The Influence of Genes and Environments on the Gap-Overlap Task
Objectives: To study genetic and environmental influences on the gap-overlap task.
Methods: Classical twin design. The sample consisted of 492 twins (120 monozygotic pairs; 126 dizygotic pairs) from the general population, ranging in age from 9-14 years, recruited from the Child and Adolescent Twin Study in Sweden (CATSS). The gap-overlap task was performed using a Tobii T120 eye tracker. In this task a central stimulus (CS) appears on a screen and is followed by a new stimulus that appears on the periphery (PS). The task has three conditions: (1) “The gap” where the CS disappears before the PS appears, (2) “The baseline” where the CS disappears simultaneously as the PS appears, (3) “The overlap” where the CS remains on screen when the PS appears. The dependent variable was the median saccadic latency of gaze arrival at the PS. Multivariate twin modelling was used to analyze the genetic and environmental contributions to the variances and covariance among saccadic latencies.
Results: A common pathway model provided the best fit for the data. The covariance between the conditions was best explained by one shared factor with a primarily genetic background (63%). Significant unique genetic effects were found for the baseline (17%) and the overlap (15%) conditions, but not for the gap condition. Shared environment did not influence the experimental conditions (neither common nor unique).
Conclusions: Performance in the gap-overlap paradigm is primarily influenced by genetic effects, most of which are shared between the three conditions. This is expected since all conditions engage basic oculomotor functions (shifting the gaze from one stimuli to another). We found unique genetic effects in the overlap condition, which is in line with the view that this condition captures a distinct attentional function in addition to oculomotor control. We found no unique genetic effects in the gap condition. Surprisingly, we found unique genetic effects in the baseline condition, suggesting that it may be problematic to operationalize “the disengagement effect” as the difference between the overlap and the baseline conditions (resulting in a less clean comparison due to different genetic factors influencing each condition), as it sometimes done, at this age. This study illustrates how twin studies can enrich our understanding of experimental measures frequently used in autism research.