How
the BRAIN Works
Paul MacLean
(1973) conceived of the brain as being three brains
in one, each with a different phylogenetic history, each with its own
intelligence, its own special memory, its own sense of time and space, and its
own motor functions.
Jim Henry
argued that the dominant left cerebral hemisphere is a
fourth, phylogenetically most recent, system peculiar to our species (Henry
& Stephens, 1977).
Based on these two theories, Stevens & Price
(2000) propose a model of human brain evolution. They argue the brain
evolved in four stages, leaving our brain with four distinct regions: the
reptilian, paleo-mammalian and neo-cortex, which reflect their ancestral roots
in terms of the types of behaviours they contribute to.
The model
resonates with the models that Freud, Jung and Plato espoused, in perceiving the mind as
possessing separate functional components, which compete with each other for
overall control of behaviour. The paradigm overlaps with Evolutionary Psychology's Modularism
and Edelman & Tononi's concept of degeneracy
in their neuroscientific model.
The model instructs evolutionary
theories of adaptive
& maladaptive depression & PTSD.
Stevens
& Price (2000)
propose four
evolutionary developmental stages of the human brain:
1. Reptilian
brain
Basal ganglia, olfacto-striatum and corpus striatum.
The most primitive cerebral component, containing algorithms vital to
maintaining life. The behavioural responses at this level are largely
governed by instinct, such as defence, dominance striving, agonistic
threat displays and mating.
2. Limbic (paleo-mammalian brain)
Subcortical structures comprising the limbic system, including the hippocampus, hypothalamus, thalamus & pituitary gland.
It controls self homeostasis, emotion, hunger, sexual desire, sleep
and memory.
MacLean draws attention to three forms of behaviour that
distinguishes the evolutionary transition from reptiles to mammals: nursing
& maternal care, audio-vocal communication between mother-offspring, and
play. MacLean suggests that the separation call, which served to maintain
mother-offspring proximity, evolved to maintain contact between members of a
group, and that play evolved as a means to promote group harmony &
affiliation.
3. Neocortex (neo-mammalian brain)
Responsible for cognition & sophisticated perceptual processes.
Behaviour originating in the neocortex is often described as voluntary and
rational, reflecting its intellectual nature.
4. Lateralisation
The lateralisation of function between the two hemispheres, with a left
dominant hemisphere responsible for language and speech.
This is a "global brain theory" that takes the position that the brain is a
Darwinian system following the principles of Natural Selection,
whose "rich" functioning is the result of its enormous variability
& individuality (Edelman
& Tononi, 2000).
The evolutionary model proposed by Edelman & Tononi shows how different proximal &
distal neural maps connect & interact with each other, producing a holistic organismic
"self" & unitary responses to the environment. These neural maps are reminiscent of the mental modules
of Evolutionary Psychology and the "central processing assemblies" of the
Triune brain.
Edelman & Tononi argue that
Darwin's "population principle" explains: (1) Natural
selection : that variations among individuals of a species that leads
eventually to the origin of other species due to differential reproduction of
those individuals with higher "fitness", and (2) Somatic selection
: selection & variation occurring in the animal's cellular systems within
an individual's body during the individual's lifespan.
The first two tenets of the Theory of Neuronal Group Selection provide the basis
for the great variability &
differentiation of distributed neuronal states that accompany consciousness
(and the states & disorders that
result from loss of differentiation). The
third tenet allows for the integration of those states.
Tenet 1: Developmental selection : The formation during brain
development of a a highly diverse, variant set of neuronal circuits, arising
from genetic constraints & somatic selection. An
important property is that "neurons that fire together wire together"
(neurons strengthen & weaken their connections according to individual
patterns of electrical activity).
Tenet 2: Experiential selection : Synaptic selection resulting
from experience changing the strength of neural
circuits, refining brain maps into internal
representations of the environment.
Tenet 3: Re-entry : This is the process
of neuronal
integration that permits coordination in space & time of brain maps,
through ongoing signalling across reciprocal connections.
Re-entry is a dynamic process
that is the central mechanism by which the spatiotemporal coordination of
diverse sensory and motor events occurs. It takes place through the
correlation of selective events across the maps of the
brain, leading to
synchronisation of the activity in the different neuronal
maps, binding them
into circuits capable of temporally coherent output. This process is made
possible by the massively parallel reciprocal connectivity of brain areas by the
brain's thalamo-cortical
system. An example of a neuronal map is the internal
representation of environmental space in the mammalian hippocampus.
