it made of glass, of metal or of cement?
4. subordinate structural info: is it smaller than a screw? (yes/no)
5. functional info: is it used for cutting, screwing or sticking nails?
6. the protypical user of the object: is it used by the painter, the carpenter, the glazer?
• Based on double dissociations, it has been proposed that stored knowledge is organized in two separate subsystems:
1. Patients with a damaged SDS but spared semantic system proper
2. Patients who performed normally on the object decision task but pathologically on tasks tapping semantic knowledge
Pattern 1: Sartori & Job 1988; Caramazza & Shelton 1998 (for animals only)
Pattern 2: Riddoch & Humphreys 1987; Stewart Parkin & Hunkin 1992; Sheridan & Humphreys 1993; Hillis & Caramazza 1995; Humphreys & Riddoch 1999; Fery & Morais 2003
SDS & Semantic System
• Agnosic deficits have been explained
in different ways, depending on which
model of semantic organization was
adopted
• Two main views:
• Multiple-semantic systems
• Amodal semantic system (also called Organized-Unitary-Content hypothesis,
OUCH by Caramazza et al.)
ASSOCIATIVE A. & SEMANTIC SYSTEM
• This view holds that the conceptual knowledge is organized in modality specific systems (e.g. verbal, visual):
– different modalities will be tapped by different stimuli (e.g. words, pictures)
• Evidence for separate systems comes from patients who showed a selective deficit in either processing words or processing pictures
Shallice1988; McCarthy & Warrington 1994
MULTIPLE SEMANTIC SYSTEM
McCarthy & Warrington 1988
Damage to the Verbal Semantic System
TOB Pictures Words
identification task % correct % correct
Living things 94 33
Inanimate things 98 89
• TOB suffered from a progressive disorder of semantic memory
that affected his ability to comprehend spoken names of
animals (except for superordinate category: “it‟s an animal”)
but spared his knowledge of named objects.
• He was able to give good definitional and associative
information about visually presented stimuli, irrespective of
their semantic category.
in McCarthy & Warrington 1994
Damage to the Visual Semantic System
PHD Pictures Words
identification task % correct % correct
animals 33 77
foods 100 96
• PHD sustained a severe closed head injury, leaving him with a
disproportionate impairment in recognizing visually presented
animals and in matching animal identity (2 different pictures of
caws) relative to objects.
• PHD was normal on the object decision task, and better when
instead of pictures he was asked to define spoken words.
• Associative visual agnosia can be interpreted in terms of a damage of the visual semantic system
• This framework does not clearly account for a difference between the SDS and semantic system
Shallice,1988; McCarthy & Warrington 1994
VISUAL ASSOCIATIVE A. & MULTIPLE
SEMANTIC SYSTEM
INPUT
Associative
Agnosia
OUTPUT
V
I
S
U
A
L
V
E
R
B
A
L
hammer
/ hammer /
X
ORGANISATION OF SEMANTIC
KNOWLEDGE (ALLPORT, 1985)
• There is only one abstract representation of a given concept
• One can access it from different modalities (visual, verbal, tactile etc.), after a pre-semantic processing (SDS)
• There are different modality-specific outputs
(Riddoch et al. 1988; Caramazza et al. 1990)
AMODAL SEMANTIC SYSTEM
• Within this framework, visual associative
agnosia corresponds to a deficit in
accessing a unitary semantic system
from the visual modality only
• The SDS is held to be intact:
– i.e. normal performance on the Object
Decision and Head Test (e.g. patients JB).
VISUAL ASSOCIATIVE A.
&
AMODAL SEMANTIC SYSTEM
Structural Description System
INPUT
SEMANTIC
SYSTEM
Presemantic
Deficit
Associativa Agnosia
(access deficit)
OUTPUT
visual/tactile/auditory
visual/tactile/auditory X
OPTIC APHASIA (Freund 1889) • The patient showed a deficit in confrontation naming of objects •
• In contrast, he could name them when they were presented in other modalities (tactile, on definition, characteristic sound) and he seemed to have preserved semantic knowledge about objects
• Lesion → Left Occipital + Splenium of Corpus Callosum
• Anatomical explanation → the visual processing is carried out in the spared RH which is disconnected from speech areas in the LH
LH RH
W
area
VISUAL VS VERBAL SEMANTICS
Lhermitte & Beauvois 1973; Beauvois 1982
• The functional breakdown in OA patients is between the visual semantic system and the verbal semantic system:
• visual semantic system is intact as demonstrated by the preserved ability to perform semantic associative matching tasks and to pantomime the use of objects (i.e. no apraxia)
• verbal semantic system is also intact since naming from other modalities is normal
INPUT
Optic Aphasia
OUTPUT
V
I
S
U
A
L
V
E
R
B
A
L
hammer
/hammer/
X
– Differently from associative agnosics, OA patients perform normally on tasks tapping visual semantic knowledge (matching, categorization)
– AO patients can recognize the objects as suggested by their spared ability to show how they would use them
– They are not sensitive to the quality of the stimulus (i.e. real objects are better recognized than line-drawings), as visual agnosics are
– They do not have difficulties in coping with everyday life as agnosic patients have
VISUAL AGNOSIA & OPTIC APHASIA
• APPERCEPTIVE A.
