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Background/Objectives: The present study aimed to investigate letter processing in children with dyslexia and typically developing readers as a function of the type of orthographic context. Methods and Results: In Experiment 1A, children performed a two-alternative forced choice task (Reicher–Wheeler paradigm) using as probes either high-frequency words, pronounceable pseudo-words, or unpronounceable non-words. The group differences in letter recognition were clearly distinguished from those present in typical word and pseudo-word reading conditions (Experiment 1B), as a global factor was present only in the latter case. In Experiment 2, the two-alternative forced choice task required the child to search for the target letter in the subsequent multi-letter string (i.e., words, pseudo-words, or non-words), thus reducing the memory load. Detecting the target letter was more difficult in a word than in a pseudo-word or non-word array, indicating that the word form’s lexical activation interfered with the target’s analysis in both groups of children. In Experiment 3, children performed the two-alternative forced choice task with symbols (Greek letters) either in the Reicher–Wheeler mode of presentation (Experiment 3A) or in the search condition (Experiment 3B). Children with dyslexia performed identically to typically developing readers in keeping with the selectivity of their orthographic difficulties. Conclusions: The present data indicate that children with dyslexia suffer from an early deficit in making perceptual operations that require the conjunction analysis of a set of letters. Still, this deficit is not due to an inability to scan the letter string. The deficit is confined to orthographic stimuli and does not extend to other types of visual targets.

In our daily lives, we are constantly exposed to numbers and letters. However, it is still under debate how letters and numbers are processed in the brain, while information on this topic would allow for a more comprehensive understanding of, for example, known influences of language on numerical cognition or neural circuits shared by numerical cognition and language processing. Some findings provide evidence for a double dissociation between numbers and letters, with numbers being represented in the right and letters in the left hemisphere, while the opposing view suggests a shared neural network. Since processing may depend on the task, we address the reported inconsistencies in a very basic symbol copying task using functional near-infrared spectroscopy (fNIRS). fNIRS data revealed that both number and letter copying rely on the bilateral middle and left inferior frontal gyri. Only numbers elicited additional activation in the bilateral parietal cortex and in the left superior temporal gyrus. However, no cortical activation difference was observed between copying numbers and letters, and there was Bayesian evidence for common activation in the middle frontal gyri and superior parietal lobules. Therefore, we conclude that basic number and letter processing are based on a largely shared cortical network, at least in a simple task such as copying symbols. This suggests that copying can be used as a control condition for more complex tasks in neuroimaging studies without subtracting stimuli-specific activation.

Understanding reading is a central issue for psychology, with major societal implications. Over the past five decades, a simple letter-detection task has been used as a window on the psycholinguistic processes involved in reading. When readers are asked to read a text for comprehension while marking with a pencil all instances of a target letter, they miss some of the letters in a systematic way known as the missing-letter effect. In the current article, we review evidence from studies that have emphasized neuroimaging, eye movement, rapid serial visual presentation, and auditory passages. As we review, the missing-letter effect captures a wide variety of cognitive processes, including lexical activation, attention, and extraction of phrase structure. To account for the large set of findings generated by studies of the missing-letter effect, we advanced an attentional-disengagement model that is rooted in how attention is allocated to and disengaged from lexical items during reading, which we have recently shown applies equally to listening.

The dominant model of number processing suggests the existence of a Number Form Area (NFA) in the inferior temporal gyrus (ITG) that supports the processing of Arabic digits as visual symbols of number. However, studies have produced inconsistent evidence for the presence and laterality of digit-specific ITG activity. Furthermore, whether any such activity relates to mathematical competence is unknown. This study investigated these two issues using functional magnetic resonance imaging. Thirty-two adults performed digit and letter detection tasks and reading and mathematics tests. During digit detection, participants determined whether digits were present in a string of letters (e.g., AH3NR versus AHTNR). During letter detection, participants determined whether letters were present in a string of digits (e.g., 93R78 versus 93478). Results showed four clusters in frontal, occipital, and temporal regions for digit detection, including a left ITG cluster. Five clusters in frontal, parietal, occipital, and temporal regions were associated with letter detection, including a left ITG cluster. Digit and letter-related ITG clusters were spatially distinct; however, a direct contrast of digit and letter processing did not reveal greater activity in the left ITG for digit detection. Whole brain correlations showed greater digit-related activity in the right ITG for participants with higher calculation skills, but there was no correlation between letter activity and calculation skills. Together, our results suggest functional localization, but not specialization, for digits in the left ITG and provide the first evidence of a relationship between calculation skills and digit processing in the right ITG.

When and where phonological processing occurs in the brain is still under some debate. Most paired-rhyme and phonological priming studies used word stimuli, which involve complex neural networks for word recognition and semantics. This study investigates early (<300 ms) and late (>300 ms) orthographic and phonological processing of letters. Fifteen participants aged 20-35 engaged in three two-forced choice experiments, one letter-detection (LetterID) and two letter-rhyme (Paired-Rhyme and Letter-Rhyme) tasks. From the EEG recordings, event related potential (ERP) differences within and across task stimuli were found. We also calculated the global field power (GFP) for each participant. Accuracies and reaction times were also measured from their button presses for each task. Behavioral: Reaction times were 18 ms faster to letter than pseudoletter stimuli, and 27 ms faster to rhyme than nonrhyme stimuli. In the LetterID task, grand-mean evoked potentials (EPs) showed typical P1, N1, P2, and P3 waveform morphologies to letter and pseudoletter stimuli, with GFPs to pseudoletters being greater than letters from 160-600 ms. Across both rhyme tasks, there were greater negativities for nonrhyme than for rhyme stimuli at 145 ms and 426 ms. The P2 effect for rhyme stimuli was smaller than letter stimuli when compared across tasks. Differences in early processing of letters vs. pseudoletters between 130-190 ms suggest that letters are processed earlier and perhaps faster in the brain than pseudoletters. The P2 effect between letter and rhyme stimuli likely reflect sublexical phonological processing. Together, findings from our study fill in evidence for the temporal dynamics of orthographic and phonological processing of single letters.

