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O RI G I N A L P A P E R

Moms shadow: structure-from-motion in newly hatched chicksas revealed by an imprinting procedure

Elena Mascalzoni Lucia Regolin Giorgio Vallortigara

Received: 22 May 2008 / Revised: 25 September 2008 / Accepted: 26 September 2008/ Published online: 22 October 2008

Springer-Verlag 2008

Abstract The ability to recognize three-dimensional

objects from two-dimensional (2-D) displays was investi-gated in domestic chicks, focusing on the role of the

objects motion. In Experiment 1 newly hatched chicks,

imprinted on a three-dimensional (3-D) object, were

allowed to choose between the shadows of the familiar

object and of an object never seen before. In Experiments 2

and 3 random-dot displays were used to produce the per-

ception of a solid shape only when set in motion. Overall,

the results showed that domestic chicks were able to rec-

ognize familiar shapes from 2-D motion stimuli. It is likely

that similar general mechanisms underlying the perception

of structure-from-motion and the extraction of 3-D infor-

mation are shared by humans and animals. The present data

shows that they occur similarly in birds as known for

mammals, two separate vertebrate classes; this possibly

indicates a common phylogenetic origin of these processes.

Keywords Structure-from-motion Imprinting

Motion perception Visual system Domestic chicks

Introduction

The human visual system shows a striking ability to extract

three-dimensional (3-D) shape information from two-

dimensional (2-D) transformations of an image (Ullman

1979). A compelling demonstration of peoples ability to

extract structure-from-motion is the so-called kineticdepth effect (KDE) described by Wallach and OConnell

(1953). A KDE display consists of a 2-D projection of a

3-D form revolving about an axis other than the line of

sight. Wallach and OConnell (1953) produced such a

display by placing objects (either solid objects or wire

figures) between a light source and a translucent screen and

by rotating these objects. This produced a transforming

shadow on the screen that was perceived as a 3-D form

when moving but not when stationary (Wallach and

OConnell1953).

There is evidence that non-human mammals can extract

structure-from-motion (for example, in primates, see Siegel

and Andersen1988; Perrett et al.1985). Unno et al. (2003),

showed that in both monkeys and humans correct perfor-

mance rates in discrimination of shape-from-motion using

random-dot kinematograms declined when the density or

speed of random dots was reduced. They suggested that

monkeys and humans perceive shapes defined by motion

cues in a similar manner and, most likely, by common

neural mechanisms.

A phenomenon somewhat related to the KDE, the so-

called stereokinetic effect (SKE), has recently been studied

in marmosets (Callithrix jacchus) (Clara et al. 2007). Ste-

reokinetic effects occur when certain 2-D patterns are

rigidly rotated in the plane perpendicular to the line of sight

(Musatti1924). For instance, when a disc with an eccentric

dot on its surface is slowly rotated in a plane perpendicular

to the line of sight (frontoparallel plane), after a short delay

subjects report seeing a solid 3-D cone that has all the

characteristic features of a real object (Musatti 1924 and

see for more recent research: Bressan and Vallortigara

1986a,b, 1987a,b; Zanforlin1988a,b; Vallortigara et al.

1986; Zanforlin and Vallortigara 1988). Four adult

E. Mascalzoni (&) L. Regolin

Department of General Psychology, University of Padova,

Via Venezia 8, 35131 Padua, Italy

e-mail: [email protected]

G. Vallortigara

Center for Mind/Brain Sciences, University of Trento,

Rovereto, Italy

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marmosets were trained to discriminate between a solid

cylinder and a solid cone to obtain a food reward. Once the

learning criterion was reached, the marmosets were tested

in sets of eight probe trials in which the two solid objects

used at training were replaced by two rotating 2-D stimuli.

Only one of these stimuli produced, at least to the human

observer, the stereokinetic illusion corresponding to the

solid object previously reinforced. At test, the time spentby the marmosets observing each stimulus was recorded.

The subjects stayed longer near to the stimulus producing

the stereokinetic illusion corresponding to the solid object

reinforced at training than they did near to the illusion

corresponding to the previously non-rewarded stimulus.

Hence, the common marmosets behaved as if they could

perceive stereokinetic illusions.

Little work has been done on perception of structure-

from-motion in birds, though there is evidence for a role of

motion (Spetch et al. 2006) and for the presence of a

viewpoint invariance (Watanabe 1997, 1999) in object

recognition. Watanabe et al. (1999) trained pigeons todiscriminate between 2-D images of two familiar and

unfamiliar 3-D objects and to generalize this learning to a

new set of images of the same objects taken from different

points of view. Results showed that pigeons are able to

recognize a 3-D familiar object from 2-D pictures of that

object taken from different points of view, showing view-

point invariance when the object is a familiar one.

