Visual Crowding

Crowding refers to the inability to recognize objects in clutter, setting a fundamental limit on various perceptual tasks such as reading and facial recognition. While prevailing models suggest that crowding is a unitary phenomenon occurring at an early level of processing, recent studies have shown that crowding might also occur at higher levels of representation. Here we investigated whether local and global crowding interference co-occurs within the same display. To do so, we tested the distinctive contribution of local flanker features and global configurations of the flankers on the pattern of crowding errors. Observers (n = 27) estimated the orientation of a target when presented alone or surrounded by flankers. Flankers were grouped into a global configuration, forming
an illusory rectangle when aligned or a rectangular configuration when misaligned. We analyzed the error distributions by fitting probabilistic mixture models. Results showed that participants often misreported the orientation of a flanker instead of that of the target. Interestingly, in some
trials the orientation of the global configuration was misreported. These results suggest that crowding occurs simultaneously across multiple levels of visual processing and crucially depends on the spatial
configuration of the stimulus. Our results pose a challenge to models of crowding with an early single pooling stage and might be better explained by models which incorporate the possibility of multilevel crowding and account for complex target-flanker interactions.

Crowding is the inability to identify an object among
flankers in the periphery. It is due to inappropriate
incorporation of features from flanking objects in
perception of the target. Crowding is characterized by
measuring critical spacing, the minimum distance needed
between a target and flankers to allow recognition. The
existing Bouma law states that, at a given point and
direction in the visual field, critical spacing, measured
from the center of a target object to the center of a
similar flanking object, is the same for all objects (Pelli &
Tillman, 2008). Because flipping an object about its center
preserves its center-to-center spacing to other objects,
according to the Bouma law, crowding should be
unaffected. However, because crowding is a result of
feature combination, the location of features within an
object might matter. In a series of experiments, we find
that critical spacing is affected by the location of features
within the flanker. For some flankers, a flip greatly
reduces crowding even though it maintains target–flanker
spacing and similarity. Our results suggest that the
existing Bouma law applies to simple one-part objects,
such as a single roman letter or a Gabor patch. Many
objects consist of multiple parts; for example, a word is
composed of multiple letters that crowd each other. To
cope with such complex objects, we revise the Bouma law
to say that critical spacing is equal across parts, rather
than objects. This accounts for old and new findings.

Effects of stimulus duration and inter-letter spacing were studied in a letter-in-string identification paradigm. Participants were shown strings of 5 random consonants (e.g., PGKDM) centered on fixation and were asked to identify the letter that had appeared at a post-cued location. Stimulus duration was manipulated in Experiment 1 (13 ms – 91 ms), and inter-letter spacing manipulated in Experiment 2 (for a fixed stimulus duration of 26 ms). We contrasted performance to outer-letters (positions1 and 5) with non-central inner letters (positions 2 and 4), the first-letter (position 1) with the final letter (position 5), and the central-letter (position 3) with the other inner letters (positions 2 and 4). The outer-letter advantage and the first-letter advantage were present throughout the entire range of exposure durations, whereas the central-letter advantage increased with longer exposures. On the other hand, increased spacing reduced both the outer-letter advantage and the first-letter advantage, whereas it led to a greater central-letter advantage. Changes in acuity and crowding as a function of stimulus exposure and inter-letter spacing, can account for this pattern of results.

The effect of Crowding has long been recognised by cognitive psychologists engaged in examining the reading process. Yet it is not generally taken into account by most field linguists involved in the development of tone orthographies for emerging African languages. True, there is a general recognition that diacritic overload is unhelpful, but this has never been articulated with the help of the more precise terminology already on offer from the field of cognitive psychology. Using an experimental tone orthography developed for Kabiye (Gur, Togo) as an example, I postulate that an near-exhaustive representation of tone by means of accents will trigger Crowding. This is a hypothesis that has yet to be tested under clinical conditions. But the aim of this article is to call the phenomenon by its name for the first time and thereby stimulate further research. I also hope to demonstrate by means of this single example the gulf that exists between the cognitive psychology and linguistics. Once we recognise that the gulf exists, we can begin to build bridges.

