Developmental Biology - Brain|
All too human
The price humans pay for advanced brains may be a greater tendency towards brain disorders...
Professor Rony Paz of the Weizmann Institute of Science suggests that our brains are like modern washing machines - evolved to have the latest sophisticated programming, but more vulnerable to breakdown with costly disorders. He and a group of researchers recently conducted experiments comparing the efficiency of the neural code in non-human and human primates, to find that as the neural code gets more efficient, prevention of errors is reduced. Their findings, appearing in Cell, may explain why in humans, ADHD, anxiety, depression, PTSD and autism are common.
Paz, in the Institute's Neurobiology Department, says that anatomical differences between humans and other primates have been well determined, particularly in our large pre-frontal cortex and its number of neurons. But differences in the neural code - our "software," in contrast to our "hardware" (physical structures) - hasn't been well explored.
Raviv Pryluk, a research student in Paz's group, has now devised a way to test and compare the efficiency of the neural code in several regions of the primate brain. "We define efficient communication as that which uses the least amount of energy to transmit the maximum information - to pass on complicated messages with the fewest 'words'," explains Pryluk.
Researchers recorded electric activity in single neurons both in humans and in macaque monkeys in two brain regions: the pre-frontal cortex, where decision making and rational thinking occur; and in the amygdala, a more evolutionarily ancient brain region responsible for emotions and the "fight or flight" basic survival function.
Paz and his group worked in collaboration with Professor Itzhak Fried PhD, Sourasky Medical Center, Tel Aviv and UCLA Medical School in Los Angeles. Patients with pharmacologically intractable epilepsy come to Fried to have electrodes implanted for diagnostic purposes, providing a rare opportunity to record electrical activity of single neurons in the human brain.
The findings provide support for a "washing machine" theory of brain evolution which describes neural code in the pre-frontal cortex as more efficient than in the amygdala, both in humans and monkeys. The neural code of both areas in the human brain being more efficient than the monkey counterpart. However, higher efficiency of either is at the expense of a less robust ability to prevent errors.
The amygdala is similar to the washing machine drum in that "It isn't highly sophisticated, but is less likely to fail - important to animal survival." Paz continues: "Human amygdala error may be an exaggerated survival response to inappropriate stimuli, as seen in PTSD and other anxiety disorders."
"Comparing single-cells from human and monkey brains is a large step toward answering the question of what makes the human brain unique.span>
Itzhak Fried PhD, Department of Neurosurgery, University of California, Los Angeles, CA, USA; Department of Neurology and Neurosurgery, Sackler Faculty of Medicine, Tel Aviv University and Functional Neurosurgery Unit, Tel Aviv Medical Center, Tel Aviv, Israel.
"Why on one hand, do humans have such superior learning, cognitive and adaptive abilities — but on the other, a tendency to anxiety, depression and mental diseases? These may be two sides of the same coin."
Rony Paz PhD, Department of Neurobiology, Weizmann Institute of Science, Rehovot, Israel.
• Human neurons utilize information capacity (efficiency) better than macaque neurons
• Cingulate cortex neurons are more efficient than amygdala neurons in both species
• Amygdala and monkey neurons show more synchrony and vocabulary overlap (robustness)
• There is a tradeoff between robustness and efficiency across species and regions
Many evolutionary years separate humans and macaques, and although the amygdala and cingulate cortex evolved to enable emotion and cognition in both, an evident functional gap exists. Although they were traditionally attributed to differential neuroanatomy, functional differences might also arise from coding mechanisms. Here we find that human neurons better utilize information capacity (efficient coding) than macaque neurons in both regions, and that cingulate neurons are more efficient than amygdala neurons in both species. In contrast, we find more overlap in the neural vocabulary and more synchronized activity (robustness coding) in monkeys in both regions and in the amygdala of both species. Our findings demonstrate a tradeoff between robustness and efficiency across species and regions. We suggest that this tradeoff can contribute to differential cognitive functions between species and underlie the complementary roles of the amygdala and the cingulate cortex. In turn, it can contribute to fragility underlying human psychopathologies.
Raviv Pryluk, Yoav Kfir, Hagar Gelbard-Sagiv, Itzhak Fried and Rony Paz.
This research was supported by the Adelis Foundation; the Irving and Dorothy Rom Family Discovery Endowment Fund; the Irving B. Harris Fund for New Directions in Brain Research; the Bernard and Norton Wolf Family Foundation; the Leff Family; the Oster Family Foundation; Mr. and Mrs. Gary Clayman; Rosanne Cohen; the estate of Toby Bieber; and the European Research Council.
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The tradeoff in human brains (PURPLE) and monkey brains (GREEN). Evolutionarily
advanced human brains are more efficient in design, but less robust against errors.
Image Credit: Weizmann Institute of Science