11 Covert Cognitive Abilities of a Person with Altered Consciousness
Levy Rahmani
This chapter outlines a prospective, rehabilitation-oriented investigation at the bedside of possible covert cognitive abilities of a patient in a vegetative state. This type of investigation is based on:
• Accumulating knowledge about partially conscious and controlled visual-perceptual responses induced experimentally in normal subjects.
• Extensive observations of restricted and implicit manifestations of memory registration and retrieval in patients with specific cognitive-behavioral deficiencies following restricted brain lesions who maintain a general state of consciousness.
• Recent advances in the functional study of brain structures that do not belong to the neocortex: the basal ganglia, cerebellum, and medial frontal structures.
We shall conduct the argument in the form of an imaginary dialog.
Question: Is it not strange to use an abstract term such as “loss of consciousness” for a medical diagnosis? Indeed, is it satisfactory to conclude the examination of a person who has suddenly, and in the most dramatic way, lost any manifest functional ability, with the statement that he or she has lost consciousness? Can this serve as a basis for the initiation of treatment? Consciousness is a psychological term. What is the meaning of its “loss” for therapy?
Answer: The concepts of consciousness, on the one hand, and coma and vegetative state (alterations of consciousness) on the other, are indeed very complex. Giacino wrote:
Disorders of consciousness remain enigmatic and continue to present some of the most intriguing challenges faced by neurologists and rehabilitation specialists in clinical practice. Of particular interest are those states characterized by severe alterations in consciousness such as coma and the vegetative state. Remarkably little is known about the relationship between clinical features associated with these disorders and the corresponding pathophysiologic substrates underlying them. In addition, little attention has been paid to investigating the natural evolution of disorders of consciousness, although this seems a logical and necessary first step toward establishing greater diagnostic accuracy, better prognostic specificity and more effective treatment interventions [1].
No wonder that neuroscientists have avoided involvement in the field of consciousness. Searle [2] was puzzled by the resistance of neuroscientists to tackling the problem of consciousness. To him, “one of the chief functions of the brain is to cause and sustain conscious states. Studying the brain without studying consciousness would be like studying the stomach without studying digestion, or studying genetics without studying the inheritance of traits.”
Q: Has it been possible to briefly yet meaningfully characterize a state of altered consciousness such as a vegetative state?
A: Persons with an altered consciousness resulting from extensive brain damage would pass through a sequence of phases from full loss of consciousness to a state of a mild disorder. These phases are still barely definable, and the characterization and labeling of them differ from one expert to another. Beyond the divergences that exist, the picture emerging is one of degraded or reduced input or reception of sensory to communicative stimulation; abnormal meaningfulness, lying between loss and distortion; and abnormal responsiveness, lying between disconnection from the input and minimal, undifferentiated responses to intensive stimulation and ranging from unawake-fulness to arousal deprived of awareness of environmental events. The neocortical structures known to be prevalently involved in higher cognitive performances, such as predominantly conscious perceptual experiences, explicit and voluntary recall of information, controlled executive functions, are the most compromised by this damage. Recent years have documented previously obscure cognitive abilities of supplementary cortical and subcortical structures – notably the basal ganglia and the cerebellum. It is increasingly appearing that emotions play a modulating role in memory and learning. The amygdala has quite extensive connections with cortical structures.
Almost as a matter of principle, the effect of a behaviorally relevant environmental stimulus can be enhanced by associating it with emotions believed to be deeply rooted. It is worth mentioning here a point made recently in favor of “overlearning”: “The shift between being nonconscious and being conscious during information processing depends upon the number of cells (and, consequently the volume of brain tissue) that participate in the underlying processes and constitute the same macro-networks, or the same networks with additional recruited neuronal populations” [3].
Q: Is it right to claim metaphorically that the traumatic brain injury which results so frequently in states of altered consciousness offers the opportunity to brain structures that do not belong to the elite neocortex to shop up and take the rostrum?
A: It is true that propensities of such brain structure are more readily demonstrable under particular circumstances. Investigators of normal behavioral capacities endeavor to design particular experimental conditions to elicit properties that are otherwise obscure. This may occur in the case of brain injury. Covert participation of such structures in normal functioning, masked by the predominant manifestation of high-level structures, becomes known at the patient’s bedside. The stimulation and promotion of these structures might be considered in a prospective tentative intervention program for patients with an altered consciousness. Limitations resulting from extensive damage could be regarded as an extreme instance of cognitive reduction, along a continuum of performances from particular induced moments of reduced awareness during a general normal performance, through modular deficits resulting from localized lesions. The management of a patient in a vegetative state requires a measurable assessment of as yet undisclosed mental life, notably the characteristics of cognitive residuals.
