Large depth differences between target and flankers can increase crowding: Evidence from a multi-depth plane display
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eLife Assessment
Using a novel multi-depth plan display, this study reveals a valuable finding regarding crowding - it decreases with small depth differences between the target and flankers but increases with larger depth differences. The evidence supporting this finding is convincing, although the explanation of the findings is somewhat speculative. This paper will be of interest to visual scientists, neuroscientists, and ophthalmologists, especially those working on visual crowding and depth perception.
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Abstract
Crowding occurs when the presence of nearby features causes highly visible objects to become unrecognizable. Although crowding has implications for many everyday tasks and the tremendous amounts of research reflect its importance, surprisingly little is known about how depth affects crowding. Most available studies show that stereoscopic disparity reduces crowding, indicating that crowding may be relatively unimportant in three-dimensional environments. However, most previous studies tested only small stereoscopic differences in depth in which disparity, defocus blur, and accommodation are inconsistent with the real world. Using a novel multi-depth plane display, this study investigated how large (0.54–2.25 diopters), real differences in target-flanker depth, representative of those experienced between many objects in the real world, affect crowding. Our findings show that large differences in target-flanker depth increased crowding in the majority of observers, contrary to previous work showing reduced crowding in the presence of small depth differences. Furthermore, when the target was at fixation depth, crowding was generally more pronounced when the flankers were behind the target as opposed to in front of it. However, when the flankers were at fixation depth, crowding was generally more pronounced when the target was behind the flankers. These findings suggest that crowding from clutter outside the limits of binocular fusion can still have a significant impact on object recognition and visual perception in the peripheral field.
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eLife Assessment
Using a novel multi-depth plan display, this study reveals a valuable finding regarding crowding - it decreases with small depth differences between the target and flankers but increases with larger depth differences. The evidence supporting this finding is convincing, although the explanation of the findings is somewhat speculative. This paper will be of interest to visual scientists, neuroscientists, and ophthalmologists, especially those working on visual crowding and depth perception.
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Reviewer #1 (Public Review):
Most previous studies investigating the phenomenon of crowding in depth use small stereoscopic differences in depth. Taken together their results suggest that a depth difference between target and flankers reduces crowding. A potential problem is that stereo displays can reduce depth perception. The studies that have used a real-depth display have provided some inconsistent findings. The present study investigated larger differences, representative of those among many objects in the real world. These larger differences increased crowding, even in the absence of diplopia (double vision).
This study is likely to be impactful in the field as it shows that crowding occurs in-depth and strengthens the importance of crowding in natural 3D environments. All existing models of crowding would need to be modified to …
Reviewer #1 (Public Review):
Most previous studies investigating the phenomenon of crowding in depth use small stereoscopic differences in depth. Taken together their results suggest that a depth difference between target and flankers reduces crowding. A potential problem is that stereo displays can reduce depth perception. The studies that have used a real-depth display have provided some inconsistent findings. The present study investigated larger differences, representative of those among many objects in the real world. These larger differences increased crowding, even in the absence of diplopia (double vision).
This study is likely to be impactful in the field as it shows that crowding occurs in-depth and strengthens the importance of crowding in natural 3D environments. All existing models of crowding would need to be modified to explain this experimental finding.
The novel multi-depth plane display that the authors used enables measurements of depth differences that are more likely to correspond to differences in the real world, and could be used by others to further investigate crowding in-depth or other perceptual processes (e.g., visual search).
In general, there are some interactions that were reported and others that were not reported, but it would be important to know if they are significant. (pages 15-16) For example, when the target is at fixation and the target is at a variable flanker depth: In Experiment 1, was there a significant interaction between (a) target-fixation depth and flanker depth (in front versus behind) and (b) target-fixation depth and target-flanker spacing? In Experiment 3, it is reported that perceptual error was higher when the target was in from or behind the flanker ring and fixation and that the greatest perceptual error occurred when the target was behind, but it is not reported if this interaction was significant. Its presence is important to know whether the data should be independently analyzed for 'in front' and 'behind'. In Experiment 5, was the interaction between target-flanker spacing and depth significant?