• Stroke of the posterior cerebral artery affecting visual associative areas bilaterally (sparing the primary visual area, BA 17)
• Tumor lesions of the occipital cortex
• Traumatic focal lesions of the occipital cortex
• Post-anoxic syndromes
- carbon monoxide intoxication
- hart attack
• Degenerative pathologies
- AD and focal, slowly progressive dementias
ETIOLOGY AND BRAIN CORRELATES OF
VISUAL AGNOSIAS
• INTEGRATIVE A.
• Stroke of the posterior cerebral artery affecting the temporo-occipital cortex bilaterally (including lingual & fusiform gyri)
• PERCEPTUAL CATEGORIZATION
• Stroke of the middle artery involving the parietal cortex in the right hemisphere
• ASSOCIATIVE A.
• Stroke of the left posterior cerebral artery that supplies the occipito-temporal cortex
• Bilateral stroke of the medial occipito-temporal cortex (unusual)
CATEGORY-SPECIFIC DEFICITS
• After brain damage, the ability to identify exemplars that belong to living categories (fruits, vegetables, animals etc.) or to non-living categories (tools, vehicles, clothes etc.) can result selectively affected
First observations:
• Nielsen (1937)
• Mc Crae & Trolle (1956)
Warrington & Shallice (1984)
• Described 2 patients with a selective identification deficit as affecting animals, foods and plants, but still able to recognize inanimate objects
• Many other cases followed: e.g.Sartori & Job 1988, Silveri & Gainotti 1988, Farah et al. 1989
• The opposite dissociation, i.e. a selective identification deficit of inanimate objects and spared recognition of biological exemplars has been observed too, but less frequently
• e.g. Hillis & Caramazza 1991; Sacchett & Humphreys 1992; Warrington & McCarthy 1994
Living vs Non Living Categories
The Sensory/Functional Theory Warrington & Shallice (1984)
• There are two semantic subsystems, one for concepts about living exemplars, the other for non-living ones:
– the former deals with sensory features, the other with functional features
• Living things are better characterized by sensory features and manmade objects are better characterized by their functions and their manner of usage
• Damage to the sensory subsystem leads to a deficit in identifying LT, whereas a damage to the functional subsystem leads to a deficit in identifying NLT
SOME THEORETICAL ACCOUNTS
THE DOMAIN-SPECIFIC HYPOTHESIS
Caramazza & coll.
• The evolutionary pressures have resulted in specialized (and functionally dissociable) neural circuits dedicated to processing, perceptually and conceptually, different categories of objects
• This applies only to those categories for which rapid and efficient identification could have had survival and reproductive advantages
• Plausible candidate categories are „animals‟, „fruit/vegetables‟, „conspecifics‟, and possibly „tools‟
• Herpes Simplex Virus Encephalitis
– affects the medial temporal cortex unilateral
left or bilaterally (hippocampus included)
– often associated with category specific
deficits for LT
• Semantic dementia
• Alzheimer‟s disease
Acquired disorders of category-specifc
deficits
Farah (1990)
• In an historical review of the literature, she noted that researchers reported: – Pure deficits in face recognition (prosopagnosia) and in
visual word recognition (alexia)
– No pure agnosia (for objects)
– No alexia and prosopagnosia
• She then proposed a two process-account of vision.
• There are two processing operations that take place in parallel: – coding undifferentiated global forms
– processing of parts-based representations
Objects, Faces, Words
• She predicted that patients with pure Object
Agnosia or with Prosopagnosia and Alexia but
without Agnosia could not exist
alexia agnosia prosopagnosia
Pure agnosia
• Rumiati et al 1994; Humphreys & Rumiati 1998
Prosopagnosia and Alexia without Agnosia
• Buxbaum et al 1996; De Renzi & Di Pellegrino 1998
Farah was wrong
BATTERIES FOR ASSESSING VISUAL
OBJECT AND SPACE PERCEPTION
BORB (Riddoch & Humphreys, 1993)
Birmingham Object Recognition Battery
VOSP (Warrington & James, 1991)
Visual Object and Space Perception Battery
EARLY VISUAL PROCESSING
Benton Test object
early visual processing
image viewer-dependent
image object-centered
(episodic structural description)
structural description system
semantic knowledge
output phonological lexicon
FROM VIEWER-DEPENDENT TO OBJECT-
CENTERED REPRESENTATION
Matching unusual views
object
early visual processing
image viewer-dependent
image object-centered
(episodic structural description)
structural description system
semantic knowledge
output phonological lexicon
STORED STRUCTURAL DESCRIPTION Object Decision
object
early visual processing
image viewer-dependent
image object-centered
(episodic structural description)
structural description system
semantic knowledge
output phonological lexicon
SEMANTIC SYSTEM
• On visual presentation
–Confrontation naming (semantic errors)
–Pantomiming the use of objects
–Sorting items into categories
–Semantic matching tasks • Which 2 items are used together
• Which 2 items are found together
• Which 2 items are associated sem.
–Questions concerning different aspects of semantic knowledge (Capitani, Laiacona etc.).
object
early visual processing
image viewer-dependent
image object-centered
(episodic structural description)
structural description system
semantic system
output phonological lexicon
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