Journal of Vision (2007) 7(2):13, 1–10
http://journalofvision.org/7/2/13/
1
Horizontal and vertical asymmetry in visual spatial
crowding effects
Chengzhi Feng
Department of Psychology, University of Minnesota,
Minneapolis, MN, USA, &
Department of Psychology, Soochow University,
Suzhou, China
Yi Jiang
Department of Psychology, University of Minnesota,
Minneapolis, MN, USA
Sheng He
Department of Psychology, University of Minnesota,
Minneapolis, MN, USA
The crowding effect is a ubiquitous phenomenon in spatial perception that has been extensively investigated. A notable
property of spatial crowding is the radial and tangential asymmetry in which crowding is much more severe for items
arranged on a radial line originating from the fixation point compared with items arranged tangentially with regard to the
fixation point. Here, we report another asymmetric property of the crowding effect between horizontally and vertically
arranged spatial layouts. In multiple experiments using a number of different stimulus patterns (e.g., letter Cs, letter Ts, and
parallel/nonparallel line segments), results consistently showed that the crowding effect was significantly stronger when the
target and distractors were horizontally rather than vertically arranged. Such a horizontal versus vertical asymmetry was
found in all four quadrants of visual space. Importantly, a further visual acuity test did not reveal a horizontal versus vertical
asymmetric effect.
Keywords: crowding, asymmetry, horizontal, vertical, visual field, Vernier acuity
Citation: Feng, C., Jiang, Y., & He, S. (2007). Horizontal and vertical asymmetry in visual spatial crowding effects. Journal of
Vision, 7(2):13, 1–10, http://journalofvision.org/7/2/13/, doi:10.1167/7.2.13.
Introduction
The crowding effect is a ubiquitous phenomenon in visual
spatial perception. When a target stimulus is surrounded by
other distractors (flankers), visual discrimination of the
target becomes more difficult. This effect has been studied
extensively with letters, digits, and gratings (Bouma, 1970;
Chung, Levi, & Legge, 2001; Felisbert, Solomon, &
Morgan, 2005; Strasburger, 2005; Strasburger, Harvey, &
Rentschler, 1991; Stuart & Burian, 1962). Many factors
have been found and determined to have significant
influences on the crowding effect. For example, the crowding effect can be strengthened by increasing the number of
distractors (Felisbert et al., 2005) and the eccentricity of the
target (Bex, Dakin, & Simmers, 2003; Bouma, 1970;
Jacobs, 1979) or be reduced by increasing the spatial
separation (Bouma, 1970; Felisbert et al., 2005; Toet &
Levi, 1992) and the dissimilarity between the target and the
distractors (Andriessen & Bouma, 1976; Kooi, Toet,
Tripathy, & Levi, 1994; Leat, Li, & Epp, 1999; Nazir,
1992). The exact neural mechanism for crowding remains a
subject of research. Explanations for this phenomenon
include a lateral inhibition between the target and the distractors at the visual feature level (Bjork & Murray, 1977),
doi: 1 0. 11 67 / 7 . 2 . 13
improper spatial pooling of the features (Parkes, Lund,
Angelucci, Solomon, & Morgan, 2001; Wilkinson, Wilson, &
Ellemberg, 1997), and insufficient visual attention resolution (He, Cavanagh, & Intriligator, 1996, 1997; Strasburger,
2005; Strasburger et al., 1991).
The spatial property and interaction across visual fields
have also been extensively examined. For example,
several studies revealed better visual performance
(Hofmann & Hallett, 1993; Talgar & Carrasco, 2002)
and finer attentional resolution in the lower compared with
upper visual field (He et al., 1996, 1997; Intriligator &
Cavanagh, 2001). Another well-established property of
crowding is the radial–tangential asymmetry (Chambers &
Wolford, 1983; Toet & Levi, 1992). Toet and Levi (1992)
found that the zone of spatial interaction appears to be
more or less elliptical in shape in the peripheral visual
field, with the long axis along a radial line connecting the
fovea to the peripheral location of the test. They also
found that, in the lower visual field, the spatial interaction
zones were considerably larger when flanking Ts were
above and below a test T (vertical alignment) than when
the flanking Ts were on either side of the test T (horizontal
alignment; Toet & Levi, 1992). We believe that their
manipulation of the target presented a horizontal versus
vertical bias in that the target was intrinsically more
Received February 15, 2007; published July 17, 2007
ISSN 1534-7362 * ARVO
Journal of Vision (2007) 7(2):13, 1–10
Feng, Jiang, & He
difficult to discriminate in the vertical configuration (more
on this in the General discussion section).
