a Pion publication
i-Perception (2013) volume 4, pages 347–351
dx.doi.org/10.1068/i0595rep perceptionweb.com/i-perceptionISSN 2041-6695
Graduate School of Letters, Kyoto University, Kyoto 6068501, Japan; e-mail: email@example.com
Graduate School of Letters, Kyoto University, Kyoto 6068501, Japan; e-mail: firstname.lastname@example.org
Graduate School of Letters, Kyoto University, Kyoto 6068501, Japan; e-mail: email@example.com
Received 5 March 2013, in revised form 1 June 2013; published 10 July 2013
Abstract. A square lled with horizontal stripes is perceived as thinner than one with vertical stripes
(Helmholtz illusion). This is not consistent with a common belief that horizontally striped clothing
makes a person look fatter, and studies on this problem have shown inconsistent results. Here,
we demonstrate three factors that could have complicated the issue. First, the Helmholtz effect is
stronger for a thin gure than for a fat one, with possible reversal for the latter. Second, we found
large variability across participants, suggesting dependence on features to attend. Third, there was
strong hysteresis as to the order of testing fat and thin gures, suggesting the effect of surrounding
people in daily life. There can be yet other factors, but we should note that this apparently simple
case of application of a geometrical illusion in daily perception should be taken as a rather complex
Keywords: Helmholtz illusion, fashion.
1 Results and discussion
It is commonly believed that horizontal stripes on clothing make them look fatter. This, however,
conicts with a known geometrical illusion found by Helmholtz (1867); namely, a square lled with
horizontal stripes looks taller and a square lled with vertical stripes looks wider. He mentioned the
same effect on ladies’ frocks. Thompson and Mikellidou (2011) examined the effects of stripes on
clothing in a series of experiments and concluded that Helmholtz was correct. However, subsequently
in 2012, an amateur scientist Val Watham in the United Kingdom won the prize in a BBC radio pro-
gram, demonstrating that the common belief is true in an experiment of watching videos of people
walking with horizontally or vertically striped clothes (http://www.bbc.co.uk/radio4/features/sywtbas/
nalists/stripes/), with the supervision of Thompson himself. We may conclude that the Helmholtz
illusion could be overridden by other factors in a realistic scene. There could be, however, a number
of other factors that may also affect the results. Here, we demonstrate some of such factors, lying on
both the stimulus and the observer sides.
One can notice that the main female gure used in Thompson and Mikellidou (2011) is relatively
thin and tall (Figure 1a). This contrasts with the fat men in the gure of Imai (1982), as shown in
Figure 1(b), with which he supported the common belief; the width of the man with vertical stripes
was judged thinner than the one with horizontal stripes by 15.37% (137 participants, SD 5 9.24%).
This large difference has led us to the possibility that the effect of stripes might depend on the shape
of the person, which is also suggested by Experiment 3 in Thompson and Mikellidou (2011), where
they found fading of the effect of stripes on simple cylinders as they became fatter. To test this idea,
we conducted an experiment in which we compared the points of subjective equality (PSEs) in terms
of the body width (i.e. when gures with horizontal and vertical stripes look equally fat) for relatively
thin and fat gures (Figure 2).
The results are summarized in Figure 3, where a positive PSE supports Helmholtz (a person
with horizontal stripes appeared thinner than one with vertical stripes), whereas a negative PSE sup-
ports the common belief (a person with horizontal stripes appeared fatter). This gure highlights three
Helmholtz illusion makes you look t only when
you are already t, but not for everyone
348 Ashida H, Kuraguchi K, Miyoshi K
major aspects of the results that demonstrate complexity underlying this phenomenon. First, the over-
all effect is positive and in accordance with the Helmholtz illusion. Second, the thin body yielded
signicantly larger Helmholtz illusion than the fat body (F(1, 26) 5 6.573, p 5 0.0165, by a three-way
mixed-design analysis of variance for block order gender body type). This result is consistent
with Experiment 3 of Thompson and Mikellidou (2011) using cylinders, and supports our hypothesis.
Third, the block order had a signicant effect (fat-rst vs. thin-rst; F(1, 26) 5 4.262, p 5 0.0491).
The effect of participants’ gender was not signicant (F(1, 26) 5 1.439, p 5 0.241), but note that
gender difference was not our primary interest and the numbers of participant were not matched. Also,
there was no signicant two-way or three-way interaction (p > 0.2 for all).
