Interletter Spacing Research Articles

The effects of interletter spacing in visual-word recognition

Despite the importance of determining the effects of interletter spacing on visual-word recognition, this issue has often been neglected in the literature. The goal of the present study is to shed some light on this topic. The rationale is that a thin increase in interletter spacing, as in c a s i n o, may reduce lateral interference among internal letters without destroying a word's integrity and/or allow a more precise encoding of a word's letter positions. Here we examined whether identification times for word stimuli in a lexical decision task were faster when the target word had a slightly wider than default interletter spacing value relative to the default settings (e.g., c a s i n o vs. casino). In Experiment 1, we examined whether interletter spacing interacted with word-frequency, whereas in Experiment 2, we examined whether interletter spacing interacted with word length. Results showed that responses to words using a thin increase in interletter spacing were faster than the responses to words using the default settings—regardless of word-frequency and word length. Thus, interletter spacing plays an important role at modulating the identification of visually presented words.

Basic processes in reading: The effect of interletter spacing.

Reading is acutely sensitive to the amount of space between letters within a string. In the present investigation, we explore the impairment caused by increasing interletter spacing when reading single words and nonwords aloud. Specifically, 2 hypotheses are tested: (a) whether increasing interletter spacing induces serial processing while reading aloud and (b) whether this serial processing results from an increased reliance on a serial sublexical mechanism similar to that implemented in dual route models of reading. Implications of the present results for understanding basic processes in reading are discussed with particular reference to different types of serial processing in reading aloud and the role of attention in reading.

Spatial attention modulates feature crosstalk in visual word processing

One major idea about spatial attention is that it serves to modulate crosstalk between features during reading. Two reading aloud experiments are reported in which a cue-validity manipulation was combined with manipulations that are thought to increase the likelihood of feature-level crosstalk: interletter spacing and the presence or absence of irrelevant features. Both experiments yielded an interaction between the effects of spatial cuing and each of these factors. These results are taken to support the hypothesis that when spatial attention is focused on the word, it provides protection against crosstalk among features in the context of reading aloud.

Effects of ocular transverse chromatic aberration on near foveal letter recognition

Transverse chromatic aberration (TCA) smears retinal images of peripheral stimuli. In reading, text information is extracted from both foveal and near fovea, where TCA magnitude is relatively small and variable. The present study investigated whether TCA significantly affects near foveal letter identification. Subjects were briefly presented a string of five letters centered one degree of visual angle to the left or right of fixation. They indicated whether the middle letter was the same as a comparison letter subsequently presented. Letter strings were rendered with a reddish fringe on the left edge of each letter and a bluish fringe on the right edge, consistent with expected left periphery TCA, or with the opposite fringe consistent with expected right periphery TCA. Effect of the color fringing on letter recognition was measured by comparing the response accuracy for fringed and non-fringed stimuli. Effects of lateral interference were examined by manipulating inter-letter spacing and similarity of neighboring letters. Results demonstrated significantly improved response accuracy with the color fringe opposite to the expected TCA, but decreased accuracy when consistent with it. Narrower letter spacing exacerbated the effect of the color fringe, whereas letter similarity did not. Our results suggest that TCA significantly reduces the ability to recognize letters in the near fovea by impeding recognition of individual letters and by enhancing lateral interference between letters.

The relationship between visual crowding and letter confusability: Towards an understanding of dyslexia in posterior cortical atrophy

Visual crowding is a form of masking in which target identification is hindered by excessive feature integration from other stimuli in the vicinity. It has previously been suggested that excessive visual crowding constitutes one specific form of early-visual-processing deficit, which may be observed in individuals with posterior cortical atrophy (PCA). This study investigated whether excessive visual crowding plays a significant role in the acquired dyslexia of two PCA patients, whose reading was characterized by visual paralexias. The patients were administered a series of letter, flanked letter, and word recognition tasks, and the effects of letter spacing and letter confusability upon response accuracy and latency were measured. In both patients, the results showed (a) evidence of excessive visual crowding, (b) a significant interaction between letter spacing and confusability on flanked letter identification tasks, and (c) effects of letter confusability affecting flanked but not unflanked letter identification. However, only mild improvements in reading accuracy were achieved in the experimental manipulations of interletter spacing within words because these manipulations had a dual effect: Increasing spacing improved individual letter identification but damaged whole-word form and/or parallel letter processing. We consider the implications of these results for the characterization of dyslexia in PCA, the design of reading rehabilitation strategies, and the relationship between visual crowding and letter confusability. In particular, we argue that the reading deficits observed in our patients cannot be accounted for solely in terms of a very low signal-to-noise ratio for letter identification, and that an additional crowding deficit is implicated in which excessive integration of fundamental letter features leads to the formation of incorrect letter percepts.

