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

A promising overarching viewpoint: Reflexive versus reflective processes as two fundamental specialized brain-behaviour systems.

As I wrote in a previous post (Some strategic problems with the scientific endeavour) “A particular problem with respect to psychology, and perhaps to some other sciences, is that there is no general theory, to connect the latest advances in the many sub-disciplines of psychology...” However one such general approach was discussed briefly by Mathew Lieberman in 2007. He reviewed the long held view (e.g. Schneider and Shiffrin, 1977) that there are two “broad organizing principles of social cognition within the human brain...” each with a distinct underlying neural basis and evolutionary history:
Reflexive (the X system, x for the x in reflexive)
Reflective (the C system for the c in reflective)
Although Lieberman focussed on human cognition I feel these principles can be useful to understanding how information is processed and turned into adaptive behaviour in all sentient life forms.

Reflexive responding. We are responding reflexively when we quickly pull our hand from a hot stove. We don’t have to think about doing so, it just happens automatically, effortlessly, and with great speed. Such reflex actions can be highly adaptive and protect us from injury or even death. They are so adaptive in fact, that the neural circuits required for reflexes have evolved as innate genetic mechanisms in even the most primitive invertebrates and vertebrates .

Reflective behaviour. If the environment changes a lot or is relatively complex, a more flexible kind of responding became more adaptive than rigid reflexive action. It is in humans that a new type of responding called reflective responding evolved most highly although it is also present in non-human primates and arguably in other species. In the case of humans, we evolved in a complex society of cooperative hunting and gathering with lots of novel challenges. More flexible responding was required. Today, if we act automatically, without reflecting on the consequences of our actions, social or otherwise, we are likely to be ostracized by our social group or rejected by our mate. For our highly interdependent species, rejection is extremely dangerous not only to our happiness but to our physical health and even survival. Therefore we have evolved brain mechanisms to inhibit automatic responding if appropriate and instead respond more adaptively. The brain mechanism underlying reflective responding permitted the individual to tolerate some pain or to delay immediate gratification in order to obtain longer term goals.

An example of reflective responding is the situation in which we grasp a plate that is so hot it hurts us but we don’t drop it because it is loaded with our favourite food. We force ourselves to hold on to the plate long enough to put it down safely. This is a much more complicated process than reflexively just letting go of the hot plate; it requires us to inhibit the reflexive response and take time to think of a more suitable response and then with deliberation and effort to put the plate down safely.

Reflective responding also allows one to adapt in a much more flexible way when presented with a challenge of some kind. Hence reflective responding increases our problem solving ability in general. This aspect of reflective responding is at the heart of the higher more sophisticated kinds of responses humans can make. In future posts I will argue that such flexibility is intimately related to the evolution of the cerebral cortex.

An additional characteristic of reflective responding is that, in situations of high stress and arousal, it can be superseded by reflexive behaviour. An example of this is when humans in a theatre fire situation panic and stampede for the exit thereby perhaps injuring others as well as themselves. However awareness of this inhibiting effect of high stress on reflection can be used positively. For example, one way to increase reflective responding is to decrease stress and to increase positive affects such as feeling serene or grateful. This is likely the neuroscientific mechanism underlying some of the processes focussed on in positive psychology.

For information about how to utilize knowledge about the two systems to improve your interpersonal or social life I suggest two posts on my other blog called Psych-matters entitled “How to move yourself and others from reflexive to reflective responding. Part One.” and “Ten ways to improve your relationship by learning to respond reflectively” As Lieberman (2007) points out, 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). In a previous post, "Some wild and crazy hypotheses", I proposed the hypothesis that “Evolution itself acts in accordance with the energy consideration...” such that energy is used efficiently. I believe that energy utilization is an important consideration in understanding the differences between the reflexive versus the reflective system and this will be the subject of a future post.


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

Schneider, W. & Shiffrin, R. M. (1977). Controlled and automatic human information processing: I. Detection, search, and attention. Psychological Review, 84, 1-66.


Copyright © 2008 Brian S. Scott