Letter Crowding Research Articles

The effect of inter-letter spacing on the N170 during visual word recognition: An event-related potentials experiment

Previous behavioral studies have shown that inter-letter spacing affects visual word recognition and reading. While condensed spacing may hinder the early stages of letter encoding because of increased crowding effects, the impact of expanded inter-letter spacing is still unclear. To examine the electrophysiological signature of inter-letter spacing on visual word recognition, we presented words in three different inter-letter spacing conditions (default, condensed [−1.5 points] or expanded [+1.5 points]) in an event-related potentials go/no-go semantic categorization task. Our focus was on the N170, an event-related potentials component associated with the early encoding of orthographic information, which also is sensitive to crowding effects. Results revealed that the N170 amplitude reached the largest values for the condensed condition than for the default and expanded spacing conditions, which did not differ. While increased crowding impacted the early encoding of orthographic information, extra letter spacing (compared with default spacing) did not. This outcome is consistent with the Modified Receptive Field hypothesis, in which letter receptors adapt their size to cope with letter crowding. These findings reveal that reducing the space between letters more than the default spacing impairs the ability to process written words, whereas slightly expanding the space between letters does not provide any additional benefit.

Offline transcranial direct current stimulation improves the ability to perceive crowded targets

The deleterious effect of nearby flankers on target identification in the periphery is known as visual crowding. Studying visual crowding can advance our understanding of the mechanisms of visual awareness and object recognition. Alleviating visual crowding is one of the major ways to improve peripheral vision. The aim of the current study was to examine whether transcranial direct current stimulation (tDCS) was capable of alleviating visual crowding at different visual eccentricities and with different visual tasks. In the present single-blind sham-controlled study, subjects were instructed to perform an orientation discrimination task or a letter identification task with isolated and crowded targets in the periphery, before and after applying 20 minutes of 2 mA anodal tDCS to visual cortex of the hemisphere contralateral or ipsilateral to visual stimuli. Contralateral tDCS significantly alleviated the orientation crowding effect at two different eccentricities and the letter crowding effect. This alleviation was absent after sham or ipsilateral stimulation and could not be fully explained by the performance improvement with the isolated targets. These findings demonstrated that offline tDCS was effective in alleviating visual crowding across different visual eccentricities and tasks, therefore providing a promising way to improve spatial vision rapidly in crowded scenes.

Offline transcranial direct current stimulation (tDCS) can improve the ability to perceive crowded targets

The deleterious influence of nearby flankers on target identification in the periphery is often referred to as visual crowding. Studying visual crowding can advance our understanding of the mechanisms of visual awareness and object recognition. Alleviating visual crowding (e.g., through perceptual learning) is one of the major ways to improve peripheral vision. Although there is a rapidly growing interest in using tDCS to modulate visual perception in humans, it remains unknown whether tDCS can alleviate visual crowding effects. We performed three experiments to investigate this issue. In Experiment 1, subjects were asked to perform an orientation discrimination task with the isolated and crowded targets in the periphery, before and after applying 20 minutes of 2 mA anodal tDCS to early visual cortex (P1 or P2) of the hemisphere contralateral or ipsilateral to the visual stimuli. We found that, electrical stimulation of the hemisphere contralateral to the visual stimuli could significantly reduce the crowding effect. This reduction was absent after the sham stimulation and could not be explained by the performance improvement with the isolated target. In Experiment 2, using the same behavioral task and the same tDCS protocol, we found that the contralateral DC stimulation remained effective in alleviating crowding at a smaller eccentricity. In Experiment 3, we adopted a letter recognition task and found that the alleviation of the letter crowding effect still existed after tDCS. In all the three experiments, no reduction was observed when tDCS was applied to the hemisphere ipsilateral to the visual stimuli. Taken together, we conclude that offline tDCS is effective in alleviating visual crowding across different visual eccentricities and perceptual tasks, which sheds new light on the mechanisms of visual crowding and possible practical applications.

Spatio-temporal properties of letter crowding.

