Feature contingencies when reading letter strings

Parafoveal letter identification in Russian: Confusion matrices based on error rates

In the present study, we introduce parafoveal letter confusion matrices for the Russian language, which uses the Cyrillic script. To ensure that our confusion rates reflect parafoveal processing and no other effects, we employed an adapted boundary paradigm (Rayner, 1975) that prevented the participants from directly fixating the letter stimuli. Additionally, we assessed confusability under isolated and word-like (crowded) conditions using two modern fonts, since previous research showed that letter recognition depended on crowding and font (Coates, 2015; Pelli et al., 2006). Our additional goal was to gain insight into what letter features or configurational patterns might be essential for letter recognition in Russian; thus, we conducted exploratory clustering analysis on visual confusion scores to identify groups of similar letters. To support this analysis, we conducted a comprehensive review of over 20 studies that proposed crucial properties of Latin letters relevant to character perception. The summary of this review is valuable not only for our current study but also for future research in the field.

The effect of serifs and stroke contrast on low vision reading

PURPOSE

Patients with low vision are generally recommended to use the same fonts as individuals with normal vision. However, we are yet to fully understand whether stroke width and serifs (small ornamentations at stroke endings) can increase readability. This study's purpose was to characterize the interaction between two factors (end-of-stroke and stroke width) in a well-defined and homogenous group of patients with low vision.

METHODS

Font legibility was assessed by measuring word identification performance of 19 patients with low vision (autosomal dominant optic atrophy [ADOA] with a best-corrected average visual acuity 20/110) and a two-interval, forced-choice task was implemented. Word stimuli were presented with four different fonts designed to isolate the stylistic features of serif and stroke width.

RESULTS

Font-size threshold and sensitivity data revealed that using a single measure (i.e., font-size threshold) is insufficient for detecting significant effects but triangulation is possible when combined with signal detection theory. Specifically, low stroke contrast (smaller variation in stroke width) yielded significantly lower thresholds and higher sensitivity when a font contained serifs (331 points; d' = 1.47) relative to no serifs (345 points; d' = 1.15), E(μ_{sans, low} - μ_{serif, low}) = -14 points, 95 % Cr. I. = [-24, -5], P(δ > 0) = 0.99 and E(μ_{serif, low} - μ_{sans, low}) = 0.32, 95 % Cr. I. = [0.16, 0.49], P(δ > 0) = 0.99.

CONCLUSION

In people with low visual acuity caused by ADOA, the combination of serifs and a uniform stroke width resulted in better text legibility than other combinations of uniform/variable stroke widths and presence/absence of serifs.

Multidimensional feature interactions in visual crowding: When configural cues eliminate the polarity advantage

Crowding occurs when surrounding objects (flankers) impair target perception. A key property of crowding is the weaker interference when target and flankers strongly differ on a given dimension. For instance, identification of a target letter is usually superior with flankers of opposite versus the same contrast polarity as the target (the "polarity advantage"). High performance when target-flanker similarity is low has been attributed to the ungrouping of target and flankers. Here, we show that configural cues can override the usual advantage of low target-flanker similarity, and strong target-flanker grouping can reduce - instead of exacerbate - crowding. In Experiment 1, observers were presented with line triplets in the periphery and reported the tilt (left or right) of the central line. Target and flankers had the same (uniform condition) or opposite contrast polarity (alternating condition). Flanker configurations were either upright (||), unidirectionally tilted (\\ or //), or bidirectionally tilted (\/ or /\). Upright flankers yielded stronger crowding than unidirectional flankers, and weaker crowding than bidirectional flankers. Importantly, our results revealed a clear interaction between contrast polarity and flanker configuration. Triplets with upright and bidirectional flankers, but not unidirectional flankers, showed the polarity advantage. In Experiments 2 and 3, we showed that emergent features and redundancy masking (i.e. the reduction of the number of perceived items in repeating configurations) made it easier to discriminate between uniform triplets when flanker tilts were unidirectional (but not when bidirectional). We propose that the spatial configurations of uniform triplets with unidirectional flankers provided sufficient task-relevant information to enable a similar performance as with alternating triplets: strong-target flanker grouping alleviated crowding. We suggest that features which modulate crowding strength can interact non-additively, limiting the validity of typical crowding rules to contexts where only single, independent dimensions determine the effects of target-flanker similarity.

