Agnosia

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DEFINITION

  • Agnosia: From Greek word, gnosis, or knowledge, so means absence of knowledge
  • Fundamentally defined as a disorder of recognition; inability to recognize the meaning of info conveyed w/in a given sensory system from external environment to the brain
  • Crucial feature is that it exists only in single sensory modality
  • NOT attributable to elementary sensory defects, mental deterioration, nonfamiliarity, aphasic misnaming of the thing or impaired consciousness or attention

HISTORY

  • Visual agnosia also called “mind blindness” (by Lissauer) or “psychic blindness” (i.e., seelenblindheit – Munk) – first demonstrated in a dog by Munk in 1881; “imperception” by Jackson
  • Term “agnosia” originally coined by Freud (1891)
  • Over years, lots of debate and confusion
    • “More mechanisms proposed for visual agnosia than number of reported cases”
    • Some folks argue agnosia is simply a combination of primary sensory loss and cognitive deterioration
  • One of the basic questions is whether agnosia is a sensory/perceptual or memory related disturbance
    • Proponents of both sides assume that two processes separable
    • Memory (Teuber): impairment of access to memory “located” at the interface between perception and memories
    • Sensory/Perceptual: (e.g., Bender) Agnosia is a perceptual impairment or the result of dementia
  • The most lasting distinction was proposed by Lissauer (1890), who differentiated apperceptive from associative agnosias

TYPES OF AGNOSIA

  • General distinction
    • Apperceptive: damage fairly early on in recognition assembly line, before perception is properly constructed; pts do not perceive objects normally so can’t recognize them
    • Associative: fault in later stages of recognition; perception may be ok, but access to memory or meaning not; Teuber – “a normal percept stripped of its meaning”
  • Classic distinction is that pts who can successfully copy but not identify are associative agnosics
    • probably not accurate since pts w/ severe deficits in visual perception can accurately copy things thru slavish, line-by-line approach

VISUAL AGNOSIAS (and related disorders)

Re: if patient can indicate recognition by verbal description or gesture, it’s an anomic disorder rather than agnosia

Apperceptive Visual Agnosia

  • Rare d/o in which pt has difficulty recognizing objects because of failure to perceive them
  • Commonly say vision is “blurred” or “foggy”; can’t describe what they see – but elementary sensory functions are relatively intact
  • Distinct from cortical blindness cuz intact visual acuity, but forms and shapes not recognized
  • Many pts recovering from cortical blindness
    • often seen with patchy visual field defects
  • Unable to draw misidentified objects or match them to sample
  • Lesions: typically bilateral damage to lateral parts of occipital
    • ass’d w/ path processes such as CM poisoning, mercury, cardiac arrest, bilateral strokes, basilar artery occlusion, or bilateral posterior cortical atrophy

Associative Visual Agnosia

  • Defect in recognizing visual stimuli that are well perceived (recent research suggests that these pts actually do have abnormal perceptual abilities, which has led to growing discontent w/ apperceptive-associative distinction)
  • Defect occurs beyond early stages of perception but before the stage of multimodal memory activity on which recognition depends (Damasio et al., 2000)
  • Particular difficulty in arriving at meaning of stimuli
  • Again, impairment of visual recognition is present, but visual acuity clearly good
  • CAN make drawings of pictures they can’t recognize and CAN match drawings/pictures to samples (although cannot put into categories)
  • Lesions: typically bilateral occipitotemporal lesions – from strokes in PCA
  • Commonly ass’d w/ right homonymous hemianopia

Visual Object Agnosia (ventral association processing disruption)

  • Specific inability to recognize, name, or demonstrate use of object such as pencil, chair, or clock
  • General visual object agnosics have an inability to recognize even the generic classes to which objects belong (as opposed to category-specific problems); don’t know face is face or car is car
  • Sometimes if object is moved or rotated can recognize (“static visual agnosia”)
  • Lesions: depend on either left unilateral or bilateral occipitotemporal (lingual, fusiform, and parahippocampal gyri), and infarction in cortex and underlying white matter of these areas
  • Optic Aphasia: visually presented objects can be recognized but not named, and auditory and tactile naming ok
    • Some folks think just milder form of visual object agnosia
    • Others think it’s separate entity

Prosopagnosia (ventral association processing disruption)

  • Inability to recognize previously known faces and failure to learn new ones
  • Generally know eyes, nose, mouth, etc., just can’t recognize particular face
  • NOT limited to human faces; farmer won’t recognize cows individually
  • Can recognize objects in environment as long as don’t require recognition of specific object w/in grp
  • Thus, can perform generic recognition but not specific recognition (car’s manufacturer)
  • Lesions: typically bilateral lesions of occipitotemporal cortex and underlying white matter; if unilateral, results from right hemisphere lesion

