A human can identify hundreds of thousands of unique colors and forms visually, but how does the brain process all of this information? Previously, scientists assumed that the visual system records shape and color separately with discrete groups of neurons and then integrates them much later. According to the Salk researchers, there are neurons that respond preferentially to specific color and form combinations.
“New genetic sensors and imaging technologies have allowed us to more thoroughly test the link between visual circuits that process color and shape,” says Edward Callaway, senior author, and professor in Salk’s Systems Neurobiology Laboratory. “These findings provide valuable insight about how visual circuits are connected and organized in the brain.”
Light-sensitive cells in the eye (photoreceptors) detect wavelengths of light within certain ranges and at precise places, similar to a digital camera sensor. This information is subsequently sent by the optic nerve to neurons in the visual cortex, which process it and begin to comprehend the contents of the image. Color and form were previously considered to be extracted independently and then merged only at the highest brain areas, but a recent Salk study reveals that they are integrated far earlier.
“The goal of our study was to better understand how the visual system processes colors and shapes of visual stimuli,” says co-first author Anupam Garg, who is a University of California San Diego MD/PhD student in the Callaway lab. “We wanted to apply new imaging techniques to answer these longstanding questions about visual processing.”
The researchers studied the activity of hundreds of individual neurons involved in color and form processing in the primary visual cortex using imaging technologies coupled with genetically produced sensors. Approximately 500 different color and shape combinations were examined over extended recording periods to determine the stimulus that best activated each visually-responsive neuron.
Contrary to long-held beliefs about how visual processing works, the team discovered that visual neurons selectively responded to color and shape along a continuum: while some neurons were only activated by a specific color or shape, many other neurons were responsive to a specific color and shape simultaneously.
“Our brain encodes visual information efficiently using circuits that are smartly designed. Contrary to what is taught in the classroom — that color and form are processed separately in the early visual cortex and then integrated later by unknown mechanisms — the brain encodes color and form together in a systematic way,” says Peichao Li, co-first author and postdoctoral fellow in the Callaway lab.
This finding sets the groundwork for understanding how brain circuits do the computations that contribute to color perception.