Cook and Katz (1999) trained pigeons to discriminate

between computer-generated projections of cube and pyr-

amid objects. The authors found that pigeons showed

evidence of being able to recognize the structure of these

objects from the pattern of their motion and were some-

times better at discriminating when all contour and surface

information had been removed (e.g., with 2-D monochro-

matic colored blobs moving consistently with the rigid

projective geometry of either a cube or a pyramid).

Recently, Clara et al. (2006) used the domestic chick

(Gallus gallus) to check whether the SKE is experienced

by birds, 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 or of a stereokinetic

cylinder. 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 non-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 to which 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. The results thus suggested

that domestic chicks experienced stereokinetic illusions.

Using the imprinting procedure or taking advantage of

spontaneous approach preferences occurring before

imprinting takes place, it has also been proved that newly

hatched chicks can recognize biological motion, being

seemingly able to reconstruct a 3-D shape on the basis of

the relative motion of a few points of light strategically

located on a conspecifics body (Regolin et al. 2000;

Vallortigara et al. 2005; Vallortigara and Regolin 2006;and see Johansson 1973for introduction of this technique

in human visual perception).

The present study was designed to investigate, using an

imprinting procedure, the ability of newly hatched chicks

to extract structure-from-motion information.

Experiment 1

In Experiment 1 the ability of newly hatched chicks to

recognize a solid object according to the shadow it casts on

a translucent screen was investigated. Na ve, newly hat-ched chicks were first imprinted for 48 h on a solid 3-D

object. They were then allowed to choose between two 2-D

stimuli, one being the shadow of the familiar (imprinting)

object, the other being the shadow of an object never seen

before. In order to minimize differences between the two

shapes of the imprinting objects, we used two 3-D objects

of controlled dimensions, differing from each other only in

the position of a single oriented element (see Fig.1).

Chicks were tested either with stationary or moving shad-

ows. Moreover, we also used two types of movement:

stimuli could rotate around either their vertical axis (rota-

tory motion) or they could rotate on the frontoparallel plane

around a central point (translatory motion). In the first case

the shadow stimuli underwent continuous changes in

shape, whereas in the second case the shadow stimuli

moved rigidly, thus maintaining their shape.

Fig. 1 The stimuli used in Experiment 1, the H and theP

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Materials and methods

Subjects

Subjects were 369 nave Hybro domestic chicks (Gallus

gallus), coming from eggs hatched in the laboratory under

controlled conditions. Fertilized eggs (at the 14th day of

incubation) were delivered weekly to the laboratory from alocal commercial hatchery (Agricola Berica, Montegalda,

Vicenza, Italy). On arrival, eggs were placed in an incu-

bator MG 70/100 (45 9 58 9 43 cm, 100 eggs capacity)

until day 19 of incubation. Temperature was constantly

monitored and maintained at 37.5C; a hygrometer was

used to control the humidity in the incubator, maintaining it

at 5560%, providing standard conditions for optimal

incubation. Then, from day 19 to 21, eggs were placed in a

hatchery (60 9 65 9 66 cm) with the same temperature as

the incubator, but at a lower humidity, an ideal condition

for hatching. The incubator, the hatchery and the hatching

room were maintained in the dark until day 21 (thehatching day), to prevent the embryos and the newly hat-

ched chicks gaining any visual experience before the

exposure to the imprinting object. This is a standard pro-

cedure in this type of experiment (see Vallortigara et al.

2005) because light exposure of the eggs in the last phase

of incubation has been shown to affect chicks neural

development and behavior (see Rogers and Bolden 1991;

Rogers et al.2004).

Imprinting stimuli and apparatus

The imprinting cage consisted of a standard metal home

cage (40 9 28 9 30 cm) with one single imprinting

stimulus hung in the middle on a fine thread (about 5 cm

from the floor), so that the chick could walk 360around it

and make it move by touching or pecking at it (see Val-

lortigara and Andrew1991).

The imprinting stimuli consisted of two solid objects

made of wood and painted in red (an attractive hue to

newly hatched chicks; Hess 1956). They were made of

three bars, 1 9 1 cm of base (two of them 5 cm long and

the third one 2 cm long) made to represent a letter U

positioned upside-down (which will be called P) and a

letter H. Hence the only difference between the two

stimuli was the position of the horizontal bar (see Fig. 1).

All other features (height, width, section, and volume) were

identical in the two stimuli. A total of 186 chicks were

imprinted on the H stimulus and 183 on the P stimulus.

