INTRODUCTION Tests of visual perception are commonly used for diagnosis and guiding effective treatment. However, few studies have attempted to ascertain the contributions of genetics or environment to visual-perceptual ability. Results of these studies have been equivocal. Based on casual observation, a hypothesis was developed that monozygotic (MZ) twins would have more similar visual-perceptual skills than dizygotic (DZ) twins. The purpose of this study is to compare visual-perceptual test results of monozygotic versus dizygotic twins to see if a genetic link could be demonstrated. Versions of Gardner's Test of Visual-Perceptual Skills were chosen due to its ease of administration, common utilization, and because it, "is a non-language test and is not biased according to race, culture, gender or education."'
METHODS The Test of Visual-Perceptual Skills (non-motor) Revised (TVPS-R) or Test of Visual- Perceptual Skills (non-motor) Upper Level-Revised (TVPS-(UL)-R) was administered to seven sets of MZ twins, 12 sets of DZ twins, and 32 family members of these twins. (The 17 years, 11 months-norms were used for all adults2) Identification of twins as MZ or DZ was based on information from reports the twins themselves or their parents. All subjects wore their habitual correction while taking the test.
RESULTS Overall Sum of Standard Scores, and scores for each subtest were analyzed using both Fixed Effects Regression Analysis and Difference of Means Test with Student T-Test. In addition to comparisons between MZ and DZ twins, results from dizygotic-different gender twins (DZDG) and dizygotic-same gender twins (DZSG) were also analyzed. For all twin pairs, scores of one twin closely matched the other (p < 0.05) for Visual Memory, Visual Form Constancy, Visual Sequential Memory, Visual Closure subtests and Sum of Standard Scores. Same-Gender twins' scores were even more similar than non-twin scores (p < 0.01 or less) for all subtests except Visual Discrimination and Visual Sequential Memory. Spatial Relationships scores and Sum of Sandard Scores were more similar for MZ than DZ twins (p < 0.01 and pC0.05). With same-gender twins, other than Spatial Relationships, no difference existed between MZ and DZ (p < 0.05). Spatial Relationships, Form Constancy, and Sum of Standard Scores (all p < 0.001) were more similar with MZ twins than DZDG twins. Same-gender DZ twins were also more similar than different-gender DZ twins for the above three areas (pc0.02; 0.01 and 0.001 respectively).
DISCUSSION The hypothesis that MZ twins would score with more similarity on a visual-perceptual test than DZ twins was partially supported by these results; notably for Spatial Relationships and Sum of the Standard Scores. Of interest is that the overall Sum of Standard Scores differentiation was no longer seen when different gender twins were removed from the sample. In other words, the Sum of Standard Scores was more dissimilar for different gender twins. Although the sample size is too small for generalization, this discovery implies that gender may be a dominant factor in the visual-perceptual differences that were found. Since the TVPS is thought to be gender-blind, this should be addressed in future investigations.
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