Behavioural Brain Research, 49 (1992) 133-139 9 1992 Elsevier Science Publishers B.V. All rights reserved. 0166-4328/92/$05.00

133

BBR 01323

Texture segregation in infants and children Ruxandra

Sireteanu and Christiane Rieth

Max-Planck-lnstitute for Brain Research, Frankfilrt]Main (FRG) (Received 14 June 1991) (Revised version received 30 September) (Accepted 23 October 1991)

Key words: Visual development; Line orientation; Texture grouping

Segregation of textures based on differences in line orientation and blob size was tested in adults, infants and children, with a forced-choice preferential looking technique. Preference for a figure defined by differences in blob size already occurs in 2-month-old infants. In contrast, preference for a figure differing from the background by the orientation of its elements emerges at the end of the first year of life and becomes adult-like by school age.

INTRODUCTION

The ability of the brain to extract figures out of a visual scene is called 'segmentation'. Segmentation of a figure from its surround is a preattentive process operating automatically, effortlessly, and in parallel across the visual field 12"z5. Segmentation may be based upon differences in contrast, colour, disparity, motion and surface texture. In this study, we investigated the development of texture segregation in infants and children. Segregation based on differences in line orientation and blob size was tested with a forced-choice preferential looking technique z3. When presented with pairs of equiluminant stimuli, one containing contours and the other not, infants tend to look at the stimulus with contours 1~ Measurements of preferential looking can thus be used to assess the subjects' ability to resolve patterns z3. Preferences are also seen when two different patterns, both containing contours, are presented ~~ A preference for one pattern is assumed to indicate that the infant discriminates the two patterns. The goal 0fthe present study was, first, to determine whether subjects show a spontaneous preference for a pattern containing a figure defined by textural differ-

Correspondence: R. Sireteanu, Max-Planck Institute for Brain Research, Deutschordenstr. 46, W-6000, Frankfurt/M. 71, Federal Republic of Germany.

ences, and,second, if there are such preferences, to investigate their development. We found that 2-month-old infants already have a preference for a figure defined by differences in blob size. In contrast, preference for a figure differing from the background by the orientation of its elements emerges at the end of the first year of life and becomes adult-like by school age.

METHODS

The subjects sat at a distance of 57 cm in front of a large wooden screen containing two circular apertures with a diameter of 15 ~ centered 20 ~ from the midline. Four small red blinking lamps, two of which could be lit at a time (Fig. I, upper panel), served as a centering stimulus in between the trials. On each trial, a pair of slides was projected from behind onto the two apertures (Fig. 1, lower panel). The room was otherwise darkened. A video camera was positioned behind the peephole. A n adult observer naive to the side of presentation of the stimulus containing the figure looked at the subject's face on a video screen. The observer had to decide on the basis of the subject's looking behaviour (side and duration of the first fixation, longest fixation, interested scanning etc.) which side the figure stimulus had been on (forced:choice preferential looking). There was no time limit for the observer's decision. Each session consisted of 20 trials. At the end of

134

Fig. 1~ Overview of the testing procedure used in the main experiment.

135 the session, percent correct responses was calculated. No preference corresponds to 5 0 ~ correct. Subjects were presented with a pair of stimuli. One stimulus contained a figure embedded in a background; the other showed the background alone. In the first two experiments, we used two stimulus configurations: a figure composed of lines of identical orientation embedded in a surround composed of lines of an orthogonal orientation (Fig. 2, upper part), and a figure composed of blobs of identical size, embedded in a surround of blobs of a different (smaller) size (Fig. 2, lower part). Both patterns have been suggested to consist of elementary features ('textons') and both are spontaneously segregated by normal adult observers ~2"15. The line stimulus does not contain global luminance differences, whereas the blob stimulus shows such differences ~5. The parameters were chosen such that the stimuli contain enough elements in both figure and background, and still be within the resolution limits of the youngest infants zt-23. For the orientation stimulus, the background stimulus contained short lines (1.2 ~ x 0.35~ all having the same oblique orientation. The figure stimulus contained the same elements, with the difference that a group of 16 neighbouring elements have an orthogonal orientation. For adult observers, this group forms a square that clearly pops-out from the background. To avoid spurious 'holes' at the boundaries of the figure, the line elements showed a positional jitter (one third of the raster width). The fundamental spatial frequency of the stimuli, measured in the direction

Fig. 2. Stimuli used in the main experiments. The apertures and stimuli are drawn to scale.

