Wednesday, January 29, 2014

Detailed mapping of the human frontal lobe


Humans and monkeys are closely related,  and even though human brains are much larger, they share a great deal of similarity in organization with those of monkeys. But humans have capabilities, especially in language and abstract reasoning, which are only modest in monkey species. It remains an ongoing question how the expanded abilities of humans are reflected in brain organization. Are human brains just bigger versions of a monkey template? Or do human brains have fundamentally new brain areas, either anatomically or by virtue of their functional connections? A recent study in Neuron by Neubert et al. [pdf link] studied this issue by using magnetic resonance imaging (MRI) to compare the brains of humans and macaque monkeys.
The frontal lobe of the human brain (source: Wikipedia).
The ventrolateral  areas, studied by Neubert et al are 44-47 and
(roughly) 10 and 11.
Neubert et al focused on the ventrolateral frontal cortex  (vlFC), because this area is associated with processes, such as language (for example, Broca's area) and cognitive flexibility, which are notably developed in humans relative to monkeys. Their study relied on two major MRI methods. To map if human brains contained new areas, not present in monkeys, they used diffusion-weighted MRI . This method extracts the magnetic resonance signals arising from differences in water diffusion, based on local tissue properties such as cell bodies, white matter fibers, or connective tissue. This type of MRI can sensitively and non-invasively reveal the neuroanatomy. 
To ask whether brain areas were differently connected in humans compared to monkeys, they used resting state functional MRI , which relies on correlated firing patterns to identify which neuronal areas are wired together.
Neubert et al. used these two methods on 25 humans and 25 macaques, and found a great deal of consistency in both the neuroanatomy and in the functionally correlated
brains. What's more, within the ventrolateral frontal cortex that they were studying, most areas in humans had a direct correlate-- anatomically and functionally-- with the macaques. They did find evidence for one area in human brain scans that did not have a direct equivalent in macaques. The lateral frontal pole region (FPl; labelled in red at the font of the brain in the image to the left), was distinctively human. It seemed to be anatomically related to area 46 (which is present in macaque, and shown in yellow in the figure) but lacking a lot of the functional connectivity of that region. 
Neubert et al. comment that the FPl is used in humans for strategic planning, multitasking, and for calibrating a correct response when many alternatives are available. 
This work is a very valuable effort to make an apples-to-apples comparison of brains in a region that is important for many of the traits that we consider most human. To really understand the significance of this area, it would be really helpful to include brain information from the nearest human relative, i.e. chimpanzees. It would also be very interesting if a specialized old-world monkey It would also be helpful to see whether niche (tree dwelling versus forest floor) or gregariousness could be correlated with the functional properties of the frontal lobes. Finally, Neubert et al noted some differences in long-range connectivity between humans and macaques from most areas in the frontal lobe. It might be that much of what we consider uniquely human might be enabled by these longer connections. 

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