Effectiveness of shading and linear perspective cues in eliciting three-dimensional perception of bidimensional images in dogs

Three-dimensionality perception relies on the integration of various depth cues, with monocular pictorial cues playing a key role in two-dimensional representations. Previous research indicates that dogs are sensitive to a combination of shading and linear perspective when perceiving three-dimensionality, yet the individual contributions of these cues remain unclear. The aim of the present study was to disentangle the specific contribution of shading and linear perspective in promoting three-dimensional perception in dogs. In a series of four experiments involving 120 dogs, subjects were presented with a ball rolling on an apparatus and either falling inside a real hole (control condition) or keeping rolling over a depicted hole (test condition). Experiment 1 and 2 assessed the individual contribution of linear perspective and shading in dogs’ perception of three-dimensionality; Experiment 3, as a replica of previous findings, investigated the combination of both cues; Experiment 4 explored the role of more intense shading. In a violation of expectation paradigm, dogs showed no sensitivity to the pictorial cue of linear perspective or low shading level alone. Conversely, the combination of linear perspective and shading, as well as intense shading alone, successfully elicited three-dimensional perception. These results confirm that dogs can successfully integrate multiple pictorial cues to perceive three-dimensionality from two-dimensional stimuli. The perception of three-dimensionality can also occur through single cues, but only if the cue is sufficiently intense, at least in the case of shading.