Abstract
Two experiments aimed to determine why adults with dyslexia have higher global motion thresholds than typically reading controls. In Experiment 1, the dot density and number of animation frames presented in the dot stimulus were manipulated because of findings that use of a high dot density can normalize coherence thresholds in individuals with dyslexia. Dot densities were 14.15 and 3.54 dots/deg2. These were presented for five (84 ms) or eight (134 ms) frames. The dyslexia group had higher coherence thresholds in all conditions than controls. However, in the high dot density, long duration condition, both reader groups had the lowest thresholds indicating normal temporal recruitment. These results indicated that the dyslexia group could sample the additional signals dots over space and then integrate these with the same efficiency as controls. In Experiment 2, we determined whether 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. We concluded that the global motion processing deficit found in adults with dyslexia can be explained by undersampling of the target motion signals. This might occur because of difficulties directing attention to the relevant motion signals in the random dot pattern, and not a specific difficulty integrating global motion signals. These effects are most likely to occur in the group with dyslexia when more complex computational processes are required to process global motion.
Highlights
Dyslexia is a neurobiological disorder that can affect multiple brain areas (Stein, 2001; Shaywitz et al, 2003; Vidyasagar and Pammer, 2010)
Regardless of reader group, lower coherence thresholds were found with presentation of the high dot density condition when presented for eight animation frames, showing temporal recruitment effects
Simple effects analysis revealed that the group with dyslexia had significantly higher coherence thresholds than the control group when the prime and test were presented in the same direction of motion, F(1, 18) = 13.34, FIGURE 3 | Global motion coherence thresholds for group with dyslexia (n = 10) and controls (n = 10) when the probe and prime are presented in the same direction, in opposite directions or when the prime is a stationary control stimulus
Summary
Dyslexia is a neurobiological disorder that can affect multiple brain areas (Stein, 2001; Shaywitz et al, 2003; Vidyasagar and Pammer, 2010). Groups with dyslexia have poorer temporal contrast sensitivity than controls but do not have poorer spatial contrast sensitivity, measured in the parvocellular visual stream (Lovegrove et al, 1986). Difficulties replicating these findings (Amitay et al, 2002; Williams et al, 2003) and evidence that poorer sensitivity is found only with use of methodologies that require either sequential processing (Ben-Yehudah et al, 2001) or prior adaptation to a stimulus (Johnston et al, 2008), indicates that groups with dyslexia have a visual sensory processing deficit only when performing tasks that use complex computational processes. The evidence indicates that these processes are impaired in the medial temporal area (MT) of the dorsal stream (Cornelissen et al, 1995; Talcott et al, 2000; Hansen et al, 2001; Conlon et al, 2004, 2009, 2012; Wilmer et al, 2004; Roach and Hogben, 2007; Benassi et al, 2010)
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