How odgcrnwi becomes crowding: Stimulus-specific learning reduces crowding

Processes underlying crowding in visual letter recognition were examined by investigating effects of training. Experiment 1 revealed that training reduces crowding mainly for trained strings. This was corroborated in Experiment 2, where no training effects were obvious after 3 days of training when strings changed from trial to trial. Experiment 3 specified that after a short amount of training, learning effects remained specific to trained strings and also to the trained retinal eccentricity and the interletter spacing used in training. Transfer to other than trained conditions was observed only after further training. Experiment 4 showed that transfer occurred earlier when words were used as stimuli. These results thus demonstrate that part of crowding results from the absence of higher level representations of the stimulus. Such representations can be acquired through learning visual properties of the stimulus.

Serifs and font legibility

Using lower-case fonts varying only in serif size (0%, 5%, and 10% cap height), we assessed legibility using size thresholds and reading speed. Five percentage serif fonts were slightly more legible than sans serif, but the average inter-letter spacing increase that serifs themselves impose, predicts greater enhancement than we observed. RSVP and continuous reading speeds showed no effect of serifs. When text is small or distant, serifs may, then, produce a tiny legibility increase due to the concomitant increase in spacing. However, our data exhibited no difference in legibility between typefaces that differ only in the presence or absence of serifs.

The relative benefit of word context is a constant proportion of letter identification time

Letter identification is reduced when the target letter is surrounded by other, flanking letters. This visual crowding is known to be impacted by physical changes to the target and flanks, such as spatial frequency content, polarity, and interletter spacing. There is also evidence that visual crowding is reduced when the flanking letters and the target letter form a word. The research reported here investigated whether these two phenomena are independent of each other or whether the degree of visual crowding impacts the benefit of word context. Stimulus duration thresholds for letters presented alone and for the middle letters of 3-letter words and nonwords were determined for stimuli presented at the fovea and at the periphery. In Experiment 1, the benefit of word context was found to be the same at the fovea, where visual crowding is minimal, and at the periphery, where visual crowding is substantial. In Experiment 2, visual crowding was manipulated by changing the interletter spacing. Here, too, the benefit of word context was fairly constant for the two retinal locations (fovea or periphery), as well as with changes in interletter spacing. These data call into question both the idea that the benefit of word context is greater when stimulus quality is reduced (as is the case with visual crowding) and the idea that words are processed more effectively when they are presented at the fovea.

Crowding Effects on Word Identification in Developmental Dyslexia

The effect of crowding on the identification of words was examined in normal readers and subjects with developmental dyslexia. In Experiment 1, a matching task was used. Words were presented either alone or embedded in other words. Vocal reaction times (RT) of dyslexics were slower and more sensitive to the presence of the surrounding stimuli than those of control subjects. Similar results were obtained in a control experiment using the same task for strings of symbols (isolated or crowded) instead of words. These data indicate that differences in crowding in control and dyslexic subjects arise at a pre-linguistic level. In Experiment 2, vocal RTs to word reading were measured. Two conditions putatively reducing the effect of crowding were tested: increasing inter-letter spacing and blurring. A moderate increase of inter-letter spacing produced faster vocal RTs in dyslexics, while no effect was present in normal controls. Moderate blurring of stimuli did not change dyslexics' RTs, while normal readers became slower. Group and individual results are discussed to evaluate the extent to which crowding contributes to the genesis of developmental dyslexia.

Apparent string shortening concomitant with letter crowding

In our previous studies of the crowding effect, we have observed that human observers tend to underestimate the length of a letter string (the number of letters in the string) when the letters are close to visual acuity, and the interletter spacings are small. In this study, we asked our observers to identify letters in randomly presented four-letter and five-letter strings. We found that, when a priori knowledge of the lengths of letter strings was not available, the probability of underestimating string length increased with decreasing interletter spacing. The causes of underestimation errors appeared to be the omission of an interior letter and the merging of two neighboring letters. Since our experiments were conducted in the foveal region, neither spatial uncertainty nor split attention can explain the underestimation errors. The effect of the point spread function of the eye on closely packed letter strings is discussed.

Design of an Arabic near visual acuity chart

This paper describes the design of an Arabic test chart for measurement of visual acuity at near. The chart was designed employing specially selected Arabic letters and was based on the logMAR principle devised by Bailey and Lovie. Ten Arabic letters of nearly equal legibility values (0.92–1.05) (mean = 1.00), (SD = 0.05) were used in the design of the chart. Each row of the chart has 5 letters and row legibility values range from 4.82 to 5.03 with a mean of 4.92 (SD = 0.06). The logMAR method of visual acuity scaling was used, hence the sizes of letters in the rows progress in a uniform step of 0.1 log unit. The inter-letter spacing is equal to the width of each letter in the row, while inter-row spacing is equal to the height of letter in the subjacent row. The height of letters ranged from 3.67 to 0.46 mm corresponding to visual acuity of 2.4M to 0.3 M which is equivalent to reduced Snellen 636 to 64.5 at 0.4m. The chart is designed for use at 40cm with a recommended luminance level of 160cd/m2.