Crowding is a phenomenon that characterizes normal periphery limiting letter identification when other letters surround the signal. We investigated the nature of the reading limitation of crowding by analyzing eye- movement patterns. The stimuli consisted of two items varying across trials for letter spacing (spaced, unspaced and increased size), lexicality (words or pseudowords), number of letters (4, 6, 8), and reading modality (oral and silent). In Experiments 1 and 2 (oral and silent reading, respectively) the results show that an increase in letter spacing induced an increase in the number of fixations and in gaze duration, but a reduction in the first fixation duration. More importantly, increasing letter size (Experiment 3) produced the same first fixation duration advantage as empty spacing, indicating that, as predicted by crowding, only center-to-center letter distance, and not spacing per se, matters. Moreover, when the letter size was enlarged the number of fixations did not increase as much as in the previous experiments, suggesting that this measure depends on visual acuity rather than on crowding. Finally, gaze duration, a measure of word recognition, did not change with the letter size enlargement. No qualitative differences were found between oral and silent reading experiments (1 and 2), indicating that the articulatory process did not influence the outcome. Finally, a facilitatory effect of lexicality was found in all conditions, indicating an interaction between perceptual and lexical processing. Overall, our results indicate that crowding influences normal word reading by means of an increase in first fixation duration, a measure of word encoding, which we interpret as a modulatory effect of attention on critical spacing.

We examined how crowding (the breakdown of object recognition in the periphery caused by interference from ‘‘clutter’’) depends on the global arrangement of target and distracting flanker elements. Specifically we probed orientation discrimination using a near-vertical target Gabor flanked by two vertical distractor Gabors (one above and one below the target). By applying variable (opposite-sign) horizontal offsets to the positions of the two flankers we arranged the elements so that on some trials they formed contours with the target and on others they did not. While the presence of flankers generally elevated orientation discrimination thresholds for the target we observe maximal crowding not when flanker and targets were co-aligned but when a small spatial offset was applied to flanker location, so that contours formed between flanker and targets only when the target orientation was cued. We also report that observers’ orientation judgements are biased, with target orientation appearing either attracted or repulsed by the
global/contour orientation. A second experiment reveals that the sign of this effect is dependent both on observer and on eccentricity. In general, the magnitude of repulsion is reduced with eccentricity but whether this becomes attraction (of element orientation to contour orientation) is dependent on observer.We note however that across observers and eccentricities, the magnitude of repulsion correlates positively with the amount of release from crowding observed with co-aligned targets and flankers, supporting the notion of fluctuating bias as the basis for elevated crowding within contours.

Can people learn complex information without conscious awareness? Implicit learning—learning without awareness of what has been learned—has been the focus of intense investigation over the last 50 years. However, it remains controversial whether complex knowledge can be learned implicitly. In the research reported here, we addressed this challenge by asking participants to differentiate between sequences of symbols they could not perceive consciously. Using an operant-conditioning task, we showed that participants learned to associate distinct sequences of crowded (nondiscriminable) symbols with their respective monetary outcomes (reward or punishment). Overall, our study demonstrates that sensitivity to sequential regularities can arise through the nonconscious temporal integration of perceptual information.

Crowding is a phenomenon that characterizes normal periphery limiting letter identification when other letters surround the signal. We investigated the nature of the reading limitation of crowding by analyzing eye-movement patterns. The stimuli consisted of two items varying across trials for letter spacing (spaced, unspaced and increased size), lexicality (words or pseudowords), number of letters (4, 6, 8), and reading modality (oral and silent). In Experiments 1 and 2 (oral and silent reading, respectively) the results show that an increase in letter spacing induced an increase in the number of fixations and in gaze duration, but a reduction in the first fixation duration. More importantly, increasing letter size (Experiment 3) produced the same first fixation duration advantage as empty spacing, indicating that, as predicted by crowding, only center-to-center letter distance, and not spacing per se, matters. Moreover, when the letter size was enlarged the number of fixations did not increase as much as in the previous experiments, suggesting that this measure depends on visual acuity rather than on crowding. Finally, gaze duration, a measure of word recognition, did not change with the letter size enlargement. No qualitative differences were found between oral and silent reading experiments (1 and 2), indicating that the articulatory process did not influence the outcome. Finally, a facilitatory effect of lexicality was found in all conditions, indicating an interaction between perceptual and lexical processing. Overall, our results indicate that crowding influences normal word reading by means of an increase in first fixation duration, a measure of word encoding, which we interpret as a modulatory effect of attention on critical spacing.