Reasons for Doubting Thought Comes from the Frontal Lobes or Prefrontal Cortex

Scientists lack any coherent explanation for how a brain could generate thought or intellect. Thoughts are immaterial things, so how could they possibly be generated by material things such as neurons? We know how physical things can generate other physical things (such as continental plates generating earthquakes), and we know how mental things can generate other mental things (such as one idea leading to a related idea). But nobody can give a coherent explanation as to how a physical thing such as a brain could produce a mental thing such as a thought or idea.

Scientists often fall back on localization claims to try to hide this shortfall. A scientist who cannot explain the how of a brain making an idea or a decision will often try to use a where as a substitute, by suggesting that specific mental capabilities come from particular parts of the brain. A common claim is that higher thought comes from the frontal lobe of the brain. More specifically, someone may claim that higher thought comes from the front-most part of the frontal lobe, what is called the prefrontal cortex. But the evidence fails to strongly support such claims, and the evidence often conflicts with such claims.

We certainly do not know from brain scans that higher thought comes from the frontal lobe or the prefrontal cortex. With the exception of the auditory and visual cortex, which show clear signs of “lighting up” during visual or auditory perception, there is no part of the brain that shows more than about a 1 percent increase in activity when humans think, decide, or remember. As a technical paper states, “cognitive effects give signal changes on the order of 1%.”

Those visuals showing “activating regions” of the brain in red are typically making use of a deceptive data presentation technique in which mere 1 percent differences in activity (or less) are represented in red, making them looking like big differences when they're really tiny differences. When you run, your heart gives a very clear signal of being involved in such a thing – for your heart rate may increase by 50 percent. But when you think, decide, or remember an old memory, there is no part of your brain that gives any clear sign of shifting into high gear or being crucially involved in such a thing.

Interestingly, a recent scientific paper notes that "neuroimaging studies have shown that intelligent individuals, despite their larger brains, tend to exhibit lower rates of brain activity during reasoning." So here we have an inverse correlation between brain activity and thinking. 

Let us look at general intelligence and the frontal lobe. It is part of the dubious folklore of neuroscientists that the prefrontal cortex is some center of higher reasoning. But the scientific paper here tells us that patients with prefrontal damage "often have a remarkable absence of intellectual impairment, as measured by conventional IQ tests." The authors of the scientific paper tried an alternate approach, using a test of so-called "fluid" intelligence on 80 patients with prefrontal damage. They concluded "our findings do not support a connection between fluid intelligence and the frontal lobes." Table 7 of this study reveals that the average intelligence of the 80 patients with prefrontal cortex damage was 99.5 – only a tiny bit lower than the average IQ of 100. Table 8 tells us that two of the  patients with prefrontal cortex damage had genius IQs of higher than 140.

In a similar vein, the paper here tested IQ for 156 Vietnam veterans who had undergone frontal lobe brain injury during combat. If you do the math using Figure 5 in this paper, you get an average IQ of 98, only two points lower than average. You could plausibly explain that 2 point difference purely by assuming that those who got injured had a very slightly lower average intelligence (a plausible assumption given that smarter people would be more likely to have smart behavior reducing their chance of injury). Similarly, this study checked the IQ of 7 patients with prefrontal cortex damage, and found that they had an average IQ of 101.

It also should be remembered that brain-damaged patients taking standard IQ tests may have higher intelligence than the test score suggests.  A standard IQ test requires visual perception skill (to read the test book) and finger coordination (to fill in the right answers using a pencil). Brain damage might cause reduced finger coordination and reduced visual perception unrelated to intelligence; and such things might cause a subject to do below-average on a standard IQ test even if his intelligence is normal.  

The 1966 study here states, "Taken as a whole, the mean I.Q. of 95.55 for the 31 patients with lateralized frontal tumors suggests that neoplasms in either the right or left frontal lobe result in only slight impairment of intellectual functions as measured by the Wechsler Bellevue test."  In this paper (page 276), scientist Karl Lashley noted that you can remove 50% of the cortex of an animal without having any effect on the retention of mazes learned by the animal.  Lashley noted on page 270 of this paper something astonishing, that the smartest animal he had tested was one in which the fibers of the cortex had been severed:

"The most capable animal that I have studied was one in which the cortex and underlying association fibers had been divided throughout the length of each hemisphere. His I.Q., based on ten tests, was 309."