Test stimuli and apparatus

The test stimuli consisted of two 2-D images obtained by

casting the shadow of each object (the imprinting object

and the novel object) on a vertical translucent screen. The

test apparatus (Fig.2) consisted of a white-plywood run-

way (45 9 20 9 30 cm), with the testing stimuli presented

at the opposite ends of it. The runway was subdivided

virtually into a central area and two side areas (each one

15 cm long), each side area ending in a translucent screen.

The translucent screen was obtained by opposing a trans-

parent glass sheet (19.5 9 27 cm; 0.5 cm thick) to a

tracing paper. One object was hung 5 cm beyond eachscreen, 5 cm from the floor, with a 25-W lamp placed

behind each stimulus (45 cm from the translucent screen).

Stationary images were obtained by placing the stimuli

in a position maximizing the difference in the objects

projected shadows (as shown in Fig. 1); rotating shadows

used for the second group were obtained by setting the two

solid objects in slow rotation about their vertical axis, at 13

revolutions per minute (rotational movement was operated

by two E.R.G. POLARIS electric engines; Art. 96208).

A group of chicks (50 females and 54 males) was tested

with static shadows (cast on the screen using stimuli in the

frontal position), of which 52 chicks had been exposed to

the H (25 females and 27 males) and 52 to the P (25

females and 27 males). A second group of chicks (58

females and 55 males) was tested with rotating shadows

(moving around their vertical axis; rotational movement

was operated by an E.R.G. POLARIS electric engine; Art.

96208), of which 57 chicks had been exposed to the H

(29 females and 28 males) and 56 to the P (29 females

and 27 males). A third group of chicks (76 females and 76

males) was tested with shadows cast by stimuli rotating

around a central point in the frontoparallel plane (a con-

dition we shall indicate as translatory movement; this

translatory movement was performed by an E.R.G.

POLARIS electric engine; Art. 96208). A total of 77 chicks

of the third group had been exposed to the H (38 females

and 39 males) and 75 to the P (38 females and 37

males).

Procedure

On day 1, in the early morning, a sub-sample of nave

chicks (n = 72, 36 females and 36 males) was randomly

Fig. 2 Schematic representation of the test apparatus

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selected and tested so that spontaneous preferences for

either static or rotating (only around their vertical axes)

shadows could be assessed. Immediately after this test

(in the early morning of day 1) these chicks, as well as

all of the remaining chicks, were placed singly in an

imprinting cage, with food and water, and continuously

exposed for 48 h to an imprinting stimulus hung on a

fine thread in the middle of the cage. On day 3 in theearly morning, each chick was carried, in a closed small

cardboard box, to the experimental room (located near

the imprinting room, and kept at 2930C and with a

humidity of 68%) and placed in the central area of the

test apparatus. The chicks position at the starting point,

as well as the position of the two stimuli, was balanced

across animals.

The procedure used for the test was the same as that

used to assess spontaneous preference. Chicks behavior

was observed for a total of six consecutive minutes.

Permanence of the chick in the mid compartment indi-

cated no choice, whereas presence of the chick in one ofthe end-side compartments was regarded as a preference

for the object placed at that end of the runway. A

computer-driven event recorder allowed the experimenter

to score the time (seconds) spent by the chick in each of

the three compartments. The proportion of time spent

near the shadow of the imprinting object was thereafter

computed as:

Time near the shadow of familiar object

Time near the shadow of the familiar of the novel object

100

The index values ranged from 0 (full choice for the

novel shadow) to 100 (full choice for the familiar object).

Exactly 50% represented chance level. Data were analyzed

with analysis of variance; departures from random choice

were estimated by one sample (two-tailed) t tests.

Results and discussion

Spontaneous preferences

Data for nave chicks (not imprinted) did not reveal anystatistically significant effects (sex: F(1,70) = 0.941,

P = 0.335; time:F(5,350) = 1.061,P = 0.382; time 9 sex:

F(5,350) = 0.617, P = 0.687; see Fig. 3). Chicks did not

show any significant preference for either object

(mean SEM = 50.960 2.989; one-sample t test

t(71) = 0.321, P = 0.749). These results revealed that

nave chicks do not exhibit any spontaneous preference for

a particular shadow, approaching similarly the two test

stimuli.