orthogonal to the line elements, was 0.8 c/deg. Oblique orientations were used to avoid artifacts due to the transient astigmatism of infancy, which usually affects the horizontal and vertical meridians 3. For the blob stimulus, the background stimulus was made of randomly arranged small black blobs of the same size (0.3 ~ diameter). The figure stimulus contained a group of 16 neighbouring blobs of larger size (0.8 ~ diameter). This group stands out from the background of small blobs as a dark square. The fundamental spatial frequency of this stimulus was 0.5 c/deg. The stimuli of the third experiment were presented on cardboard cards, 25 cm x 56 cm in size. For the orientation stimulus the whole cardboard card was covered with short oriented lines (2.1~ x 0.6~ all having the same oblique orientation. On one side, a field of 9 neighbouring elements had the orthogonal orientation. The fundamental spatial frequency of the stimulus, measured orthogonal to the line elements, was 0.5 c/ deg. For the blob stimulus, the card was covered with small black blobs of identical size (0.45~ with a group of 9 larger blobs (1.1 ~ embedded on one side. The fundamental spatial frequency of this stimulus was 0.3 c/deg. The elements of both cards showed a positional jitter, similar to the previous experiments. The procedure was adapted from ref. 24. The procedure took place in a well-illuminated room. The child was held in front'of a grey screen containing a rectafigular opening in which the card was presented, hand-held by the observer. The observer, naive to the side and the identity of the stimulus, looked at the child's face through a small peephole at the center of the card. Based on the reaction of the child, she made a judgement on the side ofpresentation of the figure. Each card was presented 2-3 times. The judgements were made independently by 2 observers. Interobserver agreement was 9 5 ~ . A response was scored positive only when both observers agreed. We tested 34 infants (22 males and 12females), 68 children from 1 up to 13 years (38 males and 30 females) and 9 adult subjects (5 males and 4 females). Most children were tested more than once, totalling 360 tests. Sixteen additional children were tested but failed to complete the experiment because of excessive fussiness, crying or falling asleep. Only full-term children with no developmental abnormalities were ineluded. Prior to the experimental sessions, the children had been tested with the Telleracuity cards (infants) or the H-test and the Lang test (children), to ensure that their visual acuity and stereopsis were normal. Children and adults with optical disorders were excluded from the study. The subjects were recruited by announcements in family health care centers and at local paedi-

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Fig. 3. Individual responses for texture segregation base~l on line orientation, as a function of age. The stimuli were presented on projected slides. The results are based on 112 tests (30 infants, 73 children and 9 adult observers). Subjects tested more than once were connected by lines.

atrician offices. Informed consent was obtained from the parents after the procedure was explained fully. All subjects were naive to the procedure and the purpose of the study. The older children and the adults were only instructed to look at the stimuli.

RESULTS

To test whether texture segregation can be detected by the forced-choice preferential looking technique, we first applied this method to naive adult observers. Using

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Fig. 4. glean + S.E.M. of the percentage correct responses for texture segregation based on line orientation, as a function of age. One asterisk: mean is significantly different from chance ( P < 0 . 0 5 : one sided Student's t-test vs. chance); two asterisks: P < 0 . 0 0 5 ; three asterisks: P < 0.0005.

137 the orientation stimulus, the adults showed a highly significant preference (91 ~o, P < 0.0005) for the stimulus containing the figure (Fig. 3). Fig. 3 also shows the percentage correct responses for all tested subjects. A large variability was seen not only between subjects, but also for children tested longitudinally. Fig. 4 shows the percentage correct responses averaged over all children in each age group. Infants aged 3-6 and 6-9 months do not show a significant preference for any stimulus. This conclusion holds true for mean preference, tested with a one-sided Student's ttest, as well as for the individual values, tested with a Sign test. Only infants from the age of 9-12 months significantly prefer the stimulus containing the figure. This preference increases slowly until the age of 6-7 years, and more rapidly afterwards. At 8-13 years, children show a strong, consistent preference, comparable to that of naive adult observers (Fig. 4). To check the subjects' state of attention in each session, four additional stimuli (drawings of faces, animals and toys, paired with a blank slide) were included as controls. The contours of these drawings were comparable in width with the individual lines in the main experiment. The responses to these control slides averaged 97~o for children younger than 4years, and 100~o afterwards. In a second experiment, we found that a very different developmental course is followed by segmentation BLOB

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Fig. 5. Mean + S.E.M. and statistical significance for texture segregation based on blob size, as a function o f age. The stimuli were presented on projected slides. The results are based on 78 tests (39 infants and 39 children). Symbols as in Fig. 4.

based on blob size. Infants aged only 1-3 months showed a very clear preference (83~o, P

Texture segregation in infants and children.

Segregation of textures based on differences in line orientation and blob size was tested in adults, infants and children, with a forced-choice prefer...
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