It is sometimes claimed that the dorsolateral prefrontal cortex is the "CEO" of the brain. This study examined six patients with damage to the dorsolateral prefrontal cortex, and found that they had an average IQ of 104, above the average of 100. The study here tells us that 37 patients with damage to the dorsolateral prefrontal cortex had an average IQ of 97.4, only slightly below average (Table 1 and Table 2).  The same study tells us that 25 patients with damage to the ventromedial prefrontal cortex had an average IQ of 105.7 (Table 3 and 4).   This study says, “We have studied numerous patients with bilateral lesions of the ventromedial prefrontal (VM) cortex” and that “most of these patients retain normal intellect, memory and problem-solving ability in laboratory settings.” 

In the paper "Neurocognitive outcome after pediatric epilepsy surgery" by Elisabeth M. S. Sherman, we have some discussion of the effects on children of hemispherectomy, surgically removing half of their brains to stop seizures. Such a procedure involves a 50% reduction in the frontal lobe of the brain, and a 50% reduction of the prefrontal cortex. We are told this:

Cognitive levels in many children do not appear to be altered significantly by hemispherectomy. Several researchers have also noted increases in the intellectual functioning of some children following this procedure....Explanations for the lack of decline in intellectual function following hemispherectomy have not been well elucidated. 

Referring to a study by Gilliam, the paper states that of 21 children who had parts of their brains removed to treat epilepsy, including 10 who had surgery to the frontal lobe, none of the 10 patients with frontal lobe surgery had a decline in IQ post-operatively, and that two of the children with frontal lobe resections had "an increase in IQ greater than 10 points following surgery." 

The paper here gives precise before and after IQ scores for more than 50 children who had half of their brains removed in a hemispherectomy operation.  For one set of 31 patients, the IQ went down by an average of only 5 points. For another set of 15 patients, the IQ went down less than 1 point. For another set of 7 patients the IQ went up by 6 points. 

A writer at Slate.com states the following: 

And victims of prefrontal injuries can still pass most neurological exams with flying colors. Pretty much anything you can measure in the lab—memory, language, motor skills, reasoning, intelligence—seems intact in these people.

Now let us look at whether there is good evidence that decision making is generated by the prefrontal cortex. It should be first noted that the evidence discussed above discredits such an idea, because you can't perform well on an IQ test unless you have a good decision-making ability. Each IQ test question requires you to make a decision; none are tests of learned knowledge. For example, when an IQ test asks which of 5 figures most closely resembles a particular figure, that is something that requires you to make a decision rather than just remember something you have learned.

A 2002 scientific paper was entitled “Decision-making processes following damage to the prefrontal cortex.” The scientists who wrote the paper identified 19 patients with damage to the prefrontal cortex, and had them do various tests. Some of the results are below:

• Patients with local orbitofrontal lesions performed normally (at control levels) on three-decision making tasks.
• There was no statistically significant difference among the four frontal subgroups and controls on letter fluency or category fluency.
• Pattern recognition performance (percentage correct) was not significantly impaired in either the combined frontal group or the five subgroups.
• On spatial recognition (percentage correct), the combined frontal group were unimpaired relative to controls.
• On a gambling test to determine decision making, “The combined frontal group did not show poorer decision making than controls... and there were no significant differences among the five subgroups.”

Based on the results above, you would have to conclude that the idea that the prefrontal cortex generates decisions or thoughts is false. But there's another test that neuroscientists use in cases such as these – a kind of very subtle and sneaky test. We might put this test under a category of “desperately seeking evidence of performance deterioration.

The test is called the “Iowa gambling task.” A person will sit in front of a computer screen that shows four card decks. The person can pick from any of the decks, and is told that when you pick a card, your money can be either increased or decreased. Normally decks A and B give you a much higher money reward, compared to decks C and D. For example, it might be that picking from deck A will normally give you about $100, and picking from decks C and D will normally give you only about $10. But there's a sneaky catch. Occasionally decks A and B will cause you to lose a large amount such as $1200.

So a person doing this test has to recognize a very subtle rule that can be detected only after 40 or 50 trials – that even though decks A and B normally give more money, they can cause big money subtractions, which means that it's really better to keep picking from decks C and D.