Preferences following exposure

The results are shown in Fig.4. A repeated measures

Anova with sex (males, females), Motion condition (sta-tionary, rotatory motion, and translatory motion) and

Imprinting (H shape, P shape) as between-subject

factors, and Time (minutes from 1 to 6) as within-subject

factor revealed a significant main effect of motion condi-

tion (F(2,357) = 6.507, P = 0.002) and of the interaction

sex 9 motion condition (F(2,357) = 3.508, P = 0.031). No

other main effects or interactions were significant (sex:

F(1,357) = 0.092, P = 0.762; imprinting: F(1,357) = 1.085,

P = 0.298; sex 9 imprinting: F(1,357) = 0.274, P = 0.601;

motion condition 9 imprinting: F(2,357) = 2.488, P =

0.085; sex 9 motion condition 9 imprinting: F(2,357) =

0.615, P = 0.541; time: F(5,1785) = 1.656, P = 0.142;time 9 sex: F(5,1785) = 0.405, P = 0.846; time 9 motion

condition: F(10,1785) = 1.608, P = 0.099; time 9 imprint-

ing: F(5,1785) = 0.593, P = 0.706; time 9 sex 9 motion

condition: F(10,1785) = 0.718, P = 0.708; time 9 sex 9

imprinting: F(5,1785) = 0.396, P = 0.852; time 9 motion

condition 9 imprinting: F(10,1785) = 1.064, P = 0.387;

time 9 sex 9 motion condition 9 imprinting: F(10,1785) =

1.708, P = 0.074).

Overall, chicks tested with the shadows projected by static

or translating objects did not show any significant preference

for thetwo stimuli (static:mean SEM = 49.252 1.925;

one-sample t test t(103) = -0.389, P = 0.698; translating:mean SEM = 47.240 2.383; one-sample ttest t(151) =

-1.158, P = 0.249). In contrast, chicks tested with the

shadow cast by rotating objects chose to approach the

familiar stimulus (mean SEM = 58.007 2.218; one-

samplettestt(112) = 3.609,P\ 0.0001).

Separate analyses for the two sexes revealed that males

showed a significant main effect of motion condition

(F(2,179) = 9.070, P\0.0001; there were no other

significant effects: imprinting: F(1,179) = 1.244, P = 0.266;

Fig. 3 Time (group mean SEM) spent by male and female chicks

near the shadow cast by the H (arbitrarily chosen)

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motion condition 9 imprinting:F(2,179) = 1.631,P = 0.199;

time: F(5,895) = 1.135, P = 0.340; time 9 motion condi-

tion: F(10,895) = 0.943, P = 0.492; time 9 imprinting:

F(5,895) = 0.969, P = 0.436; time 9 motion condi-

tion 9 imprinting: F(10,895) = 0.596, P = 0.818), whereas

this was not so for females (F(2,178) = 0.874, P = 0.419).

Females, however, showed a significant interaction of

time 9 motion condition 9 imprinting: (F(10,890) = 1.917,

P = 0.040; there were no other significant effects:

Imprinting: F(1,178) = 0.132, P = 0.717; motion condi-tion 9 imprinting: F(2,178) = 1.524, P = 0.221; time:

F(5,890) = 0.959, P = 0.442; time 9 motion condition:

F(10,890) = 1.319,P = 0.215; time 9 imprinting: F(5,890) =

0.180, P = 0.970), due to a significant main effect of time

which was apparent in the rotation condition [time

(F(5,280) = 2.361, P = 0.040)], but not in the stationary

(F(5,240) = 0.997, P = 0.420) and in the translatory

(F(5,370) = 0.281, P = 923) conditions.

As shown in Fig. 5, which summarizes the data, males

significantly chose to approach the familiar object when this

was the shadow cast by the rotating object (mean

SEM = 61.130 3.615; one-sample t test t(54) = 3.078,P = 0.003). When allowed to choose between static stimuli,

males did not exhibit any significant preference (mean

SEM = 49.810 2.392; one-samplettestt(53) = -0.081,

P = 0.935). Intriguingly, however, when tested with trans-

lating stimuli they showed a significant preference for the

unfamiliar stimulus (mean SEM = 42.320 3.198;

one-sample t test t(75) = -2.400, P = 0.019). Female

chicks, on the other hand, did not exhibit any significant

preference either in the static or in the translating condition

(static: mean SEM = 48.650 3.082; one-samplettest

t(49) = -0.437, P = 0.664; translating: mean SEM =

52.160 3.462; one-sample t test t(75) = 0.623, P =

0.535), while they showed a preference for the familiar

shadow in the last 3 min of test when it was cast by the

rotating stimulus (mean SEM = 58.977 3.747; one-

samplettestt(57) = 2.396,P = 0.020).

