How can cells in the anterior medial face patch be viewpoint invariant

In a recent paper, Freiwald and Tsao [1] found evidence that the responses of cells in the macaque anterior medial (AM) face patch are invariant to significant changes in viewpoint. The monkey subjects had no prior experience with the individuals depicted in the stimuli and were never given an opportunity to view the same individual from different viewpoints sequentially. These results cannot be explained by a mechanism based only on the temporal association of experienced views.