Several studies have demonstrated a visual attention span deficit hypothesis in dyslexia, the paradigm which was initially coined by Valdois and colleagues (e.g. Valdois et al., 2004; Bosse et al., 2007; Bosse & Valdois, 2009). In order to entirely separate the influence of both visual and phonological processing in dyslexia, visual processing is this setting has normally investigated using change detection tasks using non-verbal material. For instance, Bosse & Valdois (2009) approximated the visual attention span by having the participants subjected to five different lettered consonant strings for a short space of time, where the participants were then required to respond by naming either as many letters as possible which was previously shown to them (global report) or just the matter of reporting a single target letter displayed on a validly-cued position (partial report). Findings from this study generally showed the same pattern whereby CwDys, in comparison to age-matched controls, had a significantly worse performance on both tasks. In addition to this, the CwDys showed severe difficulties when recalling the target digits concerning the global report task, with the deficit being attributed to possible demands in the maintenance and retrieval process (based on stored information within the working memory) rather than demands to cope with the partial report task. In addition to this, no sooner the authors controlled for phonemic awareness and verbal short-term memory (using multiple regression analysis), they immediately spotted that recalculated figure explained performance similar to that in the global report task. This finding alone was sufficient enough for the authors to confirm that the CwDys suffered from a reduced visual span. A large body of Valdois and his colleagues reported the exact same finding in other tasks which assessed visual attention span, for instance, simultaneous vs. sequential comparisons of global report (Lassus-Sangosse et al., 2008).
Studies which employed recognition thresholds along the letter strings resulted with somewhat mixed findings. In Hawelka et al (2006), the output they witnessed based on
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the recognition thresholds was an M-shaped serial function, indicative of both groups requiring even shorter task duration for presentation of the letter/digit at both the initial and final string positions. This pattern was regarded to be inconsistent given that dyslexics suffered from a narrowed visual attentional window. In fact, the W-shaped function was far more pronounced in the dyslexic group suggesting that they needed more time to view the targets letters at positions two and four, further suggesting that the dyslexics had undergone crowding effects based on the interference arriving from the surrounding letters. This was along the lines of Martelli et al (2009) having found poor performances in Italian CwDys (similar to that with controls) during single letter identification requiring larger spacing for target identification when surrounded by two distracting letters. Moreover, by means of partial reports using both forward and backward masks in a group of French children, Zeigler, Pech-Georgel, Dafau, & Grainger (2010) did not replicate the significant deficits at positions two and four as Hawelka and colleagues did. This resulted in Zeigler and colleagues having to decline the idea of dyslexics undergoing excessive crowding related difficulties. It was thought that this inconsistency could have come down to the operational mode of responding to a target. For instance, Hawelka and colleagues employed the reporting of a cued digit/letter which was actually open-ended. Comparing this with Ziegler and colleagues study, it was down to a choice where in which the participants were able to choose any one of two alternatives to make a response, with the incorrect alternative gotten rid of from the stimulus array, suggesting the a slimmest of chances in have detected an error as a result of mistakenly viewing it off the string. Nevertheless, a follow-up study by Collins, Kohnen, & Kinoshita (2012) took all these methodological concerns while putting a partial report task with five-letter, digit, and symbol strings to a test in a group of AwDys. This time round, there were more alternative response options, with findings revealing a poor overall performance in AwDys compared to matched controls in both letter and digit strings except for symbol strings, in addition to both groups producing the characteristic W-shaped serial function with letter and digit strings. As expected the deficits in the dyslexic group concerning letter string were limited with respect to the number of position errors (two and four). Furthermore, the observed findings correlated with letter transposition errors unlike with the RAN task, further suggesting a deficit in parallel processing of single letter strings.
Prado, Dubois, & Valdois (2007) took this one step further in studying whether eye movements of dyslexic and control readers were recorded in both the text reading and visual search tasks for immediate relevance for reading. The study was carried out on a subgroup of French CwDys known for a visual attention span disorder with intact phoneme awareness. Findings from this study reported a higher number of rightward and
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leftward fixations in the dyslexic group unlike the control group. Consistent with previous accounts, the findings did not disclose any group differences concerning eye movement patterns during visual search, with the exception of the consonant strings having the same visuo-spatial characteristics as text reading. More importantly, the visual attention span abilities with regards to the dyslexic group were equivalent to the number of letters processed concurrently during reading. Nevertheless, the abnormal eye movements of some (not all) dyslexic readers while reading did indicate that dyslexics do have difficulties when increasing their visual attention span based on tasks which demands them to do so. Findings along these lines were also observed in tasks assessing reading and copying (Bosse, Kandel, Prado, & Valdois, 2014) in addition to naming digits and dice (Pan, Yan, Laubrock, Shu, & Kliegl, 2013).
Thus, given the inconsistent findings above, it comes down to the type of paradigm utilized since string processing has been investigated in countless of ways in dyslexia, hence yielding contrasting results. It is crucial when considering studies of this nature to opt for a multi-element parallel visual processing strategy since not only will it need to process many items, the nature of visual processing needs to run in parallel. In studies that do not conform to this, there appears to be no limit on the presentation time (e.g. Hawelka & Wimmer, 2008). In one of the most recent studies conducted by Schneps, Thomson, Chen, Sonnert, & Pomplun (2013), they carried out what was a visual span task using a reading method called span limited tactile reinforcement (SLTR) where text messages gets displayed on a smartphone with larger fonts such that the text extends a few words per line. Using this method, AwDys were inspected to see if they could benefit from SLTR method, i.e. by overcoming difficulties in phoneme decoding and sight word processing. It turned out that they performed well than Schneps et al (2013) expected, given this method presented letters which were wide apart, in which case there was no sign of crowding affecting the results. The AwDys who found it hard to comprehend items were better when using the SLTR method than the conventional paper reading. This finding alone supports the presence of a poorly distributed attention span in dyslexia. The next sub-section should tell us how good individuals with dyslexia are at controlling and distributing their attention.