It is unclear why in the condition of translatory motion,

when information on the shape is maintained over time

because the shape undergoes a rigid transformation, maleschose the unfamiliar stimulus and females did not. A pos-

sible explanation may be related to evidence suggesting that

sex differences in imprinting tests seem to be associated

with a stronger tendency of males to explore novelty and of

females to maintain a more strict social attachment

(Vallortigara and Andrew1991). Shadows that underwent

translatory motion, and that therefore maintained the same

shape as during imprinting in the homecage, might have

represented to the chicks a condition of more reduced

novelty as compared with shadows that underwent rotation

around their vertical axes, and thus continuously changed

their 2-D projections on the screen. Exploration of the novelstimulus could be expected in this condition by males,

whereas lack of choice by females could represent an

imbalance between preference for familiarity and prefer-

ence for novelty. Whether or not this interpretation is

correct, the crucial result of the experiment obviously

concerns the rotation of stimuli around their vertical axis.

When chicks were exposed to stimuli that were very similar

to each other (and that did not elicit any spontaneous pref-

erence in the animals before imprinting), the information

Fig. 4 Time (group mean SEM) spent by male and female chicks near the shadow cast by the familiar (imprinting) object ina stationary,brotatory, andc translatory conditions

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provided by the rotating motion of the stimuli enables themto discriminate between the two objects. Still, however, the

possibility that cues available in the stationary pattern

could have been used to perform the discrimination cannot

be completely excluded (for some discrimination was

observed, at least in males, even under translatory motion

conditions). We thus turned in the next experiment to the

use of random-dot kinematograms.

Experiment 2

The results of the previous experiment suggested that

chicks may be capable of discriminating between two

objects on the basis of the shadows that they cast. None-

theless, the results did not provide conclusive evidence of

pure structure-from-motion perception, because there was

information to distinguish between the two shapes in the

2-D images. In Experiment 2 we tried to investigate whe-

ther motion cues alone would suffice in allowing the

detection and recognition of the imprinting object by using

random-dot kinematograms (Julesz1971). In these displays

no shape information is available in the 2-D image to

distinguish 2-D shapes, until they are set in motion.


Subjects

A total of 125 Hybro domestic chicks (Gallus gallus) were

used. They were incubated and hatched in standard con-

ditions as described for the previous experiment.


The imprinting stimuli consisted of video animations

reproducing a cube or a sphere by a random-dot display on

a static background (for a single frame see Fig. 6). These

stimuli were generated by Blender (Version 2.41), an

open-source software released for modelling, animating

and rendering 3-D scenes. Video-stimuli were created by ablack solid cube/sphere (about 4 cm of height) with a

texture of white dots on it, placed on a black screen with

the same texture.

Each dot was composed of nine pixels on a 17 inc,

1,024 9 768 pixel resolution screen, presented at a view-

ing distance of 40 cm (about the distance from the center of

the apparatus, i.e., the starting point for the chicks during

the test). The solid cube/sphere was placed in slow rotation

(at 10 rev/min) about its vertical axis (this rotation speed

was chosen by reference to previous experiments (Clara

et al. 2006) and due to the early age at which the chicks

were tested). During the imprinting phase, the shape of the

objects could be perceived even from a single static frame

(see Fig. 6) because of the difference in size between the

dots in the objects and the dots in the background and

because of occlusion information. The cube was slanted at

a 20 visual angle with respect to the line of sight, seen

from above. Videos were produced by looping a 60-frames/

sec animation, saved with greyscale data and quality 75

AVI jpeg. Stimuli were presented on two identical Compaq

V700 screens with a refresh rate of 85 Hz. Apart from the

light arising from the monitor screens, the room was

maintained in complete darkness (this, together with the

high refresh rate of the screens, was aimed at preventing

any flicker detection by the chicks, because it was shown

that they can discriminate in dimlight a maximum Critical

Fusion Frequency of 105 Hz; Nuboer et al. 1992).

The imprinting apparatus (Fig. 7) consisted of a set of

plywood-boxes (10 9 10 cm), each one housing one chick,

with the front made of a transparent glass sheet. An

imprinting stimulus was projected on a computer screen

placed 30 cm away, with the center of the stimulus

approximately coaxial to the center of the imprinting


near the shadow cast by the familiar (imprinted) object

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apparatus, to guarantee an appropriate point of view for all

of the chicks. The computer screen was placed at the end of

a black-plastic tunnel, to prevent chicks from seeing any

other visual stimulus.

Sixty chicks were imprinted on the sphere (27 females and

33 males) and 65 on the cube (29 females and 36 males).


The test stimuli consisted of two video images reproducing

the cube and the sphere by a moving random-dot display on

a static background. This time, however, no differences in

size between dots or occlusion information was provided,

so that the shapes could not be perceived in single static

frames (Fig. 8). These stimuli were again generated by

Blender (Version 2.41), by a black solid cube/sphere

(height: about 4 cm) with a texture of white dots on it,

placed on a black screen with same texture.