As a test of executive ability, the Iowa gambling task is dubious indeed. One reason is that it may be largely testing short-term memory or prolonged concentration rather than executive ability. Another reason is that it is debatable whether the assumption of the people applying this test (that picking from decks C and D is a wiser decision) is correct. It can be argued that the person who picks from decks A and B has made a correct short-term decision. Such a person is like an investor who continues to invest in the stock market because of nice annual gains even though he knows that about every 8 years or so, stock markets have nasty downturns in which investors lose 30% or so of their money. This wikipedia page on the Iowa gambling task gives some scientific papers that argue it is flawed, and should not be used to judge executive ability.  In the paper I referred to above, the patients with prefrontal damage did worse on the Iowa gambling task, although whether that actually was inferior executive ability is debatable. We can summarize the paper by saying its tests provided no clear evidence that decisions are produced by the prefrontal cortex, and no clear evidence that damage to the prefrontal cortex significantly impairs executive ability.

Another dubious test used on some patients with frontal lobe damage is called the Wisconsin Card Sorting Test. Subjects are asked to put a card in one of 4 card stacks. As soon as they make a choice, they are told whether their choice was correct. We are told in 1:34 of this video that “After ten consecutive correct matches, the classification principle changes without warning.” So this test is also a subtle, sneaky type of test, not a straightforward test of executive ability. What it tests is the ability to discard a principle you have already adopted when the evidence no longer supports that principle. One paper says, “These findings strongly suggest that WCST scores cannot be regarded as valid nor specific markers of prefrontal lobe function.” 

The studies above are studies involving small numbers of unusual subjects with damage in the frontal lobes. Perhaps a much better way to consider the issue of how much cognition depends on the frontal lobes (or the prefrontal cortex) is to consider a much larger class of subjects: the many millions of people older than 60.

This scientific paper states this: “General linear model analyses revealed that the greatest age effects occurred in prefrontal cortex gray matter... with an average rate of volumetric decline of 4.9% per decade” after age 18. This should result in a decline in the prefrontal cortex gray matter of more than 20% by the time someone reaches 70. But we see nothing like a 20% decline in intelligence or decision-making ability in those who have reached the age of 70. People older than 70 still serve as presidents, congressmen, senators and CEO's. 

I could cite some statistics comparing the IQ tests of 20-year-olds and 70-year-olds, but then we would run into the confounding factor known as the Flynn Effect. The Flynn Effect is that for many decades, the performance of young people on IQ tests has been improving, with the improvement being about 3 points per decade. The study here states the following:

The Flynn effect was large enough to account for 100% of the variance in performance between age groups for cross-sectional analyses. After accounting for the Flynn effect, IQ was found to be relatively stable across the adult portion of the lifespan. Verbal abilities remain stable and even show gains through a large segment of the lifespan, while abilities measured by the Performance scale show modest declines from younger to older samples.

So the study finds that after we adjust for the Flynn effect, the IQ of people about 70 is about the same as people about 20. This finding is not at all what we should expect if the prefrontal cortex is responsible for intellectual capabilities, given a decline of about 20% that should occur in the prefrontal cortex between the age of 20 and 70.

I may note that the very fact of the Flynn effect is inconsistent with the dogma that our intelligence is a product of our brain. The Flynn effect, which involved an increase in IQ scores of about 3 percent per decade, went on for at least seven decades (although some think it is wearing off). During this time there was no change in human brains that could account for such a change.

Another relevant point is that the human brain is currently much smaller than it was previously. A science article in the mainstream Discover magazine tells us this: “Over the past 20,000 years, the average volume of the human male brain has decreased from 1,500 cubic centimeters to 1,350 cc, losing a chunk the size of a tennis ball.” But most people would guess that humans are smarter, or as least as smart, as those who lived 20,000 years ago.

A recent article on Aeon mentions how there is little correlation between brain size and intelligence, or a correlation between intelligence and the size of a frontal cortex. The article states the following:

Some of the most perspicacious animals are the corvids – crows, ravens, and rooks – which have brains less than 1 per cent the size of a human brain, but still perform feats of cognition comparable to chimpanzees and gorillas. Behavioural studies have shown that these birds can make and use tools, and recognise people on the street, feats that even many primates are not known to achieve. ….Among rodents, for instance, we can find the 80-gram capybara brain with 1.6 billion neurons and the 0.3-gram pygmy mouse brain with probably fewer than 60 million neurons. Despite a greater than 100-fold difference in brain size, these species live in similar habitats, display similarly social lifestyles, and do not display obvious differences in intelligence.

