Charting a new path for the organization of visual space

Demanding a keen eye, mathematical precision and a keen eye for aesthetics, card making is a unique application of art and science. Although the scale may differ, neuroscientists who study vision are like mappers of the brain; study and map how our brains represent and make sense of what we see in the world. The visual cortex, a specialized region responsible for visual processing, contains complex neural circuits that evaluate information from our eyes and preferably respond to distinctive visual characteristics such as color, edges, movement, and location in the eye. visual space. Despite the complexity of this information, our brains do a remarkable job of efficiently organizing neurons together, helping us better understand our visual landscapes.

A major organizational property that the visual cortex uses is called retinotopic mapping, where the neurons inside are arranged in an orderly fashion that preserves spatial information from the retina (the light sensing part of the eye). Just as the Mercator projection is for mapping, it is believed that retinotopic maps of the visual cortex follow a widely adopted and well characterized pattern. The dominant theory is that brain areas like the primary visual cortex (V1) follow a simple and fluid mapping method. What you see is what you get; objects in visual space that activate parts of the retina, will illuminate neurons in an identical pattern in the brain.

Despite the wealth of evidence in several species supporting this type of linear mapping, small clues and discrepancies existed in previous studies that suggested the possibility of other arrangements. The question remained: are there additional methods of spatial mapping in the brain?

Shedding light on this question and challenging the mainstream theory, researchers at MPFI’s Fitzpatrick Lab have discovered for the very first time a new type of spatial mapping in the secondary area (V2) of the visual cortex. Recently published in Neuron, the team used a combination of functional single-cell imaging, computer modeling, and connectivity studies, to reveal sinusoidal or wave organization in the V2 zone of the tree shrew. Their surprising insight deepened our understanding of neuronal representations of visual space and underscored the importance of accurate retinotopic mapping in the visual cortex.

Madineh Sedigh-Sarvestani, Ph.D., postdoctoral researcher at MPFI and first author of the publication, joined the Fitzpatrick lab interested in understanding the organization, function and behavioral link of visual areas beyond V1. His investigation began in V1’s closely related neighbor, V2, a visual area that has been studied extensively in primates, but less in animals susceptible to recent genetic tools developed in mice. The tree shrew fits this criterion perfectly, as it is a close relative of primates and has a smooth brain ideal for imaging. Using high-resolution calcium imagery, Sedigh-Sarvestani expected to find a visual space map very similar to the gold standard of V1.

Presenting visual stimuli to tree shrews that varied in position in the visual field, the team mapped the corresponding neurons in V2 that illuminated in response to the location of a visual stimulus in space. What they found were two very different maps in V2. The map of an object’s elevation, how high or low it is, followed closely by the smooth linear map found in V1, but mapping the azimuth, its horizontal position to the left or right of center , revealed a radically different sinusoidal or oscillating pattern. But why would simple spatial maps exist in V1 and more complex in V2, could differences in the shapes of regions play a role? To answer this question, MPFI researchers turned to computer modeling to recreate the conditions present in the brain, with the aim of producing a spatial map that optimizes visual field coverage. By varying only the shape, the algorithm found that the optimal spatial map for the square region V1 followed the smooth and linear arrangement, but for the thin and elongated V2, a sinusoidal map appeared to corroborate the previous results. To cement this idea, the MPFI team, led by laboratory histology coordinator Nicole Shultz, used colored dyes to trace the connections of V1 to the V2 region, finding that the neuronal projections of V1 were perfectly aligned with the sinusoidal map of V2.

“Our results demonstrate that an orderly organization of visual space in the brain does not necessarily have to follow the guiding principles we are used to thinking,” notes David Fitzpatrick, Ph.D., CEO and Scientific Director of MPFI. “While this organization may be less straightforward than we originally thought, it still has a remarkable and magnificent order.”

Beyond this intriguing discovery, researchers at the Fitzpatrick lab made another critically important discovery with broad implications for the field of visual neuroscience; neuronal preference for certain visual characteristics is directly related to the retinotopic map of visual space. Considered primarily as independent organizational principles, the MPFI team demonstrated their interdependence by studying the response properties of neurons in V2 for binocular or monocular stimuli. They found that the oscillating map of visual space completely overlapped with the map of functional characteristics, illustrating that the sensitivity of neurons to visual features is not uniform but can vary depending on the location of the features in visual space. .

“This kind of synergy between these two principles, the preference for visual characteristics and their location in space, begins to reveal unique information about the behavior or environment of certain animals,” describes Sedigh-Sarvestani. “The wiring of the visual circuits that determine the patterns that are found in the brain is influenced by our visual experience; What you see and where you see it.

In the future, Sedigh-Sarvestani plans to study whether other visual features are related to visual space mapping in different regions of the visual cortex and whether this organization can be traced back to the retina and, possibly, the native environment of an animal and its movements within. this environment; leading to a more complete understanding of visual perception.

“What we found really required us to rethink how maps of visual space are formed and to recognize that neural circuits in the visual cortex can be functionally specialized for different regions of visual space,” says Fitzpatrick . “Our results open the door to a different way of thinking about how cortical circuits are organized, how they contribute to visual perception and ultimately behavior.”

This work was supported by the National Institutes of Health Grants and the Max Planck Florida Institute for Neuroscience. The contents of this article are the sole responsibility of the authors and do not necessarily represent the official views of the funding agencies.

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