Letter Detectors Research Articles

Examining Letter Detector Tolerance through Offset Letter Halves: Evidence from Lexical Decision.

Neurobiological models of reading assume that the specialized detectors at the letter level (e.g., the arrays of detectors for the letter 'n') possess a certain degree of tolerance (e.g., Local Combination Detectors model, Dehaene et al. 2005). In this study, we designed two lexical decision experiments that examined the limits of tolerance of letter detectors by introducing a novel manipulation involving shifting letter halves (e.g., in Experiment 1; in Experiment 2) relative to intact items. This manipulation alters the transition between upper and lower parts of the letters, adding junctions that do not exist in the intact letter forms. We included high- and low-frequency words in the stimulus list to investigate whether letter distortion affects processing beyond the letter level, reasoning that interactive effects would signal top-down lexical feedback. In Experiment 1, which employed a subtle letter shift, we observed a minimal cost of letter distortion that did not interact with word frequency. Experiment 2, employing a larger letter shift, revealed an overall greater reading cost that affected differentially high- and low-frequency words. Overall, these findings offer insights into the limits of resilience in letter detectors to distortion during word recognition and introduce a novel manipulation of letter distortion.

Breaking the boundaries: the power of ligatures in visual-word recognition.

Current neurobiological-inspired models of visual-word recognition propose that letter detectors in the word recognition system can tolerate some variations in the visual form of the letters. However, it is unclear whether this tolerance extends to novel ligatures, which combine two letters into a single glyph. To investigate this, the present study utilized a masked priming experiment with a lexical decision task to examine whether primes containing novel ligatures are effective in activating their corresponding base word, relative to omitted-letter primes, in the initial stages of word processing. For each target word (e.g., VIRTUAL), were created an identity prime (virtual), a prime containing a novel ligature of two of the letters (e.g., virtual; "ir" in a single glyph), and an omitted-letter prime where one letter was removed (e.g., vrtual [omitted-vowel] in Experiment 1; vitual [omitted-consonant] in Experiment 2). Results showed that the presence of a novel ligature in the prime resulted in faster lexical decision times compared to a prime with an omitted vowel (Experiment 1), but not with an omitted consonant (Experiment 2). Furthermore, the performance with the primes containing the novel ligature was not different from that of the identity primes. These results suggest that the word recognition system can quickly enable separate letter detectors for novel ligatures. These findings have important implications for our understanding of the front-end of visual-word recognition.

Rëâdīńg wõrdš wîth ōrńåmêńtš: is there a cost?

Recent research has reported that adding non-existent diacritical marks to a word produces a minimal reading cost compared to the intact word. Here we examined whether this minimal reading cost is due to: (1) the resilience of letter detectors to the perceptual noise (i.e., the cost should be small and comparable for words and nonwords) or (2) top-down lexical processes that normalize the percept for words (i.e., the cost would be larger for nonwords). We designed a letter detection experiment in which a target stimulus (either a word or a nonword) was presented intact or with extra non-existent diacritics [e.g., amigo (friend) vs. ãmîgô; agimo vs. ãgîmô]. Participants had to decide which of two letters was in the stimulus (e.g., A vs. U). Although the task involved lexical processing, with responses being faster and more accurate for words compared to nonwords, we found only a minimal advantage in error rates for intact stimuli versus those with non-existent diacritics. This advantage was similar for both words and nonwords. The letter detectors in the word recognition system appear to be resilient to non-existent diacritics without the need for feedback from higher levels of processing.

Perceptual Contiguity Does Not Modulate Matched-Case Identity-Priming Effects in Lexical Decision.