In the current study, we focused our effort on
investigating the asymmetric property associated with
crowding, especially between horizontal and vertical
configurations. In three experiments, we used different
discrimination tasks with letter Cs, letter Ts, and parallel/
nonparallel line segments as target stimuli. The results
consistently revealed a stronger crowding effect in the
horizontal compared with vertical configuration.
Experiment 1: The crowding
effect for horizontal and vertical
configurations
We examined the crowding effect for horizontally and
vertically arranged spatial layouts using three types of
stimuli: letter Cs, letter Ts, and parallel/nonparallel line
segments.
Methods
Observers
The three types of stimuli were run in different sessions.
Eight observers (three women) participated in the experiment using letter Cs; six observers (one woman) were
tested using letter Ts; and six observers (one woman)
participated in the experiment using parallel/nonparallel
line segments. All participants had normal or correctedto-normal vision. Participants gave written informed consent in accordance with procedures and protocols approved
by the human subjects review committee of the University
of Minnesota.
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Apparatus
Stimuli were coded using the psychophysical toolbox
(Brainard, 1997; Pelli, 1997) in Matlab and were presented
on a 17-in. Sony monitor (1,024 768 pixels at 100 Hz).
Stimuli and design
In Experiment 1.1, the target and distractor stimuli were
both letter Cs, as shown in Figure 1. Each letter C
subtended a visual angle of 1.79 1.79-, with an angular
gap of 45-. The gap could point to any of the four
directions (left, right, up, or down). Another two letter Cs
served as distractors (with their orientations randomly
selected from the four possible orientations, excluding the
possibilities where both distractors were in the same
orientation as the target), aligned either horizontally or
vertically to the central target. The center-to-center
distance between the target and the distractor was 1.97(Figure 1 and the top row of Figure 2). Experiment 1.2
was the same as Experiment 1.1 except that the target was
the letter T and the distractors were replaced with squaredtheta. Each T subtended a visual angle of 1.1 1.1- and
could be presented in any of the four orientations (left,
right, up, or down). Squared-thetas were used as distractors because they were effective in interfering with the
identification of the target due to their similar contours
with the letter T (Tripathy & Cavanagh, 2002). The centerto-center distance between the target (T) and the distractor
(squared-theta) was 1.25- (Figure 2, middle row). The
target in Experiment 1.3 was a pair of line segments,
either parallel or nonparallel. Each line segment was tilted
10- from true vertical or horizontal orientations, resulting
in eight different combinations (four parallel and four nonparallel), as shown in the bottom row of Figure 2. The
upper four pairs were used as the target in the horizontal
alignment with distractors, and the lower four pairs were
Figure 1. Spatial parameters of stimuli used in Experiment 1 using the letter C as an example. The target letter C is shown in the same
location (Quadrant 4) in Panels a and b, but the distractors were either horizontally (a) or vertically (b) flanking the target.
Journal of Vision (2007) 7(2):13, 1–10
Feng, Jiang, & He
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Figure 2. Stimuli used in Experiment 1: (a) Target. (b) Distractors. (c) Examples of the combination of distractor and target. The three rows
show stimuli for the three subexperiments respectively. For Experiment 1.3, the upper four pairs of line segments were used for the
horizontal alignment, and the lower four pairs were used for the vertical alignment.
used as the target in the vertical alignment with distractors.
The distractors were the same as in Experiment 1.2
(squared-theta).
