Scientists at the National Eye Institute (NEI) have found that neurons in the superior colliculus, an ancient midbrain structure found in all vertebrates, are key players in allowing us to detect visual objects and events. This structure doesn’t help us recognize what the specific object or event is; instead, it’s the part of the brain that decides something is there at all. By comparing brain activity recorded from the right and left superior colliculi at the same time, the researchers were able to predict whether an animal was seeing an event. The findings were published in the journal Nature Neuroscience. NEI is part of the National Institutes of Health.
Perceiving objects in our environment requires not just the eyes, but also the brain’s ability to filter information, classify it, and then understand or decide that an object is actually there. Each step is handled by different parts of the brain, from the eye’s light-sensing retina to the visual cortex and the superior colliculus. For events or objects that are difficult 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 be the difference between tripping over the chair or successfully avoiding it. 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 superior colliculus.
The process of deciding to take an action (a behavior, like avoiding a chair) based on information received from the senses (like visual information) is known as “perceptual decision-making”. Most research into perceptual decision-making – in humans, non-human primates, or in other animals – uses mathematical models to describe a relationship between a stimulus shown to an animal (like moving dots, changes in color, or appearance of objects) and the animal’s behavior. But because visual information processing in the brain is highly complex, scientists have struggled to demonstrate that these mathematical models accurately mimic a biological process happening in the brain during decision-making.
In their new study, Krauzlis, Herman, and colleagues used an “accumulator threshold model” to study how neuronal activity in the superior colliculus relates to behavior. This commonly used model assumes that information builds up over time until it hits a certain threshold, whereupon 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 edges might become available, slowly convincing you that an object is there.) Because individual neurons can slowly build up information in this way, Herman and Krauzlis elected to use neuronal signals (instead of the experimental stimulus) as the input for their behavior-prediction model.
The researchers trained two monkeys to undertake a perceptual decision task: while holding down a lever, the monkeys would respond to subtle color changes in their peripheral vision. The researchers would indicate with a cue to the monkey which side they should pay attention to. If the cued side changed color, the monkey should release the lever, but if the non-cued (foil) side changed color, the monkey should ignore it.
During the task, the researchers recorded signals from the superior colliculi on both the right and left sides of the monkey’s brain at the same time. While both sides of the brain activated in response to color changes, the researchers discovered that if the difference in neuronal activity between the two sides reached a specific threshold (e.g., neurons in the right superior colliculus fired more strongly than the left), the monkey would release the lever.
To find out if this differential neuron activation was causing the monkey’s behavior, the researchers directly altered the neuronal activation only on one side, either injecting a drug to slightly lower neuron activity, or stimulating the neurons with a tiny electrical charge to raise their activity. When the neuronal signal was stronger on the side reacting to cued color changes, the monkeys were better at reporting cued changes and rejecting foil color changes. When the neuronal signal was lower on that side, the monkeys were poorer at reporting cued color changes. These alterations in behavior could be predicted by a simple computational adjustment to the threshold model.
One reason for using the color change stimulus, Krauzlis said, was that the superior colliculus doesn’t itself process this information. Instead, other parts of the brain process the changing color, and transmit that information to the superior colliculus in order for a decision to be made. In essence, this simple differential threshold of neuronal activity in the superior colliculus triggers the animal to report the presence of something in the visual field.
