Holistic perception of visual configurations in young domestic chicks.

Perceptual organization involves the grouping of visual elements in a coherent scene. The most relevant case is the segmentation of a visual scene in biologically relevant units, which are composed by elements that can be analysed separately (local processing), but also integrated in a unitary percept by extracting the overall configuration (global processing). A peculiar example of global processing is the perception of artificially figures (the “impossible objects”). These are represented by 2D drawings giving the impression of 3D objects; each part of the figure is acceptable as a correct representation of a 3D object, nevertheless the whole figure results “impossible” due to false connections between parts. The identification of an impossible figure requires the integration of local and global processing, providing a unique possibility to study implicit knowledge about objects’ properties. The general principles of perceptual organization are shared by different species. Indeed domestic chicks, similarly to human infants, proved able to discriminate between possible and impossible figures and also showed spontaneous preferences for two-dimensional depictions of possible rather than impossible objects, suggesting an innate ability to recognize global coherence in two-dimensional representations of three-dimensional objects. We also investigated another example of the predominance of global-configural processing. In the Ebbinghaus illusion, a stimulus surrounded by smaller stimuli appears larger than an identical stimulus surrounded by larger inducers. The global configuration of the stimulus is essential to perceive the illusion. This illusion has been extensively investigated in human adults, for whom it seems to be modulated by cultural factors, though data on pre-verbal infants or children are either scanty or contradictory. In animal species, the Ebbinghaus illusion has been studied only in adult baboons and pigeons. We devised a procedure to test the Ebbinghaus illusion in 3-day old domestic chicks. This species offers the advantage of allowing to test very young animals and of controlling previous visual experience, providing fruitful comparisons with data obtained in human infants and children. In the main experiment, during the first two days of life chicks were food reinforced in their home cages for approaching either a small orange circle (half of the sample, N=8) or a bigger circle (N=8). Subjects were then tested with Ebbinghaus-stimuli: two identical orange target circles were presented, one surrounded by larger and the other by smaller gray inducer circles. The percentage of correct choices (e.g. approach of the perceptually bigger target in chicks reinforced on the bigger circle) performed by each animal was computed. Results suggest that the Ebbinghaus-illusion may be perceived at least by part of the chicks (those reinforced on the smaller target, t(7) =2.646, p=0.033). Our data call for more in depth investigations concerning the role of cultural/experiential factors in the perception of the Ebbinghaus Illusion, since they support the possibility that global processing of visual configurations can be performed by animals, early in life, and in conditions of controlled visual experience.