Q: Are there currently any specific, documented pieces of information about the connection between neural structures known to be involved in some cognitive or motor function and structures that have a role in emotion?
A: One of the most recent findings about this type of integration regards the functioning of the medial frontal lobe, notably the anterior cingulate cortex (ACC): “Conceptually, the ACC is a prime example of a brain structure in which a regulatory network, composed of cells from the modulatory brain-stem nuclei, interacts with an executive network …” [4]. It is believed that the ACC has a triple role in behavioral control. It is involved in motor control due to its connections with both the motor cortex and the spinal cord. Patients with medial frontal lobe lesions involving the ACC are described as showing deficits in spontaneous initiation of movement and speech. Akinetic mutism, which is caused by bilateral lesions, is an extreme example. In addition, it is thought to have a role in cognition through the reciprocal corticocortical connections with the lateral prefrontal cortex. Last but not least, there is strong evidence for a role of the ACC in arousal. The extensive afferents from the thalamus and the brain stem contribute to the ACC’s involvement in an arousal drive state. In this context, the intriguing idea has come up that “different cognitive-affective events may lead to a transient synchronization of neural activity in the ACC.”
Q: Are we already in a position to point clearly to phases of transition from one degree of altered consciousness to a milder degree?
A: We have to distinguish between an appreciation of the patient’s emergence from unconsciousness from the neurological and neurorehabilitation perspective and the subsequent neurobehavioral or neuropsychological perspective. Wide agreement on the gradation at the first level has been achieved with the Glasgow Coma Scale. A patient’s gradation on the 15-point scale is mentioned in his chart. Essentially, this is a scale of the patient’s responsiveness to stimulation. At a higher level of assessment, many different terms have been used to describe the constituent brain activities that support consciousness; however, the application of terminology has not been consistent. The terms “arousal,” “orienting,” “alertness,” and “wakefulness” have been used interchangeably to refer to the continuum between sleep and the awake state. At the diagnostic level, any effort to group individual signs of coma into syndromes associated with severe alterations of consciousness faces difficulties due to the lack of clinical methods for assessing consciousness, particularly self-awareness.
Q: Do the prospects for further investigating the mind of the person in a vegetative state appear gloomier now, in view of the difficulty of analyzing and grading the manifestations of the alteration in consciousness in the “hard” terms of neuropathology and clinical neurology? Returning to the concept of consciousness as a mental state or process, why shouldn’t we be able to extrapolate from the way in which definite cognitive processes are defined? After all, perception, memory, thinking, and attention, are components of consciousness, as is language, and these have quite clear and accepted definitions. For instance, visual perception seems to be widely defined as a process of recognizing the features of things in the environment – their color, shape, size, position in space, and so on – by vision, as a sensory system. We recognize sounds using another sensory system. We are perfectly well aware that as normal individuals we use a definite and limited number of sensory systems. We even take it for granted that the brain operates in such a way that there are different regions separately involved for each type of sensory information. We can somehow easily define memory as our ability to register information, store it for short or long periods of time, and retrieve it when needed. These may not be simple processes at all, particularly when it involves recognizing objects, pictures, scenes, or remembering life episodes distinctly from fragments of knowledge. However, even laymen use understandable definitions of perception and memory. We even have a reasonably comprehensible definition for “thinking.” Can this knowledge and the way in which it has been reached not be extended to the domain of consciousness? Is consciousness so clearly distinct, due to being an integrated entity, that an extrapolation of this type is not possible?
