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Lorusso, M.L.; Borasio, F.; Panetto, P.; Curioni, M.; Brotto, G.; Pons, G.; Carsetti, A.; Molteni, M. Developmental Dyslexia, Dyslexia-Friendly Fonts and Text-to-Speech Technologies. Encyclopedia. Available online: (accessed on 17 June 2024).
Lorusso ML, Borasio F, Panetto P, Curioni M, Brotto G, Pons G, et al. Developmental Dyslexia, Dyslexia-Friendly Fonts and Text-to-Speech Technologies. Encyclopedia. Available at: Accessed June 17, 2024.
Lorusso, Maria Luisa, Francesca Borasio, Paola Panetto, Mariangela Curioni, Giada Brotto, Giulio Pons, Alex Carsetti, Massimo Molteni. "Developmental Dyslexia, Dyslexia-Friendly Fonts and Text-to-Speech Technologies" Encyclopedia, (accessed June 17, 2024).
Lorusso, M.L., Borasio, F., Panetto, P., Curioni, M., Brotto, G., Pons, G., Carsetti, A., & Molteni, M. (2024, February 02). Developmental Dyslexia, Dyslexia-Friendly Fonts and Text-to-Speech Technologies. In Encyclopedia.
Lorusso, Maria Luisa, et al. "Developmental Dyslexia, Dyslexia-Friendly Fonts and Text-to-Speech Technologies." Encyclopedia. Web. 02 February, 2024.
Developmental Dyslexia, Dyslexia-Friendly Fonts and Text-to-Speech Technologies

Developmental dyslexia (DD) is one of the most common neurodevelopmental disorders diagnosed in children who fail to develop normal reading skills in spite of normal intelligence. A recent study conducted in Italy showed a prevalence of 3.5%. Traditional approaches considered DD as a phonological processing impairment; however, several other functions were shown to be specifically impaired, such as long-term and short-term verbal memory or working memory, visual attention, and visual and auditory perception. Over the past years, research has been looking for solutions to alleviate deficits in the visuospatial processing of letters and words, and to improve reading in individuals with DD using specific fonts or changing visualization parameters such as size, spacing and line spacing. Moreover, text-to-speech technology (TTS) has been used to support understanding of the text. However, apart from a general recommendation about slower pace, much less research has been devoted to the specific characteristics of TTS that may lead to clearer perception and better understanding in students with DD.

dyslexia personalization font spacing size web app online reading visualization text-to-speech pitch

1. Background: Developmental Dyslexia and Perception

The attentional systems that form part of the magnocellular system (i.e., the dorsal system or dorsal visual stream), control the ability to concentrate visual attention in a restricted area of the visual field and to shift it when needed. They also control so-called visual crowding, an automatic mechanism of our perceptual system producing a sort of “blurring” of the visual areas surrounding the target object, resulting in the masking of further visual elements present in those areas [1][2]. The visual attention systems play a role in the early stages of grapheme-to-phoneme conversion, and they were shown to be involved in visual searches and in graphemic parsing through neuronal-oscillation modulation mechanisms very similar to those involved in phonological processing[3][4]. The Temporal Sampling Framework (TSF) proposed by Goswami et al.[4] could be applied to the different stages of processing within the visual system as well, before the stage of phonological processing. Visual crowding makes stimulus identification more difficult: in the case of text, single letters may be considered as target stimuli, and crowding effects can be observed also in central vision[5]. Visual crowding thus leads to the inability to recognize letters when surrounded by other letters, resulting in slower reading and mistakes [3][6][7].
More generally, Bouma’s law of crowding describes an uncrowded central window through which we can read and a crowded periphery through which we cannot. Reading rate is determined by crowding and eccentricity. During text reading, typical readers are limited by letter spacing much more than by font size [8]. Children with DD, moreover, show reduced accuracy for letter identification in and near the foveal field in comparison with typical readers, and a peculiar spatial distribution of lateral masking across central and peripheral vision [9].

