Which part of the brain is the site for intelligence?

You are a diligent mother with three loved children, and you are astonished at realizing that while all are healthy, well-nurtured, and equally treated with respect and warmth, each one seems to have a different type of intelligence. The oldest child, a 13-year-old boy, is highly analytical and good at reasoning. The second is a ten-year-old girl, who spends most of her leisure time making clothes for her dolls.   The third is an eight-year-old boy who easily fixes his broken toys.     

You wonder why, if they share the same genetics and raising environment, they are so different. You even ask yourself if they have different brains.  You go further and wanders: Which part of the brain is the site for intelligence if any.

The three different bits of intelligence of your children fit well with the research of Dr. Robert Stenberg, who coined the terms practical, creative, and analytical types of intelligence. In a different but related model, two components of intelligence were identified: general intelligence, processed in the left prefrontal, temporal and parietal cortexes, and the executive functions located in the prefrontal cortex. 

Where is intelligence located in the brain?

University of Illinois scientists have mapped the physical architecture of intelligence in the brain in one of the largest and most comprehensive analyses so far of the brain structures vital to general intelligence and specific aspects of intellectual functioning, such as verbal comprehension and working memory.

“We found that general intelligence depends on a remarkably circumscribed neural system,” said Neuroscience professor Aron Barbey of the Decision Neuroscience Laboratory, who led the study. “Several brain regions, and the connections between them, were most important for general intelligence.

Locations of general intelligence and executive function: red: common areas, orange: general intelligence, yellow: executive function (credit: Aron Barbey)

“These structures are located primarily within the left prefrontal cortex (behind the forehead), left temporal cortex (behind the ear) and left parietal cortex (at the top rear of the head)” and in the “white matter association tracts” that connect them.

These researchers thus described two main components of intelligence: general intelligence and the so-called executive functions. 

Let us describe these two components of intelligence. This will allow us to better understand the somehow surprising statement that intelligence is located in specific areas of the brain, but at the same time is located in all the brain.     

What is meant by general intelligence?

General intelligence, also known as g factor, refers to the existence of an extensive mental capacity that influences performance on cognitive ability measures. Charles Spearman first described the existence of general intelligence in 1904. According to Spearman, this g factor was responsible for overall performance on mental ability tests.

Spearman noted that while people certainly could and often did excel in certain areas, people who did well in one area tended also to do well in other areas; this makes comprehensible the term “general”. For example, a person who does well on a verbal test would probably also do well on other tests.

Those who hold this view believe that intelligence can be measured and expressed by a single number, such as an IQ score. The idea is that this underlying general intelligence influences performance on all cognitive tasks.

Many modern intelligence tests, including the Stanford-Binet, measure some of the cognitive factors that are thought to make up general intelligence, as follows.

• Visual-spatial processing: Abilities such as putting together puzzles and copying complex shapes
• Quantitative reasoning: The capacity to solve problems that involve numbers
• Knowledge: A person’s understanding of a wide range of topics
• Fluid reasoning: The ability to think flexibly and solve problems
• Working memory: The use of short-term memory (such as being able to repeat a list of items)

What is meant by executive functions?

Executive function is a set of mental skills that include working memory, flexible thinking, and self-control. We use these skills every day to learn, work, and manage daily life. Someone who has a problem with the executive functions can find it hard to focus, follow directions, and handle emotions, among several other mental processes, such as counteracting disturbing thoughts and/or emotions.

The eight key Executive functions are Impulse control, Emotional Control, Flexible Thinking, Working Memory, Self-Monitoring, Planning and Prioritizing, Task Initiation, and Organization.

The Role of the Executive System

The role of the executive system is to handle novel situations outside of the domain of some of our more automatic psychological processes. Norman and Shallice (1992) outlined five types of situations in which routine activation of behavior would not be sufficient for optimal performance, and where executive functions must kick in.

1. Situations that involve planning or decision making
2. Situations that involve error correction or troubleshooting
3. Situations where responses are not well-rehearsed or contain novel sequences of actions
4. Dangerous or technically difficult situations
5. Situations that require overcoming strong habitual response or resisting temptation

The executive functions are often evoked when it is necessary to override responses that might otherwise be automatically elicited by stimuli in the external environment. For example, when being presented with a potentially rewarding stimulus, such as a piece of the pie, a person might have an automatic response to take a bite. However, where such a response conflicts with internal plans (having decided not to eat pie due to being on a diet), the executive functions might engage and inhibit the response.

