Isolating the impact of visual perception on dyslexics’ reading ability

1. Isolating the impact of visual perception on dyslexics’ reading ability Mark M. Shovman & Merav Ahissar Vision Research 46 (2006) 3514-3525

2. Dyslexia • Consensus neurological disorder with a genetic origin • Diverse associated cognitive deficits • Co-morbidity with other learning disabilities • Several parsimonious theories of single cause • Most broadly accepted cognitive theory • core deficit at level of phonological representations • impact on decoding written script “relatively understood” • Impact of potentially impaired visual abilities • open and debated question • In general, process of reading taxing for visual system • demands fine spatial discrimination and rapid processing (Vidyasagar, 2004)

3. Visual Deficits • Magnocellular hypothesis = low-level visual deficit • Focus of recent studies • dynamics of spatial visual attention (Geiger et al., 1994) • deficit associated with higher levels of processing along dorsal stream (Vidyasagar & Pammer, 1999) • Attentional shifts “sluggish” (Hari & Renvall, 2001) • resemble minor case of neglect • Visual deficits only when comparing between • spatial or temporal aspects of serially presented stimuli (Ben-Yehudah & Ahissar, 2004)

4. Relevance for Dyslexics • Implications of visual deficits on reading abilities scarcely addressed • Designed experiments to assess “adequacy of visual routines that play important role in single word reading” • Stimuli similar to single words in graphical characteristics • no other aspects of natural reading (e.g. phonological, morphological) • If dyslexics’ reading-related visual routines mildly impaired • weakness revealed when relevant visual requirements increase and visual routines challenged

5. Visual Manipulations • Reduced letter size • affects spatial frequencies used to identify symbols (Majaj et al., 2002) • impaired performance of dyslexic children with unstable binocular control (Cornelissen et al., 1991) • Crowding (added distracting letters - flankers) • crowding effects near fixation for dyslexic children (Atkinson, 1991) • but may be confounded by verbal memory processes • Visual noise • examined contrast sensitivity for letter identification • letters presented on uniform gray background or embedded in ‘white noise’ • Linear Additive Model • discuss later

6. Methods SET-UP • Participants seated 1 metre from screen • Response time neither limited nor measured STIMULI • Symbol set = subset of Georgian alphabet • letter-like symbols graphically similar to know alphabets • not resemble English or Hebrew scripts (no phonological interference) • but similar graphical complexity • “very similar” to pseudowords (3-4 letters) in visual aspects and lack of semantic content

7. Trial Sequence A Fixation bar “suggested by Dr R Shillcock in private communication” B Single-symbol stimulus on uniform gray background (47.4 cd/m²) C Letter-triplet in noise (0-94.4 cd/m², 2′ x 2′ square grain) D Masking screen (random scatter of symbols presented for 500 ms) E Response options

8. Test Conditions • Eight conditions, comprising up to 100 trials each • Thresholds assessed with 2-up, 1-down staircase method • SOA adaptive steps: 30 ms decreasing to 10 ms after 5 reversals • Contrast: 2.4 (no-noise) / 3.7 cd/m² decreasing to 0.7 / 1.7 cd/m² after 5 reversals, and to 0.2 / 0.4 cd/m² after additional 4 reversals • Step size increase possible after 3 changes in consistent direction

9. Participants • Mainly students (aged 21 – 27) • 20 dyslexics (14 female, 6 male) • self-selecting + exclusion criteria • well below average cognitive & within average pseudoword reading scores • some participants from previous studies • 20 controls (13 female, 7 male) • excluded two for well above average cognitive scores, one for slow task completion and one because he was male! • Reading and Cognitive Tests included: • Hebrew pseudoword and paragraph reading • Rapid Automatic Naming of digits • Questionnaire on history of learning disabilities • WAIS-III Block Design and Similarities • Raven’s Standard Progressive Matrices • Digit Span (forward and backward)

10. Results: Assessments • Cognitive abilities similar, dyslexics’ digit span poorer • Dyslexics’ reading related measures significantly lower • Both groups – para reading speed correlated with RAN-D • Controls only – pseudoword reading --------- “ ------------ • dyslexics performance limited by e.g. phonological processing

11. Results: Symbol Identification • Second symbol most accurately identified • Dyslexics less accurate for fifth (not reported)

12. Results: Contrast & Duration • Performance of two groups similar in all conditions

13. Results: Second-order Effects • Similar performance for SOA and contrast thresholds • Large effect of white noise on contrast thresholds • Intermediate effects of size and flankers on SOA • Negligible effects of flankers on contrast threshold

14. Results: Visual Measures • Dyslexics • no performance difference for two visual ‘subgroups’ • visual measures and reading-related scores not significantly correlated • Controls • correlation between Contrast threshold and Pseudoword reading speed • dyslexics scores not normally distributed • Most visual thresholds highly correlated (particularly controls) • suggests common hidden factor - overall grapheme processing • primary factor “almost-equally-weighted average of all thresholds’ z-scores”

15. Discussion • Manipulations reduced performance similarly both groups • substantial differences in reading abilities • lack of differentiation by relatively broad battery or visual ability tests • suggests dissociation reading difficulties and visual skills • Tasks tap common visual mechanisms • presumably related to grapheme identification • Pseudoword reading speed : contrast threshold • control’s correlation implies visual measures tap reading related visual abilities • no dyslexic correlation suggests visual abilities not limit reading ability • Alternative explanation = experimental population • dyslexics tested not have substantial visual deficits • university students • specific reading difficulties • above average general cognitive abilities

16. Discussion (cont’d) • Complaints of visual discomfort • greater in dyslexics, but not related to visual task performance • interpreted as visual stress being consequence rather than cause of reading difficulties • reading puts heavier load on visual attention for dyslexics • Conclude • visual problems may be prevalent • could be used as markers for reading deficits • probably not relevant for any amelioration program • do not seem to pose any functional bottleneck • Final message from Abstract • difficulties with single word reading not visual processing deficit

17. Criticisms • Task supposedly similar to single word reading, but • stimuli not letters • matching task (not letter identification) • viewing distance not realistic (1 metre) • Crowding condition not really crowded • Contrast sensitivity controversial measure • previous research shows not reliably different between groups • cited articles (Dosher & Lu, 2005; Gold et al, 1999) relate to perceptual learning in non-clinical participants • No explanation/predictions for SOA condition • Experimental tasks not timed • Tasks not sensitive enough to tap dyslexics’ visual deficits

18. Efficiency THRESHOLD Equivalent Internal Noise EXTERNAL NOISE Linear Additive Model • Linear Additive Model irrelevant • mentioned in Introduction, not referred to in Discussion, not implemented • Model specified by Pelli (1999)comprises two factors Efficiency: “Rates the computation underlying our perceptual decisions on the absolute performance scale defined by the ideal observer” Equivalent Noise: “Specifies how much noise the observer’s visual system adds to the display”

19. Shovman & Ahissar’s LAM • Interpretation • simple assumption only one source of inner noise (additive to signal) • observer’s discriminatory ability (D) and equivalent inner noise (Neq) define overall efficiency of visual system for this type of letter identification • so measured contrast thresholds for identification at two different levels of noise (one with, one without)… • Lack of implementation • confusion re nature of noise: white noise, SOA, contrast, crowding, size • white noise, crowding and size not additive • no measure of Equivalent Internal Noise • baseline = performance of control group (not ideal observer)