Brain changes that enable good visual discrimination learning
Our visual perception of the world is often considered relatively stable. However, like all of our cognitive functions, visual processing is shaped by our experiences. During development and adulthood, learning can change visual perception. For example, enhanced visual discrimination of the same pattern is a learned skill that is essential for reading. In a new research study published in Current Biology, scientists have now discovered the neural changes that occur during learning to increase discrimination of closely related visual images.
This study, led by first author Dr. Joseph Schumacher and senior author Dr. David Fitzpatrick at Florida’s Max Planck Institute of Neuroscience, sets out a transformative approach to studying perceptual learning in the brain. The researchers imaged the activity of a large number of single neurons over days to track changes that occur when the visual discrimination task is studied, performing these experiments in a new animal model, the tree mouse.
The tree rat is a small mammal with visual properties similar to humans, including a high degree of visual acuity and a similar spatial arrangement of visual responsive neurons in the brain. As the researchers point out, these animals can also learn complex behavioral tasks, making them ideal for understanding how experiences shape visual perception. In this study, tree rats were trained to distinguish between very similar visual images: identical black lines that differ only with a slight change in orientation (22.5 degrees). In assignments, the presentation of lines at one orientation is rewarded with a drop of juice. Over the course of days, the Tree Rat learned to distinguish between two similar visual images, licking only in response to a line in a rewarded orientation and resisting licking a line in an unrewarded orientation.
The scientists combined this behavioral task with measurements of neural activity in V1, an area of the brain important for visual processing. Neurons in this area are activated by specific features of visual input, such as light-dark edge orientation. Individual neurons show a ‘preference’ for a particular edge orientation, responding with the highest activity for this orientation and with progressively lower activity or no activity to edges oriented further away from the preferred orientation. In this way, visual scenes that have edges with different orientations activate certain subsets of neurons to produce patterns of neural activity that encode the information needed for visual perception.
Schumacher and colleagues found that visual discrimination learning in tree mice was accompanied by an increase in the difference in neural activity patterns elicited by the two visual images. This is mainly due to an increase in the amount of neural activity in response to the presentation of the rewarded stimulus orientation relative to the unrewarded orientation. But it’s not just a general increase in neural responses to rewarded stimuli. When the scientists examined the changes more closely, they found that these were mediated by changes in the activity of a very specific subset of neurons: those whose orientation preference was optimal for distinguishing the orientation of the rewarded stimulus from the unrewarded stimulus.
To fully understand the effects of learning on visual perception, the authors next investigated whether changes in neural activity that enhance visual discrimination persist outside the context of the task being studied. Interestingly, they found that the neural changes not only persisted but were accompanied by changes in the trained tree rat’s ability to discriminate against others. These included both improvement for some stimulus orientations and disruption for others—changes in behavior that were exactly as expected given the changes in the responses of this specific subset of neurons.
“This work demonstrates specific, experience-driven changes in neuronal activity that affect the perception of visual stimuli, increasing task performance-relevant discrimination at the expense of other related discrimination,” explains first author Joe Schumacher. Now the lab has set its sights on combining this approach with new technology to unlock the sequence and changes that occur in several types of neurons to mediate perceptual learning. By investigating these questions in the tree mouse’s visual system, scientists in Fitzpatrick’s lab uncovered fundamental new insights into perceptual learning that could impact our understanding of a variety of learning disorders.
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