extracts from 'From sensation to cognition' M.-Marsel Mesulam

From: Wade T. Smith (wade.t.smith@verizon.net)
Date: Wed 09 Jul 2003 - 18:36:01 GMT

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     From sensation to cognition M.-Marsel Mesulam

    The Cognitive Neurology and Alzheimer’s Disease Center, Departments of Neurology and Psychiatry and Behavioral Sciences, Northwestern University Medical School, Chicago, USA

    Correspondence to: M. Mesulam, Cognitive Neurology and Alzheimer’s Disease Center, Northwestern University Medical School, 320 East Superior Street, 11–450, Chicago, IL 60611, USA. E-mail: mmesulam@nwu.edu


    The neural substrate of mental relativism would appear to be based on the ability to establish multiple representations of the same fact, and to realize that such representations (for example, the reflection of the self in the mirror) constitute alternative manifestations of the same basic phenomenon. This type of neural computation is not automatic. A male turtle, for example, will fight its own reflection in the mirror from dawn to dusk; only specially trained monkeys give any sign of rudimentary self-recognition in front of a mirror; and many demented patients will react to their reflections as if they were intruders (Ajuriaguerra et al., 1963; Harless, 1979; Gallup et al., 1980; Hauser et al., 1995).

    A tolerance for multiple alternative representations may provide the critical ingredient that sets the special flavour of human consciousness. It is reasonable to assume that animals have a relatively simple sort of consciousness, the content of which is closely determined by the here-and-now of immediate needs and sensations. A more complex form of consciousness would be expected to emerge if some critical mass of neurons, freed from the household chores of sensation and action, could afford to form alternative and annotated representations of ambient events.13 One consequence of this process could be the emergence of an observing self who becomes differentiated from the sensory flux and who can therefore intentionally comment (introspect) on experience.14 Such a capacity for introspection and intentionality may have generated first the sense of a ‘commenting self’ separate from the experiencing body, then the belief that others also have commenting selves, and, ultimately, that these other commenting selves believe that others also have commenting selves.

    (Preprogrammed behaviours such as the broken wing display of a plover trying to lure predators away from her offspring do not fit the description of voluntary shifts of perspective, although they do depend, in an automatic sort of way, on assumptions concerning the point of view of the predator.)

    These additional dimensions of human consciousness are likely to have created the driving force for the development of symbolic communications, including language. Thought, defined as the cerebral activity that intervenes between sensation and action, is likely to arise in all experiencing organisms of sufficient CNS complexity, and may therefore be said to exist in many animal species. Animals can also communicate, but in a concrete and reflexive fashion where a specific set of emotive states or external events triggers a specific call or gesture. The development of a communication system based on consensually sanctioned arbitrary labels such as words, however, would appear to require not only the ability to transcend concrete stimulus–response linkages, but also the presence of an intentional, observing self and the belief in the existence of other self-conscious individuals who will be able to decipher the message. Language, in this sequence of events, becomes a consequence of thought, not its cause. As Vygotsky (1962) has said, a thought may be compared to a cloud shedding a shower of words.

    (The role of prefrontal cortex in this development needs to be given considerable prominence. It is probably the only part of the brain that makes no essential contribution to routine sensory, skeletomotor or autonomic function. It is also the part of the brain that shows the greatest expansion in the primate line of evolution. The prefrontal cortex could be conceptualized as a luxury dividend that phylogeny offered primates and that catalysed the ability to transcend a stimulus-bound existence.)

    The development of language is likely to have shaped the course of cognitive evolution by enabling the emergence of a uniquely human phenomenon known as civilization. Frogs may have consciousness, brilliant apes may display occasional bursts of symbolic communication, but only humans build civilizations. The impact of this distinction becomes particularly significant within an evolutionary context. Humans and their closest cousins, apes, parted ways ~5 million years ago. During the subsequent 4 million years evolutionary pressures led to expansion of cranial volume, the assumption of an erect posture, and the descent of the larynx into a position that enabled a richer vocal output (Donald, 1991).

