Edelman’s Neural Darwinism and Reentrant Neural Circuitry

Nobel laureate biologist Gerald Edelman presents a purely biological theory of consciousness, founded on Darwinian natural selection and complex brain morphology. His foundational commitment is that “the neural systems underlying consciousness arose to enable high-order discriminations in a multidimensional space of signals,” that “qualia are those discriminations” and that “differences in qualia correlate with differences in the neural structure and dynamics that underlie them” (Edelman & Tononi, 2000; Edelman, 2003; “Gerald Edelman,” 2024).

Rejecting theories that the brain is like a computer or instructional system, Edelman proposes that “the brain is a selectional system, one in which large numbers of variant circuits are generated epigenetically, following which particular variants are selected over others during experience. Such repertoires of variant circuits are degenerate, i.e., structurally different circuit variants within this selectional system can carry out the same function or produce the same output. Subsequent to their incorporation into anatomical repertoires during development, circuit variants that match novel signals are differentially selected through changes in synaptic efficacy. Differential amplification of selected synaptic populations in groups of neurons increases the likelihood that, in the future, adaptive responses of these groups will occur following exposure to similar signals” (Edelman, 2003).

Edelman's way of thinking is motivated by his work on the immune system (for which he was awarded the Nobel), and his theory is developed in two domains: Neural Darwinism (neural group selection) and Dynamic Core (reentrant neural circuitry).

Neural Darwinism

Neural Darwinism is “the idea that higher brain functions are mediated by developmental and somatic selection upon anatomical and functional variance occurring in each individual animal” (Edelman, 1989). Neural Darwinism has two aspects: (i) development selection, which controls the gross anatomy and microstructure of the brain, allowing for great variability in the neural circuitry; and (ii) experiential selection, especially of the synaptic structure where functional plasticity is essential given the vast number of synapses (estimated at over 100 trillion, possibly 600 trillion or more).

Edelman notes that a child's brain contains many more neural connections than will ultimately survive to maturity—estimates go as high as 1000 trillion—and he argues that this redundant capacity, this functional plasticity, is needed because “neurons are the only cells in the body that cannot be renewed and because only those networks best adapted to their ultimate purpose will be selected as they organize into neuronal groups” (“Gerald Edelman,” 2024).

According to Edelman's theory of neuronal group selection (TNGS), “selectional events in the brain are necessarily constrained by the activity of diffuse ascending value systems. The activity of these systems affects the selectional process by modulating or altering synaptic thresholds” (Edelman, 2003).

Dynamic Core and Reentrant Neural Circuitry

Dynamic Core is Edelman's term encompassing reentrant neural circuitry, the ongoing process of recursive signaling among neuronal groups taking place across networks of massively parallel reciprocal fibers, especially in the connections between thalamus and cerebral cortex. This dynamic, relentless activity in thalamocortical circuits generates a continuing sequence of different metastable states that change over time, yet each of which has a unitary phenomenology at any given moment. Edelman asserts that “there is no other object in the known universe so completely distinguished by reentrant circuitry as the human brain” (Edelman, 2003, “Gerald Edelman,” 2024).

Edelman stresses that reentry is “a selectional process occurring in parallel” and that “it differs from feedback, which is instructional and involves an error function that is serially transmitted over a single pathway.” As a result of the correlations that reentry imposes on diverse, interacting neuronal groups, “synchronously active circuits across widely distributed brain areas are selectively favored.”

This, Edelman suggests, “provides a solution to the so-called binding problem: how do functionally segregated areas of the brain correlate their activities in the absence of an executive program or superordinate map?” Binding of the outputs of every sensory modality, each generated by segregated cortical areas, is essential for our commonly perceived but underappreciated unity of consciousness (Edelman, 2003).

It is worth noting the close relationship between the Dynamic Core and Global Workspace hypotheses, as jointly suggested by the authors of each, Edelman and Baars—each hypothesis having been put forward, independently, “to provide mechanistic and biologically plausible accounts of how brains generate conscious mental content.”

Whereas “the Dynamic Core proposes that reentrant neural activity in the thalamocortical system gives rise to conscious experience,” the “Global Workspace reconciles the limited capacity of momentary conscious content with the vast repertoire of long-term memory.” The close relationship between the two hypotheses is said to allow “for a strictly biological account of phenomenal experience and subjectivity that is consistent with mounting experimental evidence.” The authors suggest that “there is now sufficient evidence to consider the design and construction of a conscious artifact” (Edelman et al., 2011).

