Mario Zanforlin

Object and spatial representations in detour problems by chicks

Abstract Two-day-old chicks, Gallus gallus domesticus , were tested in a detour situation requiring them to abandon a clear view of a desired goal (a small red object on which they had been imprinted) in order to achieve that goal. The chicks were placed in a closed corridor, at one end of which was a barrier with a small window through which the goal was visible. Two symmetrical apertures placed midline to the corridor allowed the chicks to adopt routes passing around the barrier. After entering the apertures, chicks showed searching behaviour for the goal and appeared able to localize it, turning either right or left depending on their previous direction of turn. Thus, in the absence of any local orienting cues emanating from the goal, chicks were aware of the existence of an object that was no longer visible and could represent its spatial localization in egocentric coordinates.

Detour behaviour in the domestic chick: searching for a disappearing prey or a disappearing social partner

Abstract Chicks,Gallus gallus domesticus, of 2 and 6 days of age were presented with a goal-object that was made to disappear behind one of two screens opposite each other. Chicks proved able to choose the correct screen when the goal-object was a social partner (i.e. a red ball on which they had been imprinted), whereas they searched at random behind either screen when the goal-object was a palatable prey (i.e. a mealworm). Chicks, however, appeared able to make use of the directional cue provided by the movement of the mealworm when tested in the presence of a cagemate. These results suggest that previous failure to obtain detour behaviour in the double screen test in the chick was not due to a cognitive limitation, but rather to the evocation of fear responses to the novel environment that interfered with the correct execution of the spatial task.

Delayed search for a concealed imprinted object in the domestic chick

Abstract Five-day-old chicks were accustomed to follow an imprinted object (a small red ball with which they had been reared) that was moving slowly in a large arena, until it disappeared behind an opaque screen. In experiments, each chick was initially confined in a transparent cage, from where it could see and track the ball while it moved towards, and then beyond, one of two screens. The screens could be either identical or differ in colour and pattern. Either immediately after the disappearance of the ball, or with a certain delay, the chick was released and allowed to search for its imprinted object behind either screen. The results showed that chicks took into account the directional cue provided by the ball movement and its concealment, up to a delay period of about 180 s, independently of the perceptual characteristics of the two screens. If an opaque partition was positioned in front of the transparent cage immediately after the ball had disappeared, so that, throughout the delay, neither the goal-object nor the two screens were visible, chicks were still capable of remembering and choosing the correct screen, though over a much shorter period of about 60 s. The results suggest that, at least in this precocial bird species, very young chicks can maintain some form of representation of the location where a social partner was last seen, and are also capable of continuously updating this representation so as to take into account successive displacements of the goal-object.

Right - Left asymmetry in position learning of male chicks

Two-week-old chicks were trained to discriminate between two identical boxes on the basis of their position. The positive box was placed against the wall facing the entrance of the test cage, whereas the negative box was placed either on the right or on the left wall. Results showed that male chicks learned the task faster when the negative box was placed on the right than when it was placed on the left, whereas no right-left asymmetry appeared in females.

The height of a stereokinetic cone: a quantitative determination of a 3-D effect from 2-D moving patterns without a "rigidity assumption".

SummaryWhen a circular disc with an eccentric dot painted on it is set in slow circular motion, a three-dimensional solid object appears to the observer: the stereokinetic cone. The cone shows a well-defined height and tilt. All current theories of how the visual system can extract 3-D percepts from 2-D moving patterns, are based on a “rigidity assumption” hypothesis. But this assumption cannot explain why the stereokinetic cone appears to have a well-defined height. An alternative hypothesis is proposed here, which avoids the rigidity assumption and is based on a minimization process of the relative velocity differences between all the points of the rotating pattern. The hypothesis allows a quantitative prediction of the stereokinetic cone height both when the rotating disc is circular and when it is elliptical. The predictions are in good agreement with previously reported experimental results and with our own observations.