    In contrast to this relatively brisk pace of biological evolution, the pace of cognitive change during these 4 million years was painfully slow and seems to have been confined predominantly to the transition from the use of accidentally sharpened stones by Australopithecus to the use of deliberately sharpened ones by Homo erectus. Modern humans entered the scene only 50 000 to 100 000 years ago. During the short interval since then, our species has leaped from caves to skyscrapers, from the drawings of Lascaux to the mosaics of Pompei, and from cannibalism to nouvelle cuisine.15

    Brain structure, however, has probably shown no substantial change during this interval. How can brain function change so radically when brain structure has remained relatively stable?

    At least part of the answer lies in the transition of the bottleneck from individual to group cognition through the mediation of language and civilization. Civilization depends on two major ingredients: adaptability to rapid change, and the ability to transfer knowledge of procedures and relationships from one individual to another and from one generation to the next.

    The former ingredient, the ability to tolerate change, is a luxury that only the most advanced brains can afford. It might take a very long time, for example, to convince a frog to change its eating habits. In contrast, adaptability to change, in fact an overwhelming urge to seek novelty and alternative vantage points, is a major characteristic of the human CNS and the principal determinant of the cognitive relativism described above. The second ingredient of civilization, the inter-individual transfer of knowledge, is a by-product of the capacity to communicate complex and abstract relationships through the mediation of language. Such a transfer of knowledge would have fostered the development of civilization by liberating each individual member of a social group from the need to re-invent the wheel and by promoting a gradual transgenerational accumulation of knowledge. A pivotal outcome of this process would have been to shift the pacemaker for cognitive advance from the painfully slow process of physical brain evolution to the far more rapid and dynamic pace of computational evolution where each individual intelligence could potentially become a constituent link in a massively parallel network. In preliterate societies, the contribution of each link to this process was undoubtedly limited by the synaptic capacity and lifespan of individual biological memories. The invention of writing is likely to have overcome this limitation by introducing a new and infinitely expandable, extracorporeal memory.

    (Cultural evolution has obviously not been uniformly positive. The same neural template that enabled the serenity of Ryoanji has also enabled Auschwitz. It goes without saying that purpose in biology is constrained by survival, not by value. The neural connections in Fig. 2D enable the human mind to transcend stimulus–response bonds but do not specify the contents or consequences of the resultant activities.)

    (A rudimentary and stimulus-bound form of inter-individual transfer of information can be discerned in other species in the form of
    ‘observational learning’ but its scope is very limited, usually confined to consummatory behaviours.)

    Through the advent of writing, progress would no longer be contingent on a few wise men with phenomenal memories and communicative skills. Even those of modest talent could have access to vast libraries containing the distilled wisdom of all humanity. The development of writing would thus have triggered the second and even steeper acceleration of cognitive evolution. The hypothetical sequence of events outlined above is based on the following milestones: the horizon and complexity of consciousness became augmented by the spatio-temporal dilation of the pathways that lead from sensation to cognition; the enrichment of consciousness and the resultant ability to step back from real-time events enabled the emergence of intentionality and introspective commentary; the realization that other members of the species were similarly disposed encouraged symbolic communication; the resultant transgenerational accumulation of knowledge led to the establishment of culture and civilization; civilizations then became the vehicle for further accelerations in the pace of change through additional inventions such as writing. Through these steps, Homo sapiens has been able to make giant cognitive leaps without the need for parallel changes in the biological hardware of the brain. Clearly, this sequence of events represents a great oversimplification. Many of these developments could have proceeded simultaneously and numerous intermediate steps could have existed. The purpose in this account, however, has been to show that neurological scenarios based on the template in Fig. 2D could become relevant to the exploration of even the most complex achievements of the human mind, in principle if not in detail.

    (Writing was apparently invented by a Sumerian king, some 5000 years ago, who was loath to fire his loyal but forgetful messenger.)

    In the frog, sensation becomes cognition at the first synaptic level. The horizons of consciousness are defined by a rigidly filtered sensory reflection of the immediate present. In the human, sensation merely begins to kindle the creative processes of the mind, and the present is only one point along a continuum of consciousness that extends from the dim past to the distant future. In the eyes of evolution, the frog has a much more extensive record of credibility. However, its future is completely constrained by its past. The human brain has followed a different kind of adaptation based on a continuous pressure to seek and create novelty and change. The resultant texture of human existence, based on the flexibility afforded by the neural pathways that link sensation to cognition, is immensely rich but also potentially quite fragile since not all that is novel is necessarily good. The future of the human race, while almost unlimited in its potential, is therefore also more uncertain than that of the frog.

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