Just finished a lovely book by George Mather, who is Professor of Vision Science in the School of Psychology at the University of Lincoln in the UK. His deepest area of interest in research is the study of visual cues to biological motion, and his website has some interesting demonstrations.
There is a short but useful discussion in his book about energy demand in the brain (pages 142-3). The brain is a small and valuable structure, but a very expensive organ for the body to support. For the average person it represents only 2% of your total body weight, yet consumes 20% of a resting body’s overall energy requirement. Most of this energy consumption is used as the power source for electrical signals generated by its millions of neurons. The visual system ranks among the brain’s highest energy consumers.
The activity of neurons place a premium on vascular and metabolic supply, and following its own laws of conservation of energy only about 3% of the neurons in the brain can be highly active at one time. This places a fundamental limit on the capacity to process information, conserves attention, and constrains multi-tasking.
Visual processing accounts for 44% of the brain’s energy consumption. That is a huge fact (and a great sound bite), so I’ll repeat it again. Visual processing accounts for 44% of the brain’s energy consumption. Here is another great conversation stopper at a party: Just opening one’s eyes eats up a surprising amount of energy: glucose consumption in the visual cortex increases up to 50% when the eyes open onto a complex, dynamic visual scene. Selection pressure therefore favored individuals who possessed a visual system that processes natural images in the most energy-efficient manner possible.
This helps explain why certain individuals find visual processing so stressful or limiting at various times. Patients with compromised brain function can feel like they just want to throw the off switch to the visual system, or need to have it filtered through tints or other means to wedge incoming information. And sometimes brain injury simply makes that switch sticky, and complicated …
The VisionHelp Blog 
Very interesting. The 2%/20% statistic was in the lay press a few weeks ago. This explains the abnormal/increased ‘cognitive’ fatigue experienced by those with mTBI, and others with various types of brain injury. Maybe a better phrase is ‘neurocognitive’ fatigue, and related ‘neurocognitive overload’.
And, the 44% statistic is similarly so important!
I will now included these, and other related statistics, in my lectures at the college to the optometry students, and in my outside talks, on TBI!
KEN
Glad you found it useful, Ken. The article that Mather pulled much of his data from is The Cost of Cortical Computation by Lennie in Current Biology (2003). Here is link to the paper:
Click to access Lennie03a.pdf
If normal binocular visual processing consumes 44%, then what percent is sucked up in strabismic visual processing of monocular depth cues while driving? Perhaps this is why I don’t listen to the radio or use an iPod, or even have enough brain power left over to carry on a conversation with a passenger? Methinks so …
Good point, Lynda!