Why do adults with dyslexia have poor global motion sensitivity?

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/deg(2). 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.

The influence of contrast on coherent motion processing in dyslexia.

The aim of the experiments was to investigate how manipulating the contrast of the signal and noise dots in a random dot kinematogram (RDK), influenced on motion coherence thresholds in adults with dyslexia. In the first of two experiments, coherent motion thresholds were measured when the contrasts of the signal and noise dots in an RDK were manipulated. A significantly greater processing benefit was found for the group with dyslexia than a control group when the signal dots were of higher contrast than the noise dots. However, a significant processing disadvantage was found for the group with dyslexia relative to the control group when the signal dots were of lower contrast than the noise dots. These findings were interpreted as supporting evidence for the noise exclusion hypothesis of dyslexia. In Experiment 2, the effect on coherent motion thresholds of presenting a cue that alerted observers to which stimuli, high or low contrast contained the signals dots was investigated. When the cue directed attention to low contrast signal dots presented in high contrast noise, coherent motion thresholds were only enhanced for the group with dyslexia. This manipulation produced equivalent coherent motion thresholds in the reader groups. In other conditions, the group with dyslexia had significantly higher coherent motion thresholds than the control group. It was concluded that adults with dyslexia who show evidence of a coherent motion deficit (37% of the dyslexia group in each experiment), have a specific difficulty in noise exclusion. This appears to occur as consequence of a sensory processing deficit in the magnocellular or dorsal stream.

Visual search deficits are independent of magnocellular deficits in dyslexia.

The aim of this study was to investigate the theory that visual magnocellular deficits seen in groups with dyslexia are linked to reading via the mechanisms of visual attention. Visual attention was measured with a serial search task and magnocellular function with a coherent motion task. A large group of children with dyslexia (n = 70) had slower serial search times than a control group of typical readers. However, the effect size was small (η(p)(2) = 0.05) indicating considerable overlap between the groups. When the dyslexia sample was split into those with or without a magnocellular deficit, there was no difference in visual search reaction time between either group and controls. The data suggest that magnocellular sensitivity and visual spatial attention weaknesses are independent of one another. They also provide more evidence of heterogeneity in response to psychophysical tasks in groups with dyslexia. Alternative explanations for poor performance on visual attention tasks are proposed along with avenues for future research.

Does a sensory processing deficit explain counting accuracy on rapid visual sequencing tasks in adults with and without dyslexia?

The experiments conducted aimed to investigate whether reduced accuracy when counting stimuli presented in rapid temporal sequence in adults with dyslexia could be explained by a sensory processing deficit, a general slowing in processing speed or difficulties shifting attention between stimuli. To achieve these aims, the influence of the inter-stimulus interval (ISI), stimulus duration, and sequence length were evaluated in two experiments. In the first that used skilled readers only, significantly more errors were found with presentation of long sequences when the ISI or stimulus durations were short. Experiment 2 used a wider range of ISIs and stimulus durations. Compared to skilled readers, a group with dyslexia had reduced accuracy on two-stimulus sequences when the ISI was short, but not when the ISI was long. Although reduced accuracy was found on all short and long sequences by the group with dyslexia, when performance on two-stimulus sequences was used as an index of sensory processing efficiency and controlled, group differences were found with presentation of stimuli of short duration only. We concluded that continuous, repetitive stimulation to the same visual area can produce a capacity limitation on rapid counting tasks in all readers when the ISIs or stimulus durations are short. While reduced accuracy on rapid sequential counting tasks can be explained by a sensory processing deficit when the stimulus duration is long, slower processing speed in the group with dyslexia explains the greater inaccuracy found as sequence length is increased when the stimulus duration is short.

Auditory and visual processing in children with dyslexia.

This study investigated the temporal stability and longitudinal replicability of visual and auditory sensory processes found to be poor in children with dyslexia. Seventy children with dyslexia and 52 normal readers were tested twice, 9 months apart, on measures of visual and auditory sensory processing and on phonological and orthographic skills. About 30% of children with dyslexia were found to have sensory deficits. Associations were found between sensory and cognitive skills. Based on analyses of agreement, the temporal stability of the sensory tasks was poor. Future research should develop sensory measures with high temporal stability that can control for maturation.

Relationships between global motion and global form processing, practice, cognitive and visual processing in adults with dyslexia or visual discomfort.

The aim of the first of two experiments was to investigate the effect of practice on sensitivity to global motion and global form in a group of adults with dyslexia, a group of normal readers with visual discomfort, a group with dyslexia and visual discomfort, and a control group. In comparison to the control group, and regardless of the effect of practice, the group with dyslexia was significantly less sensitive to global motion. No differences in global motion sensitivity were found when individuals with dyslexia, with or without visual discomfort, were compared. The normal reading group with visual discomfort was less sensitive to global motion than the control group at baseline, but not when a second estimate of motion sensitivity was obtained. About 30% of the group with dyslexia had a global motion deficit on each threshold estimate. In contrast, there were no significant effects of practice or group on sensitivity to global form. In Experiment 2, performance on a number of cognitive and visual processing tasks was measured in four groups: two with dyslexia; one with and one without a global motion deficit, a normal reading group with visual discomfort, and a control group. The group with visual discomfort had reduced visual processing speed only. Regardless of whether a global motion processing deficit was present or absent in individuals with dyslexia, reduced accuracy was found on the language and visual processing measures, and on a rapid temporal sequencing task. Individuals with dyslexia and a global motion deficit had poorer accuracy than individuals with dyslexia and no motion deficit on the phonological processing and verbal short term memory tasks. We concluded that some adults with dyslexia have a persistent deficit when processing global motion but not global form. This is consistent with reports of a magnocellular pathway deficit in this group. Individuals with visual discomfort do not have a magnocellular processing deficit, but have perceptual difficulties when performing complex visual processing tasks.