Sex differences in the response to social stimuli in young chicks

Ambulation latencies of pair-reared 6-day-old male and female chicks were measured (i) in the presence of a cagemate; (ii) in the presence of an unfamiliar chick; (iii) in isolated chicks. Results showed that a) in animals tested with a cagemate there were no differences in ambulation latencies between males and females; b) in animals tested with an unfamiliar chick ambulation latencies were higher in females than in males; c) in animals tested in isolation ambulation latencies were higher in males than in females. It is suggested that gender effects in the open-field behaviour of young chicks might be due to a stronger social reinstatement motivation in females than in males.

Some observations on Gregory's theory of perceptual illusions

Gregory (1963, 1965, 1966) has explained the optical iIlusions of the Muller-Lyer type as a misapplication of a perceptual mechanism which he calls “primary constancy scaling.” This mechanism is brought into action by the depth features of the drawing, despite the fact that the texture of the paper makes the observer see the figure as flat. It is from the conflicting information about the figure, namely, the depth features (three-dimensionality) and the texture (flat), that the well-known phenomenon of these illusions arises. As Gregory’s replies to some criticisms (Brown and Houssiad, 1964; Humphrey and Morgan, 1965; Day, 1965; Hamilton, 1966; Wallace, 1966) do not appear to satisfy all doubts about his theory, it is opportune to consider in detail certain basic points of the theory which have not been raised by previous critics. It seems obvious that if primary constancy scaling is set by the depth features of flat figures (Gregory, 1963), then in order to predict in which figures constancy scaling will enter into action and produce illusion, we need an exact definition of depth features. But no formal or precise definition is given, nor does Gregory determine unequivocably the circumstances in which they occur. What depth features are is not at all self-evident. Gregory (1963) speaks of a drawing as having depth features when it is a perspective representation or a projection of a three-dimensional object. This does not help us very much as any drawing whatsoever may be regarded as the projection of at least one other figure in “depth.” If, for example, one supposes that Figure I b has depth

Depth effect from a rotating line of constant length

When a straight line is slowly rotated in the frontoparallel plane, subjects report seeing a compelling depth effect. The line appears slanted into 3-D space, with its end parts located at different distances from the observer and describing a sort of circular trajectory with respect to the frontal plane. As a result of this depth effect, the line appears longer than when it is located on the frontal plane, and the perceived length depends on the amount of tilt perceived. An explanation of this effect is presented, based on the hypothesis that the visual system minimizes differences in linear velocities in the 2-D velocity field. The results of an experiment in which subjects were required to evaluate the length of the tilting line were in close agreement with the expected values computed on the basis of the model. The results are discussed in relation to current theories of visual perception of structure from motion.

Domestic chicks perceive stereokinetic illusions

Stereokinetic illusions occur when certain 2-D patterns are set in slow rotation in a plane perpendicular to the line of sight. Such phenomena have never been investigated in animal species other than our own. We used the domestic chick (Gallus gallus) to check whether these illusions are experienced by non-human species, taking advantage of filial imprinting. Newly hatched visually naive chicks were individually exposed for 4 h to 2-D stimuli producing, to a human observer, the perception of a stereokinetic cone (experiment 1) or of a stereokinetic cylinder (experiment 2). Thereafter, each chick underwent a free-choice test between a solid 3-D cone and a solid 3-D cylinder. A control group of newly hatched but not imprinted chicks underwent the same testing procedure, to check for the presence of any spontaneous preference for one or other of the two solid objects. Imprinted chicks approached the 3-D stimulus closely resembling the stimulus they had been exposed to during imprinting (the cone in experiment 1 and the cylinder in experiment 2). Non-imprinted chicks did not show any preference. These results suggest that domestic chicks experience stereokinetic illusions.

The saturn illusion: A new stereokinetic effect

When a 2-D pattern composed of a solid ellipse with two symmetrical semi-rings (corresponding to the visible parts of a contour ellipse whose major axis is perpendicular to that of the solid ellipse) is slowly rotated about an axis coincident with the line of sight, a compelling 3-dimensional impression occurs. Subjects report seeing an egg-shaped object which is inserted into a circular ring: the two objects move solidly into 3-D space and a moving visual phantom is generated so that the ring appears completed by an illusory curved segment in the region nearer to the observer during rotation. A sequence of nonrigid and rigid percepts (both 2-D and 3-D) precedes this Saturn-like configuration.