Subjects were 157 female domestic chicks (Gallus gallus) either purchased from a commercial hatchery when they were a few hours old (n = 91), or coming from eggs incubated and hatched in controlled laboratory conditions (n = 66). Controlled hatching was required for testing naive chicks. The fertilized eggs were purchased at the 14th day of incubation and were kept in an incubator (MG70/100 Rurale). On day 17, eggs were placed into a hatchery (MG100), where they were maintained in total darkness, until hatching on day 21.
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 (a) by black drawing pins (diameter 1 cm). Owing to the process of amodal completion, human adults see these stimuli as three-dimensional cubes.
(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 (b), 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 (c), 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.
(c) Rearing conditions
All chicks obtained from the commercial hatchery were familiarized with the rearing stimulus (i.e. the occluded version of the cube) over 3 days post-hatching before the test. During this time, they were housed singly in standard metal cages (28 cm width × 40 cm depth × 32 cm height) at controlled temperature (28°–31°C) and humidity (68%), with food and water available ad libitum. A subgroup also had experience of physical occlusion of objects and of environmental features (occlusion condition, n = 42) during the 3 days of rearing. This was provided by printing the rearing stimulus onto both sides of two identical vertical screens (8 × 14 cm) that were placed within the rearing cage, at the corners. The food jar was positioned behind one screen and the water jar behind the other screen, so that birds learned to associate food and water with the stimulus. Each screen (together with its food/water jar) was moved to a different corner within the cage twice a day, to prevent positional learning. The remaining chicks that had become familiarized with the rearing stimuli did not experience any sort of physical occlusion of one object onto another (no occlusion condition, n = 49) as such stimuli were printed onto little (6 × 6 cm) rigid flags highly visible on top of the food or water jars. The two flagpoles (diameter 0.1 cm) holding each flag were bound directly on the edge of the glass jars, so that the base of stimuli was just over the rim of the jar. Jar positions, at the corners of the cage, were changed randomly within the cage twice a day.
A separate group of chicks came from dark-incubated and dark-hatched eggs (naive condition, n = 66). They underwent a spontaneous preference test between possible and impossible figures at 24 h post-hatching, during which time they were kept in a thoroughly dark environment, and therefore lacked any visual experience prior to the test.
(d) Test apparatus and procedure
The experimental room, located near the rearing room, was kept at controlled temperature (25°C) and humidity (70%); the only light came from a lamp (40 W), placed 25 cm above the apparatus. The apparatus consisted of a cage identical to the rearing ones, with the floor and internal walls lined with uniform white plastic sheets. On one of the short walls were hung the two testing stimuli, spaced 10 cm apart, at 3 cm from the floor and 5 cm from the side walls (d). The left/right position of the stimuli was balanced across individuals.
At the beginning of the test, each chick was positioned at the starting point, i.e. at the exact midline of the short wall (about 3 cm away from it) opposite to where the stimuli were, so that it faced them. Each chick was then observed for 6 consecutive minutes, during which time it could freely move within the apparatus and approach either stimulus.
The internal area of the apparatus was virtually divided into three areas: two identical and symmetrical choice areas situated by each stimulus (14 cm width × 25 cm depth) and a no-choice area located away from the stimuli, by the opposite wall (28 cm width × 15 cm depth), which contained the starting point. Choice of the possible (or impossible) figure was scored, using a computerized event recorder: every time the bird entered (with its head and at least half its body) a certain choice area, the time (s) counter for that area was set until the chick had walked out of it. The overall number of seconds spent by the chick within the choice area located by the two stimuli during the whole test was considered.
Since we expected a preference for the perceptually coherent stimulus, an index of choice of the possible cube was computed according to the formula: (time by the possible cube/total time spent by either stimulus) × 100. Values at around 50 per cent indicated no preference; values greater than 50 per cent indicated a preference for the possible cube and those less than 50 per cent indicated a preference for the impossible cube.