Title:

Visual and tactile perception in normal and mongoloid (Down's syndrome) infants

This thesis examines some aspects of visual and tactile perception in young mongol and normal children of similar mental ages. In Chapter 1 the mongol person is introduced as a member of a discrete group which is well defined clinically and which is characterised by several chromosome aberrations. It is seen that the mongol can be identified at birth on the basis of clinical characteristics and his condition can be confirmed by chromosome analysis. Mongols therefore provide the research worker with the opportunity to study the very early development of a relatively homogeneous group of mentally handicapped children. In the remainder of Chapter 1 the development of the mongol is considered. Four aspects of his development are examined: intellectual ability, motor ability, verbal ability and perceptual ability. It is argued that the areas of visual and tactile perception provide the most exciting prospects for research with the young mongol, since the visual abilities of the adult mongol seem to be similar to those of persons of the same mental age, whereas his tactile abilities seem to be deficient. Therefore in Chapter 2 the development of visual and tactile abilities of normal children and the ways in which these are studied in the young normal child are reviewed. It is seen that the normal child is able to perceive and remember many features of visually and tactually presented patterns from an early age. Chapter 3 describes the subject populations of mongol and normal children who took part in the experiments. It is demonstrated that the sample of mongol children was fairly representative of the mongol population in terms of birth weight, parental ages, birth order and number and age of siblings, although, the effects of maternal age and birth order were smaller than reported in other studies. It is proposed that these differences could be accounted for by the observed shift towards earlier childbearing ages in the general population. The first experiment (Experiment 1) is described in Chapter 4. Normal and mongol children were matched for mental age, sex and social class, and their visual exploration and memory of simple two dimensional patterns were compared. There was little difference in the way in which the mongol and normal children responded visually to two such patterns. Both groups showed less visual interest when the two patterns were identical compared with when they were of different shapes and colours. In a similar way, the children's interest decreased when one pair of patterns was presented repeatedly and increased following a change in either the positions of the two different patterns or the shape and colour of the two identical patterns. It was also found that the older mongol and normal children made more eye movements than the younger children did. This experiment therefore established a baseline for the study of visual perception in young mongol children. In Chapter 5 (Experiment 2) the finding often reported in the literature that the adult mongol has a tactile deficit was investigated in the young mongol. For procedural reasons a direct test of the ability of the young mongol child to distinguish between three dimensional objects by touch alone was not possible. Instead two tactilevisual crossmodal tasks and two visual withinmodal tasks using a pair of ellipsoids and a pair of cubes were employed. The mongol children were matched with normal children for mental age, sex and social class. There were two age groups with mean mental ages of about 12 and 17 months. The mongol children performed at chance level on both tactilevisual crossmodal tasks whilst the normals performed above chance level with at least one pair of objects. The older mongols performed above chance level on the visual within modal task with the ellipsoids: therefore their failure on the crossmodal task with this pair is due either to a tactile deficit or to a failure to integrate tactile and visual information. The chance level performance of the younger mongols on the withinmodal tasks could account for their poor performance on the cross modal tasks. This second experiment demonstrated that the young mongol child may have some form of tactile deficit and also showed that his ability to distinguish visually between three dimensional objects is inferior to that of the normal child of similar mental age. The normal child is capable of distinguishing visually between three dimensional objects; also his ability to perform above chance level on a tactilevisual crossmodal task with different pairs of objects increases as he gets older. The next experiment (Experiment 3) which is described in Chapter 6 investigated the possibility that the mongol T s poor visual discrimination of three dimensional objects was a result of the way in which he looked at them. Normal and mongol children were presented on successive trials with two dimensional patterns which could not be touched, and with three dimensional patterns which could be touched. A number of measures were taken of the way in which the children responded to the patterns. These were the total length of fixation, the number of fixations, the number of comparisons made between the patterns, the number of repeated fixations of one pattern, the times taken to fixate both patterns, the average fixation length, the lengths of the first fixation to both patterns and the amount of time which the children touched the three dimensional patterns. Both groups of children showed greater visual interest in the three dimensional patterns than they did in the two dimensional patterns. However, the normal children, especially at a mean mental age of six and a half months, fixated the three dimensional patterns in a different way from the way in which they fixated the two dimensional patterns. In particular these younger normal children made more comparisons between three dimensional patterns than they did between two dimensional patterns. The younger mongol children made fewer comparisons between three dimensional patterns than did the younger normal children. However, the mongol children, but not the normal children, made more repetitions of fixation to two dimensional patterns than to three dimensional patterns. The normal children made more repetitions of fixation to three dimensional patterns than did the mongol children. Thus this experiment provided some evidence for a difference in the way in which normal and mongol children look at three dimensional patterns, and confirmed the similarity in their visual responses to two dimensional patterns. The normal children seemed to be more active perceptually, particularly when they were looking at three dimensional patterns, since they moved their eyes more extensively than did the mongols. One possible problem with this experiment was that the three dimensional patterns could be touched but the two dimensional patterns could not. The next experiment (Experiment 4) which is reported in Chapter 7, investigated the possibility that the difference in the tactile abilities of the mongol and the normal children which was found in Experiment 2, could account for the difference in the ways in which the mongol and the normal children fixated the three dimensional patterns in Experiment 3, in that three dimensional patterns offer more tactile information than do two dimensional patterns. This possibility was tested in this experiment (Experiment 4) by allowing the children to touch and look at the patterns on half of the trials, and on the remaining trials only allowing them to look at the patterns. On each trial two patterns were presented side by side, and these were sometimes both two dimensional, sometimes both three dimensional and sometimes on.e of each.
