Brain processes and their energy efficiencies.

Life cannot exist without energy. In a previous post, I hypothesized that “Evolution itself acts in accordance with the energy consideration...” such that energy is used efficiently. Although it is obvious that energetic processes are at the very core of life, relatively little attention has been paid to them in the neurosciences. However, some researchers have started down the energy path as will be discussed more fully in future posts. Some examples of such studies of energy are, with respect to brain structure and function, Laughlin (2004) and Warrant (2009), and with respect to evolution Niven & Laughlin (2008).

One very broad approach to studying energy in the nervous system would be to compare energetic processes in various discrete and specialized functions, for example, photoreception as Laughlin and his group have done so well. What I wish to introduce here is the idea of comparing energetic processes in more general functions of the brain especially those of the human brain. In my last post, “A promising overarching viewpoint: Reflexive versus reflective processes as two fundamental specialized brain-behaviour systems.” I briefly described Lieberman’s (2007) argument that the brain functions in two fundamentally different ways: reflexive and reflective. Lieberman points out that one of the features that differentiates the reflexive from the reflective system is that the former acts in a relatively effortless manner while the latter requires significant effort (energy). Additional evidence information about energy and brain function comes from the work of Baumeister (e.g. Baumeister et al (1998)) and Gailliot (2008). They have presented evidence that the mental processes usually subsumed under the phrase “executive function” are relatively energy demanding and cause mental fatigue. Since these executive functions would fall into the classification of being reflective they support Lieberman’s hypothesis about the reflective system being relatively more energy demanding.

It can be argued that the brain functions are not just divisible into two systems: reactive (automatic) and reflective (controlled). Aunger and Curtis (2008) have argued for three not two basic kinds of neural processing or what they refer to as levels of control over behaviour:
1. Reactive
2. Motivation (But see Berridge (2004) for evidence that there are probably different kinds (levels of complexity) of motivation. This suggests that this division into just three levels might be an over simplification.
3. Executive control

Aunger and Curtis state that “Behaviour can also be classified by the type of evolutionary benefit it bestows: it can deliver either immediate benefits (food, gametes), improvements in the individual’s position with respect to the world (resource access, social status), or improvements in the ability to secure future benefits (knowledge, skill).” They refer to these as respectively “Physiological, situational, and aptitudinal”. Aunger and Curtis conclude “Combining history and function implies the existence of seven types of behaviour production systems in human brains responsible for reflexive, instinctual, exploratory, driven, emotional, playful and planned behaviour.” I would like to propose that this list may also reflect increasing levels of required energy and that a comparison of energetic processes among them would be revealing and useful.

The above remarks are speculative but I will now introduce some even more speculative comments, suggestions and proposals. It would be interesting to compare energy consumption in the nervous system across the complete life cycle of the organism from early embrogenisis to senescence. I remember some classical evidence that there is a gradient in energy consumption from anterior to posterior end of the embryonic neural tube. Also during embryogenesis the nervous system can engage in overproduction of nerve fibres followed by a pruning back to the minimum required for function. Perhaps here some kind of energy homeostatic regulation is at work. As well, the current view is that during postnatal development the brain can develop using neuroplastic changes. It would seem likely that neuroplasticity is relatively highly demanding in terms of energy consumption since both neurons and glia must engage in the anabolic processes involved in neuroplastic changes.

A final “wild and crazy” bit of speculation is that energetic processes play an essential role in the well recognized complexity of the nervous system. It has been said that the number of connections in the adult human brain is in the quadrillion range putting it in the same order of magnitude as the number of stars in our galaxy. And what I am wondering is how much metabolic energy is required just to maintain this enormous complexity. I also wonder if, in senescence, less energy is available for this maintenance function and hence this complexity begins to degrade. I remember a colleague of mine, Dr. Ted Petit, years ago, speculating that with age, it is the most terminal dendritic synaptic structures which deteriorate first leaving intact the larger and more proximal synapses. The latter have been present for a longer period of time perhaps and perhaps because they have been used more over their longer existence, they have been strengthened structurally in some way perhaps by extracellular matrix elaborated by the glia so that less energy is required for their maintenance.

Research on energy and the brain would seem to offer an exciting new way to understand brain structure and function, how the brain evolved, and how it develops ontologically. In a future post I will propose some ideas about why during development, tasks initially processed by one system e.g. the reflective system may become incorporated into another system e.g. the reactive in order to save the organism’s vital energy supply.

References:
Aunger, R. & Curtis, V. (2008). Kinds of behaviour. Biology and Philosophy, 23, 317-345.

Baumeister, R. F., Bratslavsky, E., Muraven, M., & Tice, D. M. (1998). Ego depletion: Is the active self a limited resource? Journal of Personality and Social Psychology, 74, 12521265.

Gailliot M. T. (2008). Unlocking the energy dynamics of executive functioning. Perspective on Psychological Science, 3, 245-263.

Berridge K. C. (2004). Motivation concepts in behavioral neuroscience. Physiology and Behavior, 81, 179-209.

Laughlin, S.B. (2004). The implication of metabolic energy requirements for the representation of information in neurons. In: The Cognitive Neurosciences III, ed MS Gazzaniga, Cambridge, Mass., MIT Press.

Lieberman, M. D. (2007). Social cognitive neuroscience: a review of core processes. Annual Review Psychology, 58, 259-289.

Copyright © 2008 Brian S. Scott

Some wild and crazy hypotheses

“I cannot give any scientist of any age better advice than this: the intensity of the conviction that a hypothesis is true has no bearing on
whether it is true or not.” (Medawar, 1979, p. 39). Medawar, P.B., 1979. Advice to a Young Scientist. Harper & Row, New York).

I remember that, during my years of doing research, unexpected research results proved most of my hypotheses were wrong. However this wrongness was a stepping stone to a deeper and more valid understanding of the problem or phenomenon. It is with this in mind that I pose the following “wild and crazy” hypotheses.

I propose that there are some very basic principles or laws underlying life forms on this planet:

Life forms maximize energy availability for reproduction and associated processes and minimize the amount of energy expended in carrying out reproduction and related functions such as food digestion. I call this the energy consideration principle.

Evolution itself acts in accordance with the energy consideration with adaptation being a structural/functional change in a species which increases the ratio of energy available to energy consumed.

Adaptation is a function which depends on the interaction of genetic variables and environmental variables.

For many species this interactivity between genetic and environmental factors has resulted in social living to enhance resource (energy) availability and the efficiency of energy expending processes such as hunting.

The human brain represents the pinnacle of interactivity with postnatal development of the cerebral cortex through neuroplasticity facilitated by a relatively long period of nurturing and both informal and formal education/learning.


It is impossible for any one person to be cognizant of the latest advances in the varied sub disciplines of psychology ranging from neuropsychology to evolutionary psychology. Indeed I think that one reason psychology is in a kind of disarray right now, with so many disparate sub disciplines and competing explanatory theories, is that there are too few, if any, general principles or “laws” which could be used to integrate findings and theories. I think the development of such fundamental principles would help psychologists synthesize the vast amount of research data being reported today in scientific journals.

You are invited to make comments about these hypotheses or to suggest alternative ones. It is hoped that the speculations, reflections, and ongoing discussions in Sci-mat will contribute in some small ways to solutions for the many problems faced by modern human civilization.