There seems to be a strong hysteresis effect, because the results in the second block are pulled
towards those in the rst block. The reason for this effect is unknown, but it may reect response
biases learned through the rst block. We therefore analysed the results of the rst block (block 1)
separately that should reect more naïve responses of each participant (between-participant compari-
sons). In block 1, the difference between the thin and the fat gures is more pronounced (t(28) 5 4.526,
Figure 1. (a) The typical visual stimuli used in Thompson and Mikellidou (2011).1 (b) A demonstration of
fattening by horizontal stripes in Imai (1982).
Figure 2. Stimuli used in our experiment: (a) a pair of thin gures and (b) a pair of fat gures.
1 The gure was not reproduced correctly in the published paper, and this was copied from the original
(P. Thompson, personal communication).
Helmholtz illusion on clothing 349
p 5 0.00047). Averaged PSE was positive for the thin gure (t(14) 5 3.976, p 5 0.00056), but it was
negative for the fat gure, although it was not signicantly below zero (t(14) 5 0.917, p 5 0.258).
There is another factor that has not been much attended to in the literature, namely, very large vari-
ability across participants. As seen in Figure 4, PSEs of individual participants in block 1 are broadly
distributed, spreading from negative to positive for both thin and fat gures. This could be partly
because we did not instruct participants to judge on the basis of a particular criterion. Giving a more
precise instruction could have reduced the variability, but we preferred to see more natural judgements
as people would do in daily life.
In summary, we have identied three factors that may underlie the inconsistency over the effects
of stripes on clothing. First, we provided striking evidence that the effect of stripes depends on the
body shape. Outts with horizontal stripes can make a slender person even tter, but the effect is not
ensured or could be reversed for others (to be consistent with what is commonly conceived). This sug-
gests that one practical way to make the Helmholtz illusion more effective would be to wear a long
dress as in Figure 1(a), which might be less useful for men. The reason is yet to be investigated, but it
might be a general sensory or perceptual effect as the result parallels the nding that the Oppenl-Kundt
illusion (overestimation of the interval lled with vertical lines) became weaker when the lling lines
were longer than a certain maximum (Wackerman & Kastner, 2010). Second, the effect can be widely
variable across people, making it of even less practical use. Third, the strong effect of order may imply
that the effect of striped outts could depend on the clothing and/or tness of surrounding people.
We still have not solved the entire problem, as there are other potential factors that could have
characterized the results in the Watham experiment. First, the effect of 3D cues with the vertical stripes
(Taya & Miura, 2007) may have been pronounced in videos (kinetic depth effect). Second, a direct
comparison between two gures, also adopted by Thompson and Mikellidou (2011), may emphasize
Figure 3. Mean PSEs for the two body conditions: all averaged, block 1, and block 2. Bars show 1 SEM across
Figure 4. A histogram that shows the distributions of individual PSEs in block 1.
350 Ashida H, Kuraguchi K, Miyoshi K
geometrical measures compared with the case of rating one person at one time, although our pre-
liminary study did not support this idea. Finally, we could speculate on an intriguing possibility that
motion should be rather crucial; horizontal stripes would yield weaker motion signals than vertical
stripes when people walk around upright. In such a case, we would expect more blurring with horizon-
tal stripes while more effective motion deblurring could operate with vertical stripes that would lead
to faster (stronger) motion signals (e.g. Castet, Lorenceau, Shiffrar, & Bonnet, 1993). This could result
in relative fattening of a person with horizontally striped clothing. These possibilities remain specula-
tive, but we should consider multiple factors and individual differences, which allows us with further
interesting questions for future studies.
Thirty-one undergraduate students participated for a course credit. One of them was omitted from the
analysis because the overall psychometric function lay below 0.4 and the estimation of PSEs was unre-
liable. The results from remaining 30 (19 females) were analysed. The participants observed a com-
puter screen (Mitsubishi 23-inch or Apple 24-inch Cinema Screen LCDs) from a distance of 48 cm
by using a chin rest. They were naïve to the exact purpose of the experiment, and were given detailed
explanations later during the class.
