IUBio

The Survival of the Wittiest

Lance Sherman lancesherman at home.com
Mon Jan 14 12:21:01 EST 2002


Thanks for the amusingly titled, informative message.

sorry I cant provide answers to your concluding questions - hopefully
another participant can.

I, on the other hand, have only another question.

what is "re-entrance" in the sense used by Edelman?


"Marko Amnell" <marko_amnell at hotmail.com> wrote in message
news:f6852717.0201110420.7884ec5e at posting.google.com...
> minsky at media.mit.edu (Marvin Minsky) wrote:
>
> > marko_amnell at hotmail.com (Marko Amnell) wrote:
>
> > > HOW BRAINS THINK: THE EVOLUTION OF INTELLIGENCE
> > > by William Calvin is a good exposition of the
> > > "neural Darwinism" thesis by the man who coined
> > > the term "Darwin machines". Occasional RAB contributor
> > > Marvin "The Society of Mind" Minsky gets a mention in
> > > a footnote for his 1994 Scientific American article
> > > "Will robots inherit the earth?".
> >
> > I've put that on my home page at
> > http://www.media.mit.edu/people/minsky/.
> > Actually it's more about the prospects for human immortality.
>
> Thanks. _Consciousness: How Matter Becomes Imagination_ by
> Gerald Edelman and Giulio Tonori is another good book on
> Neural Darwinism. It's more comprehensive than William Calvin's
> _How Brains Think_ (and even contains a few mathematical
> equations). I believe Edelman actually was the first to propose
> the Neural Darwinism hypothesis in the late 1970s. What is it?
> Well, just listing a few principles doesn't explain much, but
> this is how Edelman and Tononi introduce Neural Darwinism
> (pp. 79-85):
>
> "In considering the origin of species, Charles Darwin made a
> great contribution that centered on population thinking: the
> idea that variation or diversity among individuals in a population
> provides a basis for competition during natural selection. Natural
> selection is reflected in the differential reproduction of fitter
> individuals in a species. In principle, selective events require
> the continual generation of diversity in repertoires of individual
> variants, the polling by environmental signals of these diverse
> repertoires, and the differential amplification or reproduction
> of those repertoire elements or individuals that match such
> signals better than their competition. Could it be that the
> brain follows such principles? We believe it does, and in this
> chapter we briefly review some aspects of neuronal group selection,
> or Neural Darwinism. This theory embraces these selective principles
> and applies them to the functioning brain. Its main tenets are
>
> (1) the formation during brain development of a primary repertoire
> of highly variant neuronal groups that contribute to neuroanatomy
> (developmental selection),
>
> (2) the formation during experience of a secondary repertoire of
> facilitated neural circuits as a result of changes in the strength
> of connections or synapses (experiental selection), and
>
> (3) a process of reentrant signaling along reciprocal connections
> between and among distributed neuronal groups to assure the
> spatiotemporal correlation of selected neural events.
>
> Together, the three tenets of this global brain theory provide a
> powerful means for understanding the key neural interactions that
> contribute to consciousness. ...
>
> This theory of neuronal group selection (TNGS), or Neural Darwinism,
> has three main tenets ...
>
> 1. Developmental selection. During the early development of
> individuals in a species, formation of the initial anatomy of
> the brain is certainly constrained by genes and inheritance.
> But from early embryonic stages onward, the connectivity at the
> level of synapses is established, to a large extent, by somatic
> selection during each individual's ongoing development. For
> example, during development, neurons extend myriads of branching
> processes in many directions. This branching generates extensive
> variability in the connection patterns of that individual and
> creates an immense and diverse repertoires of neural circuits.
> Then, neurons strengthen and weake their connections according
> to their individual patterns of electrical activity: Neurons
> that fire together, wire together. As a result, neurons in a
> group are more closely connected to each other than to neurons
> in other groups.
>
> 2. Experiental selection. Overlapping this early period and
> extending throughout life, a process of synaptic selection
> occurs within the repertoire of neuronal groups as a result
> of behavioral experience. It  is known, for example, that
> maps of the brain corresponding to tactile inputs from the
> fingers can change their boundaries, depending on how much
> different fingers are used. These changes occur because
> certain synapses within and between groups of locally
> coupled neurons are strengthened and others are weakened
> without changes in the anatomy. This selectional process
> is constrained by brain signals that arise as a result of the
> activity of diffusely projecting value systems, a constraint
> that is continually modified by successful output.
>
> 3. Reentry. The correlation of selective events across the
> various maps of the brain occurs as a result of the dynamic
> process of reentry. Reentry allows an animal with a variable
> and uniquely individual nervous system to partition an
> unlabeled world into objects and events in the absence of a
> homunculus or computer program. As we have already discussed,
> reentry leads to the synchronization of the activity of
> temporally coherent output. Reentry is thus the central
> mechanism by which the spatiotemporal coordination of
> diverse sensory and motor events takes place.
>
> The first two tenets, developmental and experiental selection,
> provide the bases for the great variability and differentiation
> of distributed neural states that accompany consciousness. The
> third tenet, reentry, allows for the integration of those states. ...
>
> It is important to emphasize that reentry is not feedback. Feedback
> occurs along a *single* fixed loop made of reciprocal connections
> using previous *instructionally* derived information for control
> and correction, such as an error signal. In contrast, reentry
> occurs in selectional systems across *multiple* parallel paths
> where information is not prespecified. Life feedback, however,
> reentry can be local (within a map) or global (among maps and
> whole regions).
>
> Reentry carries out several major functions. For example, it can
> account for our ability to discern a shape in a display of moving
> dots, based on interactions between brain areas for visual movement
> and shape. Thus, reentry can lead to the construction of new
> response properties. It can also mediate the synthesis of brain
> functions by connecting one submodality, such as color, to another,
> such as motion. It can also resolve conflicts among competing
> neural signals. Reentry also ensures that changes in the efficacy
> of synapses in one area are affected by the activation patterns
> of distant areas, thereby making local synaptic changes context-
> dependent. Finally, by assuring the spatiotemporal correlation
> of neuronal firing, reentry is the main mechanism of neural integration.
>
> Since the formulation of the TNGS, considerable evidence to support
> the theory has accumulated."
>
> Unfortunately, Edelman and Tononi are not very explicit about
> the nature of this evidence. Does anyone know what is the best
> current evidence in support of the hypothesis of Neural Darwinism?
> Or is there some crucial evidence against the theory? Can anyone
> recommend other good books on Neural Darwinism, for or against?
> Are there good articles or papers on the subject that are
> available on the world wide web? Thanks in advance.
>





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