Adults with dyslexia exhibit large effects of crowding, increased dependence on cues, and detrimental effects of distractors in visual search tasks

Difficulties in visual attention are increasingly being linked to dyslexia. To date, the majority of studies have inferred functionality of attention from response times to stimuli presented for an indefinite duration. However, in paradigms that use reaction times to investigate the ability to orient attention, a delayed reaction time could also indicate difficulties in signal enhancement or noise exclusion once oriented. Thus, in order to investigate attention modulation and visual crowding effects in dyslexia, this study measured stimulus discrimination accuracy to rapidly presented displays. Adults with dyslexia (AwD) and controls discriminated the orientation of a target in an array of different numbers of - and differently spaced - vertically orientated distractors. Results showed that AwD: were disproportionately impacted by (i) close spacing and (ii) increased numbers of stimuli, (iii) did use pre-cues to modulate attention, but (iv) used cues less successfully to counter effects of increasing numbers of distractors. A greater dependence on pre-cues, larger effects of crowding and the impact of increased numbers of distractors all correlated significantly with measures of literacy. These findings extend previous studies of visual crowding of letters in dyslexia to non-complex stimuli. Overall, AwD do not use cues less, but they do use cues less successfully. We conclude that visual attention is an important factor to consider in the aetiology of dyslexia. The results challenge existing theoretical accounts of visual attention deficits, which alone are unable to comprehensively explain the pattern of findings demonstrated here

Adults with dyslexia demonstrate large effects of crowding and detrimental effects of distractors in a visual tilt discrimination task

Previous research has shown that adults with dyslexia (AwD) are disproportionately impacted by close spacing of stimuli and increased numbers of distractors in a visual search task compared to controls [1]. Using an orientation discrimination task, the present study extended these findings to show that even in conditions where target search was not required: (i) AwD had detrimental effects of both crowding and increased numbers of distractors; (ii) AwD had more pronounced difficulty with distractor exclusion in the left visual field and (iii) measures of crowding and distractor exclusion correlated significantly with literacy measures. Furthermore, such difficulties were not accounted for by the presence of covarying symptoms of ADHD in the participant groups. These findings provide further evidence to suggest that the ability to exclude distracting stimuli likely contributes to the reported visual attention difficulties in AwD and to the aetiology of literacy difficulties. The pattern of results is consistent with weaker and asymmetric attention in AwD

Demographic and psychometric group data.

a<p>The level of education represents years of schooling from year 1 (infant school) to year 13 (college/sixth form).</p>b<p>The composite standard scores (SS). For the TOWRE these were calculated using the norms 17∶0–24∶11 (years∶ months).</p><p>Demographic and psychometric group data.</p

Schematic representation of the stimulus sequence for the tilt discrimination task.

<p>The two possible target locations (left and right sides of the screen) were indicated on the display screen with arrows. Targets could be presented either alone, or surrounded by two, four or eight distractors arranged symmetrically above and below the target. In conditions in which two distractors were presented they could be presented with an interstimulus distance of either 3.5 degrees (spread) or 1.6 degrees (crowded). Participants responded whether the target tilted left or right using the z and m keys on the computer keyboard.</p

Scatter plots showing the relationships between measures of WIAT-II Spelling, WIAT-II Reading, TOWRE-PDE and TOWRE-SWE with measures of crowding, set size and asymmetry.

<p>Controls (empty dots). AwD (filled dots).</p

Interaction plots indicating performance accuracy for both controls (left panel) and AwD (right panel) plotted as a function of display type (crowded vs. spread) and task difficulty (solid lines - easy conditions and dotted lines - hard conditions).

<p>Interaction plots indicating performance accuracy for both controls (left panel) and AwD (right panel) plotted as a function of display type (crowded vs. spread) and task difficulty (solid lines - easy conditions and dotted lines - hard conditions).</p

Descriptive statistics showing performance accuracy for both controls (top horizontal panel) and AwD (bottom horizontal panel) for the stimulus display side conditions (left vs. right-VF) plotted as a function of set-size and task difficulty (solid lines representing easy conditions and dotted lines representing hard conditions).

<p>Descriptive statistics showing performance accuracy for both controls (top horizontal panel) and AwD (bottom horizontal panel) for the stimulus display side conditions (left vs. right-VF) plotted as a function of set-size and task difficulty (solid lines representing easy conditions and dotted lines representing hard conditions).</p