Perception of illusory contours in children and adults: An eye-tracking study.

The eye-tracking study investigated the perception of subjective Kanizsa and Ehrenstein figures in adults and in children aged 3-4, 5-6, 7-8, and 9-11 years of age. More specifically, the distribution of looking at the inner stimulus part versus the inducing elements was measured for illusory figures, figures with real contours, and control displays. It was hypothesized that longer looking at the inner area of the illusory figures indicates global contour interpolation, whereas longer looking at the inducing elements indicates a local processing mode. According to the results, participants of all ages looked longer at the illusory Kanizsa and Ehrenstein contours than at the figures' inducing elements. However, performance was lowest in the children aged 3-4 years and increased during the preschool period. Moreover, the illusory contour displays elicited comparable visual responses as did the real contour displays. The use of the control displays that contained no contour information ensured that the participants' looking behavior was not driven by a spontaneous tendency to attend to the inner stimulus parts. The study confirms the view that sensitivity to illusory contours emerges very early in life.

Holistic face processing in 4- and 7-month-old infants.

Previous studies found an onset of holistic face processing in the age range between 0-4 and 7 months of age. To validate these studies, the present study investigated infants 4 and 7 months of age with a different experimental approach. In a habituation-dishabituation experiment, the infants were tested with stereoscopic stimuli in which stripes floated above a face, thereby occluding some parts of the face (amodal completion condition), and stereoscopic stimuli in which the same face parts floated above stripes (modal completion condition). Research with adults indicates that faces are processed holistically, that is as global wholes, in the amodal, but as independent parts in the modal completion condition, resulting in superior face recognition when the occluding bars are in front of than when they are behind the visible face parts. The present study found that infants regardless of whether they are 4 or 7 months old reliably recognized and differentiated the faces in the amodal but not in the modal completion condition. Moreover, the difference between the experimental conditions was statistically significant. These findings show that approximately at the age of 4-7 months of life, infants begin to holistically unify disjoint face parts into a coherent whole.

Infants' responsiveness to half-occlusions in phantom stereograms.

The present natural preference study investigated infants 4 and 7 months of age for their ability to respond to phantom contoubrs, illusory surfaces generated by half-occlusions in a stereoscopic display consisting of a pair of parallel vertical lines. The left line in the half-image for the right eye and the right line in the half-image for the left eye have a gap in the middle. The visual system accounts for the binocular unmatched gaps by perceiving an illusory contour. Infants in the experimental condition were presented with a standard phantom stereogram displaying a phantom contour versus a non-standard phantom stereogram, the half-images of which were exchanged. This stereogram evokes the impression of two small separate illusory contours. In both stereograms, the gaps moved up and down. The participants aged 7 but not 4 months preferred looking at the standard phantom stereogram. A control condition supported the hypothesis that the infants 7 months of age in the experimental condition indeed responded to the coherent illusory surface instead of simply detecting differences in the geometric arrangement of the half-occlusions. The results hence indicate that infants are able to extract spatial information from monocular regions in a binocular display.

Infant perception of von Szily contours.

This habituation-dishabituation study examined infants' perception of subjective von Szily contours, the illusory effect of which is generated by horizontal disparity and half-occlusions. In these contours, a foreground surface appears to partially occlude a background surface. In Experiment 1, participants aged 4 and 5 months were habituated to a von Szily figure and were then tested for their ability to perceive the difference between the habituation figure and the same figure with reversed depth relations. The infants displayed significant novelty preferences during the posthabituation period. This observation indicates that 4- and 5-month-olds respond to the stereoscopically specified depth difference between the two surfaces of von Szily figures. In Experiment 2, participants aged 4 and 5 months were tested for the ability to conduct modal completion, that is, to perceive the surface that is stereoscopically shifted into the foreground as a whole. The infants were habituated to a von Szily figure and then examined for their ability to distinguish between complete and incomplete versions of the foreground surface. Longer looking at the incomplete posthabituation pattern indicates modal completion; the infants recognize the complete pattern as familiar and regard the incomplete pattern as novel. Similarly, Experiment 3 investigated whether infants aged 5 and 7 months amodally complete the background surface, that is, the surface that is partially covered by the foreground surface. Experiment 2 found modal completion in 5-month-olds. Experiment 3 established that 5- and 7-month-olds have developed some ability to conduct amodal completion. In sum, infants perceive the depth information in von Szily contours and conduct modal and amodal completion.

Infants Perceive Three-Dimensional Subjective Contours.

