Attentional asymmetry between visual hemifields is related to habitual direction of reading and its implications for debate on cause and effects of dyslexia

Lateral Bias in Visual Working Memory

The present study aimed to evaluate functional cerebral asymmetries of visual working memory (VWM) in relation to language lateralization. The bilateral change detection paradigm with capital letters as stimuli and the translingual lexical decision task were used to assess VWM and language asymmetry, respectively, in a sample of 99 younger healthy participants (59 women). Participant attention was cued towards right or left visual half-field. For the VWM task, men and women were more accurate and faster when stimuli were presented in the right visual half-field compared to the left visual half-field. As expected, a significant right visual half-field advantage was demonstrated in the lexical decision task in performance accuracy (but not response time). The results also revealed no relationship between lateralization in VWM and lexical decision. VWM performance accuracy decreased significantly with increasing asymmetry. This relationship was significant for women, but not men. Taken together, the present study demonstrates that the lateral bias in visual working memory is independent from language lateralization, and less lateralized individuals perform better than individuals with larger asymmetries in both visual half-field tasks.

Visuo-spatial attention deficit in children with reading difficulties

Although developmental reading disorders (developmental dyslexia) have been mainly associated with auditory-phonological deficits, recent longitudinal and training studies have shown a possible causal role of visuo-attentional skills in reading acquisition. Indeed, visuo-attentional mechanisms could be involved in the orthographic processing of the letter string and the graphemic parsing that precede the grapheme-to-phoneme mapping. Here, we used a simple paper-and-pencil task composed of three labyrinths to measure visuo-spatial attention in a large sample of primary school children (n = 398). In comparison to visual search tasks requiring visual working memory, our labyrinth task mainly measures distributed and focused visuo-spatial attention, also controlling for sensorimotor learning. Compared to typical readers (n = 340), children with reading difficulties (n = 58) showed clear visuo-spatial attention impairments that appear not linked to motor coordination and procedural learning skills implicated in this paper and pencil task. Since visual attention is dysfunctional in about 40% of the children with reading difficulties, an efficient reading remediation program should integrate both auditory-phonological and visuo-attentional interventions.

Enhanced reading abilities is modulated by faster visual spatial attention

Research has shown improved reading following visual magnocellular training in individuals with dyslexia. Many studies have demonstrated how the magnocellular pathway controls visual spatial attention. Therefore, we have investigated the relationship between magnocellular pathway and visual spatial attention deficits in dyslexia in order to better understand how magnocellular-based interventions may help children to learn to read. Magnocellular function, visual spatial attention, and reading abilities of thirty elementary school students with dyslexia, aged between 8 and 10, were measured. The experimental group received magnocellular-based visual motion training for 12 sessions, while the control group received neutral sessions. All tests were repeated at the end of the training and after 1 month. The magnocellular functioning, visual spatial attention, and reading abilities of the experimental group improved significantly compared to the controls. Additionally, improvement in reaction time of invalid conditions predicted improvements in saccadic eye movements. We conclude that visual magnocellular training improved saccadic eye movement control, visual spatial orientation, and reading ability.

A neural index of inefficient evidence accumulation in dyslexia underlying slow perceptual decision making

Visual processing deficits have been widely reported in developmental dyslexia however the locus of cognitive dysfunction remains unclear. Here, we examined the neural correlates of perceptual decision-making using a dot-motion task and electroencephalography (EEG) and investigated whether presenting deficits were unique to children with dyslexia or if they were also evident in other, typically developing children with equally immature reading systems. Sixty-eight children participated: 32 with dyslexia (DD; 16 females); 21 age-matched controls (AM; 11 females) and 15 reading-matched controls (RM; 9 females). All participants completed a bilaterally presented random-dot-motion task while EEG was recorded. Neural signatures of low level sensory processing (steady state visual evoked potentials; SSVEPs), pre-target attentional bias (posterior α power), attentional orienting (N2), evidence accumulation (centro-parietal positive decision signal; CPP) and execution of a motor response (β) were obtained to dissect the temporal sequence of perceptual decision-making. Reading profile provided a score of relative lexical and sublexical skills for each participant. Although all groups performed comparably in terms of task accuracy and false alarm rate, the DD group were slower and demonstrated an earlier peak latency, reduced slope and lower amplitude of the CPP compared with both AM and RM controls. Reading profile was found to moderate the relationship between word reading ability, reaction time as well as CPP indices showing that lexical dyslexics responded more slowly and had a shallower slope, reduced amplitude and earlier latency of CPP waveforms than sublexical dyslexics. These findings suggest that children with dyslexia, particularly those with relatively poorer lexical abilities, have a reduced rate and peak of evidence accumulation as denoted by CPP markers yet remain slow in their overt response. This is in keeping with hypotheses that children with dyslexia have impairment in effectively sampling and processing evidence about visual motion stimuli.

