Anosognosia for hemiplegia: A Window on the Bimodal Nature of Embodied
S. David Stoney, Dept. of Physiology, Medical College of Georgia, Augusta, GA 30912,
In anosognosia following a right hemisphere stroke, one fails to acknowledge his or her left hemiplegia. Anosognosia appears to be one of a family of body schema disorders. It is usually associated with neglect of the left side of the body/world, can be temporarily ameliorated by vestibular stimulation that causes rightward nystagmus, and often resolves spontaneously. A patient with anosognosia may try to use the paralyzed left arm for tasks such as clapping, picking up a tray, or pointing to the examiner's nose. When asked whether they accomplished the task, they respond "Yes, didn't you hear me," or "Don't you see my hand there in front of your nose." When pressured about their failure to perform they confabulate weak excuses: "You know I have arthritis." Some, when confronted with their paralyzed limbs, deny ownership, claiming the limb belongs to the examiner or to a corpse that someone placed in their bed. In this presentation, I will describe anosognosia and show how it can be explained in terms of a bimodal model of perception based on Alfred North Whitehead's philosophy of organism.
Although some authors claim that anosognosia results from Freudian denial, there is no compelling evidence for such a view. Instead, it appears that the occurrence of convincing illusions of intactness and movement of the paralyzed limb are what accounts for this peculiar disorder. In fact, the failure of anosognosia patients to become aware of their paralyzed limb upon confrontation defies the sensationalist doctrine of perception and demands an alternative explanation. I suggest that the following processes account for anosognosia for left hemiplegia: 1) It only occurs in those rare individuals for whom the right hemisphere has become the sole means of updating the left body schema and left world. 2) For these individuals, a right hemisphere lesion blocks leftward perception in the mode of causal efficacy ("noncognitive apprehension"). 3) The left hemisphere then "fills in" an intact left arm so as to provide a complete, but illusory body schema (perception in the mode of presentation immediacy).
This perspective suggests that conscious sensory perception is ordinarily "tacked onto" a brain-dependent, constantly updated virtual body that is, due to a prior phase of non-local, nonsensuous interaction, normally perfectly congruent with the observable world, including the body. The virtual body has a dual locus, one embodied and one universal, thus it routinely transcends corporeal boundaries. The embodied locus, however, seems to be identical with patterns of activity in a distributed population of motorsensory neurons. Anosognosia reveals what happens when a portion of that virtual body is no longer updated via noncognitive prehensions with self and world. With bimodal consciousness, the brain comes to be seen primarily as a movement control device and, due to the existence of the virtual body, neither perception nor action require neural computation. Quantum computation is, however, required.
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1. What body/brain/mind mechanisms account for the peculiar behavior and beliefs of anosognosic patients?
2. What, if anything, does anosognosia reveal to us about human consciousness.
B. The Argument
1. In its most robust form, anosognosia involves both a loss of function (paralysis and neglect of left side) and a vivid hallucination of an intact left arm ("denial").
2. A right hemisphere injury plus a left hemisphere incapacity to specify leftward affordances account for the neglect of left side.
3. The left hemisphere's inability to specify leftward affordances arises due to its inability to make use of direct, non-sensory perception involving the left side.
4. Since neither right nor left hemispheric mechanisms seem able to account for the hallucinated left arm, a non-classical source of information may be involved.
C. Conclusion: The analysis implies that brain is first and foremost a movement control component of a dynamical action system that includes body/brain/mind in the world. Embodied human consciousness has, so to speak, one foot in the brain and one foot in the universe.
III. Dealing with Preliminary Issues. Anosognosia is inexplicable in terms of the sensationalist doctrine. A process philosophy perspective (Whitehead, 1925, 1929; Griffin et al, 1993; Griffin, 1998) is required (references are to Griffin et al, 1993):
A. Abandon the idea of bits of matter with simple location in otherwise empty space as being the fundamental stuff of the universe.
B. Instead, adopt the view that experiential events are fundamental:
1. Every experiential event is a prehension, a unification of subject and object accompanied by a sympathetic experience of feeling. The figure above represents such an event for a high grade, compound organism such as a human being.
2. "A present event, which is a subject, sympathetically prehends an antecedent event, which is a subject-that-has-become-an-object." (G, 210) In this way the "the world of (actual) objects is the world of past subjects." (G, 175)
3. Experiential events, also know as actual occasions or organisms, range from quite simple, low-grade types such as subatomic particles, atoms, macromolecules, cells, etc. to highly complex, high-grade, compound organisms, such as human beings.
4. Compound individuals are societies of experiential events, sharing values.
5. Mind is conceived of as the dominant member of such a society. Thus conceived, mind "occupies the entire spatial region of the society, overlapping the regions of the lesser members" (G, 206). As locus of consciousness, mind forms a virtual body (body schema) that is coincident with the physical body, forming an agent/subject.
