Monday, November 26, 2018
Neurons from an older part of the brain code decisions based on visual information.
Scientists from the National Eye Institute (NEI) have found that upper collumn neurons, an ancient mesencephalon structure found in all vertebrates, are key players in the detection of visual objects and events. This structure does not help us to recognize the specific object or event; instead, it is the part of the brain that decides that something is there. By comparing brain activity recorded by both the upper right and left collicles, researchers were able to predict whether an animal was attending an event. The results were published today in the journal Nature Neuroscience. NEI is part of the National Institutes of Health.
To perceive objects in our environment, we need not only the eyes, but also the ability of the brain to filter the information, to classify it, then to understand or decide that an object is really there. Each stage is managed by different parts of the brain, from the retina of vision detection to the visual cortex to the superior colliculus. For events or objects that are hard to see (a gray chair in a dark room, for example), small changes in the amount of visual information available and recorded in the brain can make the difference between tripping over the chair or avoid it successfully. This new study shows that this process – deciding that an object is present or that an event has occurred in the visual field – is handled by the upper colliculus.
"Although we had known for a long time that the upper collide is involved in perception, we really wanted to know exactly how this part of the brain controlled the perceptual choice and find a way to describe this mechanism with the help of one. mathematical model, "said James Herman, Ph.D., lead author of the study.
"The superior colliculus plays a fundamental role in our ability to process and detect events," said Richard Krauzlis, Ph.D., senior researcher at NEI's Sensorimotor Research Laboratory and lead author of the study. "This new work not only shows that a specific population of neurons directly causes behavior, but also that a commonly used mathematical model can predict behavior based on these neurons."
The process of deciding to take a measurement (a behavior, such as avoiding a chair) based on information received from the senses (such as visual information) is called "perceptual decision-making". Most research on perceptual decision making – in humans, non-human primates or other animals – uses mathematical models to describe a relationship between a stimulus shown to an animal (such as points in movement, color changes or the appearance of objects) and the behavior of the animal. But because the processing of visual information in the brain is extremely complex, scientists have struggled to demonstrate that these mathematical models accurately mimic a biological process that takes place in the brain during decision-making.
In their new study, Krauzlis, Herman, and his colleagues used an "accumulator threshold model" to study the link between neuronal activity in upper colliculus and behavior. This commonly used model assumes that information accumulates over time until it reaches a certain threshold, after which a person or animal makes a decision. (For example, as you get closer to the gray chair in the dark room, more details about shadows or outlines become available, which slowly convinces you that an object is there.) As individual neurons can slowly accumulate information in this way, Herman and Krauzlis chose to use neural signals (instead of the experimental stimulus) as input for their behavior prediction model.
The researchers trained two monkeys to undertake a perceptual decision-making task: while holding down a lever, they reacted to subtle color changes in their peripheral vision. The researchers would tell the monkey which side to pay attention to. If the cut side has changed color, the monkey should release the lever, but if the uncut side (leaf) has changed color, the monkey should ignore it.
During the task, the researchers recorded the signals of the upper colliculi of the right and left sides of the monkey brain at the same time. While both sides of the brain were activated in response to color changes, the researchers found that if the difference in neuronal activity between the two sides reached a specific threshold (for example, the neurons of the right upper colliculus were pulling more strong that would release the lever.
To determine whether this differential neuronal activation was at the origin of the monkey's behavior, the researchers directly altered the neuronal activation of one side, injecting a drug to slightly reduce the activity neurons or stimulating neurons with a tiny electrical charge to increase their activity. When the neural signal was stronger on the side responding to the reported color changes, the monkeys were better able to signal the reported changes and reject the color changes of the leaf. When the neural signal was lower on this side, the monkeys were less able to report the reported color changes. These behavioral changes could be predicted by a simple computer fit of the threshold model.
According to Mr. Krauzlis, one of the reasons for using the color change stimulus is that the upper collector does not process this information itself. Instead, other parts of the brain process the changing color and pass that information to the upper collicle so that a decision is made. In essence, this simple differential threshold of neuronal activity in the upper colliculus causes the animal to signal the presence of something in the visual field.
"It is surprising to discover that despite the sophisticated visual machinery that we have in the cerebral cortex, these older evolutionary structures remain essential for the visual perception we are used to," said Herman.
"For this kind of task, where you're not asked to say exactly what the thing was, but you just say, did that happen, then this activity in the upper collector seems to be at the necessary and sufficient, "said Krauzlis.
While the model accurately predicted upper-colliculus activity-based behavior, the upper colliculus neuron activation pattern and the signal threshold itself were unique for each monkey, which means that each monkey had its own behavioral signal code.
Reference: Herman JP, Katz LN and Krauzlis RJ. "The average brain activity can explain perceptual decisions during a task of attention." November 26, 2018. Nature Neuroscience. doi: 10.1038 / s41593-018-0271-5.
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NEI leads the federal government's research on the visual system and eye diseases. NEI supports basic and clinical science programs aimed at developing eye-saving treatments and meeting the special needs of the visually impaired. For more information, visit https://www.nei.nih.gov.
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