Each dot was made of one pixel on a 17 inc

1,024 9 768 pixel resolution screen and presented at a

viewing distance of 40 cm. The solid cube/sphere was

perceived as rotating about its vertical axis, at ten revolu-

tions per min, and they were visible only during motion,

dissolving into a random-dot pattern in a single frame. The

Fig. 6 The stimuli used in Experiment 2, a cube and a sphere

Fig. 7 A schematic representation of the imprinting apparatus used

in Experiment 2

Fig. 8 A single static frame of the test stimuli used in Experiments 2

and 3

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cube was slanted at a 20 visual angle with respect to the

floor of the apparatus. Videos were produced by looping a

60-frames/s animation, saved with greyscale data and

quality 75 AVI jpeg, as for the imprinting ones, presented

on two identical Compaq V700 screens with a refresh rate

of 85 Hz in a thoroughly dark room.

The test apparatus was the same runway of the first

experiment, with the two monitors placed at the oppositeends of it, at about 40 cm from the center of the apparatus

(i.e., the starting position of the chicks), to guarantee

binocular vision.

Procedure

On day 1, in the early morning, a group of nave chicks

(n = 115, 67 females and 48 males) was tested so that a

spontaneous preference for either video animations in the

absence of imprinting could be assessed. Immediately

after the test for spontaneous preference, or in the early

morning of day 1 for the other chicks, each animal wastaken from the dark hatchery, placed individually in an

imprinting box and continuously exposed to the imprint-

ing stimulus for 3 h. At the end of the exposure each

chick was placed back in the hatchery in total darkness to

avoid any further visual experience and allow for memory

consolidation (it is known, in fact, that chicks affiliative

behavior is enhanced after a period in the dark as com-

pared to chicks tested immediately after exposure;

Johnson and Horn 1988). Chicks were tested on day 3,

48 h post-hatching (due to yolk reserves, chicks can sur-

vive without food for the first 72 h after hatching; Hogan

1973; Rogers 1995); each chick was carried, in a closed

small cardboard box, to the experimental room and placed

in the central area of the test apparatus. Procedure of both

spontaneous preference test and the test following

imprinting as well as the computation of the behavioral

index were identical to that described in previous

experiments.


Spontaneous preferences

Data from spontaneous preferences scored in nave chicks

(not imprinted) did not reveal any statistically signifi-

cant effect (sex: F(1,113) = 0.869, P = 0.353; time:

F(5,565) = 0.995, P = 0.420; time 9 sex: F(5,565) = 1.733,

P = 0.125; see Fig. 9). Chicks did not exhibit any spon-

taneous preference for either display (mean SEM =

48,510 2.448; one-sample t test t(114) = -0.610, P =

0.543), approaching similarly the two test stimuli.

Preferences following exposure

Theresults are shown in Fig.10. A repeated measures Anova

with sex and imprinting object (sphere vs. cube) as between-

subject factors, and time (minutes from 1 to 6) as a within-

subject factor revealed a significant time 9 imprinting

object interaction (F(5,625) = 3.600, P = 0.003). There were

no other statistically significant effects (sex: F(1,125) =

0.987, P = 0.322; imprinting object: F(1,125) = 0.496,

P = 0.483, time: F(5,625) = 0.220, P = 0.954; sex 9

imprinting object: F(1,125) = 1.508, P = 0.222; time 9 sex:

F(5,625) = 0.153, P = 0.979; time 9 sex 9 imprinting

object:F(5,625) = 0.121,P = 0.988).

The interaction time 9 imprinting object suggested that

in the last minutes of test a significant preference was

apparent. An Anova limited to the last two minutes of test,

in fact, revealed a significant main effect of Imprinting

object (F(1,125) = 4.354, P = 0.040). No other main effect

or interactions were significant (sex: F(1,125) = 0.584,

P = 0.447; sex 9 imprinting object: F(1,125) = 0.656,

P = 0.419). Data revealed, however, different behaviors in

chicks imprinted on the cube and chicks imprinted on

the sphere (independent-samples t test: t(125) = 2,014,

P = 0.046). In the last 2 min of test, in fact, chicks

imprinted on the sphere showed a significant preference for

the imprinting stimulus (mean SEM = 59.650 4.731;

one-sample t test: t(63) = 2.039, P = 0.046), while chicks

imprinted on the cube did not show any significant

preference (mean SEM = 46.850 4.237; one-sample

t test: t(64) = -0.742, P = 0.461).

