There are hundreds of thousands of distinct colors and shapes that a person can distinguish visually, but how does the brain process all of this information?
Scientists previously believed that the visual system initially encodes shape and color with different sets of neurons and then combines them much later.
But a new study from Salk researchers, published in Science on June 27, 2019, shows that there are neurons that respond selectively to particular combinations of color and shape.
“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.”
Similar to a digital camera sensor, light-sensitive cells in the eye (photoreceptors) detect wavelengths of light within specific ranges and at particular locations.
This information then travels through the optic nerve to neurons in the visual cortex that interpret the information and begin to decipher the contents of the picture.
Scientists long thought that color and shape were extracted separately and then combined only at the highest brain centers, but the new Salk research shows that they are combined much 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/Ph.D. student in the Callaway lab. “We wanted to apply new imaging techniques to answer these longstanding questions about visual processing.”
Visualizing responses of single neurons in primate primary visual cortex. Credit: Anupam Garg, Peichao Li, Mohammad Rashid, Edward Callaway, Salk Institute for Biological Studies
The researchers used imaging technology combined with genetically expressed sensors to study the function of thousands of individual neurons involved in processing color and shape in the primary visual cortex.
During long recording periods, roughly 500 possible combinations of color and shape were tested to find the stimulus that best activated each visually-responsive neuron.
The team found that visual neurons selectively responded to color and shape along a continuum—while some neurons were only activated by either a specific color or shape, many other neurons were responsive to a particular color and shape simultaneously, contrary to long-held notions about how visual processing works.
“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.
“For the last 20 years, I have wanted to know how the visual system processes color, so this finding is truly exciting for me,” says Callaway, who holds the Vincent J. Coates Chair in Molecular Neurobiology.
“This discovery lays a foundation for understanding how neural circuits make the computations that lead to color vision. We look forward to building on these findings to determine how the neurons in the visual cortex work together to extract colors and shapes.”
The visual cortex : Structure and Function
The primary function of the visual cortex is to process visual information.
The visual cortex has been divided up into five different areas based on structural and functional classifications.
It is hypothesized that as visual information gets passed along, each subsequent cortical area is more specialized than the last.
Neurons in the visual cortex often respond to stimuli within a fixed receptive field, the area of the visual field that they respond to, and the neurons in each visual area respond to different types of stimuli.
One of the best-studied examples of specialized cells is that of simple and complex cells.
Simple cells, which are found mostly in V1, respond to specific types of visual cues such as the orientation of edges and lines.
Complex cells, which can be found in V1-V3, are like simple cells in that they respond to edges and orientations, but they do not appear to represent a single receptive field.
Instead, they respond to the summation of several receptive fields which are integrated from many simple cells.
In addition, complex cells respond preferentially to movement in certain directions.
Other examples of specialized cells include end-stopped cells, which detect line endings, and bar and grating cells.
V1 is the first of the cortical regions to receive and process information and also the best-understood portion of the visual cortex.
V1 is divided up into six distinct layers, each comprising different cell-types and functions.
Notably, layer 4 is the location that receives information from the lateral geniculate.
Layer 4 is also the layer that has the highest concentration of simple cells.
Complex cells, on the other hand, can be found in layers 2, 3 and 6.
V1 responds to simple visual components such as orientation and direction.
The summation of this information provides the foundation for more complicated pattern recognition later in the visual stream.
V2 receives integrated information from V1 and subsequently has an increased level of complexity and response patterns to objects.
Cells in this region have been recorded responding to differences in color, spatial frequency, moderately complex patterns, and object orientation. 
V2 sends feedback connections to V1 and has feedforward connections with V3-V5.
Information leaving the second visual area is split up into the dorsal and ventral streams which specialize in processing different aspects of visual information.
The former is often described as being concerned with object recognition while the latter is focused on spatial tasks and visual-motor skills.
As visual information is disseminated throughout the brain, more specialized cells can be found.
It is theorized that there are special cells or groups of cells that learn to respond to certain objects.
This would allow for the immediate recognition of things that have been seen previously. 
In addition, similar cells could be responsible for other important visual information such as spatial orientation.
Blood Supply and Lymphatics
The visual cortex is supplied by branches of the posterior cerebral artery. These branches include the posterior temporal, parietooccipital, and calcarine arteries.
More information: A.K. Garg el al., “Color and orientation are jointly coded and spatially organized in primate primary visual cortex,” Science (2019). science.sciencemag.org/cgi/doi … 1126/science.aaw5868.
Journal information: Science
Provided by Salk Institute