Spontaneous discrimination of possible and impossible objects by newly hatched chicks

1. Introduction

The human visual system seems to be equipped with mechanisms to establish whether or not a two-dimensional image represents a view of a structurally possible three-dimensional object. An example is provided by the so-called impossible objects, first described by Penrose & Penrose [1]. These consist of two-dimensional figures at first perceived as representing a projection of a three-dimensional object, but then immediately and effortlessly recognized as an object that cannot actually exist in the form interpreted by the visual system.

Several local cues need to be integrated to detect global inconsistencies in structural information, allowing discrimination between possible and impossible objects; a long-standing debate exists as to the role played by experience with real three-dimensional objects in the development of these abilities [2,3].

Recently, Shuwairi et al. [4] reported that four-month-old infants can discriminate between structurally possible and impossible objects. However, four months leaves plenty of time for visual experience with objects, and the issue of the relative contribution of maturation of biologically predisposed mechanisms and experience with real objects remains unsolved.

Animal models allow for controlled-rearing experiments that would be impossible with human newborns. In particular, the hatchlings of precocial species (such as the domestic fowl) show adult motor behaviour soon after birth, making it possible to combine adequate control of specific experiences and behavioural testing in a newborn animal [5]. Domestic chicks display at birth, well-documented predispositions towards preferentially detecting and approaching selected object features [6–8]. Early predispositions are likely to have been evolutionarily shaped in order to maximize the chances that the animal is exposed to, and therefore imprints onto, adequate objects (which typically are possible three-dimensional objects in a natural environment). Further considerations make the domestic chick a suitable model system for the purpose of our study. This species has been shown to possess perceptual mechanisms that are implicated in the evaluation of a two-dimensional image as a view of a structurally possible three-dimensional object. Chicks successfully recognize objects (geometric shapes such as triangles or squares) even when they are partly occluded [9,10]. They also react to objects according to their relative distance, as defined by pictorial cues such as occlusion [11,12].

Here, we aimed to check whether discrimination of structurally possible from impossible three-dimensional objects may be possible soon after birth, and to assess the role of past experience with real objects in the emergence of this ability. Our hypothesis is that newly hatched chicks shall be precociously capable of detecting, and would preferentially approach, a three dimensionally possible object as compared with a three dimensionally impossible object. If this is the case, it provides novel evidence concerning the developmental (and possibly innate) nature of coherent shape processing.

2. Material and methods

(b) Stimuli

Stimuli were static two-dimensional images, printed on a 6 × 6 cm white plastic paper, representing solid edges constituting a 3.5 cm size solid that might or might not be perceived as a three-dimensional cube, depending on the overlapping of intersections between edges that in three-dimensional cubes should lie in different perspective planes.

(i) Rearing stimulus

The stimulus used for familiarization provided no information concerning the critical intersections, these being occluded (figure 1a) by black drawing pins (diameter 1 cm). Owing to the process of amodal completion, human adults see these stimuli as three-dimensional cubes.

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(ii) Testing stimuli

All chicks at testing were faced with the choice between a possible and an impossible version of the non-occluded cube. In the possible version (figure 1b), interposition cues of the edges in the critical points were coherent, so that the edges that should be in the first plane were not interrupted by edges underneath. To a human observer, edges appear as correctly overlapping and the stimulus is perceived as a structurally coherent two-dimensional representation of a three-dimensional cube. In the impossible version of the cube (figure 1c), the interposition cues caused the edges lying on the second plane to interrupt those on the first plane. The perceptual result to the human observer was an incoherent two-dimensional representation of a three-dimensional solid cube.

3. Results

Results are shown in figure 2. A one-sample two-tailed t-test was performed to compare chicks' responses with chance level (50%), separately for the three experimental conditions: occlusion: mean = 63.48; s.e.m. = 4.45; t₄₁ = 2.86, p = 0.006; no occlusion: mean = 57.99; s.e.m. = 3.08; t₄₈ = 2.58, p = 0.013; naive: mean = 58.01; s.e.m. = 2.06; t₆₅ = 3.89, p = 0.0001.

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In all conditions, chicks significantly preferred to approach the ‘possible’ cube.

An analysis of variance considering the three experimental conditions as between factors did not reveal any effect owing to different rearing (F_2,154 = 0.95; p = 0.39).

4. Discussion

Newly hatched chicks are able to discriminate between two-dimensional images depicting possible and impossible three-dimensional objects. Chicks selectively approached the possible cube, which they probably regarded as more similar to the partly occluded object experienced during familiarization. The lack of difference between the first two experimental conditions strongly suggests that experience of occluding events is not necessary for the emergence of such discrimination. Even more convincing, visually naive chicks, tested soon after hatching as they emerged from an incubator in which they had been kept in darkness, showed a spontaneous preference for the possible version of the cube.

It is known that, as a result of exposure to a particular object in early life, nidifugous birds develop a social and exclusive attachment to a particular object, a process dubbed ‘filial imprinting’ [13]. It is also known that some stimuli are more effective than others in eliciting imprinting. For example, naive chicks at their first exposure to point-light animation sequences exhibit a spontaneous preference to approach biological motion patterns rather than other kinds of motion [14]. Here the fact that visually inexperienced chicks spontaneously preferred to approach the possible cube suggests that at birth, chicks not only possess mechanisms to establish pictorial depth perception (§1) but that they use them to integrate image fragments into spatially coherent three-dimensional object representations. Stimuli that cannot be successfully processed as globally coherent three-dimensional objects may be regarded as less attractive by chicks.

It may be argued that evidence from precocial species cannot easily be transferred to altricial (slowly developing) species, and in particular to the most altricial of the species, the human one. Nonetheless, from a computational point of view, these results show that, in principle, natural selection can produce organisms perfectly able to deal with the problem of global coherence in three-dimensional objects as recovered from two-dimensional images in the absence of any specific experience of real three-dimensional objects. This has obvious implications with regards to the current debate (e.g. [3]) about the role of innateness and learning on the origins of knowledge.