I saw a movie yesterday called Lucy. In this movie, a girl called Lucy absorbs a large amount of CPH4, and her brain capacity slowly increases.

So that brings me to my question which is :

Do we humans use 100% of our cerebral capacity or do we just use 15%?

  • 2
    $\begingroup$ No: wired.com/2014/07/… $\endgroup$ – kmm Sep 24 '15 at 14:47
  • 3
    $\begingroup$ First of all, no device can work 100% of the theoretical capacity. Regarding maximal realistic capacity of the brain: Most people project a vague picture of the nervous system as if it is a room with a lot of independently operating machines. It is not like that. To explain how nervous system works would be too broad for this site. Please start some basic reading on wikipedia. $\endgroup$ – WYSIWYG Sep 24 '15 at 14:47
  • $\begingroup$ If you roughly know how a neural network responds to continuous input. You can easily figure out some facts: 1. It is impossible to use 100 % of your brain at the same time, 2. It is nearly impossible to not use all of it over some time, 3. It is impossible to use non of it at any time. $\endgroup$ – AstronAUT Sep 24 '15 at 23:08
  • 1
    $\begingroup$ "We only use 15% of our brains in the same way that we only use 33% of a traffic light." $\endgroup$ – user253751 Sep 25 '15 at 5:31

The idea that we only use 10% of our brain capacity is a myth. There is a great article at wired.com that discusses the myth and it's history.

There is really no reason to evolve an entire brain of which only 10% is used. One great point that they make is that minor brain damage can cause devastating effects, not what you would expect if you had 90% spare capacity waiting around.

The entire brain is rarely firing all at once, and it is hard to average how much is being used because it changes with activity.

  • 1
    $\begingroup$ That doesn't take away the fact that massive brain damage can result in relatively minor effects on cognition due to the fact that the functions are re-distributed to other areas. Meaning, there is available space. For example, hemispherectomy may result in remarkably little cognitive deficits in young folks. $\endgroup$ – AliceD Sep 24 '15 at 15:01
  • 3
    $\begingroup$ A quibble: that minor brain damage can cause devastating effects is also consistent with the mythical 10% view. If it always caused devastating effects, it isn't. I can have a 1 TB hard drive with almost all of it free, but if I somehow lose just the boot sector... $\endgroup$ – Chelonian Sep 24 '15 at 15:17
  • 1
    $\begingroup$ "One great point that they make is that minor brain damage can cause devastating effects" That's not a great point at all. I probably use less than 10% of my body when writing but injuries to a small part of my body (say, a broken finger) can have a devastating effect on my ability to write. $\endgroup$ – David Richerby Sep 24 '15 at 18:44
  • $\begingroup$ There are more potential neuronal connections in the brain than there are atoms in the known universe. $\endgroup$ – PCARR Sep 24 '15 at 18:54
  • 2
    $\begingroup$ @PCARR: That's not saying much, "number of connections" tend to explode in size very fast. There are also more ways to group 80 students into 20 different groups than there are atoms in the observable universe. $\endgroup$ – BlueRaja - Danny Pflughoeft Sep 24 '15 at 21:10

There is definite truth in the notion that we do not use the full capacity of our cortex. It is generally accepted that there is a reserve present in the brain that can act as a backup for cerebral damage. Brain reserve can be defined in terms of the amount of damage that can be sustained before reaching a threshold for clinical expression (Stern, 2002).

A notable example is the removal of nearly half of the cortex (referred to as a hemispherectomy) in children with intractable epilepsy. Although behavioral changes may occur and motor skills may become compromised (Van Empelen et al., 2004), these children generally recover remarkably well and overall intellectual performance may in fact improve as compared to before surgery. (Note, however, that the quality of life and brain function of these patients was severely compromised by their medical condition) (Pulsifier et al., 2004). Another example where the brain is shown to have a reserve is the fact that cognitively normal elders sometimes are diagnosed with advanced Alzheimer’s disease pathology in their brains at death (Stern, 2002). In other words, while their cortex was severely damaged by Alzheimer's, they showed no clinical signs of cognitive deficits.

In all, the brain has a remarkable amount of reserve, which may be interpreted as an incomplete use of total brain capacity.

While this reserve is critical for brain plasticity in response to injury and aging, it is questionable whether it is available to enhance cognition as depicted in the movie.

- Pulsifier et al, Epilepsia (2004); 45(3):243-45
- Stern, J International Neuropsychol Soc (2002); 8: 448–60
- Van Empelen, Brain (2004); 127: 2071–79

The linked wire article in the accepted answer is popular science and I wouldn't base a conclusion on a '60-second-all-you-need-to-know' pseudo-scientific web link.

  • 1
    $\begingroup$ There is definite truth in the notion that we do not use the full capacity of our cortex. I think this is is not a meaningful statement. For me the problem here is that comparing the brain to a hard disk storage is very problematic. What exactly is capacity of my brain? The number of bits of information can I store before it is "full"? I don't think the brain can be full. Your examples are awesome, but IMO they show that the brain has an amazing plasticity and can adapt to various changes very well. $\endgroup$ – cel Sep 26 '15 at 7:13
  • $\begingroup$ @cel, I am not comparing anything to a hard disk. My examples show that 50% of the cortex can be removed and that, indeed, the brain is plastic and can reroute many functions to other parts. Hence, the remaining cortex can perform additional tasks and as such there is a reserve, and hence we do not use 100% capacity. $\endgroup$ – AliceD Sep 26 '15 at 8:21
  • $\begingroup$ I guess I just don't like the term reserve here. It sounds as if there is some backup storage that is currently unused, but can be activated if necessary. IIRC, after a damaging event such as a stroke, restructuring often leads to the loss of skills, such as being able to speak a foreign language. So for me it's not a reserve, but a severe restructuring. I think the problem of the question is that capacity in terms of the brain is ill defined. $\endgroup$ – cel Sep 26 '15 at 8:32
  • 1
    $\begingroup$ @cel I definitely agree capacity might have needed more explanation. The term reserve is an accepted term as far as I can see in the scientific literature, more so than "capacity", which is indeed ill-defined. I therefore have sticked to appropriate terminology, i.e. reserve. $\endgroup$ – AliceD Sep 26 '15 at 9:41

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.