Color and orientation are jointly coded and spatially organized in primate primary visual cortex

Abstract

Previous studies support the textbook model that shape and color are extracted by distinct neurons in primate primary visual cortex (V1). However, rigorous testing of this model requires sampling a larger stimulus space than previously possible. We used stable GCaMP6f expression and two-photon calcium imaging to probe a very large spatial and chromatic visual stimulus space and map functional microarchitecture of thousands of neurons with single-cell resolution. Notable proportions of V1 neurons strongly preferred equiluminant color over achromatic stimuli and were also orientation selective, indicating that orientation and color in V1 are mutually processed by overlapping circuits. Single neurons could precisely and unambiguously code for both color and orientation. Further analyses revealed systematic spatial relationships between color tuning, orientation selectivity, and cytochrome oxidase histology.

Fig. 1.

In vivo GCaMp6f two-photon calcium imaging in primate V1. (A) Schematic of experimental setup (see Methods). (B) Average fluorescence of one imaging region following presentation of colored drifting gratings. Four cells are indicated and their corresponding traces shown in C. Scale bar: 200μm. (C) Sample fluorescence traces, indicated by the color of stimulus to which they responded most strongly. Colored bars indicate the hue of the stimulus displayed at each timepoint.

Fig. 2.

Orientation, 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: 20μm. (B) The location of each visually responsive neuron is plotted. Neurons are colored according their CPI. This figure contains two superimposed cortical imaging depths. Color scale (bottom) for CPI values. (C) Same region as B, with the color of individual neurons plotted based on 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, while 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 1–5 depicted in I to R are indicated with arrows. (I to M) Mean (± S.E.M.) change in fluorescence to twelve 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) Average change in fluorescence to eight stimulus directions at each neuron’s preferred hue (or achromatic) and spatial frequency.

Fig. 3.

Population statistics demonstrating 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 (responded twice as strong to achromatic or equiluminant colored stimuli, respective). (B) Relationship between CO intensity and OSI. Trendline fit using least-squares linear regression. (C) Same as C for CO intensity vs. CPI. (D) Histograms of neurons in each region of A, based on CO intensity. Due to geometric considerations the numbers of cells sampled are not equal in each bin. The actual sampling distributions are shown in Figure S2G.

Fig. 4.

Spatial organization of hue selectivity and relationship to CO histology. (A) Histogram of distance between simultaneously recorded cell pairs and correlation coefficient of their hue tuning curves. Black markers indicate the average of all points within 25μm bins, while gray markers 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 (± S.E.M.) OSIs for cells that prefer red, green, and blue hues, and neurons that are non-hue selective but visually responsive to achromatic stimuli (gray). Populations of included red, green, blue, and non-hue selective neurons are indicated with asterisks in Fig. S3C.