The addition of crossed horizontal disparity enhances the clarity of illusory contours compared to pictorial illusory contours and illusory contours with uncrossed horizontal disparity. Two infant-controlled habituation-dishabituation experiments explored the presence of this effect in infants 5 months of age. Experiment 1 examined whether infants are able to distinguish between a Kanizsa figure with crossed horizontal disparity and a Kanizsa figure with uncrossed horizontal disparity. Experiment 2 tested infants for their ability to differentiate between a Kanizsa figure with crossed horizontal disparity and a two-dimensional Kanizsa figure. The results provided evidence that the participants perceived the two- and the three-dimensional illusory Kanizsa contour, the illusory effect in which was strengthened by the addition of crossed horizontal disparity.

Infants perceive two-dimensional shape from horizontal disparity.

Previous studies observed that responsiveness to horizontal disparity as such emerges at approximately 2 months of age. Moreover, 3- to 4-month-old infants utilize stereoscopic information to perceive object variations in depth. The present study investigated infants' ability to respond to crossed horizontal disparity information that defines two-dimensional shape. Infants 4 and 5 months of age were habituated to either a cross or the outline of a square. During the posthabituation period, they were presented with both shapes. The stimuli were dynamic random dot stereograms shown on an autostereoscopic monitor. The participants 5 but not 4 months of age displayed significant novelty preferences for the unfamiliar shape during the posthabituation period. Five-month-old infants are hence sensitive to horizontal disparity information that specifies shape.

Binocular vision in infancy: Responsiveness to uncrossed horizontal disparity.

This research explored the onset of stereopsis, the ability to perceive depth from the different views provided by the two eyes. In a longitudinal study, infants were tested weekly from 6 to 20 weeks of age. The primary goal of the study was to establish the onset and the early development of sensitivity to uncrossed horizontal disparity. The infant participants were shown dynamic random dot stereograms displaying two squares, one with uncrossed horizontal disparity (0.5°) and one with vertical disparity (0.5°). The stimuli were presented on an autostereoscopic monitor. We used two methods, the forced-choice preferential looking (FPL) method and the classical natural preference (CNP) method, to measure whether the infants preferred the uncrossed over the vertical disparity display. According to the FPL data, the mean relative preferences for horizontal over vertical disparity were significantly greater than chance probability (0.50) from 13 weeks of age onward. With the CNP method we found significant preferences for uncrossed horizontal disparity from 15 weeks onward. The FPL method was hence more sensitive than the CNP method as it indicated an earlier onset of responsiveness to stereoscopic information.

Infants Perceive Three-Dimensional Illusory Contours as Occluding Surfaces.

The study assessed the contribution of stereoscopic depth cues to infants' perception of a Kanizsa rectangle as a surface that temporarily occludes a moving object. In Experiment 1, the Kanizsa figure was shifted into the foreground by enriching it with stereoscopic depth information. According to the results, perception of a three-dimensional Kanizsa figure as an occluding surface emerges between 5 (n = 16) and 7 (n = 16) months of age. Experiment 2 demonstrated that 7-month-old (n = 16) infants performed similarly to the 7-month-olds who participated in Experiment 1 if the moving object was shifted into the background. These findings suggest that 7-month-old infants respond to stereoscopic depth cues and that they exploit it to perceive subjective contours as occluders.

Infants' discrimination of crossed and uncrossed horizontal disparity.

In a series of preferential-looking experiments, infants 5 to 6 months of age were tested for their responsiveness to crossed and uncrossed horizontal disparity. In Experiments 1 and 2, infants were presented with dynamic random dot stereograms displaying a square target defined by either a 0.5° crossed or a 0.5° uncrossed horizontal disparity and a square control target defined by a 0.5° vertical disparity. In Experiment 3, infants were presented with the crossed and the uncrossed horizontal disparity targets used in Experiments 1 and 2. According to the results, the participants looked more often at the crossed (Experiment 1), as well as the uncrossed (Experiment 2), horizontal disparity targets than at the vertical disparity target. These results suggest that the infants were sensitive to both crossed and uncrossed horizontal disparity information. Moreover, the participants exhibited a natural visual preference for the crossed over the uncrossed horizontal disparity (Experiment 3). Since prior research established natural looking and reaching preferences for the (apparently) nearer of two objects, this finding is consistent with the hypothesis that the infants were able to extract the depth relations specified by crossed (near) and uncrossed (far) horizontal disparity.

The impact of stereoscopic depth on the Munker-White illusion.

The current study investigated the impact of stereoscopic depth information on adults' perception of a coloured version of the Munker-White illusion. In one half of the illusory figure red patches were embedded in black stripes and flanked by yellow stripes. In the other half of the illusory figure red patches were embedded in yellow stripes and flanked by black stripes. The red patches either remained in the same depth plane as the black and yellow inducing stripes (zero horizontal disparity condition) or were shifted into the foreground (crossed horizontal disparity condition) or into the background (uncrossed horizontal disparity condition). According to the results, the illusory effect was robust across all viewing conditions. The illusion mainly consisted of a subjective darkening of the red patches superimposed on the yellow stripes, a perceived hue shift of the red patches superimposed on the black stripes toward yellow, and a subjective saturation decrease in both kinds of red patches. Moreover, the study established a partial confirmation of Anderson's scission theory, according to which the Munker-White illusion should be largest in the crossed horizontal disparity condition, intermediate in the zero horizontal disparity condition, and smallest in the uncrossed horizontal disparity condition.

