Ancient Brain Region's Important Role in Vision Discovered
A highly conserved brain structure is more important for visual processing than we previously thought.
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As a neuroscience undergraduate, I was fortunate enough to spend time in the university’s dissection room. This was when I laid eyes on a human brain for the first time – and I was fascinated.
The beauty of the gyri and the sulci – the folds and grooves of the brain – and the contrasting colors of the grey and white matter were undeniable. A few deft cuts from the technician’s scalpel revealed the midbrain, which serves as an important connection point between the forebrain and the hindbrain. Here there were some – honestly, quite alien – bulging structures: the superior and inferior colliculi.
The name colliculi, our instructor told us, derives from the Latin word for “hill”.
The superior colliculus is a paired, highly conserved brain structure, which means it has remained relatively consistent across different species throughout evolution. Generally, if a brain region is conserved, its function is considered critical for the survival of an organism.
In fish and birds, the superior colliculus is quite large, but in humans, it’s relatively small. So what does this ancient, strange-looking structure do?
How do we distinguish an object from its background?
Neuroscientists have known for a long time that the superior colliculus plays an important role in the visual system. It receives input from the retina and the primary visual cortex and helps to mediate behavioral responses to visual stimuli in different organisms.
When you look at an object, it’s quite easy to distinguish it from its background. How the brain achieves this feat, however, is not yet known. For many years, it was assumed that the visual cortex was the key brain structure in this process. The issue here is that some organisms have a smaller, less-developed visual cortex than humans – yet they are still able to distinguish objects from their background.
A team of scientists from the Netherlands Institute for Neuroscience wanted to explore whether the superior colliculus is important for object detection. Their research is published in eLife.
Using optogenetics to explore the role of the superior colliculus
To investigate the role of the superior colliculus, Dr. Leonie Cazemier, a former PhD student in the at the Netherlands Institute for Neuroscience, and colleagues applied optogenetic techniques in mouse models.
What is optogenetics?
Optogenetics utilizes light to control neurons that have been engineered to be sensitive to illumination. By applying light, neurons can be “switched on” or “switched off”, allowing scientists to study their function.
“Previous research already showed that a mouse can still complete the task if you turn off its visual cortex, which suggests that there is a parallel pathway for visual object detection. In this study, we switched off the superior colliculus using optogenetics to see what effect that would have,” Dr. Alexander Heimel, an expert in the neurobiology of vision and the study’s lead author, said.
Heimel and colleagues specifically targeted a superficial part of the superior colliculus. “Interestingly, to our knowledge, this is one of only few studies that specifically inhibited the superficial part of superior colliculus bilaterally. Previous studies have inhibited or ablated superior colliculus, but often this was done in a unilateral fashion and/or targeting the deeper layers or the entirety of the colliculi,” they said.
The mice were subjected to a behavioral task where they were trained to distinguish a figure from a background. “The animals were trained to indicate the side on which a figure appeared by licking the corresponding side of a custom-made y-shaped lick spout,” the researchers explained.
When the superior colliculus neurons were “switched off”, the animals’ performance on the behavioral task declined. “The mice became worse at detecting the object, indicating that the superior colliculus plays an important role during this process,” said Heimel. “Our measurements also showed that information about the visual task is present in the superior colliculus, and that this information is less present the moment a mouse makes a mistake. So, its performance in the task correlates with what we’re measuring.”
The superior colliculus is important for visual detection of objects
The research provides evidence that a neural code in the superficial layers of the mouse superior colliculus is important for normal visual detection of objects. Heimel and colleagues said that they cannot rule out the possibility that deeper parts of the superior colliculus were recorded or affected by the optogenetic techniques, a limitation that must be addressed in future studies.
Though the research team are not clear on how humans distinguish an object from its background, they emphasize that many functions of the superficial superior colliculus are shared between rodents and primates.
“Although humans also have two parallel systems, their visual cortex is much more developed. The superior colliculus may therefore play a less important role in humans. It is known that the moment someone starts waving, the superior colliculus directs your gaze there. It is also striking that those who are blind with a double lesion in the visual cortex do not see anything consciously but can often still navigate and avoid objects,” Heimel said. “Our research shows that the superior colliculus might be responsible for this and may therefore be doing more than we thought.”
Reference: Cazemier JL, Haak R, Tran TL, et al. Involvement of superior colliculus in complex figure detection of mice. Wolff M, Gold JI, eds. eLife. 2024;13:e83708. doi: 10.7554/eLife.83708
This article is a rework of a press release issued by the Netherlands Institute for Neuroscience. Material has been edited for length and content.