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An Experiment to Investigate the Effects of the Centroid Bias on the Judgment of the Inclination and Separation

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An Experiment to Investigate the Effects of the Centroid Bias on the Judgment of the Inclination and Separation.


The aim of this investigation was to observe how a red dot in a cluster of blue dots would affect the way one perceived the inclination and separation of lines between the two red dots. The research hypothesis was the varying distance would affect the error judgment. The design used to test this hypothesis was experimentation. The independent variable was the placement of the red dot in the cluster of blue dots. The dependent variable was the rating of the line by personal judgment. The control variables were the dot in the center of the cluster and the hundreds of trials performed. The participants in this experiment were 37 first year psychology students. The results came to show that centroid bias affected separation, although centroid bias did not affect orientation at 10o there was reasonably convincing evidence that it could have worked at 30o. Therefore, a conclusion can be drawn that the there was more of an effect with separation than orientation.

An Experiment to Investigate the Effects of the Centroid Bias on the Judgment of the Inclination and Separation.

The perceptual ability of a human being is astounding. Through the use of perception one can judge how round a ball is, how big it is, and even how fast it is going. Sensory coding is the area of psychology in which perception is integrated. Sensory coding incorporates the five senses and how each one of them functions and allows us to understand what is going on in the external world. Perception is the process, in which the information from the external world is transmitted and interpreted by the brain for perception. It is the final product in a chain reaction, which starts from the external world and is then translated into the nervous system and then perceptual responses are made in relation to these external events. The topic in which was researched was the effect of centroid bias upon judgment of the inclination of the lines between the dots.

MÑŒller-Lyer performed an experiment, which also shows how perception works and how it may be biased due to the centroid bias. Instead of using dots and clusters as the illusion, he used what is known as the Morinaga paradox. In this paradox there are two lines equal in length parallel to each other. However, on the ends of the lines are MÑŒller-Lyer fins which are placed laterally relative to each other, but the direction of the fins on the top line are the opposite to the bottom line (Gillam, B. & Chambers, D. (1985).

When observed, the observer thinks that the bottom line is longer, but in actual fact it is the same length as the top line. It is a perceptual illusion. The significance for this paradox is to show that position and size can be incongruous (Gillam, B. & Chambers, D. (1985).

In this experiment when one observes the outward fin the eye sees the centre point as further out than the point where the fin and the horizontal line meet because that is the way in which the eyes work and how certain illusions are perceived. It is also the same with the inward fins, however with this, one's perception views the centre point as further inward than where the actual meeting point is of the inward fin and the horizontal line, hence the line looks smaller than that of the outward fins. The reason for this is because of centroid bias. Centroid bias is when the visual system is set-up to pay attention to the middle or centre of the visual stimulus (Gallate, 2004). Centroid bias skews the ability to judge the length; hence one may see the line as being longer or shorter due to the fact that the center point is viewed as being further outward or inwards depending on the direction of the fin. In this experiment the results came to show that the outward fin had more of an illusion than the smaller one because of the centroid bias.

In relation to the perception experiment performed on the 20 first year psychology students, centroid bias too affected the way in which the students perceived the inclination of the line. The students instead of judging the separation of the two dots actually judged the separation of the centers of the clusters in which the two main dots where incorporated. This again supports the evidence that centroid bias affects the way certain things are perceived by the visual system. The problem that was investigated through this experiment was if centroid bias effected the perception of separation and inclination of a line. The results predicted by the hypothesis was that there would be a centroid bias in perceived separation and orientation between two points. The Alternate Hypothesis for this experiment was that there would be a centroid bias in perceived separation and orientation between two points. Hence, the varying distance from the center will affect error judgment. The null hypothesis for this experiment is that centroid bias will not affect the judgment of the separation or orientation of two points. Hence, varying judgment from the center will not affect error in judgment.


Experiment 1. Separation

Stimuli and procedure

The standard stimulus consisted of two clusters of red dots (r=11.4 mm) containing a single blue dot each, and the comparison stimulus consisted of a pair of blue dots (Figure 1 shows the back-and-white version of stimuli). The blue dots in the standard stimulus were placed either at the centre of the cluster or at various positions off centre (referred to as 'offset'). The 5 different condition created in this manner are shown in Figure 1 (black-and-white version). Participants were instructed to decide whether the distance between the dots in the comparison stimulus, positioned to the right of the standard, was smaller or larger than in the standard.

A staircase method was used to measure the perceived separation. The initial separation between the comparison dots was equal to the standard ± 11 mm or ± 19 mm (for the smaller and the larger standard separation, respectively). The step size was 1 mm, and each staircase was terminated after 8 reversals. A staircase method is a standard psychophysical method in which a level of the stimulus is adjusted after each response. For example, the initial separation between target-dots was 46 mm in an ascending staircase for the 57 mm standard. This separation was noticeably smaller than the standard separation,



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