The onset of sensitivity to horizontal disparity in infancy: a short-term longitudinal study.

In this short-term longitudinal study, infants were examined for their natural preference of a square defined by crossed horizontal disparity (either 1° or 0.5°) over a square defined by a vertical disparity (either 1° or 0.5°). The square targets were embedded in a dynamic random dot stereogram. The stimuli were presented on an autostereoscopic monitor equipped with a face-tracking device. The infants were tested weekly between 6 and 16 weeks of age. Four experiments were conducted. In two experiments, the infants were examined with the forced-choice preferential looking (FPL) method for their ability to perceive either 1° or 0.5° horizontal disparity. In the remaining two experiments, the classical natural preference (CNP) method (measurement of looking times) was applied. According to the results of the FPL experiments, mean relative preference for the horizontal disparity square became significant at 8 weeks of age. The CNP data indicated an onset of stereopsis at 12-15 weeks. The mean relative preferences for horizontal disparity indicated by the CNP method were smaller than those found in the FPL experiments. Thus, the FPL method was more sensitive than the CNP method in the measurement of infant responsiveness to crossed horizontal disparity.

Rotary motion impairs attention to color change in 4-month-old infants.

Continuous color changes of an array of elements appear to stop changing if the array undergoes a coherent motion. This silencing illusion was demonstrated for adults by Suchow and Alvarez (Current Biology, 2011, vol. 21, pp. 140-143). The current forced-choice preferential looking study examined 4-month-old infants' sensitivity to the silencing illusion. Two experimental conditions were conducted. In the dynamic condition, infants were tested with two rotating rings of circular different-colored dots. In one of these rings the dots continuously changed color, whereas in the other ring the dots did not change color. In the static condition, the global rotary motion was eliminated from the targets. Infants preferred looking at the color-changing target in the static condition but not in the dynamic condition; they attended to the color changes in the static condition but failed to detect them in the dynamic condition. This differential looking pattern is consistent with the hypothesis that the silencing illusion can be established during early infancy. A control group of adults also responded to the silencing phenomenon. This substantiates that the stimuli generate a robust illusory effect.

The comparator model of infant visual habituation and dishabituation: recent insights.

Current knowledge of the perceptual and cognitive abilities in infancy is largely based on the visual habituation-dishabituation method. According to the comparator model [e.g., Sokolov (1963a) Perception and the conditioned reflex. Oxford: Pergamon Press], habituation refers to stimulus encoding and dishabituation refers to discriminatory memory performance. The review also describes the dual-process theory and the attention disengagement approach. The dual-process theory points to the impact of natural stimulus preferences on habituation-dishabituation processes. The attention disengagement approach emphasizes the contribution of the ability to shift the attention away from a stimulus. Moreover, arguments for the cognitive interpretation of visual habituation and dishabituations are discussed. These arguments are provided by physiological studies and by research on interindividual differences. Overall, the review shows that current research supports the comparator model. It emphasizes that the investigation of habituation and dishabituation expands our understanding of visual attention processes in infants.

The ground is dominant in infants' perception of relative distance.

When making relative distance judgments, adults attend to information provided by the ground surface and generally ignore information provided by ceiling surfaces. In the present study, we asked whether this ground dominance effect is present in infancy. Groups of 5- and 7-month-old infants viewed a display depicting textured ground and ceiling surfaces. Two toys, which were attached to vertical rods, were affixed to the display. The toys/rods were positioned so that one toy was specified as being nearer by the ground surface but farther away by the ceiling surface, while the other toy was specified as being farther away by the ground surface but nearer by the ceiling surface. Under monocular viewing conditions, the infants in both age groups reached preferentially for the toy that was specified as being nearer by the ground surface. This effect was significantly stronger than that observed under binocular viewing conditions. The findings indicate that the infants responded to the distance information provided by the ground surface to a greater extent than to information provided by the ceiling.

Infants' responsiveness to rivalrous gratings.

The study investigated the early development of responsiveness to rivalrous gratings. Infants were tested weekly between 6 and 16 weeks of age for their ability to discriminate between interocularly identical (fusible) lines and interocularly orthogonal (unfusible, rivalrous) lines. The stimuli were presented on an autostereoscopic monitor equipped with a face-tracking device. Two psychophysical techniques, the forced-choice preferential looking (FPL) method and measurement of looking times, were employed. Contrary to earlier findings, infants at all ages avoided looking at the rivalrous gratings instead of showing a developmental shift from a relative preference for unfusible, rivalrous gratings to a relative preference for fusible gratings. Avoidance of the rivalrous gratings became significant at 8-9weeks of age, suggesting that infants clearly exhibit binocular rivalry from that age onwards. Control experiments secured that the infants' preference for the fusible gratings was not governed by a natural preference for less over more complex line patterns.

