Biomedical Engineering Reference
In-Depth Information
From a modeling perspective, compartmentalization is often approached
from the perspective of metapopulation dynamics or coupled oscillators, in
which space is assumed to be discrete (implicit space) and non-local (19). An
alternative approach is based on continuous space (explicit space) with local
interactions and employs partial differential equations to study diffusion and
advection of components (42). A third approach assumes discrete space with
local interactions employing coupled map lattices and cellular automata. A
fourth approach analyzes the statistical connectivity properties of undirected
graphs and their response to node or edge elimination (2).
3.6. Distributed Processing
Distributed processing describes those cases in which an integrated set of
functions are carried out by multiple, semiautonomous units (20,40). The most
obvious example is that of nerve cells comprising the nervous system. Distrib-
uted processing, or connectionism, might be assumed to be a combination of
modularity and spatial compartmentalization, but differs in that a single function
is emergent from the collective activities of units, and correlated activity is
thereby a desired outcome.
The robustness properties of connectionist models are the ability to (1) iden-
tify incomplete patterns, (2) generalize from a subset of learned patterns, and (3)
degrade gracefully upon removal of individual nodes.
Connectionist models range from a simple application of linear algebra,
dynamical systems, and Hamiltonian representations of steady states, through to
the use of statistical mechanics models of frustrated systems such as spin
glasses.
3.7. Extended Phenotypes
The extended phenotype concept was introduced by Dawkins (11) as a
means of emancipating the gene from the discrete vehicle (often taken to be the
individual organism). Thus while the gene's most proximal effect is to encode
proteins, more distally, and as a byproduct, these participate in cells, tissues,
organs, individuals, behaviors, mental states, and on through to cultures. There
is no implication of determinism or strong causality in this statement. The ex-
tended phenotype notion merely recognizes that the boundary of physical em-
bodiment need not represent the boundary of genic action.
In nonhuman biosystems the importance of the extended phenotype to ro-
bustness is not contested: from animal artifacts (ant nests, termite mounds, bird
nests, and spider webs) and from animal behavior (policing, reconciliation, and
dominance). In human society the issue is more controversial and the evidence