Primary and Higher-Order Consciousness

The theory of neuronal group selection (TNGS), pioneered by Edelman (1987), has come to undergird a cluster of theories. As Anil Seth explains, “According to the TNGS, primary (sensory) consciousness arose in evolution when ongoing perceptual categorization was linked via reentry to a value-dependent memory creating the so-called ‘remembered present’” (Edelman, 1989). Higher-order consciousness, distinguished in humans by an explicit sense of self and the ability to construct past and future scenes, arose at a later stage with reentrant pathways linking value-dependent categorization with linguistic performance and conceptual memory (Edelman, 2003; Seth, 2007).

As Edelman's mechanism for consciousness is based on the TNGS, he first distinguishes primary from higher-order consciousness. “Animals with primary consciousness can integrate perceptual and motor events together with memory to construct a multimodal scene in the present”—what James called the “specious present” and which Edelman calls “the remembered present” (Edelman, 1989).

Such an animal with primary consciousness, Edelman says, “has no explicit narrative capability (although it has long-term memory), and, at best, it can only plan to deal with the immediate scene in the remembered present” (Edelman, 2003).

As for higher-order consciousness, Edelman is mainstream: “It emerges later in evolution and is seen in animals with semantic capabilities such as chimpanzees. It is present in its richest form in the human species, which is unique in possessing true language made up of syntax and semantics. Higher-order consciousness allows its possessors to go beyond the limits of the remembered present of primary consciousness. An individual's past history, future plans, and consciousness of being conscious all become accessible” (Edelman, 2003).

How did the neural mechanisms underlying primary consciousness arise during evolution? Edelman's proposal is as follows. “At some time around the divergence of reptiles into mammals and then into birds, the embryological development of large numbers of new reciprocal connections allowed rich reentrant activity to take place between the more posterior brain systems carrying out perceptual categorization and the more frontally located systems responsible for value-category memory. This reentrant activity provided the neural basis for integration of a scene with all of its entailed qualia … [which] conferred an adaptive evolutionary advantage” (Edelman, 2003).

Summary and the Hard Problem

In summary, according to Edelman, “consciousness arises as a result of integration of many inputs by reentrant interactions in the dynamic core. This integration occurs in periods of <500 ms. Selection occurs among a set of circuits in the core repertoire; given their degeneracy, a number of different circuits can carry out similar functions. As a result of the continual interplay of signals from the environment, the body, and the brain itself, each integrated core state is succeeded by yet another and differentiated neural state in the core … The sequences and conjoined arrays of qualia entailed by this neural activity are the higher-order discriminations that such neural events make possible. Underlying each quale are distinct neuroanatomical structures and neural dynamics that together account for the specific and distinctive phenomenal property of that quale. Qualia thus reflect the causal sequences of the underlying metastable neural states of the complex dynamic core” (Edelman, 2003).

Finally, Edelman addresses the hard problem. “The fact that it is only by having a phenotype capable of giving rise to those qualia that their ‘quality’ can be experienced is not an embarrassment to a scientific theory of consciousness. Looked at in this way, the so-called hard problem is ill posed, for it seems to be framed in the expectation that, for an observer, a theoretical construct can lead by description to the experiencing of the phenomenal quality being described. If the phenomenal part of conscious experience that constitutes its entailed distinctions is irreducible, so is the fact that physics has not explained why there is something rather than nothing. Physics is not hindered by this ontological limit nor should the scientific understanding of consciousness be hindered by the privacy of phenomenal experience.”

Edelman is confident. “At the end of our studies, when we have grasped its mechanisms in greater detail, consciousness will lose its mystery and be generally accepted as part of the natural order” (Edelman, 2003).

Personally, I [RLK] like analogizing the something/nothing ontological limit in physics to the phenomenal consciousness psychophysical privacy limit in neuroscience—the two ultimate questions of existence and sentience. But I hesitate to draw the analogy too tightly. Something/nothing is a kind of historical question of what happened, that is, explaining the hypothetical process. For example, it could be that nothing is in principle impossible. Phenomenal consciousness is a clearly contemporary question of what is, that is, explaining the actual thing. Moreover, I agree that even with its something/nothing ontological limit, physics can do its work, as with its phenomenal consciousness privacy limit, neuroscience can do its work. But that work, remember, constitutes the “easy problems.”