Studies in the literature have shown how the preference towards local or global processing can vary according to different characteristics of the stimuli involved, such as stimulus type and stimulus time duration. In the present study, we investigated whether letters and faces undergo similar or different global/local processing and the attentional mechanisms that might be linked to eventual differences. We used hierarchical, congruent, and incongruent letters and faces in different time conditions (180 and 500 ms) and we conducted three different experiments. The results of Experiment 1 showed that with stimuli shown for 180 ms, letters are processed more efficiently at the local level, with an inversion of the global interference effect. Conversely, faces are still processed more efficiently at the global level, with evidence of global advantage and global interference. The results of Experiment 2 showed that when the same stimuli are presented for longer (500 ms), they are still processed differently. Indeed, we observed faster local processing for letters but still a tendency, even if not significant, toward a global processing advantage for faces. Moreover, the cue-size effect, i.e., the ability to modulate visual focal attention based on the characteristics of the cue, was measured. In Experiment 3, the cue-size effect showed a statistically significant correlation with the local processing advantage for letters but not for faces. We conclude that during the almost automatic processing of letters it is possible to modulate focal attention on the basis of the task, narrowing the field of visual attention during the local task and neglecting the global stimulus. Conversely, during face processing, the holistic mechanism tends to prevail over focal attention modulation skills, even when it is explicitly required to focus on the local stimulus.

The sequence of Arabic numerals from 1 to 19 was taught to six chimpanzees, three pairs of mother and child. Each chimpanzee participant sat facing a touchscreen on which the numerals appeared in random positions within an imaginary 5-by-8 matrix. They had to touch the numerals in ascending order. Baseline training involved touching the adjacent numerals from 1 to X or from the numeral X to 19. Systematic tests revealed the following results: (1) The range 1 to 9 was easier than 1 to 19. (2) Adjacent numerals were easier than nonadjacent ones. (3) The “masking” (memory task) caused deterioration of performance. All these factors depended on the number of numerals simultaneously presented on the screen. A chimpanzee named Pal mastered the skill of ordering two-digit numerals with 100% accuracy. Human participants were tested in the same experiment with the same procedure. Both species showed relative difficulty in handling two-digit numerals. Global–local information processing is known to be different between humans and other primates. The assessment of chimpanzee performance and comparison with humans were discussed in terms of the possible difference in the global–local dual information processing of two-digit numerals.

Literacy affects many aspects of language and cognition, including the shift from a more holistic mode of processing to a more analytical part-based mode of processing. Here we examined whether this shift impacts the ability of preschool and primary school children to learn the rules underlying a finite-state grammar using an artificial grammar learning (AGL) paradigm implemented with either linguistic (letters) or non-linguistic (colors) materials to further examine if children’s AGL performance was modulated by type of stimuli. Both tasks involved a training phase in which half of the preschool children and half of the primary school children were exposed to a set of either letter or color strings without any information about the rules underlying the construction of those strings. Later, in the test phase, they were asked to decide whether a new set of letter or color strings conformed to those rules to test grammar learning. Results showed that only primary school children showed evidence of learning, and, importantly, only with colors. These findings seem to support the view that learning to read promotes reliance on smaller linguistic units that might hinder the ability of first-graders to learn the rules underlying finite-state grammars implemented with linguistic materials.

The present study investigated whether orthographic depth can increase the bias towards multi-letter processing in two reading-related skills: visual attention span (VAS) and rapid automatized naming (RAN). VAS (i.e., the number of visual elements that can be processed at once in a multi-element array) was tested with a visual 1-back task and RAN was measured in a serial letter naming task that introduced a novel manipulation (some letter sequences formed frequent words). Spanish-Basque and French-Basque bilingual children were tested at early (30 children in 1st and 2nd grade), and more advanced (24 children in 3rd, 4th and 5th grade) stages of reading acquisition to investigate whether they would be differently biased towards multi-letter processing due to reading in two shallow (Spanish, Basque), or a deep and a shallow (French, Basque) orthography. The French-Basque bilinguals, who read in a deep orthography, were expected to rely on larger orthographic units in reading and thus to be more biased towards multi-letter processing in both tasks. This was expected to be reflected by: (a) a uniform distribution of attention across letter strings in the VAS task, and (b) a greater interference of the embedded words on letter-by-letter naming in RAN, leading to longer naming times. The expected group differences were observed in the more advanced readers, with French-Basque bilinguals showing a wider distribution of VAS across letter strings and longer naming times in RAN.

This study builds on a specific characteristic of letters of the Roman alphabet—namely, that each letter name is associated with two visual formats, corresponding to their uppercase and lowercase versions. Participants had to read aloud the names of single letters, and event-related potentials (ERPs) for six pairs of visually dissimilar upper- and lowercase letters were recorded. Assuming that the end product of processing is the same for upper- and lowercase letters sharing the same vocal response, ERPs were compared backward, starting from the onset of articulatory responses, and the first significant divergence was observed 120 ms before response onset. Given that naming responses were produced at around 414 ms, on average, these results suggest that letter processing is influenced by visual information until 294 ms after stimulus onset. This therefore provides new empirical evidence regarding the time course and interactive nature of visual letter perception processes.