Consider the growth of intelligence in a child. A child is born with about as many neurons as it will ever have. During the period from birth to age 18, the child's intelligence seems to grow by perhaps 300%. But there is no corresponding brain growth.

There are, however, many new connections formed between brain cells. But an article at Neurosciencenews.com tells us the following:

The more intelligent a person, the fewer connections there are between the neurons in his cerebral cortex. This is the result of a study conducted by neuroscientists working with Dr Erhan Genç and Christoph Fraenz at Ruhr-Universität Bochum; the study was performed using a specific neuroimaging technique that provides insights into the wiring of the brain on a microstructural level.…. The researchers associated the gathered data with each other and found out: the more intelligent a person, the fewer dendrites there are in their cerebral cortex.

Let's put some of these facts into a table listing predictions of the theory that your intelligence comes from your brain, comparing such predictions to reality.

Prediction of theory that intelligence comes from brain, specifically the frontal lobe or prefrontal cortex	Reality
Injury to prefrontal cortex or frontal lobes should cause sharp drop in intelligence, as should hemispherectomy	This does not generally occur
Human intelligence should not have increased since 1900, because there has been no change in brain size or structure .	Since about 1930, IQ scores have risen by about 3 percent per decade (the Flynn Effect).
People about 70 should be much less intelligent than 20-year-olds, because of 5% volume decline in prefrontal cortex per decade.	Adjusting for Flynn Effect, no such drop in intelligence occurs.
Humans today should be much more stupid than humans 20,000 years ago, because our brains are smaller by about the size of a tennis ball.	Most people today would guess that humans are smarter, or at least as smart, as humans 20,000 years ago.
Elephants should be much smarter than humans, because their brains are three or four times heavier.	Humans are actually smarter than elephants.
Crows should be very stupid, because their brains are tiny, and have no neocortex.	Crows are astonishingly smart.
Greater number of connections in the brain should increase effective intelligence.	"The more intelligent a person, the fewer connections there are between the neurons in his cerebral cortex." -- neuroscience news cited above.
Men should be about nine percent smarter than women, because their brains are about nine percent bigger.	It is generally recognized that on average men are not significantly smarter than women.
Adults should not be much smarter than babies or toddlers, because they have no more brain cells than babies or toddlers.	Adults seem to be much smarter than babies and toddlers.

We see from this table that the claim that intelligence comes from the brain (specifically the frontal lobe or prefrontal cortex) massively fails to predict reality correctly.

The evidence discussed here argues against the claim that the prefrontal cortex or the frontal lobe can be identified as the source of decision making or the center of higher thought in the brain. The evidence discussed here is consistent with the claim that human higher thought capability does not come from the brain but from some unknown other source. Such a claim is also supported by many other considerations discussed at this site, including (1) convincing and well-replicated laboratory evidence (discussed here and here) for psychic phenomena such as ESP, evidence suggesting that the mind has powers that cannot be explained by brain activity; (2) evidence for near-death experiences indicating minds can continue to function even when brains have shut down because the heart has stopped.

Postscript: In 1930 a patient listed as Joe A. in the medical literature underwent a bilateral frontal lobectomy performed by Dr. Walter Dandy, who removed almost all of his frontal lobes. An autopsy in 1949 confirmed that "both frontal lobes had been removed." The paper describing the autopsy said that from 1930 to 1944 Joe A.'s behavior was "virtually unchanged." On page 236 of this source, we read that Dandy said this of three patients including Joe A.: "These three patients with the extirpation of such vast areas of brain tissue without the disclosure of any resulting defect is most disappointing." I could see how it would be disappointing for someone hoping to prove a connection between some brain area and intellectual function. Page 237 of the same source tells us that on casual meeting Joe A. appeared to be mentally normal.  Page 239 of this source states this about Joe A, summarizing the findings of Brickner.:

Nor was intellectual disturbance primary. The frontal lobes played no essential role in intellectual function; they merely added to intellectual intricacy, and "were not intellectual centers in any sense except, perhaps, a quantitative one." 