Static computer-graphics images of thin and fat gures were generated by using a female template
model in the Poser 8 software (SmithMicro Inc.). Each gure was dressed with a shirt with either
vertical or horizontal stripes with the white–black ratio of 3:1 (Figure 2). The width of the black lines
was constant (about 0.24 deg) for both body shapes. In each set, the gure with vertical stripes was
used as a standard stimulus, and the body width of the gure with horizontal stripes was varied as a
test stimulus by modifying the 3D parameters between the shoulder and the waist under Poser, from
210% to 110% of the standard stimulus in 5% steps. The shape of the horizontally striped gure was
manipulated because it yielded less local distortion that could provide a cue than the vertically striped
one. Each standard gure subtended approximately 17.8 deg vertically and 7.2 deg (thin) or 13.1 deg
(fat) horizontally. The stimulus step was relatively coarse, because our preliminary observation sug-
gested that the judgement was difcult with smaller differences. Given the PSEs that turned out to be
less than 5%, this could have reduced the precision of the PSE estimates but should not have affected
their accuracy on average.
A method of constant stimuli was used to estimate the points of subjective equality (PSE) in per-
ceived width of the gures between horizontal and vertical stripes. Superlab 4.5 on Windows (Cedrus
Inc.) was used to control the experiment. Each trial started with a xation mark. On participants’ press-
ing a key, the xation mark was replaced with a pair of gures that were presented for 1.8 s, shown side
by side with a centre-to-centre distance of 13.1 deg. The participants were asked to judge which looked
fatter and respond by pressing a key (2AFC). There was no time pressure for the response. The side
of the standard stimulus was counterbalanced across trials. Each of the ve test gures was presented
20 times on each side with a randomized order, resulting in 40 judgements per test size. Fat and thin
gures were tested in separate blocks; each participant completed one block for each body type with a
30-s break between them, with the order counterbalanced across participants. The whole session took
less than 25 min for each participant. Three or four participants were tested in a laboratory room at the
same time, using separate computers and monitors.
PSE was calculated for individual participant as the 50% point of the psychometric function that
was estimated by the Probit analysis (Finney, 1971), using the glm() function of the R language.
Acknowledgments. HA was supported by JSPS grant-in-aid for scientic research (A22243044).
Castet, E., Lorenceau, J., Shiffrar, M., & Bonnet, C. (1993). Perceived speed of moving lines depends on
orientation, length, speed and luminance. Vision Research, 33(14), 1921–1936. doi:10.1016/0042-
Finney, D. J. (1971). Probit analysis (3rd ed.). Cambridge: Cambridge University Press.
Helmholtz, H. v. (1867). Handbuch der physiologischen Optik III. Leipzig: Voss. (English translation by J. P.
C Southall for the Optical Society of America, Treatise on Physiological Optics, volume 3, New York:
Imai, S. (1982). Tateshima yokoshima no nazo (in Japanese). Psychology, 29, 12. Tokyo: Saiensu-sha.
Copyright 2013 H Ashida, K Kuraguchi, K Miyoshi
Published under a Creative Commons Licence a Pion publication
351 Ashida H, Kuraguchi K, Miyoshi K
Taya, S., & Miura, K. (2007). Shrinkage in the apparent size of cylindrical objects. Perception, 36(1), 3–16.
Thompson, P., & Mikellidou, K. (2011). Applying the Helmholtz illusion to fashion: Horizontal stripes won’t
make you look fatter. i-Perception, 2(1), 69–76. doi:10.1068/i0405
Wackerman, J., & Kastner, K. (2010). Determinants of lled/empty optical illusion: Search for the locus of
maximal effect. Acta Neurobiologiae Experimentalis, 70, 423–434.
Hiroshi Ashida received BA, MA, and PhD at Kyoto University (psychology).
After working as a postdoctoral researcher at the ATR Human Information
Processing Research Laboratories, Research Associate at Kyoto University,
and Associate Professor at Ritsumeikan University, he is now Associate
Professor at Kyoto University. His main research interests are visual
processing of motion and visual illusion in general. He also studies fMRI after
learning the basics at Royal Holloway, UK, in 2004–2005. He prefers vertical
stripes on his shirts anyway.
Kana Kuraguchi received BA and MA in psychology from Kyoto University
and is studying for PhD with Hiroshi Ashida at Kyoto University. Her main
research interest is in visual attractiveness of human faces. She is also
a licensed instructor of Ikebana, the Japanese traditional art of ower
Kiyofumi Miyoshi received BA and MA in psychology from Kyoto University
and is studying for PhD with Hiroshi Ashida at Kyoto University. His main
research interest is in re-evaluation of implicit and explicit memory.