The Ponzo Illusion

This sample essay on The Ponzo Illusion reveals arguments and important aspects of this topic. Read this essay’s introduction, body paragraphs and the conclusion below.

The purpose of the present study was to determine the affect of brain stress on ponzo illusion. In addition the study examined the affect of presence or absence of converging lines background on judgments of lines length. The study sought to test Ponzo’s (1882-1960) hypothesis, referring to it as a misjudgment in a length of two lines.

This experiment was tested among three students under two conditions, participants under stress and under no stress, each condition had three attempts and each attempt had ten trials. In stress condition, participants accomplished the following experiment after they came back from college. Participants adjusted the length of a line to match the length of another line in same page.

Five trials were with the presence of converging lines background and the other five were with absence of converging lines.

In the no stress condition, participants accomplished the experiment between 0700 – 0800 O’clock in the morning after breakfast. The study aim was to explore the correlation between stress conditions and the Ponzo illusion. The error range of length judgment in stress condition found to be higher than the no stress condition. Also the error was higher when converging lines were present. It was concluded that stress and present of converging lines have inverse affect of judgment of lines length.


The Ponzo Illusion is a visual illusion that produces misjudgment in the length of two lines; most people don’t get the length right when asked to set two lines to the same length because of Ponzo illusion.

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The illusion is named after Mario Ponzo (1882-1960) who first demonstrated it in 1913 (Vurdlak, 2008). The reason of such illusion happens when looking at lines in different length with converging lines background and how the brain analyze them; for example, if there were two boxes and they were the same size, one of them was 5 cm in front of you, the other is 5 meters in front of you, you will see that the nearby box is bigger than the far one. The same thing happens with lines in a paper where the brain thinks that one line is fare but the other is near you, so the line that is far should look smaller, but if they are the same size your brain will think the far line is bigger (Brislin, 1974). This illusion can also be observed in the moon, when the moon is on top of you it seems smaller than when it is close to the horizon, the truth is that the moon didn’t change in size but the brain thinks that it should look smaller when it is far so if it does not it must have grown in size.

Ponzo Illusion Explained

There has been a lot of research in this field. Researchers suggested that there are two forms of Ponzo Illusions; circular ponzo Illusions and the Line Ponzo illusions (Robinson, 1972). Most studies to date have explored the line Ponzo illusion, although, studying the relationship between the line and circular forms of the illusion may offer additional signs to identify the possible determinants of the illusion (Yamagami, 2007). Different theories have been suggested to explain the Ponzo Illusion. Many studies have produced data supporting either theory. For example integration field theory (IFT), suggested by Pressey and Smith Martin (1990), is based on assumption that some parts of the stimulus are more likely to be processed than others. In accordance with this, the most relevant features to be processed are the ones confined within the four end points of the two segments to be compared (Roncato et al., 1997).

Although most of the previous researches in this field are very well constructed, they are lacking the brain stability and hormones in the form of stress. This research is very important because it will explore the affect of life stress on the Ponzo Illusion. Stress in this research refers to lack of concentration and focus because of exhaustion and brain loads of life activities. The dependant variable is the difference in length between the adjustable line and the standard line. The independent variable is the status of the participants’ vision.

There are number of confounding variables that have be considered in this research like, age difference between participants, eating habits, personal life, and area where participants are living in. All these variables will affect the stress and concentration in some forms. The aim of this study is to find if there is correlation between mental stress and Ponzo Illusion. The hypothesis of this study is that the mental stress will have a negative impact on concentration and therefore judgments of sizes will be affected. Also the presence of converging lines background complicates the issue and makes it even harder to judge the lines length. It was expected that when brain is stressed that the adjusted lines will be longer. It was also expected that when brain is stressed and converging lines are present, the adjusted lines will be even longer.



Three first year introductory psychology male students at PMU participated in the experiment as a course requirement. All participants had normal or corrected to normal vision.


Internet web site ( contains illusion experiment as follow. There are two parts of the experiment. First, two lines with different sizes are laid on converging track like (figure 1). Second, two lines are laid without the converging lines (figure 2). The participants are able to adjust the length of the lower lines by using the blue control arrow below the two lines that allows participants to adjust the size of the lower line to match the size of the upper line. There were 10 trials in this experiment, five trials with present of the converging lines and the other five without.