In recent studies with the masked priming lexical decision task, matched-case identity-priming effects occur for nonwords but not for words (e.g., nonwords: ERTAR-ERTAR faster than ertar-ERTAR; words: ALTAR-ALTAR produces similar response times as altar-ALTAR). This dissociation is thought to result from lexical feedback influencing orthographic representations in word processing. As nonwords do not receive this feedback, bottom-up processing of prime-target integration leads to matched-case effects. However, the underlying mechanism of this effect in nonwords remains unclear. In this study, we added a color congruency manipulation across the prime and target in the matched-case identity-priming design. We aimed to determine whether the case effects originate at the early stages of prime-target perceptual integration or due to bottom-up activation of case-specific letter detectors. Results replicated the previous dissociation between words and nonwords regarding the matched-case identity effect. Additionally, we did not find any modulation of these effects by prime-target color congruency. These findings suggest that the locus of the matched-case identity effect is at an orthographic level of representation that encodes case information.

On the time course of the tolerance of letter detectors to rotations: A masked priming ERP investigation

A straightforward prediction of the Local Combination Detectors [LCD] model of word recognition (Dehaene et al., 2005) is that letter rotations above 40–45° should disrupt the mapping of the visual input onto orthographic representations. However, the evidence supporting this claim is scarce and not conclusive. To shed light on this issue, we conducted a masked repetition priming lexical decision experiment while recording the participants’ EEG measures. Targets were always presented in the standard horizontal format, and we rotated the individual letters of the identity/unrelated primes (0°, 45°, or 90°). Behavioral and Event-Related Potentials (ERP) results revealed that the identity priming effect decreased as a function of letter rotation. Importantly, the ERP data allowed us to examine in detail the time course of processing of words with rotated letters. Amplitude comparisons showed that identity priming followed the typical course for 0° primes (i.e., it started around 100 ms, in the visual feature encoding stage, and strengthened with processing time). The parallel effect for 45° primes emerged later, at around 175 ms. This pattern strongly suggests that letter rotations at around 45° have a processing cost, thus providing evidence in favor of the LCD model of word recognition (Dehaene et al., 2005).

Positional cueing, string location variability, and letter-in-string identification

In three experiments we measured accuracy in identifying a single letter among a string of five briefly presented consonants followed by a post-mask. The position of the to-be-identified letter was either indicated by an ordinal cue (e.g., position 2) or an underscore cue (e.g., #####). In Experiment 1 the ordinal cue was presented prior to onset of the letter string, and the underscore cue presented at string offset. In Experiments 2 and 3, both the ordinal and the underscore cues were pre-cues. In all experiments, letter strings could either appear centered on fixation or shifted randomly to the left or to the right. Participants were tested in separate blocks of trials for each of the four conditions generated by the combination of cue-type and string-location variability. In Experiment 1, letter identification accuracy was higher with ordinal cues and with fixed string locations, and ordinal cueing was more affected by string location variability. In Experiments 2 and 3, letter identification accuracy was higher with underscore pre-cues. We conclude that under conditions of brief stimulus durations (100 ms) and backward masking, letter-in-string identification accuracy is determined by read-out from location-specific letter detectors, independently of the type of cueing. Differences in the effectiveness of different types of cue are determined by differences in the ease of isolating a given gaze-centered location, and by differences in the ease with which attention can be directed to that location.

When does rotation disrupt letter encoding? Testing the resilience of letter detectors in the initial moments of processing.

Previous research has reported that both letter and word identification are slower when the stimuli are presented at rotations above 45° than when presented in their canonical horizontal view. Indeed, influential models of word recognition posit that letter detectors in the visual word recognition system are disrupted by rotation angles above 40° or 45° (e.g., Local Combinations Detector model; Dehaene, Cohen, Sigman, & Vinckier, 2005, Trends in Cognitive Sciences). However, recent experiments have shown robust masked identity/form priming effects for 90° rotated words, thus calling into question this assumption. Here we aimed to isolate the degree to which letter detectors are disrupted when manipulating letter rotation in three masked identity priming letter match experiments. Probes and targets were always presented in the canonical upright position, whereas forwardly masked primes were rotated in different angles. The rotation angles were 0° versus 45° (Experiment 1), 22.5° versus 67.5° (Experiment 2), and 45° versus 90° (Experiment 3). Results showed a sizeable masked identity priming effect regardless of the rotation angle, hence demonstrating that letter detectors are not disrupted by rotations smaller than 90° in the early moments of letter processing. This pattern suggests that letter detectors are more resilient to changes in visual form than predicted by the LCD model.

