Neural Development: Affective and Immune System Influences

Neural Development: Affective and Immune System Influences

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1: Introduction

Two recent contributions have advanced understanding of brain function: Gerald Edelman’s Neural Darwinism (Edelman 1989, 1992, Edelman and Tononi 2001), dealing with how brain development and function can be well understood in terms of a process of natural selection applied to neural connections, and Jaak Panksepp’s Affective Neuroscience (Panksepp 1998, 2001), addressing how neurobiological systems mediate the basic emotions. We point out here that these theories can complete each other in a very satisfactory way, providing a synthesis which strengthens support for each of the theories individually and also provides an extended understanding of important interactions in the brain. In brief: on the one hand, major features of the basic value system crucial to Edelmann’s Neural Darwinism but not fully elucidated by him can be provided by the affective neuroscience of Panksepp. On the other hand, important aspects of the mechanism implementing Panksepps’ proposal that `valenced affective feeling states provide fundamental values for the guidance of behavior’ can be explicated by Neural Darwinism in a way that fully takes into account current understanding of neurobiology as well as the processes of developmental biology. This proposed synthesis (which might perhaps be called `Affective Neural Group Selection’) then gives a useful standpoint from which to investigate the relations between Affective Neuroscience and Neural Darwinism, and to consider aspects of evolutionary psychology. In this synthesis the emotional centers in the brain play an important role in selection of higher cortical connections. 

The argument in this paper, together with its implications and some extensions, is developed in detail in a forthcoming paper by myself and Judith Toronchuk in the Journal Consciousness and Emotion(2).

2: Neural Darwinism

Edelman argues that generalised principles of Darwinian natural selection (`Neural Darwinism’) must apply in the developmental process controlling detailed neural connections in each individual’s brain (Edelman 1989, 1992, Edelman and Tononi 2001)(3). The theory (4) has three main elements:

1. Developmental selection,
2. Experiential selection,
3. Re-entry (Edelman 1989, pp. 4-8; Edelman 1992, pp.81-98; Edelman and Tononi 2001, pp. 79-92).
 

The key feature that concerns us here is that, after developmental processes establish a great variety of connection patterns between neurons, `a process of synaptic selection occurs within the repertoires of neuronal groups as a result of behavioural experience … these changes occur because certain synapses within and between groups of locally coupled neurons are strengthened and others 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’ (Edelman and Tononi 2001, p.84, see also Deacon 1997, p.202; Schore 1994, pp.162, 253, 257)(5). The unit of selection is neuronal groups (Edelman 1989, pp.43-69; Edelman 1992, pp.95-99).

This argument extends the Darwinian type of understanding from the evolutionary processes that historically led to the existence of the brain to also underpinning both brain developmental processes and brain functioning. This is in accord with the way that such processes are now understood to underlie the functioning of the immune system through clonal selection (Burnet 1959, Edelman 1992: pp.77-78). Thus such principles are already known to occur in human physiological functioning, giving the same benefits as discussed here: putting in place a mechanism that can deal efficiently with conditions already encountered, but that can also deal adequately with situations that have never before been encountered by the organism. Through this mechanism, “In a very literal sense, each developing brain region adapts to the body in which it finds itself” (Deacon 1997, p.205).

3: The Affective Connection

The key issue then is what provides the fitness characterisation determining whether particular connections are strengthened or not. In Edelman’s terms, this is the value system guiding the neural Darwinism, which he relates physically to a fan of connections spreading out from a relatively small number of neurons located in the brainstem and the hypothalmus, see Edelman and Tononi 2000, p.46. It is proposed here that the signals provided by the set of primitive emotional functions described by Panksepp (1998, 2001) are key signals in the value system guiding neural selection. Such a mechanism would tie brain functioning strongly in to functions developed by evolutionary processes in our animal forbears, strongly related to survival, and giving a specific mechanism to implement Panksepp’s hypothesis that `affect is a central organizing process for sentience’ (Watt 1999). 

