Familiar categories of mental functions such as perception, memory and attention reflect our experience of ourselves, but they are misleading about how the brain works. More revealing approaches are emerging.
Neuroscientists have tried to map various categories of mental function to specific regions of the brain, but recent work has shown that the definitions and boundaries of those regions are complex and context-dependent.
Neuroscientists are the cartographers of the brain’s diverse domains and territories — the features and activities that define them, the roads and highways that connect them, and the boundaries that delineate them. Toward the front of the brain, just behind the forehead, is the prefrontal cortex, celebrated as the seat of judgment. Behind it lies the motor cortex, responsible for planning and coordinating movement. To the sides: the temporal lobes, crucial for memory and the processing of emotion. Above them, the somatosensory cortex; behind them, the visual cortex.
Not only do researchers often depict the brain and its functions much as mapmakers might draw nations on continents, but they do so “the way old-fashioned mapmakers” did, according to Lisa Feldman Barrett, a psychologist at Northeastern University. “They parse the brain in terms of what they’re interested in psychologically or mentally or behaviorally,” and then they assign the functions to different networks of neurons “as if they’re Lego blocks, as if there are firm boundaries there.”
But a brain map with neat borders is not just oversimplified — it’s misleading. “Scientists for over 100 years have searched fruitlessly for brain boundaries between thinking, feeling, deciding, remembering, moving and other everyday experiences,” Barrett said. A host of recent neurological studies further confirm that these mental categories “are poor guides for understanding how brains are structured or how they work.”
Neuroscientists generally agree about how the physical tissue of the brain is organized: into particular regions, networks, cell types. But when it comes to relating those to the task the brain might be performing — perception, memory, attention, emotion or action — “things get a lot more dodgy,” said David Poeppel, a neuroscientist at New York University.
No one disputes that the visual cortex enables sight, that the auditory cortex enables hearing, or that the hippocampus is essential for memory. Damage to those regions impairs those abilities, and researchers have identified mechanisms underlying them in those areas. But memory, for example, also requires brain networks other than the hippocampus, and the hippocampus is turning out to be key to a growing number of cognitive processes other than memory. Sometimes the degree of overlap is so great that the labels start to lose their meaning.
“The idea that there’s some kind of strong parallelism between mental categories that neuroscientists use to try and understand the brain and the neural implementation of mental events is just wrong,” Barrett said.
And while the current framework has led to important insights, “it’s gotten us stuck in certain traps that are really stifling research,” said Paul Cisek, a neuroscientist at the University of Montreal — an outcome that has also directly hobbled the development of treatments for neurological and psychological conditions.
That is why Barrett, Cisek and other scientists argue that for us to truly understand how the brain works, concepts at the field’s core may need to be revised, perhaps radically. As they grapple with that challenge, they are uncovering new ways to frame their questions about the brain, and new answers: This month alone, one such approach revealed an unexpected link between memory formation and metabolic regulation. But even if a new framework succeeds in explaining the brain’s operation, some researchers wonder whether the price of that success will be a loss of connection to our human experience.
‘More Aliases Than Sherlock Holmes’
When functional magnetic resonance imaging (fMRI) and other powerful technologies made it possible to examine living brains in increasingly sophisticated ways, neuroscientists enthusiastically started searching for the physical basis of our mental faculties. They made great strides in understanding the neural foundations of perception, attention, learning, memory, decision-making, motor control and other classic categories of mental activity.
But they also found unsettling evidence that those categories and the neural networks that support them don’t work as expected. It’s not just that the architecture of the brain disrespects the boundaries between the established mental categories. It’s that there’s so much overlap that a single brain network “has more aliases than Sherlock Holmes,” Barrett said.
Recent work has found, for instance, that two-thirds of the brain is involved in simple eye movements; meanwhile, half of the brain gets activated during respiration. In 2019, several teams of scientists found that most of the neural activity in “perception” areas such as the visual cortex was encoding information about the animals’ movements rather than sensory inputs.
This identity crisis isn’t limited to neural centers of perception or other cognitive functions. The cerebellum, a structure in the brains of all vertebrates, was thought to be dedicated almost exclusively to motor control, but scientists have found that it’s also instrumental in attention processes, the regulation of emotions, language processing and decision-making. The basal ganglia, another ancient part of the brain usually associated with motor control, has been similarly implicated in several high-level cognitive processes.
Some of these confusing results may come from methodological problems. To find where the human brain performs different functions, for instance, neuroscientists typically correlate cognitive processes with patterns of brain activity measured by fMRI. But studies suggest that researchers need to be more alert to irrelevant muscle twitches and fidgets that may contaminate the readings.
“You think that your results are telling you something about high-level cognition,” said György Buzsáki, a neuroscientist at the NYU School of Medicine, “when in fact, it may reflect nothing else except that, because of the task, [the subject’s] eyes are moving differently.”
But he and other scientists believe the recent findings also highlight a deeper conceptual problem in neuroscience. “We divide the real estate of the brain according to our preconceived ideas, assuming — wrongly, as far as I’m concerned — that those preconceived ideas have boundaries, and the same boundaries exist in brain function,” Buzsáki said.[…]
August 24, 2021
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