The Effect of Simultaneously Presented Words and Auditory Tones on Visuomotor Performance

The experiment reported here used a variation of the spatial cueing task to examine the effects of unimodal and bimodal attention-orienting primes on target identification latencies and eye gaze movements. The primes were a nonspatial auditory tone and words known to drive attention consistent with the dominant writing and reading direction, as well as introducing a semantic, temporal bias (past-future) on the horizontal dimension. As expected, past-related (visual) word primes gave rise to shorter response latencies on the left hemifield and future-related words on the right. This congruency effect was differentiated by an asymmetric performance on the right space following future words and driven by the left-to-right trajectory of scanning habits that facilitated search times and eye gaze movements to lateralized targets. Auditory tone prime alone acted as an alarm signal, boosting visual search and reducing response latencies. Bimodal priming, i.e., temporal visual words paired with the auditory tone, impaired performance by delaying visual attention and response times relative to the unimodal visual word condition. We conclude that bimodal primes were no more effective in capturing participants' spatial attention than the unimodal auditory and visual primes. Their contribution to the literature on multisensory integration is discussed.

Visual search efficiency and functional visual cortical size in children with and without dyslexia

Dyslexia is characterised by poor reading ability. Its aetiology is probably multifactorial, with abnormal visual processing playing an important role. Among adults with normal reading ability, there is a larger representation of central visual field in the primary visual cortex (V1) in those with more efficient visuospatial attention. In this study, we tested the hypothesis that poor reading ability in school-aged children (17 children with dyslexia, 14 control children with normal reading ability) is associated with deficits in visuospatial attention using a visual search task. We corroborated the psychophysical findings with neuroimaging, by measuring the functional size of V1 in response to a central 12° visual stimulus. Consistent with other literature, visual search was impaired and less efficient in the dyslexic children, particularly with more distractor elements in the search array (p = 0.04). We also found atypical interhemispheric asymmetry in functional V1 size in the dyslexia group (p = 0.02). Reading impaired children showed poorer visual search efficiency (p = 0.01), needing more time per unit distractor (higher ms/item). Reading ability was also correlated with V1 size asymmetry (p = 0.03), such that poorer readers showed less left hemisphere bias relative to the right hemisphere. Our findings support the view that dyslexic children have abnormal visuospatial attention and interhemispheric V1 asymmetry, relative to chronological age-matched peers, and that these factors may contribute to inter-individual variation in reading performance in children.

Asymmetric practices of reading and writing shape visuospatial attention and discrimination

Movement is generally conceived of as unfolding laterally in the writing direction that one is socialized into. In ‘Western’ languages, this is a left-to-right bias contributing to an imbalance in how attention is distributed across space. We propose that the rightward attentional bias exercises an additional unidirectional influence on discrimination performance thus shaping the congruency effect typically observed in Posner-inspired cueing tasks. In two studies, we test whether faces averted laterally serve as attention orienting cues and generate differences in both target discrimination latencies and gaze movements across left and right hemifields. Results systematically show that right-facing faces (i.e. aligned with the script direction) give rise to an advantage for cue-target pairs pertaining to the right (versus left) side of space. We report an asymmetry between congruent conditions in the form of right-sided facilitation for: (a) response time in discrimination decisions (experiment 1–2) and (b) eye-gaze movements, namely earlier onset to first fixation in the respective region of interest (experiment 2). Left and front facing cues generated virtually equal exploration patterns, confirming that the latter did not prime any directionality. These findings demonstrate that visuospatial attention and consequent discrimination are highly dependent on the asymmetric practices of reading and writing.

Advantage of detecting visual events in the right hemifield is affected by reading skill

Visual perception is often not homogenous across the visual field and can vary depending on situational demands. The reasons behind this inhomogeneity are not clear. Here we show that directing attention that is consistent with a western reading habit from left to right, results in a ~32% higher sensitivity to detect transient visual events in the right hemifield. This right visual field advantage was largely reduced in individuals with reading difficulties from developmental dyslexia. Similarly, visual detection became more symmetric in skilled readers, when attention was guided opposite to the reading pattern. Taken together, these findings highlight a higher sensitivity in the right visual field for detecting the onset of sudden visual events that is well accounted for by left hemisphere dominated reading habit.