IMAGINE your life as an old animation movie in stop-motion: a sequence of static frames, scrolled quickly, produces the illusion of movement. Illusion, in fact, because the imperceptible jerky progress, devoid of the ‘trail’ that characterizes the movement, only temporarily deceives the brain and ends up generating a state of uncertainty. Fortunately, this is not the case. Our visual system is able to retain the information acquired in motion – without having to process the images of an object or a person from scratch to be able to recognize it – and therefore allows us a more stable vision of the external world.
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To these conclusions comes a study conducted by SISSA of Trieste in collaboration with the University of Pennsylvania and the Catholic University of Louvain, published in Nature Communications, which explains the neuronal origin of this phenomenon. “One of the great challenges of all sensory systems is to maintain a stable representation of the external world, despite the constant changes that occur around us. This also applies to the visual system” , said Davide Zoccolan, director of the Visual Neuroscience laboratory of the Trieste institute. To understand this, just look around: objects, animals, people are in constant motion. “This causes very rapid fluctuations in the signals acquired by the retina but until now it was not clear whether the same type of variations characterize the deeper structures of the visual cortex, where the information is integrated and processed. If so, we would live in a condition of great instability. “.
Visual stimuli
It has long been known that, following visual stimuli, the signals generated by the retina reach different stages of processing of the visual cortex, organized according to a precise hierarchy. It is this processing process that allows us to recognize an object or a face and to do so regardless of its position or angle. Although this had been demonstrated in the case of static stimuli, there was still no evidence for dynamic situations. To verify its existence, some researchers from SISSA, the University of Pennsylvania and the Catholic University of Louvain, analyzed the signals generated by the neurons present in the various visual cortical areas of some rodents following dynamic visual stimuli. “We used three distinct data sets: one acquired in Trieste, one in Louvain and one made freely available to the scientific community by the Allen Institute for Brain Science in Seattle” explains the neuroscientist, who specifies: “the visual stimuli used were of different type. In SISSA we have made special films with objects moving at different speeds. The other data were acquired using clips of various kinds of films, including cinematographic ones “.
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The processing of visual scenes
To verify that neural signals evolve according to different time scales, the researchers analyzed the signals recorded in the different areas of the visual cortex and then applied mathematical models to relate the images in the films with the activity of individual neurons. “The difficulty of this analysis is to identify a method that allows us to understand whether the processing of visual scenes becomes slower as we go deeper into the brain,” explains physicist Vijay Balasubramanian of the University of Pennsylvania. “Different stages of the brain process visual information at different time scales – some signals become more stable, others change faster. It is very difficult to tell if time scales systematically vary along the visual system. Our contribution was to develop a robust and reliable method to understand it “.
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Regardless of the nature of the visual stimulus, the results were the same: “We observed a greater persistence of the signals acquired in the deeper layers, a sort of perceptual constancy that guarantees a certain stability to the information, eliminating the fluctuations observed in the more superficial layers. “resumes Zoccolan.
The memory of the image
But not only. The researchers noted that there is a kind of inherent persistence that increases along the hierarchy of visual areas. In deeper areas, the neuronal response persists for a few hundreds of milliseconds even when the stimulus disappears, thus imposing a minimum duration on the encoding of the images which guarantees that the information is processed correctly. And therefore, that the reaction to the stimulus is also correctly calibrated. In other words, to guarantee us a more stable world, the visual system on the one hand, it reduces too rapid fluctuations but on the other hand it stalls in order not to make us lose potentially valuable information.
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