Design of an Arabic near visual acuity chart

This paper describes the design of an Arabic test chart for measurement of visual acuity at near. The chart was designed employing specially selected Arabic letters and was based on the logMAR principle devised by Bailey and Lovie. Ten Arabic letters of nearly equal legibility values (0.92-1.05) (mean = 1.00), (SD = 0.05) were used in the design of the chart. Each row of the chart has 5 letters and row legibility values range from 4.82 to 5.03 with a mean of 4.92 (SD = 0.06). The logMAR method of visual acuity scaling was used, hence the sizes of letters in the rows progress in a uniform step of 0.1 log unit. The inter-letter spacing is equal to the width of each letter in the row, while inter-row spacing is equal to the height of letter in the subjacent row. The height of letters ranged from 3.67 to 0.46 mm corresponding to visual acuity of 2.4 M to 0.3 M which is equivalent to reduced Snellen 6/36 to 6/4.5 at 0.4 m. The chart is designed for use at 40 cm with a recommended luminance level of 160 cd/m2.

Contrasting Effects of Letter-spacing in Alexia: Further Evidence that Different Strategies Generate Word Length Effects in Reading

The reading behaviour of two alexic patients (SA and WH) is reported. Both patients are severely impaired at reading single words, and both show abnormally strong effects of word length when reading. These two symptoms are characteristic of letter-by-letter reading. Experiment 1 examined the pattern of errors when the patients read large and small words. Further experiments examined the effects of inter-letter spacing on word naming (Experiments 2a and 2b) and the identification of letters in letter strings (Experiment 3). For both patients, letter identification was better for widely spaced letters in letter strings, and this effect was most pronounced for the central letters in the strings. This is consistent with abnormally strong flanker interference in letter identification. However, inter-letter spacing affected word reading behaviour in the two patients in different ways. SA's word reading improved with widely spaced letters; WH's word reading was disrupted. This suggests that these patients adopted different strategies when reading words. We conclude that several reading behaviours can elicit word length effects, and that these different behaviours can reflect strategic adaptation to a common functional deficit in patients. We discuss the implications both for understanding alexia and for models of normal word identification.

Spatial and/or temporal adjustments of scanning behavior to visibility changes

This paper continues a series of studies focussing on elementary processes controlling eye movements in visual search and reading. It investigates the question of how scanning behavior in a letter search task adjusts to changes in spatial visibility created by different visual factors, such as viewing distance, inter-letter spacing and target-background similarity. Each of the three visual factors is found to affect scanning behavior in a different way, either by changing its spatial parameter (saccade size) or its temporal (fixation duration), or both of them at the same time. Possible mechanisms underlying these effects are discussed.

Eye-movement control in visual search: how direct is visual span control?

In order to distinguish between the effects of low-level sensory mechanisms and those of higher level factors on eye-movement control processes, a simple letter search task was used in which cognitive load was reduced to the very minimum. The special purpose of this study was to test the visual span control hypothesis, which considers that, in such a task, eye movements are controlled as a direct function of spatial visibility limits (O’Regan, Levy-Schoen, & Jacobs, 1983). In a first psychophysical experiment, three methods were used to manipulate the spatial visibility limits (visual span), as measured by a psychophysical procedure: changing viewing distance, interletter spacing, and target-background similarity. The results of this experiment then were used as a reference for predicting mean saccade sizes and fixation durations in a visual search task in which the same visibility changes were made. About 80% of the variance of mean saccade sizes could be accounted for by adjustment of saccades to changes in visual span, so the visual span control hypothesis was confirmed. As to the temporal characteristics of scanning behavior, less than 50% of fixation duration variance seemed to be determined by visual span changes. Other, higher level factors, possibly related to decisional processes intervening in the triggering of saccades and the computation of their spatial parameters, might play an important role in determining fixation durations in a simple search task. The results are interpreted in relation to recent models of eye-movement control by two largely independent subsystems functioning in parallel.

Further evidence for priming in perceptual matching: temporal, not spatial, separation enhances the fast-same effect.

Sequential (vs. simultaneous) presentation of two letters in a physical-identity matching task enhanced the fast-same effect, but did not reduce the preponderance of false-“different” errors or the effect of visual similarity. Thus, the sequential enhancement of the fast-same effect involves an increased efficiency in encoding (d’) owing to letter repetition, as Proctor claimed, rather than a criterion shift (β), and it involves the visual code rather than the name code. The increased efficiency in detecting sameness with sequential presentation might result from spatial separation (e.g., reduced lateral interference and self-termination), though, rather than temporal separation (e.g., priming), However, such spatial factors as letter size and interletter spacing had no consistent effect on the speed advantage forsame pairs, and it was concluded that temporal, not spatial, separation enhances the fast-same effect on sequential trials, Consistent with the response-competition model (Eriksen, O’Hara, & Eriksen), responses were slower and more errorful to similar than to dissimilardifferent pairs, and were equally fast to dissimilar andsame pairs on simultaneous trials.

Perceived size of letters depends on inter-letter spacing: A new visual illusion

Perceived size of letters depends on inter-letter spacing: A new visual illusion