Two new studies show how spatial parts of the brain are already functioning in infancy, revealing that not everything we understand about our surroundings is learned
By Katherine Harmon
Not everyone has a perfect sense of direction, whether they would like to admit it or not. But two new studies have found that even baby rats have a basic spatial framework in their brains ready to use as soon as they leave the nest for the first time—which is much earlier than had previously been documented.
The findings reveal that not all sense of space is learned. They show that at least some of that sense is innate, "that the basic constituents of the cognitive map develop independently of spatial experience or might even precede it," noted the authors of one of the new studies, both published online June 17 in Science.
For the two independent studies researchers record rats' neuronal firings as soon as newborn pups opened their eyes and began to explore their surroundings. Both teams were surprised to find adult-level cell function in some of the directional regions.
At this age, "the animals would not yet have had a chance to explore the environment beyond their nest," Francesca Cacucci, a researcher at the Institute of Behavioral Neuroscience at University College London and co-author of one of the papers, writes in an e-mail. "This suggests strongly that sense of direction is independent of spatial experience."
And because the mammalian hippocampus is relatively consistent in its make-up across species, these lab rat–based findings likely mirror a similar developmental trajectory in humans.
Other abilities, such as face perception or language use, are thought to be innate. But "space is such a basic cognitive function, and to have it be partly innate is really interesting and groundbreaking work," says Linda Palmer, a project scientist at the Department of Anatomy and Neurobiology at the University of California, Irvine, who coauthored a perspectives essay accompanying the two studies in Science.
Three-celled mapping
To navigate a space, mammals seem to need only three basic types of cells, and the new finding suggests that they are ready to fire up in the brain at a very early age.
These cells reside in the hippocampus and record direction (where the head is pointing), place (location in an environment), and grid (distance covered while moving).
In both studies researchers implanted electrodes in the brains of the rat pups about 14 days after birth. Before that age rat pups remain, eyes sealed shut, in the nest. As soon as their eyes open, however, the young pups begin to explore their environment.
"As soon as the baby rat is able to leave the nest it has the basic foundations of a neural map of the environment ready to use," Rosamund Langston, a lecturer in behavioral neuroscience at the Ninewells Hospital and Medical School in Scotland and lead author on the second study, notes in an e-mail.
The most developed cells were the head-direction cells, which "are there, fully fledged," as soon as the mouse opens its eyes and begins to explore, Palmer says.
Although these directional cells are functioning at near-adult levels early, there is a period of refinement over the next two weeks in which the place and grid cells develop more fully, suggesting that some spatial experience and learning might be involved. Place cells were first recorded at 16 days after birth in both experiments, but there was some discrepancy as to the grid cell emergence: Langston and her colleagues at the Kavli Institute for Systems Neuroscience at the Norwegian University of Science and Technology recorded early grid cells at day 16, whereas Cacucci and her group documented more mature ones at day 20.
That grid cells—presumed to record the distance an animal has traveled—developed after place cells was "unexpected," Cacucci noted. Both anatomically and computationally the reverse order would have seemed more likely because distance would help inform place cells, she said.
Global orientation
The new research explains the neuronal development of young rats as they explore their basic surroundings, but this process might seem much different than, say, birds navigating across continents or humans driving in an unfamiliar city.
Palmer explains that our larger sense of location and orientation (how a room fits into a building, which fits into a block, which fits into a city, etcetera) might simply be an extension of this basic, hippocampal cell–based understanding of our surroundings. As our known environment becomes more expansive, we assemble that into a larger representation, she speculates.
So why is it that some people always have to ask for directions (or refuse to even if they are lost)? This variability has not yet been pinned down at the neuronic level. In the rats there did not seem to be notable differences in the rate or onset of cell firing. "They have different characters or 'personalities' but all have these same neuronal signals," Langston noted, adding, however, that lab rats are bred to be as similar as possible for the sake of experiment replication.