Children's and adults' size estimates at near and far distances: A test of the perceptual learning theory of size constancy development.

This study tested the perceptual learning theory of size constancy development, which proposes that children younger than 9 years are relatively insensitive to monocular cues for distance and size, and that developmental changes in far-distance size estimation result from increasing sensitivity to these cues. This theory predicts that before 10 years, children will make less accurate size judgments at far distances under monocular than under binocular viewing conditions. Five age groups were tested: 5-6, 7-8, 9-10, 19-28, and 50+ years. Participants judged the size of a standard disc, from viewing distances of 6.1 and 61 m, by pointing at 1 of 9 nearby comparison discs. Testing was conducted under both monocular and binocular viewing conditions. Five- to 6-year-olds underestimated object size at the far distance, 7- to 8-, 9- to 10-year-olds, and older adults made size estimates that were close to accurate, and the young adults significantly overestimated size. At the near distance, all age groups underestimated size and no age differences were found. Contrary to predictions from the perceptual learning theory, viewing condition had no significant effect on size estimates.

Infants' sensitivity to pictorial depth cues: a review and meta-analysis of looking studies.

This paper reviews habituation-dishabituation and preferential-looking studies on the emergence of sensitivity to pictorial depth cues in infancy. This research can be subdivided into two groups. While one group of studies has established responsiveness to pictorial depth cues at 3-5 months of age, the other has found that the ability to extract pictorial 3D information emerges at about 6 months. In the former, young infants were tested for their ability to distinguish between displays that differ in spatial information provided by pictorial depth cues. The results of these studies might demonstrate that 3-5-month-old infants perceive spatial layout from pictorial cues. It is possible, however, that the infants in these studies responded primarily to low-level, two-dimensional stimulus differences. In contrast, the second group of studies controlled for the potential influence of lower-level stimulus features on the infants' experimental performance and more unambiguously demonstrated sensitivity to pictorial depth information in infants 6 months of age and older. In sum, the divergent findings of studies in this area may be resolved by assuming substantial developmental progress in infant sensitivity to pictorial depth cues during the first months of life.

Infants' responsiveness to lightness changes on a dynamic three-dimensional surface.

In a looking-time study, 24 infants 6 months of age were presented with continuously folding and unfolding patterns of stripes. The luminances in the dynamic lightness constancy pattern were changed in such way that adults attribute them to changes of the various regions' orientation relative to the light source (lightness constancy display). The "reversed" lightness constancy stimulus consisted of a continuously folding and unfolding pattern, in which the luminance changes were not consistent with a striped surface illuminated from one side. The only difference between the animations was the relationship between the change in surface orientation and the change of luminances. The infants looked significantly longer at the reversed lightness constancy animation than at the lightness constancy display. This finding suggests that the infants detected the violation of the lightness constancy rules in the reversed lightness constancy stimulus. The infants were also presented with control animations to rule out the possibility that looking preferences were based on low-level properties of the display.

Mental rotation in female fraternal twins: Evidence for intra-uterine hormone transfer?

Men outperform women in mental rotation by about one standard deviation. Prenatal exposure to testosterone has been suggested as one cause. In animals it has been shown that a female fetus located between two male ones is exposed to higher levels of testosterone. It is still unclear whether intra-uterine hormone transfer exists in humans. Therefore, the influence of an intra-uterine presence of a male co twin was studied in female fraternal twins (N=200). Women with a male co-twin outperformed women with a female co-twin by about a third standard deviation. In a no-twin control group (N=200), performance of women with a slightly older sibling did not depend upon the sibling's sex. These findings provide preliminary support for the theory of an influence of prenatal testosterone on mental rotation performance.

Visual habituation and dishabituation in preterm infants: a review and meta-analysis.

We review comparative studies of infant habituation and dishabituation performance focusing on preterm infants. Habituation refers to cognitive encoding, and dishabituation refers to discrimination and memory. If habituation and dishabituation constitute basic information-processing skills, and preterm infants suffer cognitive disadvantages, then preterms should show diminished habituation and dishabituation performance. Our review provides evidence that preterm infants' habituation and dishabituation are impoverished relative to term infants. On the whole, effect sizes indicated that the differences between preterms and terms are of a medium magnitude. We also find that preterms' performance is moderated by risk factors, stimulus materials, procedural variables, and age. These factors need to be taken into account in the construction of tests in which habituation-dishabituation tasks are employed. Overall, the habituation-dishabituation paradigm presents a promising approach in the diagnosis of cognitive status and development in preterm infants.