- Energetic basis of brain activity: implications for neuroimaging.
The complex activities of the brain need not distract us from the certainty that it uses energy and performs work very efficiently. The human brain, which claims approximately 2% of our body mass, is responsible for approximately 20% of our body oxygen consumption. In vivo magnetic resonance spectroscopy (MRS) follows the metabolic pathways of energy production (as glucose oxidation) and work (as monitored by the cycling of glutamate and GABA neurotransmitters). In the resting awake state, approximately 80% of energy used by the brain supports events associated with neuronal firing and cycling of GABA and glutamate neurotransmitters.
Wearable computing researcher Thad Starner, in a paper
he wrote (while a graduate student at MIT) on powering wearable computers with energy harvested from the human body, included a table of total body heat dissipation (the corollary of total body energy consumption):
Table 2 Human energy expenditures for selected
activities (derived from Reference 3)
Activity Kilocal/hr Watts
Sleeping 70 81
Lying quietly 80 93
Sitting 100 116
Standing at ease 110 128
Conversation 110 128
Eating a meal 110 128
Strolling 140 163
Driving a car 140 163
Playing violin or piano 140 163
Housekeeping 150 175
Carpentry 230 268
Hiking, 4 mph 350 407
Swimming 500 582
Mountain climbing 600 698
Long-distance running 900 1048
Sprinting 1400 1630
This gets us a little closer, but it doesn't answer directly questions regarding how much energy the brain uses during specific activities. As far as total energy used throughout the day, there seems to be broad agreement across multiple allied disciplines that the brain typically uses approximately 20 percent of the total body energy throughout the entirety of any given typical day. Therefore, if your total body burned 2,500 Calories (a calorie with a capital c
is a kilocalorie and is the energy unit adopted as standard for food labeling; e.g., food labels state Calories
and not calories
) in a given day, then if you are typical and that day was typical your brain burned ~500 Calories (and mean power dissipation by your brain would be ~24 watts).
According to Arthur Jensen, yes (and no researcher seems to disagree with that, judging by the contents of the abstracts returned by a combined search for the keywords glucose
). Magnetic resonance studies involving glucose doped with radioisotopic tracers show more glucose use when the brain is active and show more glucose use in areas of the brain involved inspecific activities. This has allowed researchers to see which areas of the brain are activated during given specific types of cognitive activity. For example, a test subject may be asked to perform a certain cognitive task or even take an IQ test while his brain is being scanned. The areas of his brain that are activated during performance of the task are then visible to the researchers.
- Another study  investigated glucose metabolic rate (GMR) as a function of the "mental effort" expended on a task. The investigators did not correlate GMR with the same test for each individual, but compared groups of average and high-IQ subjects (mean IQ of 104 vs. 123) on easy tasks and on difficult tasks that were equated for the same degree of either "easiness" or "difficulty" within each group. Regardless of the task's objective demands, tasks for which 90% of the responses were correct (within the average group, or within the high-IQ group) were defined as "easy" for each group, and tasks for which only 75% of the responses were correct (within each group) were defined as "difficult." In other words, the level of a task's subjective difficulty was calibrated relative to each group's ability. For example, the average-IQ group could recall 6 digits backwards on 75% of the trials, whereas the high-IQ group could recall 7 digits on 75% of the trials. The measurements of GMR during these tasks revealed a significant interaction between IQ level and "mental effort" (i.e., level of difficulty relative to the individual's general ability level). Average- and high-IQ subjects hardly differed in GMR on the "easy" items but differed markedly on the "difficult" items. The high-IQ subjects brought more "fuel" to bear on the more difficult task. This increase in GMR by the high-IQ subjects suggests that more neural units are involved in their level of performance on a difficult task that is beyond the ability of the average-IQ subjects.
27. Larson et al., 1995.
Larson G. E., Haier R. J., LaCasse L. & Hazen K. (1995). Evaluation of a "mental effort" hypothesis for correlations between cortical metabolism and intelligence. Intelligence, 21, 267-278.
(Arthur Jensen. The g Factor
. pp 158-159, 168, 616-616.)