2. Dyslexia-Friendly Fonts and Text-to-Speech Technologies

The visual characteristics of the text exert a significant influence on reading [10]. A study by Joo and colleagues [11], for instance, showed that individuals with DD read faster when a text is presented with increased inter-letter and inter-word spacing. An extra-large space (inter-letter spacing enlarged by 2.5 pt. on 14 pt. body size, +~18% of the body size) was found to improve reading in children with DD when they use the most common fonts, such as Times New Roman or Arial (regular space between two lowercase letters may vary roughly from 0 to 15% of the body size, while the regular space between two words corresponds approximately to 20–25% of the body size) [12][13]. Moreover, using sentences of the same length and extra-large inter-letter spacing was found to decrease the number of errors without increasing reading speed [12]. Also, Perea et al. [14] found that young readers with dyslexia showed faster reading times and higher comprehension scores when the text had a small increase in inter-letter spacing relative to the default settings. Non-dyslexic children and older children had reduced effects compared to younger children with dyslexia. A recent study by Łuniewska et al. [15] by contrast showed that, in both children with DD and typical readers, increased inter-letter spacing did not affect reading accuracy, speed, or comprehension but led to shorter fixations in dyslexic children with respect to normal readers, suggesting possibly different reading strategies.
Turning to font types, it is usually suggested that sans-serif fonts are easier to be read by people with DD than fonts with serif, and that monospaced fonts (where the space occupied by a single letter is constant regardless of the actual width of the letter) are more readable than proportional fonts (where space depends on the letter’s width). According to the results obtained in reading performance and to subjective preferences, the fonts recommended for children with dyslexia are Helvetica, Courier, Arial, Verdana and Computer Modern Unicode. Arial Italics, instead, seems to be difficult to read on screen. More generally, sans serif and roman font types increased the reading performance, while italic fonts did the opposite [16]. Results from previous studies indicate that subjective readability increases with increasing font size, but that there is a plateau around 18 pt–22 pt, and a decrease beyond 22 pt [17]. Also, subjective comprehension seems to be significantly better for larger font sizes (18, 22, 26 pt) than for smaller ones (10, 12 pt). The same authors, however, also found that smaller line spacing leads to better subjective understanding of the text compared to very large spacing, while intermediate spacing differences seem to have no impact [18]. It should be noted that this research used eye-tracking measures (fixation duration) for readability and no objective measure of accuracy was recorded, nor was any further measure of reading speed. On the other hand, a large study on Spanish subjects with and without dyslexia [19] and an Italian study [20] clearly show that there is no correspondence between the subjects’ objective performances and their subjective evaluations and preferences. So-called dyslexia-friendly fonts (DF) like, e.g., Dyslexie, Open Dyslexia or EasyReading are designed to help people with DD to recognize letters, distinguish between letters of similar shapes, and limit crowding effects. Characteristics of these fonts are a specific letterform, such as increased thickness near the bottom, angling and changing the height and the contours of similarly shaped letters, and increased spacing. Bachmann [21] found that children with DD read a text with a specific font (EasyReading™) faster and with less errors when compared with a standard font like Times New Roman. EasyReading™ was developed for individuals with DD and integrates particular graphic features (e.g., letterform with dedicated serifs, and longer ascenders and descenders) and enlarged inter-letter and inter-word spacing (spacing between two lowercase letters vary between 16 and 18% of the body size, and spacing between two words corresponds to 39% of the body size). This “increased spacing” effect has partially been replicated by Duranovic et al. [22]. Bosnian-speaking children with DD improved in reading accuracy, but not in speed. Other recent studies failed to find any advantage of a dedicated letterform or increased spacing effects comparing DF fonts, Arial, and Times New Roman [23][24]. Other studies did not find any differences between children with and without DD in reading speed and accuracy when they read texts written with DF letterforms or with standard letterforms [23][24][25]. Both children with DD and typically developing children showed a decrease in reading speed when text was presented with an unusual spacing condition: it seems to be difficult to segment sentences into words when the sizes of the (increased) inter-letter and the (default) inter-word spacing are similar [25]. A significant interaction between letterform and inter-word spacing emerged in the group of children with DD, whose reading speed was similar for a text with DF or with standard letterform and increased spacing [25].