A: Now we are getting to the core of the matter! Firstly, we should be aware of the complexity of the processes of perception, memory, and thinking themselves. For instance, the visual recognition of objects, as well as naming them, involves not only the brain region that specifically receives the visual information – the so-called “visual cortex.” It is believed that about 60 % of the cerebral cortex is involved, although obviously not solely, in visual perception. The definition of perception given in the question above is a simplified one. For instance, a fuller definition was offered in the MIT Encyclopedia of Cognitive Science (1999): “Perception reflects the ability to derive meaning from sensory experience, in the form of information about structure and causality in the perceiver’s environment, of the sort necessary to guide behavior” [5]. In addition, most contemporary theories of vision describe object recognition in terms of a comparison between the input image and stored models that represent known objects [6]. Finally, there is a well-documented influence of the environment on an observer’s memory for things. Memory is the name given to the ability of living organisms to acquire, retain, and use information or knowledge. The term is closely related to “learning.” The brain substrate for memory is a topic of vivid debate. A central issue is that of the “consolidation” of memory in the brain [7]. The point is thus that it is not only consciousness that is difficult to define; one should be aware that the extensive knowledge available about perception and memory – to mention only two of the more specific entities for which there are widely used “handy” or convenient definitions – has been obtained only thanks to extensive studies in both normal and pathological conditions.
Q: Now there is a bit of confusion. It turns out that everything in the human mind is complex and that when something happens to the brain, the distortion of any one single aspect is hard to grasp. Why is consciousness in particular said to be “enigmatic,” “mysterious,” eluding any attempt to capture the cerebral substrate for it? Is this because it is a monolithic entity that cannot be broken down into smaller units? Generally speaking, any entity that cannot be broken down for analysis is more difficult to comprehend. If there is no way of parceling consciousness, are there any ways of grading it, particularly with regard to the way it is transformed into more familiar entities, even though these are complex in themselves as well? Perception, thinking, memory, and attention are largely conscious activities. Their complexity is closely linked to their being part of human consciousness. Yet it seems that we are pretty advanced in investigating and understanding these. The argument boils down to the following question: since specific cognitive processes are components of consciousness – and indeed form the core of consciousness, its contents – while at the same time their structure is more accessible for analysis and grading, might this avenue of systematic clinical observation and investigation lead to better insights into coma and its gradations, before we obtain better neurological and neuropathological information?
A: This could indeed be a prospective path of investigation – the specific cognitive elements of the vegetative state. These should be looked at from the perspective of their manifestations in a normal person. We can take the example of visual perception. There is recent experimental evidence that visual perception takes a variety of forms, some of which may not be under voluntary conscious control. This depends on the conditions of presentation of the visual displays being recognized. Neuroimaging studies of visual awareness in normal subjects have shown that there at least two distinct aspects of perceptual experience: “First, the neural correlates of those mechanisms responsible for maintaining a particular level of awareness, and second, the neural correlates of the specific contents of consciousness” [8]. These functional magnetic resonance imaging studies have been able to demonstrate the different role of two mechanisms in the cerebral cortex, beyond the traditional view about the role of visual cortex, with the occipital lobe situated in the posterior part of the cortex. The studies distinguished between the neural bases of conscious experience and unconscious perception and behavior. As Rees puts it, “We all have first-hand knowledge of what it is like to be conscious, as opposed to not being conscious (e. g. dreamless sleep). Similarly, when we are conscious we experience something. Our conscious state may change, but at any moment our experience is distinguished by its phenomenal content. When phenomenal content is absent, then consciousness is (usually) absent” [8].
Q: At each and every moment that we are awake, we see, recognize, and are aware of things that are present in our environment. This seems a very natural and elementary fact. Is this an inseparable element of consciousness that is bound to disappear in a person in vegetative state? Much has been written about a lowering of this capacity in a variety of modes in patients with circumscribed brain lesions. These persons did not lose consciousness, or their consciousness was not altogether altered. What does the content of their visual-perceptual experiences consist of?
A: We now have a great deal of information about perception in general, and in particular about visual perception in normal individuals and in those who have damage in part of the brain, in one of the two halves (hemispheres) of the cerebral cortex, or in part of a hemisphere. This knowledge is expected to provide the starting-point for studies leading to better insights into possible residual perceptual capacities in a comatose patient. A few relevant points are detailed below.
It is generally believed that the processing of visual patterning information follows a hierarchical structure, proceeding along multiple parallel pathways in the brain. Recoding usually begins in the retina and proceeds through the lateral geniculate nucleus to primary visual cortex. Cortical processing is thought to begin with an elementary representation of local spatial frequency, orientation, and contrast, presumably represented by responses of simple ells in primary visual cortex (V1). Much less is known about how the early neural representations are transformed at higher levels to signal information needed to perform tasks on complex, real-world objects [9].
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