09/01/99 - Draft S. David Stoney, Ph.D.
Anosognosia: A Window on Embodied Human Consciousness
The reason anosognosia is a window on human consciousness is due to the peculiar fact that the patient's perception is incongruent with his or her actual circumstance. That is, the patient is conscious of a representation of his left arm, which he experiences as intact and movement capable. Reality is, of course, a paralyzed left arm and neither somatic sensory nor visual information of that reality is sufficient to alter his or her conscious perception of an intact arm. This means that the brain is capable, independent of sensory input, of generating an apparitional stand-in for the patient's left arm, a stand-in that the patient experiences as real and functional. I propose that this apparitional stand-in, this conception of an intact left arm that replaces the paralyzed arm for these patients, is nothing other than the left arm of the body scheme that the brain uses for planning goal-directed movement. The brain must be able to generate such a representation of the arm and, in fact, the ubiquitous occurrence of phantom limbs following amputation or denervation is ample evidence of this capability.
In fact, I shall go so far as to say that in the absence of plans for goal-directed movement there is no body scheme and no body to be perceived. This circumstance is a little like the slow degradation and ultimate disappearance of sight when the eye is immobilized. The rule is, "No movement, no perception." Thus, the primary defect in anosognosia, is the absence of a repertoire of movement for the left arm of the body scheme, which effectively eliminates the left arm from existence so far as conscious and unconscious awareness is concerned. Under these circumstances, Nature abhorring a vacuum and so forth, the left hemisphere does what it can and must, it "fills in" with an illusory conception of the arm. That filling in is an obligation of each hemisphere is quite clearly shown by studies of individuals who have undergone corpus callosotomy. Although each hemisphere is unaware of the ipsilateral world, neither is conscious of being incomplete.
Now it is tempting to jump at the idea that absence of somatic sensory input from the limb (i.e., loss of feelings such as tactile impressions, vague pressures, variations in temperature, a sense of fullness, and sense of position and movement) must be what leads to an imperception of the limb, the gap in the body scheme, and the loss of awareness of the limb, and the filling in by the left hemisphere. This idea is, however, nothing more than a restatement of the notion of inattention, and is not the case. In fact, in many anosognosics there is little or no disturbance of somatic sensation from the paralyzed limb. Touch localization and position sense can be intact in the presence of anosognosia for left hemiplegia. Thus, an anosognosic's conscious conception of his left arm is not determined by the absence of sensory input from the limb. This most important piece of information tells us that there is more to this situation than meets the eye.
What is going on then? Two facts seem clear:
1. The brain is able to generate a convincing illusion of the left arm that completely dominates consciousness.
2. Input from the left arm, even when available is not, by itself, assimilated to the conscious conception of the left arm.
This suggests that something else, another kind of interaction between self and the world is ordinarily necessary for the appropriate synchronization of sensory input and one's body scheme, and that, in anosognosics, this interaction is temporarily blocked due to right hemisphere dysfunction.
At this point, before proceeding, we must back track to establish an important point. It is customary in Western neuroscience to presume that sensory perception follows from sensory stimulation and sensory processing. According to this Standard Model of Perception (SMP), percepts are considered to be produced by sensory inputs that somehow encode stimulus features (as sensations), that are decoded, e.g., by synchronous action potential activity of neurons in distributed networks especially involving so-call association cortical regions, many of which we now know to be higher order sensory areas. Such decoding is considered be synonymous with conscious awareness. I and others are coming to believe that there are aspects of this view that are no longer tenable. Llinas, for example, suggests that the brain is a closed system, that neural inputs, instead of bringing a copy of the world to the brain, generate a copy of the world in the brain by virtue of activating neurons that already represent the world's features. A critical appraisal of neuronal electrophysiology at both ends of the neuraxis supports some such view. At the level of the cerebral cortex, the most damning evidence arguing against the SMP is that the firing frequency of neocortical neurons though to represent higher order stimulus features is just too low (usually less than 100 Hz, often around or less than 50 Hz, when they are supposed to be doing their neural processing) for them to be involved in any robust form of computation. Unless, that is, by 'computation' one means something entirely different than what computers do. At the input end of things, the occurrence of filtering action at axon branch points of primary afferent neurons, which greatly reduces the bandwidth of the neural signals that any neuron can actually expect to reach its axon terminals (Stoney, 1990), also makes neuronal computation unlikely. The time is ripe for a new conception of perception.
I propose that we take anosognosia seriously as a window to human consciousness. Let's accept for the sake of argument that anosognosia clearly shows that the SMP is untenable. It is after all, hard to deny that anosognosics have a conscious conception of their left arm that is independent of somatic sensory and visual input. If we take this road, where does it lead us?
1. Well, clearly we are capable of being aware of a representation of the world.
2. At any given moment, the electrical activity of distributed networks of neurons is the basis for our conscious awareness of self and world.
3. Normally the representation of the world is seamlessly synchronized with the objects of the world.
4. Anosognosics, however, seem to simultaneously entertain a representation of the left arm that is not synchronized with reality (an illusory conception) and a representation of the right arm that is.
5. This implies that in anosognosics the right hemisphere lesion has led to a dysfunction that prevents the synchronization of inputs from the left arm with the representation of the left arm in the body scheme.
6. What is lost due to the lesion in anosognosics is the capacity for a subconscious prehension between the person and his world. A prehension is a non-local interaction between the knower and the object of knowledge that unites them into actual occasion, a "droplet of experience" that has been called an instance of "noncognitive apprehension" (Whitehead).
7. Each actual occasion creates a standing wave in the implicate order?????
8. For an individual, an actual occasion is a kind of sympathetic resonance between him or herself and his or her nervous system and the world
9. Each hemisphere of most individuals is normally capable of handling prehensions for either side of the body and space. However, in a few individuals the left hemisphere, presumably in conjunction with the development of orality and literacy, sacrificed that part of its neural apparatus that used to be able to handle prehensions for the left side of the body. For these few individuals a RH stroke will temporarily prevent prehensions involving the left side of the body and space. This accounts for the rarity of anosognosia.
10. In the course of life, through the unceasing formations of prehensions we become part of world and the world becomes part of us. It is by virtue of this fact that our conscious conceptions are accurate representations of the world. Neurons, which no doubt are genetically limited with regard to the scope of their representational capacity, as suggested by Llinas, come to accurately represent the world without any computation at all.
11. How to explain that "spontaneous" recovery from anosognosia is common. First of all, this indicates that anosognosia is generally not due to the destruction of one or another specific set of neurons. What is suggested is that convergence at some site from widespread sites in the right hemisphere is necessary and that, as recovery occurs, some critical level of activity is achieved that again allows the right hemisphere to participate in prehensions. This site is sure to be closely associated with the limbic system and the medial prefrontal cortex.
Llinas, R. and D. Pare, The brain as a closed system modulated by the senses, In: The Brain-Mind Continuum, Llinas, R. and P. Churchland (Eds.), Cambridge, MA: The MIT Press, 1996.
Stoney, S. D. Limitations on impulse conduction at the branch point of afferent axons in frog dorsal root ganglion, Exp. Brain Res. 80:512-524, 1990.