The pupil size of the eyes is not only affected by light, but also by a person's inner state of the brain. An international research team consisting of neuroscientists from the Universities of Göttingen and Tübingen in Germany, and the Baylor College of Medicine in Houston, USA, has now been able to provide initial answers to the question of why the pupil size changes with the inner state and whether these rapid, state-dependent changes in the pupil change the way we perceive our surroundings. The results were published in the journal "Nature".
The emotional state determines preception, as a research team found out. Picture: Konstantin Willeke
Bernstein members involved: Konstantin Willeke, Fabian Sinz
Internal state changes pupil size
The eyes are also often referred to as the “windows to the soul”. In fact, there is a grain of neurobiological truth to this. Pupil size is not only influenced by sensory stimuli such as sunlight, but also by our current internal state such as fear, excitement or attention. Interestingly, these state-dependent changes in pupil size are observed not only in humans but in many other vertebrates.
Artificial intelligence for data analysis
In experiments, an international team of researchers from the universities of Tübingen and Göttingen in Germany together with scientists from Baylor College of Medicine, investigated how state-dependent changes in pupil size affect the vision of mice. While the eyes convert light to neural activity, it is the brain which is crucial for the interpretation of visual scenes. In their experiments, the researchers showed mice different colored images and recorded the activity of thousands of individual neurons within the visual cortex, a particularly relevant brain area for visual perception. Based on these recordings, they used deep neural networks to create a computer model as a digital twin of the cortex, simulating the responses of large numbers of neurons in the brain. They then used this computer model to identify the optimal visual light stimulus for each neuron – each neuron’s “favorite image”.
Effects on visual perception
This model revealed something quite interesting: When the mice dilated their pupils due to an alert state of mind, the color sensitivity of the neurons to change from green to blue light within seconds. This was particularly true for neurons that sample stimuli from the upper hemisphere used to observe the sky. In subsequent experiments they were able to verify that this also happens in the real biological neurons.
With the help of eye drops that dilate the pupil, the researchers were then able to simulate the higher sensitivity to blue light even for a quiet brain state. “These results clearly demonstrate that pupil dilation due to an alert brain state can directly affect visual sensitivity and probably visual perception as well. The mechanism here is that a larger pupil lets more light into the eye, recruiting different types of photoreceptors in our retina and thus indirectly changing the color sensitivity in the visual cortex,” explains Dr. Katrin Franke, research group leader at the Institute for Ophthalmology Research at the University of Tübingen and first author of the study.
But what are the benefits of this change in visual sensitivity? Konstantin Willeke, shared first author of the study and member of the research group led by Dr. Fabian Sinz explains: “We were able to show that the higher neuronal sensitivity to blue light probably helps the mice to better recognize predators against a blue sky.” The computer model that the researchers created can also prove useful in many ways: “We assume that our model can be used for further experiments to understand visual processing.”
“Combining high throughput experimental data with AI modeling is opening a new era in neuroscience research. They enable us to extract accurate digital twins of real-world biological systems from data.” Willeke adds. “With these digital twins, we can perform an essentially unlimited number of experiments in the computer. In particular, we can use them to generate very specific hypotheses about the biological system which we can then verify in physiological experiments.”
The finding that brain state-related changes in pupil size affect visual sensitivity has implications for our understanding of vision well beyond predator detection in mice. Further research questions now arise as to how perception in numerous other animals is influenced by this effect. The pupils in our eyes could thus not only be a window into the soul, but also change the way we perceive the world from moment to moment depending on our inner state of mind.
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