[Submitted for presentation at the 92nd Annual Meeting of the Southern Society for Philosophy and Psychology, Atlanta, GA, April 20-23, 2000.]
Anosognosia for Hemiplegia: A Window on Embodied Human Consciousness, S. David Stoney, Dept. of Physiology and Endocrinology, Medical College of Georgia, Augusta, GA 30912
Challenges to classical theories of consciousness have lately been raised by various authors (e.g., Bohm, 1990; Stapp, 1993; Chalmers, 1996; Schwartz et al, 1999). During the same period indisputable evidence supporting the critical role of the nervous system in feelings, perception, cognition, and action has rapidly accumulated (Kandel et al, 1999). In this presentation I will show how a process philosophical approach to perception (Whitehead, 1925, 1929; see also Griffin, 1998) provides a coherent explanation of anosognosia for left hemiplegia and suggests a dual locus for embodied human consciousness.
In anosognosia following a right hemisphere stroke, one fails to acknowledge his or her left hemiplegia. 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 (Gerstmann, 1942; Schilder, 1950; Bisiach et al, 1992; Ramachandran et al, 1996; Ramachandran & Blakeslee, 1998; Damasio, 1999). 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 patients, 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.
Some authors claim that anosognosia results from Freudian denial, breakdown of a hypothetical "discrepancy detector," or inattention (Ramachandran & Blakeslee, 1998; Heilman & Valenstein, 1993). However, the evidence for such views is neither compelling nor able to account for many of anosognosia's peculiar features. A more plausible hypothesis is that anosognosia patients deny their left hemiplegia because they experience a convincing, involuntary illusion of an intact, movement-capable left arm. The fact that sensory input sufficient for awareness of the paralyzed limb fails to update the conscious body schema, described by Damasio (1999) as "nothing less than astounding," defies the sensationalist doctrine of perception and demands an alternative explanation. I suggest that the following processes account for anosognosia for left hemiplegia: 1) The brain lesion due to the stroke has impaired the mode of direct perception ("noncognitive apprehension" [Whitehead, 1925; Griffin, 1999] or "quantum entanglement" [Bohm, 1990; Stapp, 1999]) for the right hemisphere. This mode is necessary for the conscious body schema to be congruent with the actual body. 2) The left hemisphere is unable to perform this function (i.e., causally efficacious perception) for the ipsilateral limbs in these individuals (Parsons et al, 1998). The rarity of this neural arrangement is what makes anosognosia for left hemiplegia so rare. In most individuals the left hemisphere can do this task, so a right hemisphere stroke produces an acknowledged left hemiplegia. 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 presentational immediacy [Whitehead, 1925; Griffin, 1999]).
This perspective suggests that the locus of conscious sensory perception is ordinarily a constantly updated 'virtual body' that is, due to the phase of non-local, nonsensuous interaction, normally very congruent with the observable world, including the body. Damasio (1999) has recently described a "proto-body" and McGlynn & Schacter (1989) a "conscious awareness system" that share many features with the 'virtual body' that I propose. One difference between our points of view is that the virtual body appears to require a dual locus (Stapp, 1999), one embodied and one universal, thus it can potentially transcend corporeal boundaries. The embodied pole, however, seems to be identical with patterns of activity in a distributed population of sensorimotor neurons in the cerebral cortex.
In summary, anosognosia for left hemiplegia reveals what happens when a portion of the virtual body is no longer updated via noncognitive prehensions with the world. With the realization of consciousness' bimodal character, the brain comes to be seen primarily as a movement control device. An intriguing possibility is that the existence of the virtual body means that neither conscious perception nor action require neural computation.
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