All participants followed standardized instructions for each of the following conditions.

Before the participants start the experiment they had to do one practice test, in that test each one of the participants had to move a blue arrow to adjust the length of a black line, if the arrow was moved to the right the line gets longer, if moved to the left the line gets shorter. Then they were asked to do the experiment three times in two different conditions. All participants had practice go before the real experiment start.

The first condition is that the experiment must be done without stress (early morning). All participants must log on into the web before going to college and follow the experiment steps. All ten trials have to be completed without taking any break in between. After the first attempt wait for ten minutes and relax. Accomplish a second attempt using the same procedures in the first attempt. Participants must wait for ten minutes and relax before the third attempt by using the same procedures.

The second condition must be done under stress (after a stressed day in college). In this condition participants must accomplish the experiment under a form of stress. Three attempts must be accomplished by each participant right after coming back from long stressed day in college. Accomplish the first attempt continuously without any breaks between trials. After the first attempt take a ten minutes break and then accomplish the second attempt. Take a ten minutes break and then accomplish the last attempt.

The results of this experiment support the hypothesis’s that with the brain stressed, the error magnitude is much higher as shown in (Table 1). Participant error in the first trial was very high p1=60 and as participant relaxed the error dropped to a lower level P1=29.20. In reference to (Table 2) the first trial for the first participant error was very low from the start P1=14 and the error continued to drop. Generally all participants showed very high error at start; however, the error rate deceased as the brain was relaxing. The result of the first and third participants showed a considerable amount of drop followed by a slight increase. This minor offset could be a momentarily loss of concentration or distraction.

The results of the experiment when participants are not under stress are a lot better. It showed much lower error values for all participants. In reference to (Table 2), the error values continued to decrease for first and third participants. The second participant was slightly different because of possible noise in the house. There was a big increase in the error value in the second trial, however, a vast drop in the third trial. This is caused by the different in age between participants. Participant number one is approximately twenty years older than the other participants.

The overall result summary showed higher error margin when lines are placed on converging lines. It also showed that the mean is also very high when the converging lines are present (mean = 23.086) and the standard deviation was (StDev = 16.308). When converging lines were absent, the data were much lower (mean = 7.267) and the standard deviation was (StDev = 5.955) as shown in (figure 3)

Figure 3


The results support the hypothesis that stress affects judgment of line lengths and caused the adjusted lines to be longer. Also the present of converging lines affects judgment of lines length and caused the lines to be longer. This finding is consistent with Piaget’s theory (1969; cited in Wagner, 1977) that proposes “the law of relative centrations”. Centration is the trend to focus on one side of a situation and overlook others.

The second hypothesis that length of horizontal lines judgment was affected when the converging lines background was present. This finding was consistent with Yamagami’s theory (1976, 1978; cited in Yamagami, 2007) that the former distortion is attributed to “the framing distortion “specified as a contour interaction between a horizontal line and two vertical lines as shown in (table 3). The absence of converging lines caused the error to be lesser as shown in (table 4)

One problem with the present experiment was the sample size was very small. In future studies, a larger sample size from different age group and different sex could be used to ensure that age is directly related to the misjudgment of lines length and also to explore the Ponzo illusion affect on female.

In interpreting the present results it could be argued stress affects lines length judgment especially when converging background is present. Age is also a factor to the increasing of judgment error, with age increase the rate increases, however, the rate of change is not yet defined. Participant one has higher error rate, this is possibly because of age of the participant. Participants two and three are same age, however, participant two lives in a noisier area and this possibly one of the reasons of concentration loss and caused a higher error rate than participant three.

The present study examined the effect of stress on Ponzo illusion on male students. Future study needs to examine other forms of stress and its effects on Ponzo illusion on female students.future research also needs to use larger sample and larger age scale in order to find the error rate compare to age. It would be of value in applied situations to control the confounding variables like food, exercise, and living environment to obtain accurate results.

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The Ponzo Illusion. (2019, Dec 07). Retrieved from

The Ponzo Illusion
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