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Previous reports of improved oral reading performance for dyslexic children but not for regular readers when between-letter spacing was enlarged led to the proposal of a dyslexia-specific deficit in visual crowding. However, it is in this context also critical to understand how letter spacing affects visual word recognition and reading in unimpaired readers. Adopting an individual differences approach, the present study, accordingly, examined whether wider letter spacing improves reading performance also for non-impaired adults during silent reading and whether there is an association between letter spacing and crowding sensitivity. We report eye movement data of 24 German students who silently read texts presented either with normal or wider letter spacing. Foveal and parafoveal crowding sensitivity were estimated using two independent tests. Wider spacing reduced first fixation durations, gaze durations, and total fixation time for all participants, with slower readers showing stronger effects. However, wider letter spacing also reduced skipping probabilities and elicited more fixations, especially for faster readers. In terms of words read per minute, wider letter spacing did not provide a benefit, and faster readers in particular were slowed down. Neither foveal nor parafoveal crowding sensitivity correlated with the observed letter-spacing effects. In conclusion, wide letter spacing reduces single word processing time in typically developed readers during silent reading, but affects reading rates negatively since more words must be fixated. We tentatively propose that wider letter spacing reinforces serial letter processing in slower readers, but disrupts parallel processing of letter chunks in faster readers. These effects of letter spacing do not seem to be mediated by individual differences in crowding sensitivity.
The reading acceleration phenomenon refers to the effect that experimentally induced time constraints can generate instantaneous improvements of reading rate, accuracy and comprehension among typical and reading impaired readers of different age groups. An overview of studies applying the fading manipulation (i.e., letters are erased in reading direction), which induces the time constraints causing the acceleration phenomenon, is provided in the first part of this review. The second part summarises the outcomes of studies using a training approach called the reading acceleration program (RAP) that integrated core principles of the acceleration phenomenon to generate persistent reading performance improvements. Our review shows ample evidence for the validity of the acceleration phenomenon, since it has been replicated across various languages and populations. However, although there are several explanatory approaches for underlying mechanisms, none of them is well substantiated by empirical evidence so far. Similarly, although generally positive effects of RAP training were reported for several languages and groups of readers, the exact mechanisms causing improved reading rates and comprehension are not well understood. Our critical discussion points out several limitations of RAP that call for further research. However, we also highlight several benefits regarding RAP's potential as an intervention approach for enhancements in reading performance. Video abstract link: https://youtu.be/wO6aEXavk8w