A central dark disc with a pinhole was presented
throughout the duration of the experiments, serving
as a fixation point. During the experiment, the target
and distractors were aligned either horizontally or
vertically and were presented on a uniform gray
background. The mean luminance of the stimulus
pattern and the background was 1.08 and 25.6 cd/m2,
respectively. In each trial of Experiment 1.1, the stimulus
pattern was presented in one of the four quadrants of the
visual field with the central target at 7.6- of eccentricity
(Figure 1). Thus, each type of stimulus resulted in eight
conditions [2 (arrangements: horizontal vs. vertical) 4
(quadrants)]. In Experiments 1.2 and 1.3, the stimulus
pattern was only presented in the third and the fourth
quadrants, and all other aspects were the same as in
Experiment 1.1. All conditions were mixed randomly
within blocks.
Procedure
Observers sat 50 cm away from the monitor and viewed
the display binocularly. Observers were required to fixate
at the fixation point throughout the experiment. At the
beginning of each trial, there was a small bar pointing
from the fixation point to one of the quadrants, indicating
the spatial location where the stimuli would be presented
(Figure 3). The central cue was presented for 1,000 ms,
followed by the test stimuli presented at the cued quadrant
for 250 ms (Figure 3). In Experiments 1.1 and 1.2,
observers pressed one of four direction keys to indicate the
orientation of the letter C or letter T (left, right, up, or
down). In Experiment 1.3, observers were asked to press
Figure 3. Schematic depiction of the presentation sequence of one trial.
Journal of Vision (2007) 7(2):13, 1–10
Feng, Jiang, & He
one of two keys to indicate whether the line segments
were parallel or nonparallel to each other.
Experiment 1.1 (letter C) consisted of eight blocks with
64 trials in each block. Experiment 1.2 (letter T) and 1.3
(parallel/nonparallel line segment) each consisted of four
blocks with 64 trials in each block, and observers were
tested in either the lower left or lower right quadrant.
Results and discussion
Experiment 1.1 (C)
Percentages of correct responses to different crowding
conditions (horizontal vs. vertical) and different visual fields
are shown in Figure 4, and a repeated measures analysis of
variance (ANOVA) was carried out with spatial configuration (horizontal vs. vertical) and visual field (four
quadrants) as independent variables. Observers’ performance was significantly better when the stimuli were aligned
vertically than horizontally, as revealed by the significant
main effect of spatial configuration, F(1, 7) = 9.40, p = .02.
The main effect of visual field (different quadrants) was
also significant, F(3, 7) = 3.41, p = .04, and observers
performed better in the lower compared with the upper
visual field. There was no interaction between spatial
4
configuration and visual field, F(3, 21) = 0.036, p = .99,
suggesting that the crowding effect is more severe in the
horizontal configuration in all four quadrants.
Experiment 1.2 (letter T )
To further test whether the asymmetric horizontal and
vertical crowding effects were specific to the stimulus we
used or were a general property of crowding, we used
letter T and squared-theta as the target and distractors,
respectively, in Experiment 1.2. The stimuli were presented only in the third and the fourth quadrants; all other
aspects were the same as in Experiment 1.1.
The results from six observers are shown in Figure 5.
Consistent with Experiment 1.1 (letter C), Experiment 1.2
still showed a significant asymmetric crowding effect,
t(5) = 6.20, p = .002. In other words, the crowding effect
was significantly stronger when the target and flankers
were arranged horizontally than vertically.
As mentioned earlier, our observation of horizontal and
vertical asymmetry is inconsistent with what was reported
in a previous study (Toet & Levi, 1992). In the study of
Toet and Levi (1992), the letter T was used as the target,
but the target T could be in either upright or inverted
orientations. We suspected that because the target T is not
Figure 4. Results of Experiment 1.1 using the letter C, showing performance of detecting the orientation of C in different quadrants
(labeled 1–4) at the two spatial arrangements. Lower performance means stronger crowding effect. In each quadrant, observers
performed better under the vertical configuration. Error bars indicate T1 SEM.
