Figure 1.

Visual objects, behavioral task and the Bubbles method. (A) Default views of the two objects that rats were trained to discriminate in Alemi-Neissi et al. (2013). In the present study, these objects are referred to as Object 1 and 2 and, collectively, as Stimulus Set 1. The panel on the right shows to what extent these views of the objects overlapped, when superimposed. (B) Default views of the two objects that rats were trained to discriminate during Phase I of the present study. These objects are referred to as Object 3 and 4 and, collectively, as Stimulus Set 2. The panel on the right shows to what extent these views of the objects overlapped, when superimposed. (C) Schematic of the object discrimination task. Rats were trained in an operant box that was equipped with an LCD monitor for stimulus presentation and an array of three sensors. The animals learned to trigger the presentation of a visual object by licking the central sensor, and to associate the identity of each object to a specific reward port/sensor (right port for Object 3 and left port for Object 4). (D) A sample of the transformed object views used during Phase II of the study. Transformations included: (1) size changes; (2) azimuth in-depth rotations; (3) horizontal position shifts; and (4) in-plane rotations. Azimuth rotated and horizontally shifted objects were also scaled down to a size of 30° of visual angle; in-plane rotated objects were scaled down to a size of 32.5° of visual angle. Note that each transformation axis was sampled more densely than shown in the figure—sizes were sampled in 2.5° steps; azimuth rotations in 5° steps; position shifts in 4.5° steps; and in-plane rotations in 9° steps. The red frames highlight the subsets of object views that were tested in bubbles trials. (E) Illustration of the Bubbles method, which consists in generating an opaque mask (fully black area) punctured by a number of randomly located windows (i.e., the bubbles; shown as semi-transparent, circular openings) and then overlapping the mask to the image of a visual object, so that only parts of the object is visible through the mask. (F) Examples of the different degrees of occlusion that can be achieved by varying the number of bubbles in the masks. (G) An example of possible trials' sequence 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). (H) An example of possible trials' sequence during experimental Phase II. 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 changes and azimuth rotations in this example), whereas the latter included masked views of the most recently trained transformation (i.e., −40° azimuth rotated objects).