Detailed maps of the physical world are formed in different regions

of the brain as the central nervous system receives information from the five senses. The sense of sight helps humans develop topographic brain maps that give an accurate representation of where they are in space.

Researchers from the Scripps Research Institute in La Jolla, California, investigated whether movements animals make repeatedly in their environments communicate information to the retina, which in turn is used to organize topographic maps throughout the brain. The study, published online Nov. 10 in the Proceedings of the National Academy of Sciences, found that reversing the visual flow that tadpoles experience impaired the organization of their topographic maps by inhibiting the growth of structures in the nervous system that allow them to form normally.  

“Tadpoles and fish and many animals that move move forward,” said Hollis Cline, professor of neuroscience at the Scripps Research Institute and co-author of the study. “The predominant visual motion that these animals experience in their everyday life is that of an object moving from the front of them to behind them.”

The researchers exposed two sets of immobilized tadpoles to opposing types of optical stimulation in two separate chambers. One group was placed in front of a screen that displayed bars moving from anterior to posterior—front to back—in their visual fields, simulating normal swimming motion. The second group saw bars moving

from back to front—the opposite of how the tadpoles would normally perceive their environment. 

“What happened with these animals when they were provided with posterior to anterior visual stimulus is that spatial information about objects in [their] visual world was lost,” Cline said. “An animal would see something but wouldn’t know where it was located in space.”

Reversing the natural visual flow tadpoles experienced illustrated that the optical stimulation they normally receive activates cells in the retina in a specific sequence important to the proper formation of mental maps of the environment. This sequence, called temporal code, informs the organization of topographic maps as they are communicated from the eye to the central nervous system, according to the study. 

Objects that are close to one another in visual space will activate cells in the retina that are near one another and in turn activate cells in the brain that are close together. The adjacent cells being triggered result in the selective strengthening or weakening of the associations that form circuits associated with the building of topographic maps throughout the brain, according to Carlos Aizenman, an associate professor of neuroscience at Brown University.

“Initially you have a very rough map of the external world,” Aizenman said. “Once [visual activity occurs] you get selective stabilization of some of the connections and elimination of others. The circuits are sculpted to become better at perceiving differences in the external world.”

When it comes to human vision, the eye collects information about the world and relays it to the central nervous system through the retina. The location of objects in space is conveyed to the brain to form maps of the visual world, of which approximately 30 are distributed across different brain regions, Cline said. Retinal ganglion cells within the eye compress the images the retina compiles from the environment and transmits those signals to nerve fibers, which then pass them on to the occipital lobe—the visual processing center of the mammalian brain. The position of the cells that are activated in the eye tells the brain where the information

is coming from in the field of vision.

The process of refining the central nervous system’s relationship with visual signals from the environment still occurs for mice and other mammals that are born blind, even though they cannot see, said Michael Crair, professor of neurobiology at Yale University. 

“It seems like mice have an intrinsic activity pattern in their retina,” Crair said. “The retina generates spontaneous waves of activity which … appear to substitute for visual experience.”

Though the mice cannot see anything, the activity sweeps across the retina in a patterned fashion as if the animals were moving through their environment with their eyes open. This allows neural mapping to occur without the sense of sight, according to Crair. 

“The waves are not random,” Crair said. “There is a strong preference for them to propagate in one direction, which corresponds to [the animal] walking or running through its environment. I think they’ve substituted this spontaneous pattern of activity for

visual experience.”

Based on what researchers previously understood about the temporal information being a code for topographic map development in the brain, Cline said the prediction would have been that maps would develop whether visual stimulation was experienced normally or

in reverse.

“When you move backwards you can get confused and your topographic map doesn’t get messed up,” she said. “But if you were always exposed to an alternate visual experience, my prediction is your topographic map would

be [impaired].”