An electrophysiological investigation of orthographic spatial integration in reading

During reading, word recognition speed is influenced by the amount of orthographic overlap with surrounding words. The nature of this phenomenon is not understood: some theories attribute it to low-level visual operations (i.e., parafoveal feature detectors influencing foveal letter detectors), whereas other theories assume that orthographic processing (i.e., letter position coding and word activation) occurs across multiple words in parallel. To arbitrate between these theories, we used electroencephalography to reveal the time course of orthographic spatial integration in a lexical decision task. Foveal target words were flanked on each side by parafoveal words, manipulated across three conditions: repetition flankers (e.g. rock rock rock), unrelated flankers (step rock step) and a no-flanker condition. Linear mixed-effect models were constructed to analyze EEG data on a trial-by-trial basis. Word recognition was worse in the unrelated flanker condition than in the repetition and no-flanker conditions. This behavioral pattern was accompanied by increased negativity in the N250 and N400 windows, associated with the activation of sub-lexical and lexico-semantic representations, respectively. Crucially, the absence of effects prior to 200 ms post-stimulus onset provides evidence against the involvement of low-level visual processes. We conclude that orthographic spatial integration is driven by parallel processing of multiple words, which leads to the activation of a larger set of sub-lexical nodes and more difficult processing at the lexical level when those words are orthographically unrelated.

Parallel graded attention in reading: A pupillometric study

There are roughly two lines of theory to account for recent evidence that word processing is influenced by adjacent orthographic information. One line assumes that multiple words can be processed simultaneously through a parallel graded distribution of visuo-spatial attention. The other line assumes that attention is strictly directed to single words, but that letter detectors are connected to both foveal and parafoveal feature detectors, as such driving parafoveal-foveal integrative effects. Putting these two accounts to the test, we build on recent research showing that the pupil responds to the brightness of covertly attended (i.e., without looking) locations in the visual field. Experiment 1 showed that foveal target word processing was facilitated by related parafoveal flanking words when these were positioned to the left and right of the target, but not when these were positioned above and below the target. Perfectly in line with this asymmetry, in Experiment 2 we found that the pupil size was contingent with the brightness of the locations of horizontally but not vertically aligned flankers, indicating that attentional resources were allocated to those words involved in the parafoveal-on-foveal effect. We conclude that orthographic parafoveal-on-foveal effects are driven by parallel graded attention.

Side flankers produce less crowding, but only for letters

Identification of isolated and crowded letter (B, D, F, G, K, N, L, S, T) and symbol stimuli (%, /, ?, @, }, <, £, §, μ) was examined across the visual field in a two-alternative forced-choice match-to-sample task (2AFC-MTS). During isolated presentation, identification accuracy did not differ between the two stimulus types. Identification rates for the central characters within the three-character strings were higher for letters than for symbols at the horizontal and vertical meridian (Experiment 1), and at diagonal locations (Experiment 2). However, this reduction of parafoveal letter crowding was present in horizontally but not in vertically oriented strings of stimuli. The same pattern of results was replicated in the periphery of the visual field (Experiment 3). The obtained results are in agreement with the proposition that the receptive fields of letter detectors are modified during reading acquisition, in order to support efficient letter identification (Tydgat & Grainger, 2009). However, the pervasive presence of the effect across the visual field suggests that it could originate from a non-retinotopic stage of visual processing.

The effect of decreased interletter spacing on orthographic processing.

