Why do adults with dyslexia have poor global motion sensitivity?

Abstract

Two experiments aimed to determine why adults with dyslexia have poorer global motion sensitivity than typically reading controls. In Experiment 1, the global stimulus had dot densities of 14.15 dots/deg2 (high) or 3.54 dots/deg2 (low) and was presented for five (84ms) or eight (134ms) animation frames. The dyslexia group had higher global motion thresholds in all conditions than controls. In the high dot density, long duration condition, both reader groups had the lowest thresholds indicating that although the dyslexia group had higher motion thresholds they were able to sample the additional signals dots over time with the same efficiency as controls. In Experiment 2, we determined whether the increased neural activity induced by briefly presenting a fully coherent prime moving in either the same or opposite direction of motion to a partially coherent test stimulus would systematically increase and decrease global motion thresholds in the reader groups. When the direction of motion in the prime and test was the same, global motion thresholds increased for both reader groups. The increase in coherence thresholds was significantly greater for the dyslexia group. When the motion of the prime and test were presented in opposite directions, coherence thresholds were reduced in both groups. No group threshold differences were found. In the latter condition, the dyslexia group were able to sample the target motion signals with the same efficiency as the control group. We concluded that the global motion processing deficit found in adults with dyslexia can be explained by undersampling of the target motion signals because of reduced neural activity in the dorsal visual stream, and not a specific difficulty integrating global motion signals. This reduced neural activity in the dorsal visual stream increases the computational problems for the group with dyslexia when processing global motion.