Journal of Vision (2007) 7(2):13, 1–10
Feng, Jiang, & He
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Figure 5. Results of Experiment 1.2 using the letter T. Individual and group average performance values are shown on the left panel for
horizontal (hatched bars) and vertical (open bars) spatial configurations. Performances are generally better for vertical configuration, both
individually and for the average. When target T orientations are separately analyzed (left/right vs. upright/inverted), results show a very
strong interaction between the target T orientation and the spatial configuration of the flankers (right panel). Upright/inverted Ts were
minimally crowded by horizontal flankers, so were sideways Ts by vertical flankers. However, sideways Ts were severely crowded by
horizontal flankers, and upright/inverted Ts were moderately affected by vertical flankers. Error bars indicate T1 SEM.
isotropic, discriminating an upright T from an inverted T
could be hampered more by distractors on top and below
the target than distractors on the left and right sides of it.
This is because the critical information for upright versus
inverted discrimination is the location of the horizontal
bar in the T: Is it on the top or is it at the bottom? A more
detailed examination of our data provides strong support
for this interpretation. When we reanalyzed our results by
separating the data into 2 (horizontal target T vs. vertical
target T) 2 (horizontal crowding vs. vertical crowding)
subgroups, we found a significant interaction between the
orientation of the target T and the orientation of the spatial
configuration (Figure 5, right). A repeated measures
ANOVA shows a significant interaction between target
orientation (horizontal vs. vertical) and spatial configuration (horizontal vs. vertical), F(1, 5) = 67.21, p G .0005.
If we only look at the data points for the upright versus
inverted Ts (the two rightmost bars in Figure 5), then our
results are consistent with Toet and Levi’s finding in that
discrimination of the central target (upright and inverted
Ts) was more difficult when flankers were arranged
vertically than horizontally, although the flanker stimuli
were different in our study (squared-theta) and their study
(upright and inverted Ts).
Overall, our results show that observers performed
equally well when the orientation of the central target T
was different from the orientation of the flankers, that is,
horizontal T in vertical flanker configuration or vertical
T in horizontal flanker configuration. But when the
orientation of the central target T was in line with the
spatial configuration of the flankers, target discrimination
with the horizontal T in the horizontal spatial configuration was more severely degraded compared with the
vertical T in the vertical spatial configuration (Figure 5,
right). The overall effect with the target T was the same as
that using the target letter C; crowding is more severe in
the horizontal direction than in the vertical direction.
Experiment 1.3 ( parallel/nonparallel line segments)
Experiments 1.1 and 1.2 revealed a robust asymmetric
crowding effect between horizontally and vertically arranged
spatial layouts. Because the targets in these experiments were
letters (C and T), it is possible that the observed asymmetry
applies to letter discrimination only. For instance, when
target and distractors were horizontally aligned, the stimuli
were more likely to be considered as a trigram, and our usual
reading experience could potentially integrate and group this
horizontal information more strongly compared with vertical
alignment. To test this possibility, we ran a third experiment
by replacing the target letters with two line segments that
could be either parallel or nonparallel from each other.
Similar to Experiment 1.2, the stimuli were presented only in
the third and the fourth quadrants. The data obtained from six
individual observers, as well as a group average, are shown
in Figure 6, and a paired-sample t test clearly showed a
significant asymmetric crowding effect, t(5) = 3.49, p = .02,
Journal of Vision (2007) 7(2):13, 1–10
Feng, Jiang, & He
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Figure 6. Results of Experiment 1.3, using parallel and nonparallel bars as targets. Performance of six individual observers and the group
average are shown. Consistent with the letter C and letter T results, performance was also poorer for the horizontal configuration. Error
bars indicate T1 SEM.
with better performance for vertical than for horizontal
spatial arrangement.