There is growing interest in how perceptual factors such as the spacing between letters within words modulate performance in visual word recognition and reading aloud. Extra-large letter spacing can strongly improve the reading performance of dyslexic children, and a small increase with respect to the standard spacing seems beneficial even for skilled word recognition in adult readers. In the present study we examined the effect of decreased letter spacing on perceptual identification and lexical decision tasks. Identification in the decreased spacing condition was slower than identification of normally spaced strings, thereby confirming that the reciprocal interference among letters located in close proximity (crowding) poses critical constraints on visual word processing. Importantly, the effect of spacing was not modulated by string length, suggesting that the locus of the spacing effect is at the level of letter detectors. Moreover, the processing of crowded letters was facilitated by top-down support from orthographic lexical representation as indicated by the fact that decreased spacing affected pseudowords significantly more than words. Conversely, in the lexical decision task only word responses were affected by the spacing manipulation. Overall, our findings support the hypothesis that increased crowding is particularly harmful for phonological decoding, thereby adversely affecting reading development in dyslexic children.

Alphabetism in reading science.

Alphabetism in reading science.

Tests of a model of multi-word reading: Effects of parafoveal flanking letters on foveal word recognition

We used the “flanking letters lexical decision” paradigm of Dare and Shillcock (2013) in order to test a model of multi-word reading. In the model, multiple words (on fixation, and to the left and right of fixation) are processed in parallel by a bank of location-specific letter detectors. These letter detectors feed information forward to a “bag of bigrams” that represents location-invariant sublexical orthographic information for all words processed in parallel. Bigrams are only formed within words (i.e., between spaces) but activate all compatible word representations. The model accounts for a finding reported by Dare and Shillcock (2013): Word recognition is facilitated when flanking letter pairs are present in the target (e.g. RO ROCK CK) compared with different letter flankers (ST ROCK EN), but independently of the position of the flanking bigrams (e.g., CK ROCK RO). In the present study we replicate this key finding and show that, as predicted by the model, although bigram position does not matter, within-bigram letter position does. Word recognition is harder when the position of letters within bigram flankers is reversed (e.g., OR ROCK KC/KC ROCK OR), but these conditions still facilitate with respect to a different letter flanker condition.

An ERP investigation of location invariance in masked repetition priming.

In an experiment combining masked repetition priming and the recording of event-related potentials (ERPs) the location of prime stimuli relative to centrally located target words was manipulated. Prime words could appear at the same location as targets or shifted one letter position to the right or to the left. Repetition priming effects (amplitude differences across the repeat vs. unrelated prime conditions) were found in a series of ERP components starting at around 100 msec posttarget onset. The earliest of these, the N/P150 component, was found to be sensitive to prime location. Repetition priming was only apparent with centrally located primes in this component. Repetition priming effects in later components (N250 and N400), on the other hand, were not affected by prime location. The results are interpreted in terms of location-specific letter detectors that map onto a higher level, location-invariant orthographic code for printed words.

Components of the location probability effect in visual search tasks.

In visual search tasks, targets are detected more rapidly when they appear in locations that commonly contain a target than when they appear in locations that rarely contain a target. Five experiments were conducted to investigate two specific properties of this location probability effect: its dependence on spatial location versus relative position and its dependence on or independence of target identity. In Experiment 1 spatial location of a stimulus row was varied to determine whether high location probability facilitates target detection in a particular location in visual space or a particular relative position within the row. Both were facilitated to approximately the same extent. In Experiment 2 an inducing target occurred with high probability in one of four display locations, and a test target occurred with equal probability in all four locations. Both targets were found more quickly in the high-probability location than in the other locations, but the advantage associated with targets in the high-probability location was larger for the inducing target than for the test target. In Experiments 3-5 the correspondence between the components observed in Experiments 1 and 2 was examined. The overall pattern of results was compatible with a model in which the location probability effect is produced partly by an attentional spotlight, which facilitates processing of any stimulus appearing in a particular location in visual space, and partly by a network of position-specific letter detectors, which facilitates detection of a particular letter in a particular relative position within a display. Models with flexible scanning strategies were also considered.