A 1939 paper you can read here was entitled "A Study of the Effect of Right Frontal Lobectomy on Intelligence and Temperament." A patient C.J was tested for IQ before and after an operation removing his right frontal lobe. He had the same IQ of 139 before and after the operation. Page 9 says the lobectomy "produced no modification of intellectual or personality functions."  On page 10 we are told this about patients having one of their frontal lobes removed:

Jefferson (1937) reported a series of eight frontal lobectomies in which the patients were observed for intellectual and emotional deficits following operation. There were five cases of right frontal lobectomy, three of whom were living and well when the article was written. It could be stated definitely that in two of the three cases there were no abnormalities which could be noted by the surgeon, patient, or family, and while the third case showed a mild memory defect, the operation had been too recently performed to judge whether or not the loss would be permanent. The three cases of left frontal excision likewise showed no significant changes, but comment was made that one patient was slightly lacking in reserve, another remained slightly facetious, and the third, who suffered a transient post-operative aphasia, appeared a trifle slow and diffident.

If the frontal cortex is some kind of "seat of reason," we might expect the human frontal cortex to be unusually large for a primate. But the paper here states, "The consistency of our results across independent data sets supports the view...that human frontal cortex, and regions and tissue subtypes within it, are no larger than expected for a nonhuman primate of our overall cortex or brain size." 

The following excerpt from a scientific paper tells us of additional cases of people who did not seem to suffer much mind damage after massive damage to the frontal lobes or prefrontal cortex. Resection is defined as "the process of cutting out tissue or part of an organ."

Several well-documented patients have been described with a normal level of consciousness after extensive frontal damage. For example, Patient A (Brickner, 1952) (Fig. 2A), after extensive surgical removal of the frontal lobes bilaterally, including Brodmann areas 8–12, 16, 24, 32, 33, and 45–47, sparing only area 6 and Broca's area (Brickner, 1936), “toured the Neurological Institute in a party of five, two of whom were distinguished neurologists, and none of them noticed anything unusual until their attention was especially called to A after the passage of more than an hour.” Patient KM (Hebb and Penfield, 1940) had a near-complete bilateral prefrontal resection for epilepsy surgery (including bilateral Brodmann areas 9–12, 32, and 45–47), after which his IQ improved. Patients undergoing bilateral resection of prefrontal cortical areas for psychosurgery (Mettler et al., 1949), including Brodmann areas 10, 11, 45, 46, 47, or 8, 9, 10, or 44, 45, 46, 10, or area 24 (ventral anterior cingulate), remained fully conscious (see also Penfield and Jasper, 1954; Kozuch, 2014; Tononi et al., 2016b). A young man who had fallen on an iron spike that completely penetrated both frontal lobes, affecting bilateral Brodmann areas 10, 11, 24, 25, 32, and 45–47, and areas 44 and 6 on the right side, went on to marry, raise two children, have a professional life, and never complained of perceptual or other deficits (Mataró et al., 2001).

Apparently patient KM got smarter after they took out most of his prefrontal cortex. That's a case helping to show that brains don't make minds. The book here discusses intelligence tests done on patients who underwent surgery on the frontal lobes:

"It was natural that the effect of an injury on the frontal lobes, said to be concerned with the higher functions of men, should be measured by these tests of intelligence. The absence of marked effects on mental ability, as measured by these intelligence tests, was, not surprisingly, felt to be puzzling." 

This paper here describes a case of a "modern Phineas Gage": a patient C.D. who suffered massive prefrontal  damage after a penetrating head injury. But C.D's IQ after the injury was measured at 113, well above average.  His verbal IQ after the injury was 119, in the 90th percentile. We read:

C.D. reported that he did not have any cognitive or emotional problems following the accident. In describing how his thinking skills were completely unaffected, C.D. stated that, "all the shattered bone was caught in the gray matter in front of the brain." 

The paper also tells us, "C.D.’s performances on memory tests were all in the average to above-average ranges in terms of the traditional measure of level of correct responses." 

A 2021 paper is entitled "Reduced decision bias and more rational decision making following ventromedial prefrontal cortex damage."  So you make more rational decisions if your front brain is damaged? That's a result incompatible with claims that your brain makes decisions. 