Overall, the studies show great heterogeneity of results, probably based on substantial differences in relevant variables: the age of the participants (children or adults), their characteristics (with/without DD), the type of text used (single words, single letters, nonwords or passages), the parameters with which the outcome was assessed (comprehension of texts or reading speed and/or accuracy, lexical decision, satisfaction questionnaires, eye movements), the type of design and analysis used (manipulation of single or multiple variables, DD/control comparisons vs. differences within groups), the language of the text (orthographically regular languages such as Italian or Spanish or irregular languages such as English). In most studies, group averages are taken into account rather than individual differences. A systematic study conducted on a large number of parameters [20] suggests that the individual effects are limited at the group level and can instead vary from subject to subject (turning out to be facilitating for some and penalizing for others), leading to the conclusion that a highly personalized approach is desirable.
Much less information is available concerning the advantages of various types of TTS (text-to-speech) technologies to support reading and comprehension in children with DD. Better learning [26] and reading comprehension [27] performances have been observed with TTS compared to no TTS for students with reading and language difficulties. However, a recent study suggested that TTS is not necessarily advantageous to all children with reading difficulties, but only for those with typical profiles where listening comprehension is less impaired than decoding skills [28].
Thanks to Italian Law 170/2010, which contains norms and guidelines on educational rights for students with specific learning disorders (SLD), a DD diagnosis brings about the right to use certain compensatory or dispensatory tools at school [29]. Research on SLD has made it possible to create increasingly versatile technologies to support reading, writing and study, and Seleggo is one of these new technologies.
The results of a recent study [30] assessing the validity of a special automatized procedure, called the “Seleggo Test”[31] designed to select the most favorable parameters for both text visualization and TTS conversion,  confirmed that font types have a significant impact on readability for both children with (atypical reading development, AD) and without (typical reading development, TD) reading disorders, as found in previous studies (e.g., [20][21][22][26]). It is immediately evident that there is no single font type giving a clear advantage in reading for children with AD. The range of selected font types is wide for both groups (although it is admittedly less wide in the AD group). The most frequently selected fonts do not reach 20% of total outcomes in both groups. Taking into consideration font size and spacing, TD and AD children behave rather similarly, even if 75% of the TD group were facilitated by size 2, while children of the AD group are more distributed in the three dimensions of the font. Moreover, most of the TD group chose standard spacing, while in the AD group the spacing allowing for the best reading performances was a larger one. The descriptive data from the study point to the fact that children without reading disorders also prefer, and read better, the fonts that people with dyslexia prefer [24]. So-called DF fonts, such as EasyReading™, were not the most frequently selected font types, but they were selected by 5 to 10% of the children in both groups. Very common font types such as Times New Roman and Roboto were selected very rarely (2–3% in both groups), a result that came rather unexpected. This actually confirms the results by Bachmann and Mengheri[32] showing significant differences in reading fluency and accuracy for EasyReading™ compared to Times New Roman. As expected, the study shows that each child improved in reading speed and accuracy and in dictation accuracy when using a font customized to their needs.

Regarding voice parameters, children in both the TD and the AD group performed better with speed 0.8 (slightly relented with respect to natural voice) and a 0.6 pitch (a deeper, less acute pitch compared to the default voice). This supports a general recommendation that the use of Assistive Technology to support reading and study in DD can be of great help. Even if very little data are reported in the literature concerning objective advantages in the use of TTS to enhance reading accuracy, the present results suggest that personalized TTS parameters may further facilitate text comprehension.


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