Color Agnosia – 3 classes

  • Central Achromatopsia (loss of color vision due to CNS disease)
    • Pts complain of gray or washed out look in affected area
    • Can be complete or just certain quadrants
    • Still perceive form and shape ok
    • Full-field ass’d w/ visual agnosia, especially prosopagnosia
    • Lesion: damage of visual ass’n cortex or subjacent white matter
  • Color Anomia (pt will succeed on visual-visual tasks and on verbal-verbal tasks, but cannot name colors)
    • pts have right-homonymous hemianopia and intact color perception of left field
    • other common correlate is pure alexia
    • lesion is usually in left hemisphere, mesially, in transition btwn occipital and temporal lobes
  • Specific Color Aphasia
    • Pt can sort colors and match them
    • seen in context of aphasia; represents disproportionate difficulty in naming colors
    • suspect left parietal damage

Simultanagnosia

  • Deficit characterized by failure to synthesize all elements of a picture or scene, even though components can be recognized in isolation
  • Often considered a variant of apperceptive agnosia
  • Varieties:
    • “Dorsal”(bilateral occipitoparietal lesions): pt can’t see more than one object at time
    • “Ventral” (left inferior occipital lesions): pt may be able to “see” more than one object at time

Balint’s syndrome

Caused by large bilateral parietal lesions – especially severe if frontal lobes affected. Clinical Triad:

  • Simultanagnosia
  • Ocular apraxia: an inability to shift gaze voluntarily from a fixation point; pts behave as though mesmerized by original object; gaze may be shifted if close eyes
  • Optic ataxia: impairment of visually guided movements as a result of a defect in stereopsis (depth perception); pts may not be able to read in methodical visual sweeps
    • Inability to manually respond to visual objects

Cortical Blindness and Anton’s Syndrome

  • Vision obliterated completely
  • Lesion: severe bilateral damage to visual cortices and to optic radiations
  • When accompanied by denial of blindness = Anton’s syndrome, form of anosognosia
    • may be caused by blindsight, where pt can see some movement but denies it; mediated by “second visual system” of superior colliculus, pulvinar, and parietal cortex
    • may also relate to impaired memory or insight because of ass’d temporal or frontal damage

Metamorphopsia

  • Very rare d/o where objects can be identified but they look odd; can appear fragmented, compressed, tilted
  • Macropsia: objects seen as larger than they are
  • Micropsia: objects smaller than they are

AUDITORY AGNOSIAS (and related disorders)

  • Impairment in ability to recognize speech or nonverbal sounds in presence of adequate hearing
  • Damage typically centers on temporal lobes
  • Severe damage results in cortical deafness
  • Lissauer’s apperceptive/associative distinction has been suggested but clinical differentiation problematic and no established anatomical correlates (Mesulam, 2000)
  • May reflect disconnection of unimodal auditory areas specialized for encoding the auditory properties of familiar objects from transmodal nodes that coordinate multimodal recognition

Pure word deafness (auditory word agnosia)

  • In the face of intact hearing and comprehension of nonverbal sounds, impaired recognition of speech
  • Voices are heard, but words do not make sense – may complain muffled, sounds like foreign language
  • Can’t repeat
  • May evolve from Wernicke’s aphasia, but spontaneous speech, reading, writing all intact
  • In aphasias, word sounds are perceived normally but they can’t link sound to meaning
  • Lesions: thought to reflect disconnection of primary auditory area on both sides from Wernicke’s area
  • may be bilateral or unilateral left temporal; CVA is most common cause

Auditory Sound Agnosia

  • Nonverbal counterpart of pure word deafness
  • Involves deficits in recognizing environmental sounds such as bell ringing, dog barking
  • Lesion: appears to be right hemisphere analog to pure word deafness

Phonagnosia

  • Reflects inability to recognize familiar voices
  • Auditory analog of prosopagnosia

Cortical Deafness

  • applied to pts whose daily activities and auditory behavior indicate an extreme lack of awareness of auditory stimuli of any kind and whose audiometric tone very abnormal
  • Most commonly seen in bilateral CV disease
  • Lesion: bilateral destruction of auditory radiations or primary auditory cortex
  • Distinguishing between auditory agnosia and cortical deafness is problematic

Receptive Amusia (sensory amusia)

  • loss of the ability to appreciate various characteristics of heard music
  • Occurs to some extent in all cases of auditory sound agnosia and in majority of pure word deafness
  • tough to study given the extreme variability in premorbid experience, skill, etc.