Which are the brain sites for the executive functions

Executive functions are located primarily in the prefrontal regions of the frontal lobe with multiple neuronal connections to other cortical, subcortical, and brainstem regions. Neuroimaging and lesion studies from a variety of neurological diseases and injury models have confirmed the findings. However, it should be noted that prefrontal injury does not directly affect specific cognitive or linguistic processes; rather it affects their regulation and effective use, likely through alteration of the numerous neuronal connections between the prefrontal cortex and other brain regions.

This figure shows how the prefrontal regions of the frontal lobe radiate to all the brain to coordinate the mental functions. 

The general intelligence and the executive functions have different sites in the brain

 Now we can have a better understanding of Figure 1. The executive functions are organized in the frontal lobe (yellow areas); the general intelligence processes are set in a long and rather sharp area involving the temporal and occipital lobes (orange areas), and between this two areas there is an extensive inter-connecting area (in red) that associates both sets of structures.   

In a healthy person with a balanced intelligence, the three neural substrates of intelligence are probably balanced as well. When a given type of intelligence predominates over the others, there is probably a different type of functional equilibrium in the brain. 

For example, in the three children described above, the analytical intelligence is surely related to the efficiency of the prefrontal areas, and the practical and creative ones to a more integrated relationship between the three areas, involving a strong visual and spatial arrangement, which may be related to a particular efficiency of the occipital pole of the brain.     

But intelligence also involves Emotions, and the emotions have a place in the brain as well.

Where are emotions located in the brain?

Three brain structures appear most closely linked with emotions: the amygdala, the insula or insular cortex, and a structure in the midbrain called the periaqueductal gray. A paired, almond-shaped structure deep within the brain, the amygdala integrates emotions, emotional behavior, and motivation. These areas are often named as the Emotional Brain as represented in the Figure.

Interestingly, we humans share these brain areas with other animals. This allows us to understand why and how we perceive emotions such as fear, rage, and happiness in other living beings.  

But the location of emotions in the brain depends on whether you are right- or left-handed 

Since the 1970s, hundreds of studies have suggested that each hemisphere of the brain is home to a specific type of emotion. The neural system for emotions linked to approaching and engaging with the world – like happiness, pride, and anger – lives in the left side of the brain, while emotions associated with avoidance – like disgust and fear – are housed in the right.

But those studies were done almost exclusively on right-handed people. That simple fact has given us a skewed understanding of how emotion works in the brain, according to Daniel Casasanto, associate professor of human development and psychology.

That longstanding model is reversed in left-handed people, whose emotions like alertness and determination are housed on the right side of their brains, Casasanto suggests in a new study. Even more radical: The location of a person’s neural systems for emotion depends on whether they are left-handed, right-handed, or somewhere in between, the research shows.

“The old model suggests that each hemisphere is specialized for one type of emotion, but that’s not true,” Casasanto said. “Approach emotions are smeared over both hemispheres according to the direction and degree of your handedness. The big theoretical shift is, we are saying emotion in the brain isn’t its system. Emotion in the cerebral cortex is built upon neural systems for motor action.”

So, which part of the brain is the site for intelligence?

It is now clear that what we call “intelligence” is not a single process in a specific brain area, but a complex set of functions involving many if not all parts of the brain.

Just think you being a diligent doctor who must make a fast decision: you must have a clear perception of your patient’s body and complaints (through your senses); then you have to translate that information into a concept (diagnosis), retrieve the stored information that you have about that specific disease; then, you have to choose one among many plausible options, this last step involving a high-level integrative set of mechanisms, either mental or combined with manual and/or instrumental maneuvers.

The speed, efficiency, consistency, and safety to do your job as a doctor will be thus, associated with your general intelligence, with your executive functions, and with all your emotions. Not a single brain area executes all these functions; each one is necessary but requires the others for a harmonious outcome.  Intelligence is not in a single site; it is in all the brain.           

References (in addition to linked text above)

Norman, D. A., & Shallice, T. (1986). Attention to action: Willed and automatic control of behavior. In R. J. Davidson, G. E. Schwartz, & D.Shapiro (Eds.), Consciousness and self-regulation (Vol. 4, pp. 4-18). New York: Plenum.

Miller, E. K., & Cohen, J. D. (2001). An integrative theory of prefrontal cortex function. Annual Review of Neuroscience, 24, 167-202.

Levine, B., Robertson, I.H., Clare, L., Carter, G., Hong, J., Wilson, B.A., Duncan, J., & Stuss, D.T. (2000). Rehabilitation of executive functioning: an experimental-clinical validation of goal management training. Journal of International Neuropsychological Society, 6(3), 299-312.