The outside world appears to us, immediate, in detail, and seemingly without any effort. We generally don’t question that this is the actual world we live in. However, what we see depends on what we know and what we are searching for.
For example, when looking at the neighboring image, you might just see a bunch of colored lines. However, if you search for the yellow line elements, you will find them all within an instance, and see that they form a square. Also, if you now look for a circular contour, you will quickly find it as well (in the bottom left). This is because in your mind, you can have an abstract representation of yellow, or a circle, that helps you find it.
Moreover, you can now switch between seeing the circle and seeing the square. You might appreciate how your perception gets warped when switching for one to the other. This illustrates how our perception is an active process that relies on what is relevant for us at a given moment in time.
In my lab, we seek to understand the neuronal mechanisms supporting this process, both the encoding of the abstract mental models in the prefrontal cortex, as well as the effects on perception in early visual areas. We're mainly using three-photon imaging in macaque monkeys that are trained to perform visual search tasks, like finding a circle.