The brain transforms and discards pixels.
Pixels strike the rods and cones in the eye.
They are transformed to smoothed differences of contrast before being sent to the brain.
In the primary visual cortex, signals are further transformed to include the orientation.
The receptive field describes individual nerve cells in visual cortex.
Each nerve cell responds to just a small region of the image.
Multiplying each point in the image region by the receptive field and summing gives the amount of response.
Thus patches of image are correlated by a smoothed derivative filter.
The receptive field is the impulse response for a cortical filter.