Overall, chicks seemed to discriminate the two stimuli

from one another and selectively prefer the sphere, the shape

of which is known to be a very attractive stimulus for this

species (Fantz1957). To better show this, we recomputed

chicks choice in order to obtain for all chicks a score of their

preference for the sphere rather than for their own familiar

Fig. 9 Time (group mean SEM) spent by nave male and female

chicks near the random-dot kinematogram depicting the sphere

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object. An Anova run on these data revealed only a

main effect of time (F(5,635) = 3.638, P = 0.003, sex:

F(1,127) = 1.534, P = 0.218; time 9 sex: F(5,635) = 0.121,

P = 0.988). As shown in Fig. 11, chicks showed a prefer-

ence to approach the sphere, that became significant in the

last 2 min of the test (mean SEM = 56.372 3.173;

one-samplettest:t(128) = 2.008,P = 0.047).

Chicks seem therefore to be able to discriminate objects

from coherently moving point-light patterns requiring a

sheer structure-from-motion extraction. Although they

expressed a preference for the sphere rather than a pref-

erence for their own familiar stimulus, the extraction of the

shape was not possible in the random dot display in thestatic conditions and could only be obtained through

motion information.

Experiment 3

Results of Experiment 2 showed that chicks were capable

of a pure structure-from-motion discrimination, discrimi-

nating the two test stimuli from one another, though they

selectively preferred the sphere rather than the familiar

object. The aim of Experiment 3 was to investigate whetherchicks would be capable of a higher-level generalization in

a structure-from-motion task, namely that of recognizing

an imprinting 3-D solid stimulus in a random-dot display

which has lost all features of the solid imprinting object.

In order to do that, after 48 h of exposure to a real 3-D

object, chicks were allowed to choose between two

random-dot animations, reproducing, by the coherent

movement of the dots, the familiar 3-D stimulus and a 3-D

stimulus never seen before, respectively.


Subjects

A total of 144 Hybro domestic chicks, incubated and hat-

ched in standard conditions as in the previous experiments,

were used.


The imprinting stimuli consisted of two solid 3-D objects, a

cube (edge = 3.5 cm) and a sphere (4.3 cm in diameter, so

that it approximated to the cube both in volume andheight), made of wood and painted red (see Fig. 12). Sixty-

nine chicks were imprinted on the sphere (36 females and

33 males) and 75 on the cube (35 females and 40 males).


The test stimuli and apparatus were the same as in

Experiment 2.


near the random-dot kinematogram of the familiar object

Fig. 11 Time (group mean SEM) spent by chicks imprinted on the

sphere or the cube near the most attractive object, i.e., the sphere

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Procedure

In the early morning of day 1, each chick was placed

individually in an imprinting cage, with food and water,

and continuously exposed for 48 h to an imprinting stim-

ulus hung on a fine thread in the middle of the cage. On day

3 each chick was carried, in a closed small cardboard box,to the experimental room and placed in the central area of

the test apparatus. Procedures at test were identical to that

described in previous experiments.


The results are shown in Fig.13. An Anova with sex

(males vs. females) and imprinting (sphere vs. cube) as

between subjects factors, and time (minutes from 1 to 6) as

a repeated measure factor, showed a significant main effect

of time (F(5,700) =

6,132, P\

0.0001) and a significantsex 9 imprinting interaction (F(1,140) = 3.960, P =

0.049). There were no other statistically significant effects

(sex: F(1,140) = 0.227, P = 0.635; imprinting: F(1,140) =

1.190, P = 0.277; time 9 sex: F(5,700) = 1.242,

P = 0.288; time 9 imprinting:F(5,700) = 0.778,P = 0.566;

time 9 sex 9 imprinting: F(5,700) = 1.450, P = 0.204).

Separate analyses for the two sexes revealed that in

females there was a significant main effect of imprinting

(F(1,69) = 4.578, P = 0.036) without any time 9 imprint-

ing interaction (F(5,345) = 2.102, P = 0.065; time:

F(5,345) = 1.594,P = 0.161), whereas in males no such an

effect was observed (imprinting: F(1,71) =

0.419, P =

0.519; time 9 imprinting: F(5,355) = 0.268, P = 0.930;

time: F(5,355) = 5.501, P\ 0.0001). Males did not show

any significant preference for any display during test

(mean SEM = 49.675 2.350; one-sample t test

t(72) = -0.138, P = 0.891). Females, in contrast, when

imprinted on the cube did not show any preference when

considering overall the 6 min of test (mean

SEM = 55.740 2.965; one-sample t test t(34) = 1.936,

P = 0.061) but showed a significant preference when

considering the last minutes of test (minutes from 3 to 6:

mean SEM = 58.414 3.866; one-sample ttestt(34) =

2.177,P = 0.037); females imprinted on the sphere did not

significantly choose to approach either the familiar or the

unfamiliar stimulus (one-sample t test t(35) = 0.233,

P = 0.817).

