 Subsection "Some visual responses of pulvinar and parabigeminal nucleus are explained by selective innervation of retinal ganglion cell types" (and later in the Discussion section): I don\'t think the lack of responses to full-field stimuli requires a nonlinearity. For example, you could have a linear summation of responses with oppositely signed weights from two sets of cells with similar responses to the chirp (or other full field) stimuli. I believe that in the LGN work the ganglion cells were combined with mostly or exclusively positive signs, so I think the difference here is that you either need a nonlinearity or a combination of responses with differing signs so that you can get cancelation.  