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.
In vivo GCaMp6f two-photon calcium imaging in primate V1.(A) Schematic of the experimental setup (see supplementary materials and methods). (B) Average fluorescence of one imaging region after the presentation of colored drifting gratings. Four cells are indicated and their corresponding traces are shown in (C). Scale bar: 200 μm. (C) Sample fluorescence traces, indicated by the color of the stimulus to which they responded most strongly. Colored bars indicate the hue of the stimulus displayed at each time point.Orientation-selective, hue-selective, and color-preferring neurons in primate V1.(A) Aligned CO histology for two-photon imaging region in (B to D) (see fig. S2). Contours demonstrate normalized CO intensity. Scale bar: 200 μm. (B) The location of each visually responsive neuron is plotted. Neurons are colored according to their CPI. This figure contains two superimposed cortical imaging depths. Color scale (bottom) for CPI values. (C) The same region as (B), with the color of individual neurons plotted on the basis of each neuron’s OSI in response to its preferred stimulus. Color scale (bottom) for OSI values. (D) Neurons considered hue selective are labeled by their preferred hue, whereas neurons that were visually responsive but not hue selective are depicted in gray. Color bar (bottom) showing presented hues. (E to H) Same as (A to D), for a second imaging region. Cells 1 to 5 depicted in (I to R) are indicated with arrows. Scale bar: 200 μm. (I to M) Mean (± SEM) change in fluorescence to 12 hues at each neuron’s preferred orientation and spatial frequency. The response to achromatic gratings at each neuron’s preferred orientation and spatial frequency is plotted in black. (N to R) The average change in fluorescence to eight stimulus directions at each neuron’s preferred hue (or achromatic) and spatial frequency.Population statistics demonstrate a mutual representation of color and orientation.(A) Relationship between CPI and OSI for all cells. Neurons above the horizontal dashed line responded at least twice as strongly to their preferred versus orthogonal orientation (OSI > 0.5). Vertical dashed lines represent CPI = −0.33 and 0.33 (neurons responded twice as strongly to achromatic or equiluminant colored stimuli, respectively). (B) Relationship between CO intensity and OSI. Trend line fit using least-squares linear regression. r, correlation coefficient. (C) Same as (C), for CO intensity versus CPI. (D) Histograms of neurons in each region of (A), based on CO intensity. Because of geometric considerations, the numbers of cells sampled are not equal in each bin. The actual sampling distributions are shown in fig. S2G.Spatial organization of hue selectivity and relationship to CO histology.(A) Histogram of the distance between simultaneously recorded cell pairs and correlation coefficient of their hue-tuning curves. Black circles indicate the average of all points in 25-μm bins and gray circles indicate the average of all points when shuffled. (B to E) Histograms of the number of green (B), blue (C), red (D), and non-hue-selective (E) neurons based on CO intensity. Dashed lines indicate the median of each distribution. (F) Mean (± SEM) OSIs for cells that prefer red, green, and blue hues, and neurons that are not hue selective but are visually responsive to achromatic stimuli (gray). *P = 0.002; ***P < 0.001. Populations of included red, green, blue, and non-hue-selective neurons are indicated with asterisks in fig. S3C.
“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.
Color and orientation are jointly coded and spatially organized in primate primary visual cortex, Anupam K. Garg, Peichao Li, Mohammad S. Rashid, Edward M. Callaway