Also, the researchers only observed the rats exploring a neutral enclosure and did not test the rats by pushing their individual navigational abilities to make a comparison between neuronal functioning and development. "We do not know whether having the neuronal correlates of space is a reflection of how well you can actually find your way around," Langston explained.
Although neither group set out to test specifically for sex differences in spatial representations, Langston pointed out that they found all of the rats to be consistent. That in itself, Langston noted, was "an interesting finding that came from our results considering the debate about the navigational skills of men versus women!"
The two studies also did not uncover how the rats make use of their cognitive maps. Langston is starting work that will study how early life experience shapes map development and how they use these representations in the future.
Representing the world
The distinction between a partially innate model of spatial representation and fully learned model might seem minute or trifling—especially with the bourgeoning selection of navigational gadgetry at our fingertips that can not only pinpoint our home on a city plan but also point us in the right direction to find our car in a parking lot. But Palmer, whose background is in Kantian philosophy, finds the results quite relevant and "very encouraging."
"On the philosophical side, it's really fascinating to think about the question of how we represent the world," she says. And the difference between experientially developed and (at least partially) biologically innate means that in philosophical terms some of our understanding of the world is absolute, rather than relative, such as notions of left and right, near and far. "The thing about space is it's cognitive—it's a representation of the world, and to have something like that be innate is really interesting," she notes.
Similar studies are not possible in humans, unless they are being treated for rare brain disorders, due to the invasive nature of the procedure. Researchers can, however, use fMRI (functional magnetic resonance imaging) to look for more general neuronal activity in the brain as space is represented and correlate that to readings from individually recorded cells in rodents.
But the patterns of neuronal development in rats are likely to be similar to that of humans—if on a different timescale—Cacucci noted. The ability to use a single visual landmark for spatial orientation occurs in rats about 15 days after birth and at about six months in humans. Being able to construct a maplike concept of a space, however, does not occur until about 21 days after birth in rats—and nine months in babies—both ages around the time of weaning, she pointed out.
If these senses of space and orientation are even more hardwired that had previously been thought, it begs the question: "Where does it come from?" Palmer notes. "We would assume that the brain is shaped by evolution, so that means it must be good enough for reproductive success…but does that mean it's accurate?"
The findings reveal that not all sense of space is learned. They show that at least some of that sense is innate, "that the basic constituents of the cognitive map develop independently of spatial experience or might even precede it," noted the authors of one of the new studies, both published online June 17 in Science.
For the two independent studies researchers record rats' neuronal firings as soon as newborn pups opened their eyes and began to explore their surroundings. Both teams were surprised to find adult-level cell function in some of the directional regions.
At this age, "the animals would not yet have had a chance to explore the environment beyond their nest," Francesca Cacucci, a researcher at the Institute of Behavioral Neuroscience at University College London and co-author of one of the papers, writes in an e-mail. "This suggests strongly that sense of direction is independent of spatial experience."
And because the mammalian hippocampus is relatively consistent in its make-up across species, these lab rat–based findings likely mirror a similar developmental trajectory in humans.
Other abilities, such as face perception or language use, are thought to be innate. But "space is such a basic cognitive function, and to have it be partly innate is really interesting and groundbreaking work," says Linda Palmer, a project scientist at the Department of Anatomy and Neurobiology at the University of California, Irvine, who coauthored a perspectives essay accompanying the two studies in Science.
Three-celled mapping
To navigate a space, mammals seem to need only three basic types of cells, and the new finding suggests that they are ready to fire up in the brain at a very early age.
These cells reside in the hippocampus and record direction (where the head is pointing), place (location in an environment), and grid (distance covered while moving).
In both studies researchers implanted electrodes in the brains of the rat pups about 14 days after birth. Before that age rat pups remain, eyes sealed shut, in the nest. As soon as their eyes open, however, the young pups begin to explore their environment.
"As soon as the baby rat is able to leave the nest it has the basic foundations of a neural map of the environment ready to use," Rosamund Langston, a lecturer in behavioral neuroscience at the Ninewells Hospital and Medical School in Scotland and lead author on the second study, notes in an e-mail.
