Are chicks sensitive to the Ebbinghaus illusion?

A central circle surrounded by large circular flankers is perceived to be smaller than an identical one surrounded by small flankers: such effect is known as the Ebbinghaus illusion (Fig. 1). This size-illusion has been theorized to be caused or influenced by cultural factors (Doherty et al., 2008) as it is reduced or absent in primitive cultures (de Fockert et al., 2007), 4-5 years-old children (Doherty et al., 2010) and adult baboons (Parron e Fagot, 2007). Several papers have reported that animals share some visual illusions with humans. Animal experience, among others, the horizontal-vertical illusion (monkey species: Dominguez, 1954), the Zoellner illusion (baboons: Benhar & Samuel, 1982), the corridor illusion (baboons: Barbet & Fagot, 2002), stereokinetic illusions (chicks: Clara et al., 2006) and the Ponzo illusion (pigeons: Fujita et al., 1991; horses: Timney & Keil, 1996; macaques: Bayne & Davis, 1993; chimpanzees: Fujita, 1997). In addition, experiments using Kanizsa-type figures have shown that nonhuman animals are able to perceive illusory contours as complete units (Nieder, 2002). Relevant data with regard to the presence of visual illusion in animal species have been obtained in studies employing young domestic chicks as an animal model (e.g. Regolin & Vallortigara, 1995; Zanforlin, 1981;). This animal model has, in fact, the advantage of allowing to test very young animals (domestic chicken are a precocial species), controlling for the role of previous visual experience and allowing comparisons with data obtained in human infants/chikdren. This is a particularly relevant aspect for studies on the Ebbinghaus illusion, since the role of experiential and cultural factors has been contested for this illusion, and the illusion seems to be absent in young children (see above). However, up to the present moment, the Ebbinghaus illusion has been studied only in adult subjects of a primate species (baboons, Parron e Fagot, 2007). The aim of the present study was thus to check whether the Ebbinghaus Illusion would be experienced by three-day old domestic chicks. Methods and Procedure Subjects were 3-4 day-old domestic chicks (Gallus gallus). According to an incidental learning paradigm, during the first 3 days of life, chicks were reared in presence of 4 small plastic screens (14 X 9.5 cm), on two of which it was visible a small orange circle (0.5 cm), whereas on the other two there was a larger orange circle (0.9 cm). Moreover, both reinforced and non reinforced screens, presented above the orange target circles, an identical line composed of 4 big and 6 small black circles (1 cm and 0.4 cm respectively) (See Fig 2.). For half of the chicks food was available only behind the screens presenting large orange circles, whereas for the other half food was available behind the screens presenting the small orange circle (the position of the screens in the rearing cage was constantly swapped during the day). See Fig. 3 for the rearing apparatus. This rearing phase was identical for both experiments. In the first experiment, after this incidental learning phase, chicks underwent a free-choice test between the two rearing stimuli placed at the two sides of a rectangular apparatus. Each chick’s behaviour was observed for 6 minutes, when it approached one of the two stimuli, a preference was scored (the percentage of time spent near the positive stimulus was recorded). The aim of this phase was to verify if chicks could learn to prefer a smaller or a bigger circle if it was associated with food, in order to allow the test of the Ebbinghaus illusion in further phase. Since, the first experiment was not successful in demonstrating a preference for the positive stimulus, we devised a second experiment, using a detour procedure. In this second experiment, in a first test-phase chicks were presented with two screens identical to those used at training (one with a smaller orange circle printed on it, and the other with a bigger circle). Only chicks that demonstrated to have a reliable preference for the reinforced stimulus (i.e. choose the correct screen on at least 4 trials out of 6), were then tested with Ebbinghaus stimuli in a further test phase (identical to the previous one except for the stimuli) (see Fig. X). Chicks’ preferences were computed as the first screen detoured by the subjects. Subjects showing a preference for the stimulus that had been associated with food were thereafter tested with the Ebbinghaus-stimuli (i.e., the central circle was surrounded by either large or small gray inducer circles. 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