Lateral Inhibition in Pink

Article: How Many Colors in This Image? Here's The Science Behind The Illusion...
Source: ScienceAlert
Published: February 10, 2021

Center-surround receptive field,
including interaction between
photoreceptors and horizonal cells

The optical illusion depicted above illustrates an example of lateral inhibition. Lateral inhibition promotes increased contrast sensitivity through the detection of borders, as enhanced by feedback modulation of signals by horizontal cells. Horizontal cells come in three types, depending on the types of photoreceptors they synapse with, but that detail is less relevant to their function in lateral inhibition overall. Light detection is performed by the photoreceptors, which hyperpolarize in the presence of light and depolarize in the absence of light (an exception to most neuronal signaling). Depolarization leads to increased glutamate release, the neurotransmitter currency of photoreceptors, while hyperpolarization leads to decreased glutamate release. Whether the receptive field is on-center or off-center is relevant to the types of bipolar cells downstream, but is only really relevant to this discussion in the sense that the center and surround provide contrast to one another. The excitation or inhibition affects two types of bipolar cells (on-center and off-center), which then affect their corresponding retinal ganglion cells (also on-center and off-center). These cells downstream of the photoreceptors and horizontal cells are interesting in themselves, but not necessary for a discussion of lateral inhibition, which is mostly about contrast. Horizontal cells receive excitatory input from photoreceptors and send inhibitory input to all/neigboring photoreceptors. In this case (of an off-center receptive field), the surrounding horizontal cells are hyperpolarized in the presence of light. When horizontal cells are hyperpolarized, they send less inhibitory signals (recall that they send only inhibitory signals). The neighboring photoreceptor in an off/dark region is already depolarized in the absence of light, but because it receives less inhibition from its neighboring horizontal cells, it depolarizes even more and releases even more glutamate. It is this "even more" effect of modulation by horizontal cells that increases border contrast and causes shades to stand out more when placed next to one another.

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Further reading: Center–Surround Receptive Field