It’s Never simple...The Tidy Textbook Story About the Primary Visual Cortex is Wrong.

When I was a postdoc in the Harvard Neurobiology department in the mid-1960’s I used to have afternoon tea with the Hubel and Weisel group. These are the guys who got a Nobel prize for, among other things, finding that the primary visual cortex is organized into cortical columns of cells that responded to lines that prefer different orientations. Another grouping of columns, called ‘blobs’ responded selectively to color and brightness but not orientation. These two different kinds of groups sent their outputs to higher visual areas that were supposed to integrate the information. My neurobiology course lectures and my Biology of Mind book showed drawings illustrating these tidy distinctions.
Sigh… now Garg et al. come along with 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. They show that processing of orientation and color is combined at the earliest stages of visual processing, totally challenging the existing model. Their 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.