Figure 2.

The Bubbles method. A, Illustration of the Bubbles method, which consists of generating an opaque mask (fully black area) punctured by a number of randomly located transparent windows (i.e., the bubbles; shown as white, circular clouds) and then overlapping the mask to the image of a visual object, so that only parts of the object remain visible. B, Examples of the different degrees of occlusion of the default object views that were produced by varying the number of bubbles in the masks. C, Examples of trial types shown to the rats at the end of experimental Phase I. The object default views were presented both unmasked and masked in randomly interleaved trials (named, respectively, regular and bubbles trials). D, Examples of trial types shown during experimental Phase II, after the rats had learned to tolerate size and azimuth rotations. The animals were presented with interleaved regular and bubbles trials. The former included all possible unmasked object views to which the rats had been exposed up to that point (i.e., size and azimuth changes), whereas the latter included masked views of the −40° azimuth rotated objects.