Experiments on Perception | Experimental Psychology

To demonstrate the changes in perceptual organisation accompanying changes in selection and grouping of stimuli.

Figure ‘A’ containing 13 small X marks arranged in a square pattern on a white background (black X marks). The X marks are arranged in five parallel rows having three, two, three, two and three X marks respectively. Figure ‘B’ has 28 X marks (black X marks on a white background) arranged in a pyramid pattern with the base line having seven X marks, the next on 6 and the top there being only one X mark.

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I. Figure ‘A’ is hung on the wall at a convenient height. The subject is seated at a distance of 8 to 10 feet and instructed to focus his vision on this figure.

Then as he is looking at it, the following descriptions of his experience are obtained:

1. What does the figure represent?

2. Does he experience any change in the pattern in which the dots are grouped? If so, he has to describe whether these changes in pattern appear gradually or suddenly.

3. Whether at any moment he sees two or three patterns, i.e., whether the different parts of the figure appear to be differently organised.

II. Take Figure ‘A’ away and after a few minutes of rest, place figure ‘B’ before the subject and repeat the experiment.

1. Analyse the responses of the subject and note the different factors which come into play in the grouping and organisation of the stimuli.

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2. Find out whether shifts in perceptual organisation are sudden or gradual.

Note:

1. Repeat the experiment with figures in which we have dissimilar elements, e.g., marks and squares mixed.

2. Repeat the experiment with figures in which the distances among the dots in the same line are unequal.

3. Use figures in which the dots or X marks are not arranged in a regular shape or pattern.

Experiment # 2. Mental Set and Perception:

Our perception of a stimulus is influenced not only by its properties but also by certain other factors. Some of these are our needs, our past experiences and our present mental conditions or sets.

For example, if we are expecting a visitor, even the noise of rain drops falling on the wooden door is mistaken for footsteps. Again, if we are travelling in a train passing through a series of mountain ranges visible at a distance through the window, then even a cloud is mistaken for a hill. This illustrates influence of mental set on perception.

To demonstrate the effect of mental set on perception.

A large number of cartoon drawings falling into two categories- Category ‘A’ consisting of a number of animal figures, Category ‘B’ consisting of a large number of human figures and an ambiguous drawing.

For the purpose of this experiment, the subjects are categorised into two groups ‘A’ and ‘B’.

They are given the following instructions:

“You will be shown a series of pictures, each one for a short duration; you must judge what these pictures represent”. The pictures may be exposed through an exposure apparatus.

Procedure for Group ‘A’:

Present each of the animal pictures and note down the response for each. Then present the ambiguous picture. Note down the response, and ask the subject whether he sees anything else in this picture.

Procedure for Group ‘B’:

The procedure is similar to the above except that for this group the human pictures are shown first, followed by the ambiguous picture.

1. Count the frequency of ‘animal’ and ‘human’ responses for the ambiguous picture in group ‘A’ and group ‘B’. See whether the groups differ, significantly, in their responses to the ambiguous picture.

2. Tabulate as follows:

3. Test the significance of difference between the two groups, with reference to ‘animal’ and ‘human’ responses computing X²(Chi- square) with the above 2X2 contingency table or by computing CR for percentages.

Experiment # 3. Apparent Movement or Phi-Phenomenon:

When we go to see a movie, we see the objects and persons on the film moving continuously. However, in the actual film there in no such continuity. What appears as a continuous movement is actually a series of separate films. However, we do not see them separately and the motion has an apparent continuity.

This phenomenon was studied in the laboratory with the help of simple light stimuli by the German psychologist Max Wertheimer. From the results of this experiment, he developed an elaborate theory of perception called the Gestalt theory of perception. This theory was later extended to learning, memory and other phenomena and the supporters of this theory came to be known as Gestalt Psychologists.

To determine the optimal distance and time- interval for the occurrence of phi-phenomenon or illusion of motion.

The Phi-phenomenon apparatus, a metronome and a stopwatch.

Description of the Phi-Phenomenon Apparatus:

The Phi-phenomenon apparatus consists of a board on which there are two lights, kept in line with each other. The two lights are movable, so that the distance between them can be adjusted. There is an adjustment by which the intensities of the lights can also be varied. By connecting a metronome (double-contact) in the circuit it is possible to regulate the time interval between the appearances of two lights.

The subject is seated at a distance of 3 feet from the apparatus and the following instructions are given to the subject:

“Focus your attention on the lights. When I switch on the light you will find one of these lights illuminated first, and then the other. When I say ‘ready’ keep on looking at it. At one point you will cease to see two separate lights. Instead, you will see one wave of light following along this glass screen placed in front of the two lights. Indicate to me when this happens by raising your hand”.

