Our eyes give us a very subjective view of the world. This subjectivity leaves us susceptible to a variety of optical illusions. The fact that we are not very good at estimating the absolute brightness of a region, but have a strong ability to detect differences between neighboring regions causes simultaneous brightness contrast illusions. This type of illusion makes the perceived brightness of any region in a complex image vary depending on the brightness of the region’s immediate surroundings.
The illustration below shows how our perception of a solid grey square is influenced by the background. The two squares below are the same shade of grey, but the lighter background behind the square on the left makes that square appear darker than the one on the right.