Crowding between adjacent letters has been investigated primarily as a spatial effect. The purpose of this study was to investigate the spatio-temporal properties of letter crowding. Specifically, we examined the systematic changes in the degradation effects in letter identification performance when adjacent letters were presented with a temporal asynchrony, as a function of letter separation and between the fovea and the periphery. We measured proportion-correct performance for identifying the middle target letter in strings of three lowercase letters at the fovea and 10° in the inferior visual field, for a range of center-to-center letter separations and a range of stimulus onset asynchronies (SOA) between the target and flanking letters (positive SOAs: target preceded flankers). As expected, the accuracy for identifying the target letters reduces with decreases in letter separation. This crowding effect shows a strong dependency on SOAs, such that crowding is maximal between 0 and ∼100 ms (depending on conditions) and diminishes for larger SOAs (positive or negative). Maximal crowding does not require the target and flanking letters to physically coexist for the entire presentation duration. Most importantly, crowding can be minimized even for closely spaced letters if there is a large temporal asynchrony between the target and flankers. The reliance of letter identification performance on SOAs and how it changes with letter separations imply that the crowding effect can be traded between space and time. Our findings are consistent with the notion that crowding should be considered as a spatio-temporal, and not simply a spatial, effect.

Can substitution explain crowding? A study of error distribution in letter crowding

Can substitution explain crowding? A study of error distribution in letter crowding

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 motion on crowding: zooming text.

Crowding is the major sensory factor responsible for the slow reading speeds exhibited in peripheral vision. Past attempts to improve peripheral reading via crowding reduction have generally focused on applying spatial changes to the stimulus and have been largely ineffective. Recent evidence indicates that dynamic approaches have good potential for reducing crowding in peripheral reading. We tested this hypothesis by introducing "zooming" motion (smooth letter resizing across the presentation duration) to trigram stimuli (groups of three randomly selected letters) presented at 10° in the lower visual field and evaluating recognition of the middle letter. Crowding was alleviated in the presence of this motion, both when dynamic cues were introduced to all letters in the trigram simultaneously and when they were applied to individual letters alone. The magnitude and direction of crowding reduction depended on the amplitude and direction of motion. These results suggest that dynamic presentation may be a useful tool for improving peripheral reading through reducing letter crowding. Zooming motion, in particular, has the additional advantage of conserving text layout, making it a good candidate for such an application.

Foveal letter crowding: Is it due to contour interaction or gaze instability?

Foveal letter crowding: Is it due to contour interaction or gaze instability?

A crowdful of letters: Disentangling the role of similarity, eccentricity and spatial frequencies in letter crowding

The present study investigated the joint impact of target-flanker similarity and of spatial frequency content on the crowding effect in letter identification. We presented spatial frequency filtered letters to neurologically intact non-dyslexic readers while manipulating target-flanker distance, target eccentricity and target-flanker confusability (letter similarity metric based on published letter confusion matrices). The results show that high target-flanker confusability magnifies crowding. They also reveal an intricate pattern of interactions of the spatial frequency content of the stimuli with target eccentricity, flanker distance and similarity. The findings are congruent with the notion that crowding results from the inappropriate pooling of target and flanker features and that this integration is more likely to match a response template at a subsequent decision stage with similar than dissimilar flankers. In addition, the evidence suggests that crowding from similar flankers is biased towards relatively high spatial frequencies and that crowding shifts towards lower spatial frequencies as target eccentricity is increased.