Now the experimenter sets the metronome at a speed of 60 beats per minute and conducts the experiment after giving the subject a ‘ready’ signal. As soon as the subject indicates that s/he has perceived a continuous moving light instead of two separate lights one after the other the experimenter notes the time. Five such trials are given. Then, the metronome is set at a speed of 90,120 etc., and the experiment is repeated. The experiment is also repeated with the subject seated at 6 feet and 9 feet distance.

On each occasion, give maximum time of three to four minutes for the subject to respond. If there is no response within this period, then the response is considered negative.

Tabulate the results as follows:

1. Find what speed in general the Phi-phenomenon occurs most frequently, irrespective of the distance.

2. Find out the speed at which the least time is taken.

3. Similarly, compare the responses when the distances change.

4. Is there any particular optimal combination of speed and distance?

5. Collect the group data and study individual variations.

Experiment # 4. Optical Illusion:

In the experiment on ‘perception and mental set’, our perceptions are influenced not only by the properties of the stimuli but also by several other factors, like the surroundings or past experience, mental set, etc. To quote a classical example, a rope lying on the floor at night is often mistaken for a snake.

This phenomenon of illusion or wrong perception can be demonstrated in the laboratory by a number of experiments. Several types of illusions have been designed for experimentation in the laboratory. Out of these, the simple and the most common ones are the ‘geometrical illusions’.

Problem:

To demonstrate the occurrence of error or illusion effect in perceiving lines.

Apparatus Required:

A Muller-Lyer illusion board and a horizontal-vertical illusion board.

Description of the Apparatus:

1. Muller-Lyer Illusion Board:

The Muller-Lyer illusion board consists of two horizontal lines, side by side. One of the lines ‘A’ has its extremities flanked by two open arrowheads; the other one ‘B’ has at its extremities two closed arrowheads.

2. Horizontal-Vertical Illusion Board:

Here again there are two lines, one horizontal and the other vertical. The length of the vertical line can be increased or decreased by means of a mechanical arrangement.

This experiment is done using two conditions in two series:

(1) With ‘A’ as standard and

(2) With ‘B’ as standard; in-

(a) Descending series and

(b) Ascending series.

(1) ‘A’ as Standard:

Give the following instructions to the subject:

“Look at this board, there are two lines. These two lines as you can see are unequal in size. I will keep the length of this line ‘A’ constant and go on varying the length of ‘B’ in small units, either increasing or decreasing. At every step you should tell me whether ‘B’ is equal to ‘A’ or not. When you say, they are equal, I will stop”.

Under this condition as we have already mentioned, there are two series:

(a) Descending Series:

Here the experimenter after fixing the length of ‘A’ starts with ‘B’ perceptibly longer than ‘A’ and gradually shortens it step by step until the subject says both are of equal length. Then the actual lengths of ‘A’ and ‘B’ are measured and the difference is noted down. [Note how much shorter or longer ‘B’ is, put an appropriate minus (-) or plus (+) sign before the error value.]

(b) Ascending Series:

Here the experimenter starts with line ‘b’ perceptibly shorter than ‘A’ and goes on increasing its length until the subject says ‘A’ and ‘B’ are equal. The errors are noted down as above.

(2) ‘B’ as Standard:

The procedure here is exactly the same except that the length of line ‘B’ is kept constant while that of ‘A’ is varied. The subject is instructed to compare A’ and ‘B’ and indicate when they appear equal.

As before, the experiment is done in both the ascending and descending series. Under each of the two conditions, there will be ten trials in ascending series and ten in descending series alternately. There will then be a total of 40 trials.

Horizontal-Vertical Illusion Procedure:

1. Calculate the average errors in estimation of lines for each the subjects as below:

(a) Average error in all the 40 trials (P)

(b) Average error in condition one (Q)

(d) Average error for all ascending series put together (S)

(e) Average error for all descending series put together (T)

2. Tabulate the results for the group as follows:

1. Discuss the individual variations in all the columns.

2. Do individuals have positive or negative values in all the columns?

3. Discuss the variation among the column averages.

Here also there are two lines. One horizontal and the other vertical. The horizontal line is fixed and the vertical line is variable.

Hence, the subjects are given the following instructions:

‘Look at these two lines. They are unequal in size. I will go on changing the vertical line, increasing or decreasing its length. Tell me when you find the lines equal.’

