Fourth processing stage – Generation of impressions

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With the double opponent color cells, we have now arrived at the primary visual cortex, the so-called area V1. This 3-mm-thick, credit-card-sized area sits at the posterior ends of the two halves of the brain and has a good 200 million neurons. It integrates not only motion- and orientation-sensitive neurons, but also those that respond exclusively to wavelength stimuli but not to colors, as we know from animal experiments. In these cells, objects of different colors but with the same spectral reflectance elicited the same responses. Therefore, although knowledge of color processing starting in the primary visual cortex has been fragmentary to date, we can state that color impressions must originate not in the primary visual cortex but at a higher level of processing. As a stage of these impressions, Semir Zeki’s animal experiments suggest the area V4 located in the what-pathway leading to the temporal lobe (13). The cells of V1 project to this area via the intermediate area V2. Cells in V4 respond to the color of an object, not its spectral content, as in the visual cortex. Zeki’s later PET scans (positron emission tomography, which visualizes metabolic activity) on humans confirmed the importance of area V4 for color perception (14). A clear indication of the enormous importance of this area, which is only the size of a bean, for color vision is provided by the neurologist Oliver Sacks in his case description of the painter Jonathan I., who lost his color perception as a result of an injury to this very area following an accident. Sacks outlines this as follows:

„His brown dog appears dark gray to him. He perceives tomato juice as black. And the color television pictures are a gray mishmash for him … He was troubled … by the unappetizing, „dirty“ appearance of what he saw – every white smudgy, as if moldy or washed out, every black as if dusty. Everything looked wrong, unnatural, dirty and unclean. … The skin of other people, of his wife, even his own skin he perceived in a repulsive shade of gray; „flesh-colored“ now appeared to him „rat-colored,“ and this did not change even when he closed his eyes, for his vivid imagination had remained with him, only it had also lost all colorfulness.“

Sacks concludes:

„Patient I. saw with the cone cells of his retinas and with the wavelength-responsive cells of V1, while the color-generating mechanisms of V4 failed at a higher level. For us, the result of stimulus processing in V1 is inconceivable because it is never perceived as such, but is immediately fed to a higher level where, after further processing, it gives rise to color perception. Thus, the pure V1 output never enters our consciousness. Jonathan I., however, perceived this output. His brain damage kept him trapped in an alien in-between space, the uncanny world of V1, a world of abnormal and, as it were, pre-color sensations that could not be assigned to the category of either colorfulness or colorlessness.“ (15).

The neurons in V4 and V1 are selective for the shape of visual stimuli (their length, width, and orientation) and also respond selectively to their direction of motion and speed (16). This means that in addition to color perception, many other processing processes happen in this area that are important for recognizing objects.

At this point, we have explored color perception to the extent that current scientific knowledge permits, concluding that area V4 in our brains is clearly the primary source of color. However, we cannot yet definitively state how our brains actually generate color impressions. This is due to the integration of V4 into the comprehensive network of all brain regions. These include the hippocampus, which is of great importance for storing memory traces; the limbic system and the amygdala, which give us emotions; and a number of other areas of the cerebral cortex whose exact tasks are still unexplored. V4 acts on all of them, and they in turn act back on V4, whereby the colors generated there are fused with memories, associations, smells, tastes and sounds—in short, all other sensory impressions. This fusion ultimately makes up the final impression, which in turn creates a world that is meaningful to everyone in different ways.

Regardless of all the questions that are still open, however, one thing has become clear: the idea of colors standing alone, which we only perceive, is wrong. Objects do not possess colors themselves; they do not exist independently of our perceptual apparatus; rather, our brain constructs them through a complex processing process from the combined stimulus patterns of the three cone receptor types in the retina, which are activated by the incident wavelength patterns.

Our brain only creates the impression of a color after complex processing steps. Colors do not exist independently of us.

Next Red is better than blue – Our preference for warm colors

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