6. Taking account of its world, the agent/subject decides on one course of action from among the possibilities, it becomes what it becomes, rather than becoming something else. And so on... (c.f., Stapp, 1993, 1999; Bohm, 1990)
7. Embodied human conscious awareness is "not a continuous 'stream' of experience which is simply the same through time, but is instead a temporal 'society' of discrete occasions of experience, each of which is a distinct actuality." (G, 209)
8. The phenomenal continuity of experience reveals that memory is nothing more than the prehension, by my present experience, of "some of my previous occasions of experience." (G, 209)
C. Recognition that perception of the world requires two modes of interaction is critical for understanding anosognosia.
1. Direct non-sensory, perception that involves an information-rich, sympathetic sharing of feeling among organisms. This mode of perception is mostly non-conscious and logically prior.
2. Sensory system dominated perception that provides an information-poor, feeling poor, but articulated knowledge of the world. This mode of perception is mostly conscious and is the terminus of the act of perception.
D. The priority of direct perception explains how we are able to know an actual world.
E. The on-going prehensive unification of the feeling states of the events that constitute one's dominant occasion (mind) explains why perception is via the virtual body (body schema).
F. Neurons themselves, as organisms in prehension with other cells of the body, are seen as conduits for feeling.
G. Rather than simply capturing information from the environment, non-sensory, direct perception contributes to the formation of dynamic prehensions that include the object in the world, the neurons that specify it, and the act of knowing.
H. Sensory systems of neurons are tuned to respond to affordances: opportunities for action in accord with one's intentions. Perception is constitutively predicated on action, real or intended (e.g., Hurley, 1998; Turvey & Shaw, 1999).
I. The primary purpose of the nervous system, around which all its activities are shaped, is to allow real-time action in the animal's niche (umwelt), its affordance space in the world. The mind-brain-body-world dynamic system, a system in a state of perpetual becoming, nevertheless generates a subjective experience of self being in the world.
IV. The heart of the argument.
A. What accounts for lack of awareness of the left side?
1. Injury to the right hemisphere caused by the stroke.
2. An incapacity in the left hemisphere. This is shown by the failure of such patients to recognize and admit ownership of their own left arm even when sensation remains for the arm.
B. The sensationalist doctrine denied. The inability of sensory input to update the conscious awareness of the intact left hemisphere is described by Damasio (1999) as "nothing less than astounding." Bisiach (1997) describes the situation as a "cognitively irremediable lack of awareness."
C. What accounts for the patient's hallucination of an intact arm? Conscious awareness is littered with "fills." In this case, the right hemisphere can't be generating the hallucination because it is injured. However, the left hemisphere is not able to even perceive the left arm or world, so this means that it cannot be the source of the hallucination. This suggests that the hallucination originates at the level of mind.
D. Why is the left hemisphere unable to provide for an updated perception of the patient's paralyzed left arm, even when the input is provided to it. The Figure above below shows a first approximation of the answer:
1. In the brains of animals without language (A), each hemisphere is equally capable of forming prehensions with any part of the world via perception in the mode of causal efficacy.
2. In languaged brains (B), the capacity for perception in the mode of causal efficacy by the left hemisphere is diminished due to recruitment of that capability for language function. These left hemispheres have difficulty interacting in the direct mode with the ipsilateral limb (LeDoux et al, 1977; Parsons et al, 1998).
3. In anosognosic brains (C), the left hemispheres are completely unable to interact in the direct mode with the left arm or left world. Because of the left hemisphere's incapacity to generate embodied percepts and concepts for the left arm, the individual's perceptual world is absent a left arm and left side, and this accounts for neglect.
V. Summary Figure. A right hemisphere lesion disrupts the capacity of the brain to enter into prehensive relations either with the left side of the body or in support of leftward affordances, i.e., opportunities for leftward action. Undoubtedly, the orbitofrontal monitoring areas (Luu et al, 2000) for the affective dimensions of leftward self are inactivated. This account of anosognosia, which implicates left hemisphere specialization for language function in a contingent role, seems to be compatible with the recent findings by Meador et al (2000) that the great majority (70 - 90%) of right handed individuals undergoing the Wada Test developed symptoms of anosognosia when the right hemisphere was transiently incapacitated.
A. Anosognosia for left hemiplegia is explicable as a disorder of right hemisphere neural processing in conjunction with an inability of the left hemisphere to specify affordances for the left body/world.
B. The left hemisphere's inability to specify affordances for the left body/world stems from its having utilized part of its capacity for non-sensory, direct perception in the mode of causal efficacy for language function.
C. The inability of the left hemisphere to process information about the left body/world implicates the critical role for intention and action in perception and defies the sensationalist doctrine.
D. The anosognosic's hallucination of an intact left arm seems to require an updating of the virtual body (body schema) from a non-traditional source (e.g., see Schwartz & Russek, 1999; Sheldrake, 1995).
E. The analysis implies that brain is first & foremost a movement control component of a dynamical action system that includes body/brain/mind in the world. Embodied human consciousness has, so to speak, one foot in the brain and one foot in the world (universe).
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