Using the term "decorticate" to refer to animals that had their cortex surgically removed, the scientific paper here tells us that rats and cats seem to show relatively little behavioral effects when you remove their cortex:

"All of the behaviors just mentioned are also exhibited by experimental animals after their cerebral cortex is removed surgically, either in adulthood or neonatally. Best studied in this regard are rodents (Woods 1964; Wishaw 1990). After recovery, decorticate rats show no gross abnormalities in behavior that would allow a casual observer to identify them as impaired in an ordinary captive housing situation, though an experienced observer would be able to do so on the basis of cues in posture, movement and appearance (Whishaw 1990, on which what follows relies, supplemented by additional sources as indicated). They stand, rear, climb, hang from bars and sleep with normal postures (Vanderwolf et al. 1978). They groom, play (Pellis et al. 1992; Panksepp et al. 1994), swim, eat, and defend themselves (Vanderwolf et al. 1978) in ways that differ in some details from those of intact animals, but not in outline. Either sex is capable of mating successfully when paired with normal cage mates (Carter et al. 1982; Whishaw & Kolb 1985), though some behavioral components of normal mating are missing and some are abnormally executed. Neonatally decorticated rats as adults show the essentials of maternal behavior which, though deficient in some respects, allows them to raise pups to maturity. Some, but not all, aspects of skilled movements survive decortication (Whishaw and Kolb 1988), and decorticate rats perform as readily as controls on a number of learning tests (Oakley 1983). Much of what is observed in rats (including mating and maternal behavior) is also true of cats with cortical removal in infancy: they move purposefully, orient themselves to their surroundings by vision and touch (as do the rodents), and are capable of solving a visual discrimination task in a T-maze (Bjursten et al. 1976; see also Bard & Rioch 1937)." 

The paper "Neuropsychological outcome following frontal lobectomy for pharmacoresistant epilepsy in adults" here deals specifically with the surgical removal of the frontal lobe to treat epilepsy. Neuroscientists have made more claims about the frontal lobe than any other part of the brain. We have been told that the frontal lobe is some kind of center of judgment and memory.  The paper states the following:

"Forty-eight percent of the sample did not show decline on any of the 16 cognitive measures examined in this study. Forty-two showed decline on measures in 1 or 2 cognitive domains. In contrast, 10% of the sample showed declines in 3 or more cognitive domains." 

Elsewhere the paper states, "The vast majority of patients who undergo frontal lobectomy for treatment of pharmacoresistant epilepsy demonstrate good cognitive and motor outcomes." Using the term "frontal lobectomy" for the removal of the front part of the brain, the paper also states, "Interestingly, there was a subset of patients who demonstrated clinically meaningful improvements in confrontation naming (15% of sample), verbal intellectual function (11%), or memory (10%–17%) following frontal lobectomy."  The paper says, "Existing studies that have examined change in intellectual functioning following frontal lobe surgery have had mixed results, with some studies reporting no change on intelligence measures and others reporting apparent improvements."

A neuroscience paper says, "A series of clinical observations reports the facilitation of artistic abilities in some patients with neurodegenerative disease affecting the frontal lobes, raising the question of a possible increased creativity following frontal damage (Palmiero et al., 2012; Schott, 2012; Gretton and ffytche, 2014)."

In an article last year on the site of Forbes magazine, we read the following:

"One more bizarre thing the researchers noticed was the bigger the lesions on the cortex, the better the mice performed. 'It was a strange result…' says Hong, who hesitates before adding: 'I wouldn't say that we're confident that if we [tested] a lot more animals we would see it. It was sort of a trend that we noticed. I guess the answer is, we don't know. Basically, it implied that the less the cortex is active, the better the animal is doing and the cortex was somehow interfering with the animal's ability to learn.' "

A paper in the journal Science refers to "the large number of reports describing 'negative' findings -- that is, the absence of demonstrable 

deficits in test performance, despite the presence of large

cerebral lesions, especially in the frontal lobes." The paper compared IQ tests for 60 armed force members who had their intelligence tested before penetrating brain injuries, and also had their intelligence tested after their brain injuries. Speaking of results on IQ tests, the paper states, "These analyses demonstrated that lesions of the frontal and occipital lobes did not produces a significant decline in score, and that only lesions of parietal or temporal lobes of the left hemisphere showed a significant decrease." The soldiers with lesions in these areas actually performed higher on IQ tests after their penetrating brain injuries, with an average of about a 7% increase:

• The left nonparieto-temporal region
• The right parietal region
• The right temporal lobe
• The right parietotemporal lobe
• The right nonparieto-temporal region

The only decrease in IQ scores occurred with injuries to the left parietotemporal lobe.  Referring to removal of brain tissue (resection), the paper here states that " intellectual or memory decrements are seldom found after frontal resections."