Because the targets for the horizontal and vertical spatial
configurations also differed in the target orientations, we also
obtained the baseline for discriminating parallel/nonparallel
line segments without flankers as a control experiment. The
targets (line segments) were presented in the same spatial
location as Experiment 1.3 for 50 ms without distractors,
and the observers were asked to complete the same
discrimination task (parallel or nonparallel). There was no
significant difference in this control experiment (77.7% vs.
80.7%), t(5) = j1.23 p = .27.
in the Vernier acuity test session, and six of them also
participated in the three-dot bisection test session.
Stimuli and design
The experimental design and procedure were the same
as in Experiment 1, except that the target and distractors
were replaced with three dots, as shown in Figure 7. The
Experiment 2: Vernier acuity and
bisection
In this experiment, we aimed to investigate whether the
asymmetric crowding effect obtained in Experiment 1 was
specific to crowding or due to horizontal and vertical
asymmetries in visual acuity. Therefore, a Vernier visual
acuity test and a three-dot bisection discrimination test
were adopted in this experiment, with the same spatial
arrangement as in Experiment 1.
Methods
Observers
Eight observers were randomly recruited from those
who have participated in Experiment 1. All eight took part
Figure 7. Illustrations of the stimuli used in Experiment 2. The
diameter for each dot is 0.46-, and the center-to-center distance
between the two outside dots is 3.94-. In the Vernier acuity test
(upper row), the central dot was slightly shifted perpendicular to
the straight line connecting the two outside dots. In the three-dot
bisection test (lower row), the central dot was slightly shifted along
the straight line connecting the two outside dots.
Journal of Vision (2007) 7(2):13, 1–10
Feng, Jiang, & He
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Figure 8. Results of Experiment 2 for the Vernier acuity test (A) and the three-dot bisection test (B). Numbers (1, 2, 3, and 4) indicate the
quadrants where test dots were presented. Error bars indicate T1 SEM.
Journal of Vision (2007) 7(2):13, 1–10
Feng, Jiang, & He
dots were presented at the same spatial locations as those
of the target and distractors in Experiment 1.
In the Vernier acuity test session, the central dot was
slightly shifted perpendicular to the straight line connecting the two outside dots; that is, when the three dots
were arranged vertically, the central dot could slightly
shift either left or right, and when the three dots were
arranged horizontally, the central dot could slightly shift
either up or down. In the three-dot bisection test session,
the central dot was slightly shifted along the straight line
connecting the two outside dots; that is, when the three
dots were arranged horizontally, the central dot could
slightly shift either left or right from the center, and
when the three dots were arranged vertically, the central
dot could slightly shift either up or down from the
center. Observers were asked to press one of the four
keys to indicate the shift direction of the central dot.
Each test session consisted of 512 trials, which was
divided into eight 64-trial blocks, with the order of each
condition randomized.
Results and discussion
Vernier acuity
The results are shown in Figure 8A. ANOVA was
applied on these data with spatial configuration and
visual field as the two main factors. No significant
differences were found for spatial layout (horizontal vs.
vertical), F(1, 7) = 2.10, p = .19, and visual field (four
quadrants), F(3, 7) = 0.40, p = .75, nor was there a significant interaction between these two factors, F(3, 21) =
0.12, p = .95. Although the difference between horizontal
and vertical Vernier performance did not reach statistical
significance, we note here that it seems that Vernier acuity
was a bit better in the horizontal than the vertical
configuration. This is consistent with a recent article
showing a “somewhat” better performance for horizontal
over vertical Vernier stimuli (Westheimer, 2005). In any
case, it is unlikely that the vertical advantage in
crowding effect could be explained by a better spatial
resolution along the vertical dimension.
Three-dot bisection
Results for the three-dot bisection test are presented in
Figure 8B. Similar to the Vernier test, there were no
significant main effects for spatial layout, F(1, 5) = 0.29,
p = .61, and visual field, F(3, 5) = 0.74, p = .54, nor was
there a significant interaction between them, F(3, 15) =
0.71, p = .56. This is also consistent with previous finding
(Yap, Levi, & Klein, 1987).