The most developed cells were the head-direction cells, which "are there, fully fledged," as soon as the mouse opens its eyes and begins to explore, Palmer says.
Although these directional cells are functioning at near-adult levels early, there is a period of refinement over the next two weeks in which the place and grid cells develop more fully, suggesting that some spatial experience and learning might be involved. Place cells were first recorded at 16 days after birth in both experiments, but there was some discrepancy as to the grid cell emergence: Langston and her colleagues at the Kavli Institute for Systems Neuroscience at the Norwegian University of Science and Technology recorded early grid cells at day 16, whereas Cacucci and her group documented more mature ones at day 20.
That grid cells—presumed to record the distance an animal has traveled—developed after place cells was "unexpected," Cacucci noted. Both anatomically and computationally the reverse order would have seemed more likely because distance would help inform place cells, she said.
Global orientation
The new research explains the neuronal development of young rats as they explore their basic surroundings, but this process might seem much different than, say, birds navigating across continents or humans driving in an unfamiliar city.
Palmer explains that our larger sense of location and orientation (how a room fits into a building, which fits into a block, which fits into a city, etcetera) might simply be an extension of this basic, hippocampal cell–based understanding of our surroundings. As our known environment becomes more expansive, we assemble that into a larger representation, she speculates.
So why is it that some people always have to ask for directions (or refuse to even if they are lost)? This variability has not yet been pinned down at the neuronic level. In the rats there did not seem to be notable differences in the rate or onset of cell firing. "They have different characters or 'personalities' but all have these same neuronal signals," Langston noted, adding, however, that lab rats are bred to be as similar as possible for the sake of experiment replication.
Also, the researchers only observed the rats exploring a neutral enclosure and did not test the rats by pushing their individual navigational abilities to make a comparison between neuronal functioning and development. "We do not know whether having the neuronal correlates of space is a reflection of how well you can actually find your way around," Langston explained.
Although neither group set out to test specifically for sex differences in spatial representations, Langston pointed out that they found all of the rats to be consistent. That in itself, Langston noted, was "an interesting finding that came from our results considering the debate about the navigational skills of men versus women!"
The two studies also did not uncover how the rats make use of their cognitive maps. Langston is starting work that will study how early life experience shapes map development and how they use these representations in the future.
Representing the world
The distinction between a partially innate model of spatial representation and fully learned model might seem minute or trifling—especially with the bourgeoning selection of navigational gadgetry at our fingertips that can not only pinpoint our home on a city plan but also point us in the right direction to find our car in a parking lot. But Palmer, whose background is in Kantian philosophy, finds the results quite relevant and "very encouraging."
"On the philosophical side, it's really fascinating to think about the question of how we represent the world," she says. And the difference between experientially developed and (at least partially) biologically innate means that in philosophical terms some of our understanding of the world is absolute, rather than relative, such as notions of left and right, near and far. "The thing about space is it's cognitive—it's a representation of the world, and to have something like that be innate is really interesting," she notes.
Similar studies are not possible in humans, unless they are being treated for rare brain disorders, due to the invasive nature of the procedure. Researchers can, however, use fMRI (functional magnetic resonance imaging) to look for more general neuronal activity in the brain as space is represented and correlate that to readings from individually recorded cells in rodents.
But the patterns of neuronal development in rats are likely to be similar to that of humans—if on a different timescale—Cacucci noted. The ability to use a single visual landmark for spatial orientation occurs in rats about 15 days after birth and at about six months in humans. Being able to construct a maplike concept of a space, however, does not occur until about 21 days after birth in rats—and nine months in babies—both ages around the time of weaning, she pointed out.
If these senses of space and orientation are even more hardwired that had previously been thought, it begs the question: "Where does it come from?" Palmer notes. "We would assume that the brain is shaped by evolution, so that means it must be good enough for reproductive success…but does that mean it's accurate?"
No hay comentarios:
Publicar un comentario