As in the case of Muller-Lyer illusion, ascending and descending trials are taken. In the ascending series the experimenter starts keeping the vertical line perceptibly shorter and increases the length gradually while, in the descending series, the experiment/starts with the vertical line perceptibly longer and goes on gradually decreasing.

The errors are measured and noted with their appropriate algebraic signs.

i. Calculate the average errors for the descending and ascending series.

ii. In which series is the occurrence of error greater? Is there any difference in the positive or negative direction of the errors?

iii. Tabulate the group results and discuss the individual differences.

Experiment # 5. Binocular Fusion:

The human eye is innately equipped to perceive visual objects only in two dimensions, i.e., in length and width or in height and width. However, in daily life we respond to a third dimension also namely depth, distance, etc. We perceive objects as 3-dimensional objects. The 3-dimensional film is a classical example of this.

One of the factors which helps us in perceiving the third dimension is called retinal overlap. This overlap takes place because of an overlapping of two slightly disparate images produced on the two retina. This overlap takes place in a region called ‘horopter’.

This phenomenon can be illustrated by the following experiment:

To demonstrate the phenomenon of binocular fusion.

Materials Required:

A stereoscope, a set of stereoscopic pictures, a meter rod.

Procedure:

Slide the stereoscope on the meter rod through the groove in the stereoscope and clamp the meter rod at its two ends on two stands so that it is kept parallel to the surface of the table. Adjust the height of the two stands so that the meter rod is at level with the eyes of the subject who is comfortably seated on a chair.

Give the following instructions to the subject:

“Look through this stereoscope (fix one of the stereograms in the holder of the stereoscope). Each eye will see a different picture. I will keep moving this along the scale. Tell me when you see a single picture instead of two pictures. For example on one side of this picture now you see a parrot, and on the other side you see a cage. At one point you will find that the parrot has gone into the cage. Tell me when that happens. After some time this oneness will change and again, as before, two pictures will appear. Tell me when you experience this change also.”

The experimenter makes note of the two transition points. A total of ten trials is given. For five of the trials the experimenter starts from a point close to the eye and keeps on moving the stereoscope away from the subject (outwards). For the other five trials the stereoscope is originally fixed at a point far away and gradually moved nearer to the subject (inwards). The inward and outward trials are alternated.

Tabulate the results as follows:

i. Find out the range of fusion for each trial.

ii. Is there any difference between the inward and outward trials?

Experiment # 6. Depth Perception:

One of the important problems of the psychology of vision has been the problem of perceiving depth of distance. The earliest scientists to attempt an explanation of this were made by Galen and Leonardo da Vinci. Leonardo emphasised the importance of shadow in the depth perception.

According to him, perception of distance is mainly a mediated process, i.e., it is not an immediate perceptual fact, but depends on factors like clarity, loss of details, etc. Helmholtz attributed perception of distance to unconscious inferences.

One of the important facts established with regard to perception of distance is the role played by binocular vision. The presence of two eyes makes perception of distance more efficient than vision with one eye.

To verify whether binocular vision enables more accurate perception of depth compared to monocular vision.

The depth perception box is an elongated box which is illuminated inside and contains three concealed vertical rods or pins. These pins can be seen by a subject through a small window. Two of these rods are fixed and the one in the middle is movable, resting on a pulley bottom, wheeled on a rod running completely across the length of the box. The top of the box has a long groove-like opening throughout the length of the box.

The movable rod projects above the box through the groove-like opening and ends in a handle. By moving the handle the movable vertical rod can be shifted to any position along the length of the box. A meter-scale is fixed to the groove-like opening on the top so that the exact position of the movable rod can be read.

The experiment is done in two parts-monocular and binocular.

Monocular Vision:

Instructions:

Seat the subject in a convenient position by using a chin rest before the slit window so that he has a good view of the three vertical rods. The slit window must be adjusted so that it is only wide enough to give view of the three rods and not the rest of the box.

Blindfold the right eye of the subject. Instruct the subject as follows:

“Through the opening you will see three vertical rods. Two of them are fixed and the third one is movable. (Demonstrate) Now, I will keep on moving this rod keeping it at different points. You should ask me to stop moving the rod whenever you see the movable rod exactly in line with the other two fixed rods.”

Descending Series:

Start from a position where the movable rod is very much behind or away from the two fixed rods. Move it until the subject says that the three are in a line. Repeat the experiment about ten times, starting from different points behind the fixed pulleys.

Ascending Series:

The procedure here is essentially the same, excepting that here the movable rod starts from a position closer to the subject and in front of the fixed rods and is moved to a position away from the subject. The experiment is repeated ten times.

The ascending and descending series must be alternately carried out. Repeat the experiment by blindfolding the left eye.

