Evolutionary revelations: Positively not rhesus

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  • Published: Mar 1, 2013
  • Author: David Bradley
  • Channels: MRI Spectroscopy
thumbnail image: Evolutionary revelations: Positively not rhesus

Close, but not too close

Monkey see, monkey do. Different responses to movie watching in fMRI of human and rhesus macaque monkey. Image courtesy of Vanduffel

Functional magnetic resonance imaging (fMRI) studies have demonstrated that humans have at least two functional networks in the brain's cerebral cortex that are not present in our relatively close relative, the rhesus monkey. The findings suggest that this characteristic of the human brain emerged some time during the evolutionary process between our primate ancestor and modern humans and was probably not present in an ancestor common to rhesus monkeys and humans.

The evolution of primates has led to profound reorganization in brain anatomy and function, but little is known about how such structural changes are actually linked to changes in function. As such, studies on humans and monkeys could offer new clues and help us to understand how we reached the point where the human brain is perhaps the most complex object in the universe and yet those of our monkey cousins are not.

Totally topological

Neurophysiologist Wim Vanduffel of KU Leuven, Belgium and Harvard Medical School, Massachusetts, USA, working with colleagues in Italy and the USA, have gathered the first evidence to suggest that there are topologically and functionally correspondent networks in the sensory-motor and attention regions of the brains of humans and rhesus monkeys, based on fMRI studies. "More specifically, we revealed a possible monkey equivalent of the human ventral attention network," the team reports. "For other human networks, such as the language and the default-mode networks, we detected topological correspondent networks in the monkey, but with different functional signatures." However, they also saw two lateralized human frontoparietal networks in the cortical regions, which they say show the greatest divide, or evolutionary expansion. These regions have neither topological nor functional monkey correspondents, the team adds.

The point in evolution at which humans split from the common ancestor of the rhesus monkey occurred around 25 million years ago. In the intervening millennia, new regions of the brain have been added while others have disappeared or their functions have changed. This, the team, suggested beggars the question: "Has evolution given humans unique brain structures?" Researchers have mused on the notion that the human brain might have certain novel features not present in our ancestors nor in other living primates and monkeys. But evidence was lacking. Vanduffel and colleagues have now demonstrated that we may have something unique about our cortical brain networks after all.

Movie-watching monkeys

"We did functional brain scans in humans and rhesus monkeys at rest and while watching a movie to compare both the place and the function of cortical brain networks," explains Vanduffel. It is well known that even when we (and indeed monkeys) are inactive, apparently at rest, the brain is very active. Researchers refer to the different parts of the brain active at such times as 'resting state' networks and in general these networks are not dissimilar when observing humans and monkeys. "But we found two networks unique to humans and one unique network in the monkey," adds Vanduffel.

He explains that when watching a movie, the cortex needs to process a lot of visual and auditory information. Of course, it seems obvious that the human-specific resting state networks would react to such stimulation in very different ways to the brain of a monkey, which we might assume does not extract quite as much information from a movie as a human. The implication is that the human networks presumably have a different function to any of the resting state networks found in the monkey. "Brain structures that are unique in humans are anatomically absent in the monkey and there no other brain structures in the monkey that have an analogous function," Vanduffel says. "Our unique brain areas are primarily located high at the back and at the front of the cortex and are probably related to specific human cognitive abilities, such as human-specific intelligence," he adds.

"We aim to perform more and much better controlled comparative functional tests in our future comparative studies," Vanduffel told SpectroscopyNOW. "We will not only present visual and auditory stimuli to humans and monkeys but also sensory information from the other senses." He points out that the problem with the functional tests involving movie watching in the present study is that these are not well controlled and that humans and monkeys may perceive them quite differently despite the fact that the retinal and cochlear stimulation would be identical. He warns that all such results must be  nterpreted with care. "One of our goals is to get an as accurate image as possible of brain networks and (at smaller scale) areas which are (partially) homologous or not," he adds. "This may help learn us how evolution has shaped the primate brain."

Related Links

J Neurosci, , 2013, 33, 3259-3275: "Evolutionary-Novel Functional Networks in the Human Brain?"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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