Lateral inhibition

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Lateral inhibition is an adaptation mechanism that operates at the level of individual receptors. Without it, the wide dynamic range of photoreceptors would not be possible. Figure 16 illustrates how it works. All photoreceptors are coupled through the amacrine and horizontal cells of the retina and are thus able to influence each other’s output potentials. Each neighbor inhibits each receptor, causing its output potential to equal the logarithm of its own illumination intensity minus the inhibitory effect. If the inhibition has a value >0, the output potential will be lower than it should be due to the illumination intensity, and more light is required to reach this stimulus size. As a result, we get a larger distance between the lowest and the highest brightness intensity that the receptor can process, and thus a larger dynamic range. This type of circuitry plays a crucial role in the functioning of our visual system. Various modes of calculation have implemented it in different digital image carriers, resulting in significant increases in dynamic range.

When we apply this to the entire retina, it implies that the receptors adjust to local adaptation levels at varying brightness levels. Consequently, we can pinpoint numerous individual adaptation levels that guarantee the retina’s dynamic range consistently aligns optimally with the scene’s brightness patterns. For photography, this would mean that we would have differently sensitive areas for the shadows and the highlights of a motif within the image carrier to be exposed. And indeed, there are color-negative films from Fuji, among others, that combine a mix of high-sensitivity and low-sensitivity silver halide crystals in their layers, providing increased dynamic range and improved details in the shadows. Fuji has also applied this idea to digital technology, equipping its Super CCD SR sensors with two photodiodes within a pixel that also have different sensitivities (figure 17) Further technical implementations of this connection can be found in notes 3 and 4.

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