What separates man from monkey? (Aside from some stricter etiquette around poop-flinging, that is.) Some researchers think they’ve found one key feature that’s unique to Homo sapiens: an area of the brain that seems to have no equivalent in other primates.
“We tend to think that being able to plan into the future, be flexible in our approach and learn from others are things that are particularly impressive about humans,” Oxford experimental psychologist Matthew Rushworth said in a statement. “We've identified an area of the brain that appears to be uniquely human and is likely to have something to do with these cognitive powers.”
Rushworth and colleagues compared brain scans of 25 adult humans to the brain scans of 25 macaque monkeys.
“The brain is a mosaic of interlinked areas,” Rushworth says. “We wanted to look at this very important region of the frontal part of the brain and see how many tiles there are and where they are placed. We also looked at the connections of each tile -- how they are wired up to the rest of the brain -- as it is these connections that determine the information that can reach that component part and the influence that part can have on other brain regions.”
As the team reported on Tuesday in the journal Neuron [PDF], it found that one of those “tiles” on the brain scan seems wholly human: A certain area in the frontal cortex, called the lateral frontal pole prefrontal cortex, seems to have no equivalent in the macaque. This is particularly interesting because the surrounding brain region is thought to be involved in a wide range of cognitive functions. Damage to the ventrolateral frontal cortex affects a person's language abilities, and the brain region is also implicated in a number of psychiatric disorders.
Comparisons of the human and monkey brain scans also revealed other, subtler differences in this brain region. The way that the ventrolateral frontal cortex interacts with other brain areas associated with hearing differs between humans and macaques, the researchers found.
"This could explain why monkeys perform very poorly in some auditory tasks and might suggest that we humans use auditory information in a different way when making decisions and selecting actions," co-author Franz-Xaver Neubert said in a statement.
The work jives with other studies that have shown identifiable differences between monkey and human brain structure. Last February, a team of researchers found two functional networks in human cortexes that react to visual stimulation (in the experiments, the researchers played a movie for the subjects) much differently than rhesus monkeys. They also found one network of connections that was unique to the monkeys.
“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,” Harvard Medical School researcher Wim Vanduffel said in a statement last February. “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.”
Neubert et al. “Comparison of Human Ventral Frontal Cortex Areas for Cognitive Control and Language with Areas in Monkey Frontal Cortex.” Neuron published 28 January 2014.