The effect of dot lifetime, dot size, & percent area covered by dots on motion coherence thresholds: Implications for diagnosing reading difficulties

Abstract

Purpose: Prior research has established that individuals with reading difficulties tend to perform more poorly than controls on tasks requiring them to identify the overall direction of motion within a random dot kinematogram (RDK). However, it is unclear whether the size of the difference in performance between individuals with reading difficulties and their ‘normal’ counterparts is a function of reading ability or the stimulus parameters used. In this investigation, we examine the degree to which the lifetime of dots, the size of dots, and the percent area covered by dots affect the ability to identify the direction of motion in normal individuals. Methods: Observers (N=7) were asked to indicate whether the global direction of the dots contained within a 3 degree square region was leftward or rightward. The dots within the RDK varied in terms of lifetime (16.7 or 33.3ms), size (1 or 2 pixels in diameter), and percent area covered (1 or 21%). Results: Changes in the parameters of the RDK affected subjects' ability to identify the direction of motion. When the lifetime of the dots was short and the percent area covered was high, observers had more difficulty in judging the overall direction of movement for larger than for smaller dots. In addition, when the lifetime of the dots was short and the dot size was large, observers had more difficulty in identifying the motion direction for higher than for lower percent area covered. In contrast, these effects disappeared when the lifetime of the dots was lengthened; the overall pattern of results suggests that increasing the lifetime of the dots increased the difficulty of the task. Conclusion: Motion coherence thresholds are dependent on the parameters used during testing. One implication of this investigation is that the specific combination of parameters used may facilitate or hinder the detection of differences between those with reading difficulties and controls.