CPRG-Research

Perception of symmetry in chicks

(Zanforlin, Regolin, Clara)

In animal visual perception, detection of symmetry is a crucial ability, since symmetry is inherent to most living creatures. The published data seem to indicate that birds have developed symmetry discrimination abilities, for example pigeons can learn to distinguish symmetry from asymmetry and then generalize this discrimination to novel stimuli.

It is not clear whether such abilities reflect the existence of an innate (rather than acquired) category for symmetry. We (Clara et al., 2003) investigated naïve chicks' preferences for pecking spontaneously at symmetrical and asymmetrical food-size patterns.
Newborn chicks, soon after hatching, spontaneously peck at small, round and high contrast objects present in their environment, and representing possible food items.

The stimuli used in our work have been selected from those employed by Giurfa, Eichmann and Menzel (1996) with pigeons and by Delius and Habers (1978) with honeybees. In the first experiment, we tested chicks 24 hrs after hatching, pecks at each symmetrical and asymmetrical stimulus were recorded. A retest was administered 24 hrs later, after the same chicks had experienced standard rearing conditions. Results did not reveal any significant preference for symmetry on Day 1, whereas a significant preference for symmetry appeared on Day 2.

To disentangle between experiential and maturational effects a second experiment was set up using the same procedure on chicks hatched and kept in total darkness till Day 2; the results showed that chicks exhibited a clear preference for pecking at asymmetrical stimuli.

To investigate the nature of the visual experience affecting this time-course, in a third experiment, 2-day old chicks were tested after having experienced standard rearing conditions, with or without any food available. Results showed a trend to prefer symmetrical patterns in food experienced chicks, whereas food deprived chicks did not show any preference. It seems that visual experience per se is not the only crucial factor, whereas having performed some feeding behaviour or pecking at various types of symmetrical and asymmetrical patterns could represent the most likely key element for the development of a preference for symmetry.

Publications

- Clara E., Agrillo C., Regolin L., Vallortigara G. Spontaneous preference for symmetrical stimuli in visually naive chicks. In: Birgitta Berglund (Ed.), Fechner Day 2003: Proceedings of the 19th Annual Meeting of the International Society for Psychophysics. Cyprus, Greece, International Society for Psychophysics, 2003.