Rationale:

According to our high-level visual dysfunction hypothesis, the reading problems of some dyslexic readers are a salient manifestation of a more general deficit of visual cognition stemming from disrupted functioning of high-level visual regions. These regions play a fundamental role in the visual recognition of words and other objects. Supporting the hypothesis, our previous studies indicate that people with dyslexia have problems with tasks thought to rely on high-level regions of the ventral visual stream. Our prior work however has been strictly behavioral, and neural functioning was not directly measured. The primary research question is whether the high-level visual dysfunction hypothesis holds true, as we put the hypothesized deficient neural processing of high-level visual mechanisms in dyslexia to a direct test.

To answer this question, the experiment will be divided into four main parts: DYSERP, DYSSSVEP faces, DYSSSVEP objects and DYSSSVEP words. In DYSERP, we will record and analyze the N170 and N250 evoked related potentials (ERP) components while participants perform a matching task with faces or houses. In the three DYSSSVEPs, we will record steady state visual-evoked potentials (SSVEPs), periodic responses where the EEG frequency content is related to the frequency of stimulus presentation and identity change, to evaluate the processing of faces, objects (at the subordinate or basic level) and words (by using real words, pseudowords, nonwords and false font characters).

Group analyses (dyslexic vs. typical readers) takes precedence, but hypotheses will also be tested by correlating with continuous measures on reading problems like reading speed and reading accuracy. We will also assess to which degree our results are modulated by various background factors/covariates. For more, please refer to the preregistration https://osf.io/4dr3f.

Material and methods:

On day 1, participants answer background questions, complete an assessment of visual acuity, DYSERP (60% of the trials) and DYSSSVEP faces. On day 2, participants complete DYSERP (40% of the trials), DYSSSVEP objects, and DYSSSVEP words, and a reading test.

Participants will be assigned to one of two groups: dyslexic and typical readers. For each dyslexic reader, a typical reader will be matched based on gender, age and education, and group is accordingly treated as a repeated (paired) factor.

We will run a minimum of 86 participants, allowing us to detect an effect size dz of 0.5 with a two-tailed paired samples t-test and an effect size of r = 0.33, assuming alpha of 0.05 and 0.80 power. In order to detect an effect size dz of 0.4 with the same criteria, we will aim for a maximum of 130 total participants and we can expect to detect effect sizes of r = 0.27. We expect up to 20% subject loss due to various factors.

For DYSERP, participants will be shown a fixation point for 600–900 ms, a sample stimulus (a face or a house) containing all spatial frequencies for 3000 ms, a fixation point for 500–700 ms and a comparison stimulus of the same basic category for 200 ms. This object can be presented with LSF (LSF < 14 cpi) or HSF (HSF > 14 cpi). The participant is then presented with a question mark and has to indicate whether the two images were of the SAME or DIFFERENT identity. We include Group (dyslexic vs. typical readers), Stimuli Type (face, house), Spatial Frequency (AllSF, LoSF, HiSF), Hemisphere (left, right), and Repetition (same, different) as repeated factors.

For DYSSSVEP, stimulus will be displayed at a rate of 6 cycles per second through sinusoidal contrast modulation. A cycle begins with a uniform grey background from which a stimulus appears as its contrast increases. The contrast then decreases at the same rate. Stimulus size varies randomly from cycle to cycle between 74% and 120%. On each trial, one stimulus will be selected as the standard stimulus and repeated throughout the sequence. At fixed intervals of every 5th stimulus, a different oddball stimulus selected from the remaining possible stimuli will be presented. The mean EEG power at precisely this oddball frequency (F/5 = 1.2 Hz) and its first three harmonics (i.e., 2F/5 = 2.4 Hz, 3F/5 = 3.6 Hz, and 4F/5 = 4.8 Hz) will be used as an index of the visual system's discrimination of the standard and oddball stimuli.

DYSSSVEP faces includes two types of trials: attend-fix trials where people monitor color changes of the fixation point and attend-face trials where people monitor oddball identity changes. Each trial is followed by two 3-alternative forced choice recognition tests. We include Group (dyslexic vs. typical readers), Attention (attend to faces, attend to color) and Hemisphere (left, right) as repeated factors.

DYSSSVEP objects includes two types of trials: basic-level trials where standard and oddball images are of two different basic categories and subordinate-level trials where standard and oddball images are different exemplars of the same basic category. After each trial, two 6-alternative forced choice recognition tests are performed. We include Group (dyslexic vs. typical readers), Stimulus Level (basic-level, subordinate-level) and Hemisphere (left, right) as repeated factors.

In DYSSSVEP words, we will have 4 types of stimuli: real words (RW), pseudowords (PW) , nonwords (NW), and false font nonwords (FF) and 3 types of trials: RW in FF trials, PW in FF trials and NW in FF trials where the standard stimulus is respectively a RW, PW and NW and the oddball stimulus is a FF. A fixation point will be presented in the middle of the stimuli and participants will be asked to press the spacebar each time the fixation point turns from red to blue. We include Group (dyslexic vs. typical readers), Print Type Contrast (real words in false fonts, pseudowords in false fonts, and nonwords in false fonts) and Hemisphere (left, right) as repeated factors.