The findings are clear but the explanation(s) for the findings is not. The authors state that large interocular disparity differences likely induce diplopia, which could increase perceptual error by increasing the number of features. The authors should explain what they mean by features and how an increased perceived number of features would increase crowding. Moreover, the authors acknowledge that only a few observers reported experiencing diplopia; however, they speculate that observers may have experienced diplopia but not noticed it consciously given the short stimulus presentation time.
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Reviewer #2 (Public Review):
The authors reexamine the effects of depth on crowding, using a clever display that presents at three depths at once, and find that placing the target or flanker at far depth greatly increases crowding, contrary to what might have been expected by prior work with small depth differences. These stimuli avoid creating conflicting cues to depth and are thus the most relevant to viewing in daily life, indicating more crowding than was expected.
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Reviewer #3 (Public Review):
Smithers et al. examine the effects of large differences in target-flanker depth on peripheral visual crowding. To investigate this, they developed a novel real-depth display and measured the perceptual errors caused by the presence of flanker objects that were presented at different distances and at either the same or at different depths from a target object that the participants had to recognize.
Their primary result is that large depth differences between flanking and target objects increase the magnitude of crowding. Interestingly, it appears to be a two-faced finding: when the target is at fixation depth, crowding is more pronounced if the flankers are behind the target as opposed to in front of it. Yet, when the flankers are at fixation depth, crowding is more pronounced if the target is behind the …
Reviewer #3 (Public Review):
Smithers et al. examine the effects of large differences in target-flanker depth on peripheral visual crowding. To investigate this, they developed a novel real-depth display and measured the perceptual errors caused by the presence of flanker objects that were presented at different distances and at either the same or at different depths from a target object that the participants had to recognize.
Their primary result is that large depth differences between flanking and target objects increase the magnitude of crowding. Interestingly, it appears to be a two-faced finding: when the target is at fixation depth, crowding is more pronounced if the flankers are behind the target as opposed to in front of it. Yet, when the flankers are at fixation depth, crowding is more pronounced if the target is behind the flankers. They explain their finding in terms of increased clutter in areas outside the limits of binocular fusion. This conclusion of the study is well supported by the data and experiments. The work provides compelling evidence that real depth may affect peripheral crowding under the specific circumstances of their experiment. Whether this finding would also apply to more natural viewing conditions, in which there is much more clutter, to begin with, remains to be determined.
Strengths:
By introducing a novel multi-depth plane display authors contribute to future research on the effect of real depth differences on several visual functions and increase the potential ecological validity of their results.
By using perceptual error as their dependent variable and linear mixed models to analyze their data, authors improve their ability to represent the variability in the data.
The authors explain the discrepancies between their results and previous research with sufficient additional experiments and data.
The inclusion of a large number of participants, which is fairly uncommon in this type of experiment.Weaknesses:
1. At several points in the paper authors refer to the 'natural three dimensional scenes'. Indeed, the authors increase the ecological validity of their experiment by introducing actual depth differences, therefore allowing for depth cues such as accommodation, vergence and defocus blur. This is indeed a significant improvement over previous studies. However, they still use relatively impoverished visual stimuli in a tightly controlled psychophysical experiment requiring head stabilization by means of a chin rest. So, their experiment is still far removed from deploying actual, ecologically valid, conditions. Consequently, their stimuli mostly lack the complexity and associated clutter of natural stimuli as well as other potential depth cues that an observer might gain from parallax, aerial perspective, lighting, or shading. Therefore, their suggestion "that crowding has a more significant impact on our perception of natural three-dimensional environments than previously estimated with 2D displays." is stretching what can be concluded from their present work.
2. The inclusion of a large number of participants, in which none of the participants seemed to have performed all the conditions, is both a strength and a potential weakness. Their current approach of including (presumably) naive participants and having each do a portion of the experiments in itself is valid. But it also adds to the complexity of their study and presumably adds variability to their data. -
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