In the following sentence from "How does the brain solve visual object recognition?" by DiCarlo et al.:
In sum, our view is that the “output” of the ventral stream is reflexively expressed in neuronal firing rates across a short interval of time (~50 ms), is an “explicit” object representation (i.e., object identity is easily decodable), and the rapid production of this representation is consistent with a largely feedforward, non-linear processing of the visual input.
I'm familiar with "feed-forward" but not with the meaning of "non-linear processing" in a neuroscience context. What does "non-linear processing" mean, exactly?