General discussion

Structure-from-motion, the process whereby a 3-D shapecan be perceived solely from motion cues, has been

investigated extensively in humans (Wallach and

OConnel1953; Johansson1978; Ullman1979). Recently,

some research has been carried out on how motion cues

can influence recognition of shapes in birds, especially

pigeons, a species which has been demonstrated to be

highly sensitive to fine changes in motion patterns

(Bischof et al. 1999; Cook and Katz 1999; Spetch et al.

2006).

Fig. 12 The imprinting stimuli used in Experiment 3, the cube and

the sphere

Fig. 13 Time (group means SEM) spent by male and female

chicks by the random-dot kinematogram depicting the familiar shape

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The aim of the present research was to investigate

structure-from-motion perception in young domestic

chicks, by looking at their ability to recognize solid shapes

from impoverished displays, taking advantage of this spe-

cies susceptibility to filial imprinting.

The influence of kinematic factors in the recognition

process was investigated by providing the chicks selec-

tively with such types of cue. In Experiment 1 the onlyinformation presented to the chicks (previously imprinted

onto a solid object) was the shadows cast by the

imprinting and by a non-familiar object, in either a

static, rotating or translating condition. The results

showed that chicks tested with the shadow projected by

a static or translating object did not make a clear choice

for the shadow cast by the familiar stimulus, while

chicks tested with the shadow cast by a rotating object

chose to selectively approach the familiar stimulus.

Experiment 2 was run to investigate if motion cues

alone were enough to recognize the familiar solid object

at test, since in Experiment 1 a role of non-motion cuescould not be excluded. Chicks capability to perceive

structure-from-motion was tested when both imprinting

and test stimuli belonged to similar kind of patterns, both

of them being random-dot displays reproducing a solid

shape once in motion. Imprinting stimuli differed from

test stimuli in that 2-D cues were available to discrimi-

nate the two shapes during imprinting but were no

longer available during test. At test, motion cues only

could be used to discriminate between the two shapes.

Chicks appeared to be capable of discriminating the two-

test stimuli from one another, although this was dem-

onstrated by their preference for the sphere, which is

known to be a very attractive shape for the young of this

species (Fantz 1957).

In Experiment 3 chicks were imprinted on solid, real 3-

D objects and then tested with random-dot kinematograms

reproducing the appearance of the same 3-D objects on the

basis of motion cues alone. The results of these experi-

ments were unclear, since only females showed some

evidence of transfer. The reason for such a puzzling sex

difference will require further research.

Overall, it seems that domestic chicks, at a very early

stage of development (day 3) are able to recognize

familiar shapes using extremely impoverished two-

dimensional stimulations, such as a cast shadow or a

random dot kinematogram. Such recognition is particu-

larly sensitive to fine differences when motion is

provided (i.e., rotation along the vertical axis, allowing

the integration in time of different visual perspectives).

Given the very limited extent of previous visual expe-

rience, such results seem to add to the idea that

mechanisms of depth perception through motion are

largely predisposed in the vertebrate brain.

Ethical considerations

All of the experiments reported in the present paper comply

with current Italian and European Community laws on the

ethical treatment of animals. All experimental procedures

have been licensed by the Office of the Italian Government

(Ministero della Salute, Dipartimento Alimenti, Nutrizione e

Sanita Pubblica Veterinaria), and the present project hasbeen classified as purely behavioral testing, involving min-

imal discomfort to the animals. All of the chicks, after being

tested in the 6-min behavioral observations, were caged in

social groups with food and water available ad libitum and

immediately after they were donated to local farmers, who

provided them withfree range conditions, as approved by our

Animal House licence for observational experiments on

chicks.

In Experiment two chicks remained for 2 days post-

hatching in the hatchery in complete darkness, with no food

available. This procedure is possible since 20% of newly

hatched chicks body weight consists of the remains of yolk,and the yolk-sac, that provides the newborn with sufficient

nutrients for the first few days (4872 h after hatching,

Hogan1973; Rogers1995). Throughout the time in which

chicks were food deprived they were constantly monitored.

Acknowledgments The authors wish to thank Diego Varotto for

technical advice and support, Piero Cazzavillan for creating the ran-

dom-dot-stimuli and Stefania Tognin for the help provided with

animal care and testing. This study was supported by a grant P.R.IN.

2005 Zanforlin (to E.M.); and by grants MIUR Cofin 2004,

2004070353_002 Intellat and MIPAF Benolat (to G V.).

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