Panksepp presents in his work a careful neurologically based taxonomy of basic emotional processes, each characterised by specific neurotransmitters  and associated with activity in specific  sub-cortical brain areas. These are the evolutionary heritage we share with many members of the animal kingdom. They play a fundamental role in human behaviour: “the basic emotional states provide efficient ways to mediate categorical types of learned behavioural changes. … emotional feelings not only sustain certain unconditioned behavioural tendencies but also help guide new behaviours by providing simple value coding mechanisms that provide self-referential salience, thereby allowing organisms to categorize world events efficiently so as to control future behaviours … [they] may provide efficient ways to guide and sustain behaviour patterns, as well as to mediate certain types of learning” (Panksepp 1998, pp.14-15). That seems just what is required to explicate in detail the value system needed by Neural Darwinism (Edelman and Tononi 2001, pp.87-90). Emotions, in this context,  are then the core of pre-organized mechanisms which “help the organism classify things or events as `good’ or `bad’ because of their possible impact on survival” (Damasio 1995, p.117).

The basic emotional systems identified by Panksepp (1998) are the following:

1: The SEEKING system: general motivation, seeking, expectancy (pp.52-54 and 144-163).
2: The RAGE system: rage/anger (p.54 and pp.187-205)
3: The FEAR system: fear/anxiety (p.54 and pp.206-222)
4: The LUST system: lust/sexuality (p.54 and pp.225-245)
5: The CARE system: providing maternal care/nurturance  (p.54 and pp.246-260)
6: The PANIC system: panic/separation, need of care (p.54 and pp/261-279)
7: The PLAY system: roughousing play/joy (pp.280-299)
 

Panksepp gives a detailed characterisation in each case, including associated key brain areas and neuro-transmitters (for a summary, see Panksepp 2001, p.147). It is these neuro-transmitters that enable the overall mechanism to function in neuro-physiological terms. They embody the basic preferences or biases of the organism (Damasio 1995, p.117) that guide its further development.
These basic emotional systems underlie the higher level systems that develop in the brain (Panksepp 1998, pp.300-323). Various inputs to the seeking system to do with thermal balance, hunger, thirst, sexual arousal, etc. enable it to provide the basis of maintaining homeostasis. The seeking system also drives the basic impulse to search, investigate, and make sense of the environment. The foundation of learning is then provided by satisfaction or dissatisfaction associated with the success or failure of one’s endeavours as motivated by the seeking system. Presumably as brain development takes place this underlies the development of modules that carry out specific tasks to aid these functions, in particular modules that anticipate what may happen by means of some kind of modeling of the external world, including anticipation of the behaviors of others. This makes explicit the way in which emotions underlie rationality, extending the understanding of the significance of emotions beyond that recognised by Damasio, namely

    (i) the production of a specific reaction to an inducing situation,
    (ii) the regulation of the internal state of the organism so that it can be prepared for a specific reaction (Damasio 2000, pp.53-56),
to also including
    (iii) shaping of brain functionality, including the aspects classified as rational.
 

This proposal agrees with his statement that “emotions are curious adaptations that are part and parcel of the machinery with which organisms regulate survival” (Damasio 2000, p.54). They do so both in the short term through facilitating homeostasis, and in the long term through facilitating the development of intellect. In this way “all mammals, indeed all organisms, come into the world with a variety of abilities that do not require previous learning but which provide immediate opportunities to learn” (Panksepp 1998, p. 25). Thus what is proposed here can provide a neural mechanism underlying classical learning theory, and is therefore supported by all the body of evidence that emotion is developmentally and functionally important. In genetic terms, “the genome helps set the precise or nearly precise structure of a number of important systems and circuits in the evolutionarily old sectors of the human brain” (Damasio 1995, p. 109); these innate circuits then “intervene not just in bodily regulation but also in the development and adult activity of the evolutionarily modern structures of the brain …. whose precise arrangement comes about under the influence of environmental circumstances complemented and constrained by the influences of the innately and precisely set circuits concerned with biological regulation” (Damasio 1995, p. 109). The present proposal is that this happens through the mechanism of Affective Neural Darwinism outlined above.

