I really dig his research and was happy to chat with him about a bunch of important topics relating to intelligence, the brain, and philosophy of mind.

1. What is intelligence?

Various lengthy definitions have been proposed, and I don’t think I can do much better than any of those. But my quick and dirty interpretation would be something like “A set of abilities required to acquire, process, synthesize and to put to effective use of knowledge and abilities”. However, I think insights into the nature of intelligence will come from the gradual accumulation of knowledge rather than single, unifying, ‘perfect’ definition.

2. What do IQ tests measure?

IQ tests give an estimate of a wide range of positively correlated abilities that together summarize an individual’s ability to synthesize and purposefully use knowledge and reason. Although IQ-tests certainly don’t capture everything about cognitive abilities, or even about human intelligence, they can be useful for certain purposes. However, we should always be wary of putting too much faith in strict boundaries or cut-off criteria: Even within a person there is measurement error and considerable fluctuation over time, so it is dangerous to try to capture all of someone’s cognitive ability in a single number. 

3. How important is mental speed for intellectual functioning?

Assuming mental speed is interpreted as the time it takes to process and distribute information, I think mental speed is probably a necessary, but not a sufficient condition for (high) intelligence. The ability to balance and synthesize a lot of information is dependent on different kinds of information being available to simultaneously access and process. However, it is of course only one element of intelligence, and singular explanations haven’t fared well historically. Every proposed simple explanation or substrate or intelligence I know of has led to funny, but insightful, counterexamples. For instance, in several mental tasks, including some concerning working memory and mental speed, we loose, badly, to chimpanzees on both speed and accuracy. And in terms of the cortex-to-body ratio, humans are behind several species of rodents and some species of fish. The lesson to take from this is that there are, simply, no simple answers to understanding intelligence. Or, conversely, that we are consistently underestimating the intelligence of rodents and fish...

4. Where is intelligence in the brain?

I don’t think intelligence is somewhere in the brain, in the same way that ‘athletic ability’ is not in some particular place in your body, and the top speed of a car isn’t somewhere in your car. Firstly, intelligence reflects differences between people, so it can’t really be ‘in’ a person. Secondly, although intelligent behavior is ultimately dependent on the function of the brain, and perhaps more so on certain aspects of brain functioning than on others, I think attempting to locate intelligence somewhere is asking the wrong questions. We can learn a lot from studying the brain, but we can’t just peer in and hope for a clear, simple answer.

5. What do you think of current approaches to understanding the neural basis of g?

Traditionally, studies have looked for correlations between individual neurological measures and g. This was useful initially, but also inherently limited. Increasingly, the field is moving towards combining different kinds of data, and I think this is a good thing. Recent attempts have started to integrate both functional and structural data, to get a grip on the relationship between brain function and structure with regards to intelligence and intelligent behavior. Most importantly, there is an increasing realization that we need to focus on the dynamics of intelligence. For instance, the field of network analysis has been gaining popularity. These studies look at the dynamics and changes of brain activity, how these relate to intelligent behavior, and how these vary across people. For example, recent large-scale collaborations are moving towards studying groups of people as they develop and change over time, both behaviorally and neurologically. Such studies allow researchers to study the development of intelligence and the brain during childhood and adolescence, and during aging later in life. This is especially important, as it allows us to get a grip on the dynamical changes over time. We know already that this is not a simple, linear process, but the interaction of a range of abilities. In the past, intelligence research has learned a lot by studying changes over long periods of time, as this showed that every generation scores considerably higher on IQ-tests, the so-called Flynn-effect. The hope is that similar gains in understanding will be possible by being able to track and model the development of intelligence and the brain during childhood and old age.

6. What do you think of the Parieto-Frontal Integration Theory (P-FIT)?

Although the scope of the effort is both admirable and useful in collating a large body of research, it is ultimately not satisfying. Firstly, the P-FIT theory isn’t very specific, and the brain regions discussed include most of the cortex. As the authors themselves state, this same network has also been found to underlie cognitive functions related to perception, short-term memory storage, and language. Then again, this may simply be the way it is, but then we should perhaps ask more specific questions. For instance, my main problem with the P-FIT model is that it combines evidence from both the inter-individual study of intelligence (how do people differ from one another in terms of intelligence and/or brain structure?) and the intra-individual study of intelligence (what happens in people’s brains when they perform complex tasks?). Although these are both very interesting questions, they are also very different questions. Taking them together as ‘intelligence and the brain’ is perhaps a partial explanation of why the network they identified is so broad: It summarizes a range of answers to two complex, but distinct, questions.

7. Describe your latest research.

Generally, my research is concerned with using statistical tools to better understand the relationship between psychological behavior and the brain [paper]. For instance, one of the things we are looking at currently is integrating psychometric models to study what happens when people try to solve complex problems, both behaviorally and in terms of brain function. To really be able to understand this process, we must carefully take into account both differences between people and differences between different tasks, in terms of difficulty. Only then can we study the similarities and differences in strategies when solving complex reasoning tasks. We use certain psychometric (Rasch) models to take into account these differences, and so hope to better understand the similarities and differences in how people solve complex reasoning problems.

8. How does your own research move the field forward and overcome some prior limitations of studying the neural basis of intelligence?