Taken together, both the Vernier test and the three-dot
bisection test showed minimal difference between the
horizontal and vertical spatial configurations. These
results support the idea that the horizontal and vertical
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asymmetry observed in Experiment 1 is not a property of
low-level sensory processing, but likely of a higher level
origin.
General discussion
The current study examined the crowding effect for
horizontally and vertically arranged spatial configurations.
Experiment 1, using letter Cs, letter Ts, and parallel/
nonparallel line segments as target stimuli, consistently
found that the crowding effect was significantly stronger
when the target and distractors were horizontally rather
than vertically arranged. This effect was found in all four
quadrants of the visual field.
The horizontal and vertical asymmetry in crowding
adds to the two other well-known asymmetric properties
in spatial crowdingVstronger crowding for radial than
tangential configuration and stronger crowding from outer
than inner flanker. The stimuli in our experiments were
placed in diagonal locations in the four quadrants,
avoiding the potential confounding effect from tangential
and radial asymmetry in testing the horizontal and vertical
crowding effects. If the stimuli were centered on the
horizontal meridian either to the left or right of the
fixation, then the horizontal versus vertical asymmetry and
the radial versus tangential asymmetry make the same
prediction that the horizontal configuration (at the same
time it is a radial configuration) would suffer from a much
stronger crowding effect than would the vertical configuration (at the same time a tangential configuration).
However, if the stimuli were centered on the vertical
meridian either above or below the fixation, then radial
versus tangential asymmetry predicts a stronger crowding
effect for the vertical spatial arrangement, contradicting
the prediction based on findings reported in this article.
Under this condition, our preliminary observation suggests
that radial/tangential asymmetry is the more significant
variable. Thus, for targets placed on the vertical meridian,
vertically aligned flankers would still generate a stronger
crowding effect than horizontally aligned flankers would.
The results of our second experiment and results from
previous studies show that visual acuity (e.g., Vernier and
bisecting) was not very different in the horizontal and
vertical direction, suggesting that the asymmetric crowding
effect we observed here is likely due to factors beyond visual
spatial resolution. The Vernier acuity measurements in our
second experiment and the study of Westheimer (2005)
actually indicate a somewhat better performance for the
horizontally versus vertically aligned Vernier test.
One factor that may contribute to the horizontal versus
vertical asymmetry in crowding effect is that all of our
subjects read horizontally. It is possible that our attentional mechanism tends to organize the horizontally
arranged items into a single unit (i.e., like a trigram
Journal of Vision (2007) 7(2):13, 1–10
Feng, Jiang, & He
word), but such an organization is less likely to occur for
the vertically oriented spatial layouts. As a result, there is
stronger integration horizontally than vertically. Admittedly, this reading-experience-related explanation is quite
ad hoc. Another possibility is that our visual environment
is intrinsically more related horizontally than vertically,
and consequently, the shape of our attentional window is
more elongated horizontally. Indeed, a previous study also
suggested that attentional window (distribution) might be
elliptical in shape (Pan & Eriksen, 1993). Taking the view
that crowding is due to the lack of attentional resolution
(He et al., 1996, 1997; Strasburger, 2005; Strasburger
et al., 1991), it follows that more severe crowding would
occur horizontally than vertically.
No matter what the explanation is, our finding of a
stronger crowding effect in the horizontal configuration
raises the interesting practical possibility for people who
have to use their peripheral vision to read (such as
macular degeneration patients): It might be more efficient
to present the text vertically in the peripheral. Further
studies are obviously required to test this idea.
Acknowledgments
We thank Susana Chung, Gordon Legge, and Bosco Tjan
for their helpful discussions on the results. This research was
supported by the James S. McDonnell foundation, a U.S.
National Institutes of Health Grant (EY02934), and grants
from the Education Department (SK206414) and Education
Science Program (04SJB880014) of Jiangsu Province,
China.
Commercial relationships: none.
Corresponding author: Sheng He.
Email: [email protected].
Address: Department of Psychology, University of
Minnesota, 75 E. River Rd., Minneapolis, MN 55455.
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