Auditory affective agnosia

  • pts show impaired comprehension of prosody • ass’d w/ right temporoparietal lesions and neglect

TACTILE AGNOSIAS (and related disorders)

  • Impairment of ability to recognize objects by palpation in hand
  • Because of complexity of somatosensory system, enormous variability in presentation of pts
  • Lesion localization two views:
    • ass’d w/ contralateral primary somatosensory area in postcentral gyrus
    • more diffuse aspects of cortex (eg, posterior parietal lobe) are involved in perception

Apperceptive Variety

  • Equivalent to astereognosis
  • Impaired ability to discriminate objects based on physical characteristics of size, weight, shape, density, or textural cues
  • inability to id objects by touch
    • Operationally defined in many ways, the most common being the loss of tactile object recognition in the absence of hypesthesia
    • Usually unilateral
  • deficit in tactile recognition in basic somatosensory perception
  • Often seen with agraphesthesia, impairment in recognition of characters on skin of palm
  • Lesion: contralateral primary sensory cortex

Associative Variety

  • d/o of tactile recognition in absence of primary somatosensory dysfx
  • More subtle than astereognosis
  • Like pts w/ associative agnosia, can’t recognize object in hand, BUT can accurately draw the object even when palpation fails to elicit recognition
  • Lesion: inferior parietal cortex, where high-level tactile processing occurs

Other Tactile Agnosias

  • Amorphognosia: Impaired recognition of size and shape of objects
  • Ahylognosia: Impaired discrimination of distinctive qualities of objects such as density, weight, texture, thermal properties
  • Tactile asymboly: Impaired tactile recognition of identity of objects in absence of amorphognosia or ahylognosia

THEORETICAL PERSPECTIVES

Stage Models

  • cortex builds up percept from elementary sensory perceptions
  • recognition achieved when resulting percept matched to stored info about object
  • Lissauer’s model of apperception and association agnosias is example
  • Validity called into question, since perception is not normal in many “associative” agnosias

Lissauer’s two-stage model of perception/recognition:

  • After an elementary Sensation occurs…
  1. there is an Object Perception (apperception), in which there is a conscious perception of a form, object, or “thing”,
  2. then there is Object Recognition (association), in which there is recognition of the category or identity of this “thing”…
  • …which allows for Naming, the retrieval of the word for the category or identity for the “thing”.

Disconnection Models

  • Most associated w/ Geschwind who suggested that agnosia results from disconnection btwn visual and verbal processes
  • Eg, Pts who fail to identify objects, who later used or interacted normally with the object
  • Disconnection theory can’t, by self, account for fact that most agnosics show abnormal verbal and nonverbal processing of viewed objects

Computational Models

  • Begins by trying to explain normal perceptual phenomena
  • Recognizes the enormous complexity involved in perceptual analysis and concludes that brain must store representations in some kind of codable, symbolic form that is flexible enough to accommodate perceptual variations required in everyday recognition
  • In this model, agnosia is a d/o dependent on perceptual dysfx
  • Marr’s Model of 3 types of representation
    • Primal sketch: represents brightness changes across visual field – results in a way of specifying the geometric shape of an object
    • viewer-centered sketch, which represents spatial locations of visible surfaces from viewer perspective
    • object-centered sketch: specifies the configuration of surfaces w/in object-centered coordinate frame
  • Damasio’s Model
    • perception involves evocation of neural activity pattern in primary and first-order ass’n cortex which corresponds to various perceptual features
    • Downstream, features combined in “local convergence zones”
    • No distinguish btwn memory and perception which bind features of a pattern into an “entity”
    • Predicts there can be no disorder of object recognition without perceptual dysfunction

Cognitive Neuropsychological Models

  • only received significant attn in visual recognition
  • Ellis and Young’s model: recognition begins by comparing viewer-centered and object-centered representation to stored structural descriptions of objects known as “object recognition units”

OTHER AGNOSIA TERMS

Autotopagnosia

  • disturbed body schema involving an inability to identify parts of one’s body, either to verbal command or by imitation
  • Gross autopagnosia is rare and is not observed in isolation
  • Limited forms include left-right disorientation and finger agnosia

Finger agnosia

  • bilateral loss of the ability to name or id the fingers
  • concept is linked to Gerstmann syndrome
  • failure on tests of finger recognition and finger localization may depend on specific demands of tasks employed (eg, aphasic misnaming, sensory deficits, spatial disorientation, attention pxs)

Static Object Agnosia

  • visual object agnosia, the effect of which is reduced when object is moved

ASSESSMENT OF AGNOSIA

  • Rule out alternative explanations
    • Ensure visual fields and visual acuity ok
    • Anomia: implies that stored representation of word has been lost; naming deficit is present regardless of sensory modality; recognizes meaning of object that cannot be named; in agnosia, object can readily be named thru different sensory modality
    • Aphasia: agnosic won’t demonstrate word-finding pxs in spontaneous speech and won’t be able to identify misnamed objects by circumlocution
    • Apraxia: can pt follow commands not requiring objects (e.g., wave, salute); can pt demonstrate object when not seen
    • Multimodal deficits are indicative of amnesia, dementia, or generalized impairments in semantic access
  • Determine which modality affected
  • Check apperception – matching, drawing, copying
  • Evaluate perception – figure/ground discrim, closure, synthetic ability, route finding
  • Evaluate visual memory: designs, objects, faces, colors
  • Evaluate associations: sorting, categorizing, pairing of similar objects, spontaneous use of object
  • Confrontation naming: for faces, ask male/female? Human/animal?
  • Test color perception