Abstract: 59 words
Main Text: 998 words
References: 210 words
Total Text: 1348 words
Department of
Psychological Sciences
BraseG@missouri.edu
http://bengal.missouri.edu/~braseg/
The
tension between focusing on species similarities versus species differences (phylogenetic
versus adaptationist approaches) recurs in discussions about the nature of
neural connectivity and organization following brain expansion. Whereas Striedter suggests a primary role for response
inhibition, other possibilities include dense recurrent connectivity loops.
Computer simulations and brain imaging technologies are crucial in better
understanding actual neuronal connectivity patterns.
Striedter’s book represents an important
synthesis of ideas and approaches to brain evolution across different levels,
with general constraints and evolutionary principles clearly related to neural
structures and functions throughout the chapters. It also includes an excellent
review of the history of comparative neurobiology (Chapter 2) that concludes by
noting the existence of a “tug-of-war between those who emphasize species
differences in brain organization and those who dwell on similarities”
(p.50). It is useful to realize
that this tug-of-war is being waged today on fields beyond just neuroscience.
More generally, these contrasting views are often referred to as the phylogenetic
approach (emphasizing the continuity and similarities across species) and
the adaptationist
approach (emphasizing the adaptive specializations within each
species).
These contrasting views play out at several levels, on many topics,
across the behavioral sciences.
Most recognize and accept the phylogenetic view of species (ironically,
the most controversial aspect of evolutionary theory in the 19th
century); research with model animals such as pigeons, rats, and apes demand at
least an implicit phylogenetic view.
On the other hand, the adaptationist view, sometimes labeled as
“evolutionary psychology,” is currently quite controversial and prone to both
adamant support and vigorous opposition (e.g., Tooby & Cosmides, 1992; Rose
& Rose, 2000).
Tension between phylogenetic and adaptationist approaches
recurs within Striedter’s argument for the importance of absolute brain
size. It is an important insight
that increases in absolute brain size have implication for patterns of
connectivity and organization in general (i.e., more widely connected regions,
decreases in average connection density, and more structural and functional
modularity as a consequence). Yet
there are unaddressed issues within these general implications. How was the brain parsed itself modular
aspects in the course of evolution?
Was it cut like a cake with a chainsaw: random, messy, and in random
bits? Or was it like the fissioning
of cells, families, and academic departments: the parts already functionally
relevant to each other were maintained (relatively higher connection densities)
whereas the connections across these parts were reduced? This is precisely the
sort of adaptationist question that has been relatively neglected, and is a key
to stronger linkages between neuroscience and psychology.
Striedter addresses the core issue of what is special about
human brains (Chapter 9), and emphasizes the importance of the enlargement of
the lateral prefrontal cortex and its various associated regions, which seems
to be well-motivated due to the involvement of the prefrontal cortex in
high-level cognitive control, selective attention, working memory, and
planning, as well as standard intelligence tests, as shown by neuroimaging
studies (see Duncan, 2001; Miller
& Cohen, 2001). This chapter also suggests that “response inhibition” plays
a major role in enabling the human prefrontal cortex to mediate the production
of novel solutions to behavioral problems, but we submit that the nature of the
neural mechanisms and architectures supporting flexibility of human behavior
and cognition is not yet so clearly specified.
It is also possible that (alternatively or additionally)
dense recurrent connectivity loops in the lateral prefrontal cortex enable the
formation of stable reverberatory states in working memory, planning, goal
representation and effect anticipation. These active neural representations
would “go beyond the stimulus given”, and mediate context-sensitive input-output
associations, based on a representation of the task context (Duncan, 2001;
Miller & Cohen, 2001). Response inhibition and top-down control of
input-output associations would therefore be achieved by means of these stable
states in competition (via mutual inhibition) with bottom-up
context-independent associations (e.g. impulsive responses). Feedback
connections from dorsolateral prefrontal cortex to posterior cortical areas
would mediate control of unimodal and multimodal representational states in
perception and memory retrieval. In this view, more stable states emerging in
the lateral prefrontal cortex via extensive recurrent loops would dominate more
transient representations in the brain encoding for current stimuli, responses,
and their closer associates. For instance, sustained working memory (delay)
activity in the lateral prefrontal cortex is immune to interference, whereas
delay activity in the inferotemporal cortex is vulnerable to task-irrelevant
interfering distracters (Miller et al., 1996). In other words, we share
Streidter’s view about the crucial role of the enlargement of dorsolateral
prefrontal cortex in increasing the flexibility of human behavior and
cognition, but propose that it was the emergence of convergent recurrent loops
within the dorsolateral prefrontal cortex and between the dorsolateral
prefrontal cortex and posterior cortical areas (as well as premotor areas) to
mediate this increase in functional flexibility. Another possibility is that
the human dorsolateral cortex has evolved to support massive adaptive coding of
its neuronal populations (
Computer simulations are likely to play a crucial role in
shedding light on how different kinds of neuronal connectivity patterns can
lead to optimal function-related neuronal coherence within and between brain
regions, with special reference to the orchestrating role of the dorsolateral
prefrontal cortex. Striedter
(Chapter 7) clearly considers the importance of small-world networks, of which
the visual cortex can be regarded as an example, and related theoretical
studies. Other large-scale simulations (Tononi et al., 1996; Tononi &
Edelman, 1998), have emphasized the importance of recurrent or re-entrant
connectivity systems in binding of neural representations and the emergence of
consciousness. The dorsolateral prefrontal area may play a crucial role in
coordinating neural synchrony and multiregional cooperative signaling in the
brain (see Chapter 9) in a task-dependent fashion, and to encode action
contexts, due to the high number of convergent re-entrant circuits coding
multiple modalities and synapses mediating maintenance of stable activation
patterns, such as NMDA-synapses (Wang, 1999).
Relating structure to function by means of EEG/MEG (combined
with high spatial resolution fMRI) and single-cell recording studies, as well
as large-scale computer simulation and neuropsychological evidence, may provide
a crucial contribution to clarify the role of the dorsolateral prefrontal
cortex in making humans superior to other animals in cognition and flexible behavior.
Chapter 9 of Striedter’s book can be regarded as a good starting point for this
challenge.
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