Fonts of wider letter shapes improve letter recognition in parafovea and periphery

Most text on modern electronic displays is set in fonts of regular letter width. Little is known about whether this is the optimal font width for letter recognition. We tested three variants of the font family Helvetica Neue (Condensed, Standard, and Extended). We ran two separate experiments at different distances and different retinal locations. In Experiment 1, the stimuli were presented in the parafovea at 2° eccentricity; in Experiment 2, the stimuli were presented in the periphery at 9° eccentricity. In both experiments, we employed a short-exposure single-report trigram paradigm in which a string of three letters was presented left or right off-centre. Participants were instructed to report the middle letter while maintaining fixation on the fixation cross. Wider fonts resulted in better recognition and fewer misreadings for neighbouring letters than narrower fonts, which demonstrated that wider letter shapes improve recognition at glance reading in the peripheral visual view. Practitioner summary: Most of the text is set in fonts of regular letter width. In two single-target trigram letter recognition experiments, we showed that wider letter shapes facilitate better recognition than narrower letter shapes. This indicates that when letter identification is a priority, it is beneficial to choose fonts of wider letter shapes.

Closed letter counters impair recognition

An often-repeated piece of advice when choosing fonts for great legibility is to use fonts with large counters and apertures. To identify effects of open and closed apertures on the letters 'a', 'c', 'e', 'r', 's', 't' and 'f', we ran an experiment using the serif font Pyke as stimulus. The letters in focus were designed for this experiment with three variations of open apertures (Open, Medium and Closed). The experimental paradigm was to present a letter either with or without flankers in the parafovea at 2° eccentricity. The findings showed that participants had more trouble identifying the letter if it was set in a font variation with closed apertures.

Atypical visual field asymmetries in redundancy masking

Redundancy masking is the reduction of the perceived number of items in repeating patterns. It shares a number of characteristics with crowding, the impairment of target identification in visual clutter. Crowding strongly depends on the location of the target in the visual field. For example, it is stronger in the upper compared to the lower visual field and is usually weakest on the horizontal meridian. This pattern of visual field asymmetries is common in spatial vision, as revealed by tasks measuring, for example, spatial resolution and contrast sensitivity. Here, to characterize redundancy masking and reveal its similarities to and differences from other spatial tasks, we investigated whether redundancy masking shows the same typical visual field asymmetries. Observers were presented with three to six radially arranged lines at 10° eccentricity at one of eight locations around fixation and were asked to report the number of lines. We found asymmetries that differed pronouncedly from those found in crowding. Redundancy masking did not differ between upper and lower visual fields. Importantly, redundancy masking was stronger on the horizontal meridian than on the vertical meridian, the opposite of what is usually found in crowding. These results show that redundancy masking diverges from crowding in regard to visual field asymmetries, suggesting different underlying mechanisms of redundancy masking and crowding. We suggest that the observed atypical visual field asymmetries in redundancy masking are due to the superior extraction of regularity and a more pronounced compression of visual space on the horizontal compared to the vertical meridian.

Spatial Attention Enhances Crowded Stimulus Encoding Across Modeled Receptive Fields by Increasing Redundancy of Feature Representations

Any visual system, biological or artificial, must make a trade-off between the number of units used to represent the visual environment and the spatial resolution of the sampling array. Humans and some other animals are able to allocate attention to spatial locations to reconfigure the sampling array of receptive fields (RFs), thereby enhancing the spatial resolution of representations without changing the overall number of sampling units. Here, we examine how representations of visual features in a fully convolutional neural network interact and interfere with each other in an eccentricity-dependent RF pooling array and how these interactions are influenced by dynamic changes in spatial resolution across the array. We study these feature interactions within the framework of visual crowding, a well-characterized perceptual phenomenon in which target objects in the visual periphery that are easily identified in isolation are much more difficult to identify when flanked by similar nearby objects. By separately simulating effects of spatial attention on RF size and on the density of the pooling array, we demonstrate that the increase in RF density due to attention is more beneficial than changes in RF size for enhancing target classification for crowded stimuli. Furthermore, by varying target/flanker spacing, as well as the spatial extent of attention, we find that feature redundancy across RFs has more influence on target classification than the fidelity of the feature representations themselves. Based on these findings, we propose a candidate mechanism by which spatial attention relieves visual crowding through enhanced feature redundancy that is mostly due to increased RF density.

High letter stroke contrast impairs letter recognition of bold fonts

To make graphical user interfaces look more fashionable, designers often make use of high-stroke-contrast fonts. We are yet to understand how these fonts affect reading. We examined the effect of letter-stroke contrast on three bold fonts, one with extreme contrast between thick and thin strokes, one with no contrast, and one in between. The fonts were designed for this experiment to enable control of font variables. Participants identified the middle letter in a lowercase letter trigram in each trial, briefly presented in the parafovea (at 2° left and right of fixation) and at the foveal fixation point. There was evidence for letter recognition impairment for the font with high stroke contrast compared to the fonts with low and medium stroke contrast, while there was no significant difference in performance between the medium- and low-stroke-contrast fonts. The results suggest that bold fonts with high stroke contrast should not be considered for designs where letter recognition is a priority.