Evidence for misplaced target information with letter crowding

Evidence for misplaced target information with letter crowding

Neural correlates of letter crowding in the periphery

Neural correlates of letter crowding in the periphery

The nature of letter crowding as revealed by first- and second-order classification images

Visual crowding refers to the marked inability to identify an otherwise perfectly identifiable object when it is flanked by other objects. Crowding places a significant limit on form vision in the visual periphery; its mechanism is, however, unknown. Building on the method of signal-clamped classification images (Tjan & Nandy, 2006), we developed a series of first- and second-order classification-image techniques to investigate the nature of crowding without presupposing any model of crowding. Using an "o" versus "x" letter-identification task, we found that (1) crowding significantly reduced the contrast of first-order classification images, although it did not alter the shape of the classification images; (2) response errors during crowding were strongly correlated with the spatial structures of the flankers that resembled those of the erroneously perceived targets; (3) crowding had no systematic effect on intrinsic spatial uncertainty of an observer nor did it suppress feature detection; and (4) analysis of the second-order classification images revealed that crowding reduced the amount of valid features used by the visual system and, at the same time, increased the amount of invalid features used. Our findings strongly support the feature-mislocalization or source-confusion hypothesis as one of the proximal contributors of crowding. Our data also agree with the inappropriate feature-integration account with the requirement that feature integration be a competitive process. However, the feature-masking account and a front-end version of the spatial attention account of crowding are not supported by our data.

Isolating the impact of visual perception on dyslexics’ reading ability

A large body of data suggests that phonological deficits play an important causal role in dyslexics’ reading difficulties. The functional role of visual impairments is still highly debated. Many recent studies have shown clear visual deficits in large subgroups of dyslexics. However, the relationship between these deficits and visual routines required for reading is not clear. To assess the direct contribution of visual factors to dyslexics’ slower and less accurate reading, we composed a task that was similar to single word reading in its basic visual characteristics, but had none of the other (phonological, morphological, semantic, etc.) aspects of reading. Young adult dyslexics, with average or above general cognitive abilities, and controls matched for age and cognitive skills participated in the study. We measured both SOA and contrast thresholds for identifying unfamiliar letters. Letters were chosen from an alphabet graphically similar to Hebrew and English (a subset of Georgian letters), but unfamiliar to the subjects. Effects of decreasing letter size, increasing letter crowding (by adding a flanker letter on each side) and adding white noise, were measured. Dyslexics performed as well as controls under all test conditions, and had similar effect sizes. We thus conclude that, despite the data showing that dyslexics have marked difficulties with single word reading, the cause of these difficulties is not a visual processing deficit.

What crowds a letter in the periphery?

In the periphery, nearby letters (flankers) can impair the identification of a target letter, but white-noise patches or gratings do not (Palomares et al., ARVO99), suggesting that letter crowding is unlike contrast masking (Chung, Levi, Legge, 2001). The aim of this study was to systematically test different types of flankers that may induce letter crowding in the periphery. Subjects identified a target letter presented at 5 deg inferior to fixation. Letters were in Times New Roman, of size 0.4 log units above the subject's acuity. Six conditions were ested: target letter alone, flanked by same-polarity letter flankers, opposite-polarity letter flankers, letter- noise patches, or white-noise patches at 1 x-height spacing, or flanked by same-polarity letter flankers at 2 x-height spacing. The letternoise patch was created by scrambling the phase spectrum of a letter but retaining its power spectral density (PSD). Moreover, both types of noise had the same contrast energy and bounding box as the letter they replaced. Flankers were at 20% contrast. Threshold contrast of the target letter was determined at 79% correct. Average threshold contrast (across 3 Ss) for the target-alone condition was 12.8% (+/-0.04 log units). No significant threshold elevation was observed for white-noise patches or letter flankers at 2x spacing. All other conditions led to significant crowding (avg. threshold elevation in log units - letter: 0.35, letter noise: 0.26, reverse contrast: 0.21). Threshold elevation between letter and letter noise flankers was not significant for 2 out of the 3 Ss. Letter-noise patches caused significant crowding, while white-noise patches did not. Spatial frequency distribution thus seems to play a major role in letter crowding. Phase, however, which defines the visual form of the flankers, had only a limited role. We speculate that a main component of letter crowding may be noise masking, with noise being induced by and having a similar PSD to the flankers.

Letter crowding in the periphery is best modeled by an increase in additive equivalent noise

Letter crowding in the periphery is best modeled by an increase in additive equivalent noise