Mice have particularly well-developed hippocampi, comparable to that of humans,
although mice are particularly good at memorising space but not different
humans.
Re-entry functions to: (1) assist the
special
brain centres to categorise sensory input (2) mediate the synthesis of
brain functions by connecting sub-modalities (3) resolve conflicts among
competing neural signals (4) make local synaptic changes context-dependent
(changes in the efficacy of synapses in one area are affected by activation
patterns of distant areas) (5) it is the main mechanism for neuronal
integration,
by assuring spatiotemporal correlation of neuronal firing.
Edelman & Tononi contrast the selectional nature
of this system, as opposed to the instructional nature of computer
networks. They argue against the notion that the brain operates according
to an unambiguous set of algorithms or instructions like a computer. They
propose that "re-entry organisation is what uniquely
characterises & differentiates higher brains from all other known objects
& systems." (Edelman
& Tononi, 2000)
Loss of integration results in
distinct neuropsychiatric disorders
Other features of their theory include:
Degeneracy : the property whereby structurally different
components yield similar results (multiple paths lead to the same output).
They relate this property to the thalamocortical
system such that a large number of different neuronal groups can similarly
affect the output of a given subset of neurones (a large number of brain
circuits can lead to the same motor output). They propose that degeneracy is a
consequence of natural selection and results in increased robustness & adaptability
of biological networks.
Value
system : constraints are provided by this family of phenotypic neuronal circuits that have been
selected over evolutionary time. Each of the neural circuits modulate synaptic
function in widespread regions throughout the brain. These systems provide "constraints",
providing a basis for the development & refinement of brain-based
categorisation & action within a species.
Neuroscientist Joseph
LeDoux perceives the brain to be made up of mental
modules. His research focuses on the brain's emotional system,
specifically the organisation of the fear module of the brain.
In his seminal work The Emotional
Brain, LeDoux described his research on the limbic system. He comments on the general nature of emotions: (1) The term
"emotion" is a label referring not to something the mind or brain has
or does but to the feelings that result from activating specific neural systems
that have evolved to solve different problems that animals face. (2) There
is no single emotional system. The system we use to defend against danger
is different from the one we use in procreation. The brain systems that
generate emotional behaviours (i.e. mental emotional modules) do so to
assist to fulfil their biological imperative to pass their genes to their
offspring. Each neural system accomplishes its behavioural
goals, such as obtaining food & shelter, protection from bodily harm,
& procreation. (3) Conscious emotional experiences result from the
activation of these neural systems in the presence of a conscious
brain. Most emotional processing is generated unconsciously, as is
cognitive processing, with only the outcome of these processes entering
awareness, & only in some instances. (4) Emotional experiences are
just a part of the overall activation of the emotional modules reacting
to environmental triggers. LeDoux suggests they are not necessarily
the central function of the systems that generate them. (5)
He suggests that most mental disorders are emotional disorders. (6) The
states of consciousness that occur with conscious feelings (emotions) are no
different from other states such as awareness (& categorisation) of an
external object. He argues that states of consciousness occur when the mental
module responsible for awareness focuses on the activity occurring in an
otherwise unconscious mental module. (7) People set up situations
to modulate their emotions but have little direct control over
their emotional reactions. Emotional modules are dominant over cognitive
modules. (8) Emotions are powerful motivators of future behaviours.
home
[ EPIG ] [ Evolutionary Models ] [ EP Classification ] [ EP Seminar 2006 ]
Citation suggestion: Dr Gary Galambos, Neuroscience Models, Evolutionary
Psychiatry Interest Group Sydney Australia (http://www.ep.org.au/models/neurosc.htm)
[date accessed]
The materials provided on this website may be freely cited
but reposting on other websites, publishing or other reproductions, whole or in
part, are subject to the written permission of Gary Galambos. Images may be
reproduced provided the source is properly acknowledged.
Site Copyright (C) 1999-2004 Dr Gary Galambos M.B.B.S.
F.R.A.N.Z.C.P.
Page last updated: 05 September 2006
