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