Thursday, 16 August 2018
Human Brain Networks Operate on a Unimodal/Multimodal Gradient
This week I’d like to tell you about an article published in the journal PNAS in 2016. It is of interest because it does something that is extremely valuable in the realm of science: it shows how two bodies of data converge into a single phenomenon and thereby helps us to understand some things that were less clear before. Let me explain.
The article, by Daniel S. Margulies and no fewer than 11 co-authors, is entitled “ Situating the default-mode network along a principal gradient of macroscale cortical organization”. In slightly simpler terms, their study involved situating the brain’s default mode network along a large-scale organizational gradient within the cerebral cortex. Okay, let me explain further.
For some time, neuroscientists have known of the existence of a gradient between the more sensory parts of the cerebral cortex (the primary visual cortexes, primary auditory cortexes, and so on) and the more associative ones. One way of describing the two ends of this gradient would be to say that the more sensory end is composed of unimodal areas (whose neurons respond to stimuli from only one sensory modality) while the more associative end is composed of multimodal areas (in which the same neuron can be activated by several different sensory modalities). Yet another way to describe this distinction would be to say that one end of the gradient consists of the parts of the brain that are more involved in “online” processing (when we perceive and take action in real time) while the other consists of the parts that are more involved in “offline” processing (when we perform more abstract mental simulations without translating them into actual behaviour). In short, scientists knew that one aspect of the organization of the cortex consisted of this gradient between perception and concrete action on the one hand and abstract thought on the other, and that the latter was made possible by the disproportionate expansion of the associative cortex in the human brain.
Scientists also already knew about a number of large networks in the brain to which they assigned functional names to attempt to account for their increased activation in certain situations—for example, the visual network, the somatomotor network, the dorsal attention network, the salience network, and, perhaps most fascinating of all, the default mode network.
The default mode network was first discovered when scientists realized that all of the brain areas that form it (such as the angular gyrus, anterior cingulate cortex, posteromedial cortex, medial temporal gyrus, medial frontal gyrus and superior frontal gyrus) became active when the individual was no longer engaged in any particular task. What could a network that becomes active when you’re no longer doing anything be good for? For many things, scientists soon realized. Just because you’re not moving doesn’t mean you’re not really doing anything. You can be thinking about all kinds of things in “offline” mode: you might be recalling certain memories, or making plans for the future, or trying to understand what someone was trying to tell you, or, more broadly, trying to gain insight into other people’s mental states.
As has now been shown, the default mode network is more active in all of these situations, as well as in some more nebulous ones, such as when you’re daydreaming and making all kinds of free associations. As has also been shown recently, the default mode network may co-operate with other brain areas involved in controlling cognitive tasks that require the use of working memory. So apart from the fact that the default mode network is apparently involved in some high-level processes, there is still no consensus regarding its function.
This is where the study by Margulies et al. seems to have advanced our understanding. First, these authors have successfully confirmed the existence of a connectivity gradient (from the more unimodal regions to the more multimodal regions of the brain) that reflects structural characteristics of the brain’s anatomy with which we are already familiar (the physical position of the visual areas relative to that of the associative areas of the cortex). Second, these authors showed that the various known networks of the brain could be situated along this gradient, with the networks involved in perception and action clustered toward the more unimodal end and the networks associated with increasingly abstract cognitive functions clustered toward the more multimodal end. And can you guess which network is located farthest along this gradient of increasing abstraction? Why, the default mode network, of course.
The Margulies article makes some other interesting points as well. And I have also just learned that in January 2018, Margulies published an opinion piece on this subject in Trends in Cognitive Sciences. Once I’ve read and digested it, I’ll write another blog post to tell you what I’ve learned.
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