Isolating the impact of visual perception on dyslexics’ reading ability

Mark M. Shovman & Merav Ahissar

Vision Research 46 (2006) 3514-3525

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)

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)

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

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

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

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

Test Conditions

• Eight conditions, comprising up to 100 trials each

Set Stimulus Size Background SOA Luminance

1 Single symbol 0.5 Uniform gray Adaptive 53.3 cd/m²

2 Single symbol 1 Uniform gray Adaptive 53.3 cd/m²

3 Single symbol 1 Uniform gray 200 ms Minimal (adaptive)

4 Single symbol 1 White noise 200 ms Minimal (adaptive)

5 Triplet 0.5 Uniform gray Adaptive 53.3 cd/m²

6 Triplet 1 Uniform gray Adaptive 53.3 cd/m²

7 Triplet 1 Uniform gray 200 ms Minimal (adaptive)

8 Triplet 1 White noise 200 ms Minimal (adaptive)

• 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

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)

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

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

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

Results: Second-order Effects • Similar performance for SOA and contrast thresholds

(1) Large effect of white noise on contrast thresholds

(2) Intermediate effects of size and flankers on SOA

(3) Negligible effects of flankers on contrast threshold

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”

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

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

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

Linear Additive Model

• Linear Additive Model irrelevant– mentioned in Introduction, not referred to in Discussion, not implemented

• Model specified by Pelli (1999) comprises two factorsEfficiency: “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”

THRESHOLD

EXTERNAL NOISE

Equivalent Internal Noise

Efficiency

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)