I found this optical illusion today on the internet. This posting is accompanied by an explanation of how it works, that invokes lateral inhibition among neurons in the visual system. There is another version of this available on the internet where they show two versions. one version has red circles around the spots which (for me) destroys the illusion, presumably by altering the lateral inhibition among visual system neurons. Tilting your head when looking at the illusion can also affect what you see. This should affect which line orientation sensitive cells being activated within the overall scheme of inhibition. I like optical illusions for a variety of reasons, but one is that they demonstrate the constructed nature of our observed "reality". What you think you are seeing is not always what "is". Your nervous system (brain) constructs a an internal model based on some inputs and changes it as more information is accrued. Lateral inhibition in the nervous system works among a field of neurons, all active in parallel (such as cells in the retina), that interact among themselves to accentuate features of the environment important to the organism. Lateral inhibition is found in many places in the nervous system, not just in the visual system. A row of laterally inhibiting cells, in a field of visual cells in the nervous system, can feed into a higher level cell which can then detect edges, something developed by step-wise information processing. At least in some cases, the development of these kinds of cells is dependent upon visual experience (see the development section). Also indicative of the constructed nature of our observed "reality". The spots in the illusion above appear and disappear based on where you focus. This implicates the importance of the fovea in the retina, where the most detailed visual information is gathered and where lateral inhibition is strongest. See: "The diagram shows the relative acuity of the left human eye (horizontal section) in degrees from the fovea", in the fovea link. Saccades are eye movements that move the fovea around over informative parts of the visual field, often following edges and areas of contrast. This may also affect the illusion. As saccades focus the gathering of information from different areas of the visual field, a model is built in the brain. Areas lacking contrast are often just filled in with what surrounds them, which can be used to create another class of illusions. Saccades can be tracked and are used to analyze advertising, movies and art, and reading among other things. These kinds of phenomena tie together externally available observations available to numerous people (such as physiology and anatomy) with the internally available (introspective?) observations available only to first person individuals (which I would call the realm of pure psychology).