4: Conclusion

Several pieces of evidence have been proposed to bolster the hypothesis of Neuronal Group Selection. These include the following:

– The stored information in the human genome is far too little to control brain development by itself. The Human Genome Project has revealed (Baltimore 2001, Wolfsberg et al 2001) that there are of the order of 45,000 genes in the human genome; but there are about 1013 cells in the human body and 1011 neurons in a human brain. Consequently – remembering that this genetic information has to cover development of all other bodily structure as well as the brain – there is not a fraction of the information required to structure in detail any significant brain modules, let alone the human brain as a whole (Damasio 1995, p. 108), for there is only one gene per 107 neurons and one gene per 1011 neural connections;

– something like this is needed to account for the great variability in human brain structure, contrary to any process of construction according to a preset algorithm (Edelman 1992, pp. 27, 82);

– because this allows the brain to optimally adapt to the local physical and cultural environment (Deacon 1997, p. 206;  Siegel, 2001), while also being able to face up to new circumstances in a sensible way.
 

If the proposed syntheses of this paper were true, this would imply two further key features:

– First, immune system, higher brain, and emotional system evolution, development, and function would all be based on the same generalised Darwinian selectional principles; a significant unification of understanding and extension of application of the basic Darwinian insight in terms of allowing emergence of high level structure where there was none before through processes of natural selection operating on various timescales.

– Second, it makes concrete proposals as regards the neuro-biological functioning underlying the role of emotions and the immune system in brain evolution and development, namely providing an essential part of the fitness function that shapes the result of the natural selection process. This understanding would provide sound links between evolutionary theory, neurology, developmental biology, and aspects of psychology (particularly, learning theory) and ethology.
 

A key point is that the emotions listed above are valenced: many are negative emotions suggesting either avoidance or confrontation (`fight or flee’), but some are positive emotions suggesting reinforcement. These two valences must presumably be handled in a different way through the effects of their associated neurotransmitters, which may either strengthen or weaken neural connections (but not in simple overall correlation to the valence of the emotion). Explicating this would seem a worthwhile exercise. A second key point would be considering the relation between primary and secondary emotions (Ekman and Davidson 1994, pp. 5-48; Damasio 2000, pp.50-51). On the view put here, secondary emotions would arise through the effects of the primary emotions on the brain in the course of social interaction, the primary emotions being our genetic heritage from our animal forbears. But then it is crucial clarifying which are secondary and which are primary.  Those listed as primary must be sufficient to underlie development of present day intellectual and emotional capabilities, including the secondary emotions. Third, the key issue of language and symbolism separates humans from all other animals (Deacon 1997, Hauser et al 2002). There must be a significant difference in the way the seeking system operates in humans as opposed to in all other animals in order to allow language development in conjunction with the vocal apparatus allowing speech. This mechanism must provide the basis for brain-language-culture co-evolution (Deacon 1997, Donald 1991). As argued above there is not sufficient genetic information available to specifically determine construction of language modules (Pinker 1994), but rather the mechanisms to develop such modules must evolve, see Edelman (1992), Deacon (1997), Panksepp (1998). If Affective Neural Darwinism is valid, then the processes involved in language and symbol development should be consistent with the basic mechanism suggested here. It may well be that it develops in response to strong emotional pressures related to development of culture and social interaction between ever more conscious beings in a social context (Bonner 1980, Donald 1991, 2001). 

* * *
I thank the members of the University of Cape Town Consciousness Study Group for helpful discussions, and particularly David Kibel for drawing my attention to the writings of Panksepp. I thank Jaak Panksepp, Esther Sternberg and Judy Toronchuk for helpful communications.
* * *

Endnotes

1 Mathematics Department, University of Cape Town, Rondebosch 7701, Cape Town, South Africa. email address: [email protected]

2 see http://www.benjamins.nl/cgi-bin/t_seriesview.cgi?series=C%26E

3 See also Schore (1994), Deacon (1997), LeDoux (2002), and references therein..

4 The name Neural Darwinism has been criticised because there is no element of reproduction with genetic inheritance and variation; instead there is variation of connections created by random processes, followed by neuronal group selection. Thus this should really be called Generalised Neural Darwinism rather than just Neural Darwinism. This is however just an argument over semantics; it does not affect the nature of the theory, which centres on the core Darwinian feature of survival of the fittest.  It may be more accurate to call the proposal, `Neuronal Group Selection’

5 Schore (1994) relates it to the idea of `parcellation’ (pp.19, 250, 258).

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