The generality of the critical spacing for crowded optotypes: From Bouma to the 21st century

It is rare to find a crowding manuscript that fails to mention "Bouma's law," the rule of thumb stating that flankers within a distance of about one half of the target eccentricity will induce crowding. Here we investigate the generality of this rule (even for just optotypes), the factors that modulate the critical spacing, and the evidence for the rule in Bouma's own data. We explore these questions by reanalyzing a variety of studies from the literature, running several new control experiments, and by utilizing a model that unifies flanked identification measurements between psychophysical paradigms. Specifically, with minimal assumptions (equivalent psychometric slopes across conditions, for example), crowded acuity can be predicted for arbitrary target sizes and flanker spacings, revealing a performance "landscape" that delineates the critical spacing. Last, we present a compact quantitative summary of the effects of different types of stimulus manipulations on optotype crowding.

Geometrically restricted image descriptors: A method to capture the appearance of shape

Shape perception varies depending on many factors. For example, presenting a stimulus in the periphery often yields a different appearance compared with its foveal presentation. However, how exactly shape appearance is altered under different conditions remains elusive. One reason for this is that studies typically measure identification performance, leaving details about target appearance unknown. The lack of appearance-based methods and general challenges to quantify appearance complicate the investigation of shape appearance. Here, we introduce Geometrically Restricted Image Descriptors (GRIDs), a method to investigate the appearance of shapes. Stimuli in the GRID paradigm are shapes consisting of distinct line elements placed on a grid by connecting grid nodes. Each line is treated as a discrete target. Observers are asked to capture target appearance by placing lines on a freely viewed response grid. We used GRIDs to investigate the appearance of letters and letter-like shapes. Targets were presented at 10° eccentricity in the right visual field. Gaze-contingent stimulus presentation was used to prevent eye movements to the target. The data were analyzed by quantifying the differences between targets and response in regard to overall accuracy, element discriminability, and several distinct error types. Our results show how shape appearance can be captured by GRIDs, and how a fine-grained analysis of stimulus parts provides quantifications of appearance typically not available in standard measures of performance. We propose that GRIDs are an effective tool to investigate the appearance of shapes.

Hidden by bias: how standard psychophysical procedures conceal crucial aspects of peripheral visual appearance

The perception of a target depends on other stimuli surrounding it in time and space. This contextual modulation is ubiquitous in visual perception, and is usually quantified by measuring performance on sets of highly similar stimuli. Implicit or explicit comparisons among the stimuli may, however, inadvertently bias responses and conceal strong variability of target appearance. Here, we investigated the influence of contextual stimuli on the perception of a repeating pattern (a line triplet), presented in the visual periphery. In the neutral condition, the triplet was presented a single time to capture its minimally biased perception. In the similar and dissimilar conditions, it was presented within stimulus sets composed of lines similar to the triplet, and distinct shapes, respectively. The majority of observers reported perceiving a line pair in the neutral and dissimilar conditions, revealing 'redundancy masking', the reduction of the perceived number of repeating items. In the similar condition, by contrast, the number of lines was overestimated. Our results show that the similar context did not reveal redundancy masking which was only observed in the neutral and dissimilar context. We suggest that the influence of contextual stimuli has inadvertently concealed this crucial aspect of peripheral appearance.

Redundancy masking: The loss of repeated items in crowded peripheral vision

Crowding is the deterioration of target identification in the presence of neighboring objects. Recent studies using appearance-based methods showed that the perceived number of target elements is often diminished in crowding. Here we introduce a related type of diminishment in repeating patterns (sets of parallel lines), which we term “redundancy masking.” In four experiments, observers were presented with arrays of small numbers of lines centered at 10° eccentricity. The task was to indicate the number of lines. In Experiment 1, spatial characteristics of redundancy masking were examined by varying the inter-line spacing. We found that redundancy masking decreased with increasing inter-line spacing and ceased at spacings of approximately 0.25 times the eccentricity. In Experiment 2, we assessed whether the strength of redundancy masking differed between radial and tangential arrangements of elements as it does in crowding. Redundancy masking was strong with radially arranged lines (horizontally arranged vertical lines), and absent with tangentially arranged lines (vertically arranged horizontal lines). In Experiment 3, we investigated whether target size (line width and length) modulated redundancy masking. There was an effect of width: Thinner lines yielded stronger redundancy masking. We did not find any differences between the tested line lengths. In Experiment 4, we varied the regularity of the line arrays by vertically or horizontally jittering the positions of the lines. Redundancy masking was strongest with regular spacings and weakened with decreasing regularity. Our experiments show under which conditions whole items are lost in crowded displays, and how this redundancy masking resembles—and partly diverges from—crowded identification. We suggest that redundancy masking is a contributor to the deterioration of performance in crowded displays with redundant patterns.