Impaired filtering of behaviourally irrelevant visual information in dyslexia

Abstract

A recent proposal suggests that dyslexic individuals suffer from attentional deficiencies, which impair the ability to selectively process incoming visual information. To investigate this possibility, we employed a spatial cueing procedure in conjunction with a single fixation visual search task measuring thresholds for discriminating the orientation of a target stimulus. Replicating preliminary findings in an earlier report, we found evidence of a striking dissociation between dyslexic participants' performance in cued and uncued conditions. Whereas uncued search results were equivalent for dyslexic and normal adult readers, the majority of dyslexic individuals failed to display a comparable benefit when the location of the target was indicated by the appearance of a brief peripheral pre-cue. Using receiver operating characteristic curve analysis, we further demonstrate that the effectiveness of the cueing task at discriminating between dyslexic and normal readers surpasses that of a range of other psychophysical tasks typically used in dyslexia research. Moreover, we find that the discriminative accuracy of the task is at least on par with measures of verbal short-term memory (a core component of phonological processing), which ranks as one of the most widely accepted areas of difficulty in dyslexia. Potential mechanisms underlying the cueing effect are outlined, and the plausibility of each considered within a signal detection theory framework of visual search. It is argued that performance benefits obtained by normal readers in cued conditions most likely reflect the prioritization of target information during decision making, and could feasibly be subserved by top-down biasing effects on pooling processes in extrastriate cortex.