Binocular Vision:

Remove the blindfold and repeat the entire experiment with both eyes open.

1. Calculate the error, positive or negative, for each series for the right and the left eye and for binocular vision. The error is recorded in units of deviation in millimeters between the fixed and movable rods. If the movable rod is behind the fixed rods, that is, if the movable rod is farther than the fixed rods from the subject, the error is counted as positive, and if the movable rod is nearer than the fixed rods to the subject, the error is counted as negative.

2. Calculate the average algebraic error for each condition-the right eye, the left eye and binocular vision.

3. Tabulate the results as follows:

4. Calculate the Mean and SD for the group for all the conditions.

5. Compare the results of monocular with those of binocular vision.

Note:

The ascending and descending series are employed to overcome certain biases which may result if only one direction of movement is employed, called movement error.

Experiment # 7. Perceptual Constancies:

Perceptual Constancy is the tendency for a perceived object to resist change in spite of wide variations in the conditions of observation. In other words, we can say that the perceptual constancies influence the subject to perceive the shape, size and colour of the objects as they are, in spite of the variations in the viewing conditions or the background of the stimuli.

Psychologists have widely studied these three phenomena viz. – Shape Constancy, Size Constancy and the Colour Constancy. The former two of these constancies are grouped under object constancy.

The following two experiments are used to demonstrate the phenomena of:

A. Shape Constancy, and

B. Size Constancy.

A. Shape Constancy:

Introduction:

Shape Constancy is defined as the tendency on the part of the individual to perceive the shape of an object as having the same shape in spite of the wide variations in the conditions of viewing. For example, we consider the shape of a circular plate as circular whether we look at it, sitting or standing before the table or even from a corner of a room. Inspite of the image that is cast on the retina being elliptical, we still observe it as circular and not as elliptical. This is because of the operation of the phenomenon of Shape Constancy.

To demonstrate the phenomenon of Shape Constancy.

Materials Required:

Shape Constancy Apparatus:

A circle is mounted on an upright rod so that the disc is in the vertical plane. In addition, several ellipses differing in width, but of the same height as the circle are also mounted on vertical rods. These ellipses are mounted in such a way that the long (major) axis is vertical. Provision is made to determine the extent to which the rod and circle have been rotated by means of a pointer and protector.

The projection of the diameter of the circle is calculated by means of the following formula:

p = d Cos θ

Where p = projection; d = diameter of the circle and θ = the angle of rotation.

The circle and an ellipse are placed parallel to the line of the subject’s sight at a distance of two meters and the subject is given the following instructions:

“There are two objects mounted on the vertical rods. One of them is a circle and the other an ellipse. I will rotate the circle till you perceive it as an ellipse. The moment the circle matches the ellipse, ask me to stop rotating the circular disc.” After the experimenter assures herself/himself that the instructions are clear to the subjects, he/she repeats the experiment about twenty times.

For one half of the trials the experimenter begins with the circle parallel to the line of sight and for the other half of the trials the circle is initially perpendicular to the line of sight. The starting positions should be alternated in random order. A new ellipse may then be substituted and the entire experiment repeated. The results are tabulated.

The projections of the diameter of the circle (p) on each trial with different ellipses are tabulated as shown below:

The larger the angle of rotation, the smaller the projection of the diameter of the circle needed to match the ellipse and hence the greater the constancy. The amount of constancy may be conveniently quantified by comparing the projection with the width of the ellipse. For this purpose, the average of the measurements of projection are computed.

The ratio width of ellipse/Mean Projection provides us with the index of constancy. If there were no constancy effect, the width of the ellipse and the mean projection would be equal and the ratio would be 1. The greater the constancy, the smaller the projection and hence the greater the ratio.

B. Size Constancy:

Introduction:

One of the optical properties of the eye is that the retinal image of the size of an object decreases as the distance of the object from the eye increases in accordance with the ‘Law of Visual Angle’. This law states that the linear size of the optical images is inversely proportional to the distance of the object. It is true that distant objects look smaller than the objects of similar size.

It is equally true that perceived size does not confirm the law of visual angle, according to which a man 10 meters away should appear half as tall as he should 5 meters away. Over a considerable range of distances, perceived size decreases much less than what would be predicted by the law of visual angle. The failure of perceived size to decrease in proportion to the distance is known as Size Constancy.

To demonstrate the phenomenon of ‘Size Constancy’.

Material Required:

A square (Standard stimulus) whose sides are 10 cms each is mounted on a vertical rod. Another square (Variable stimulus) is mounted on another vertical rod and its sides can be varied in length with the help of a pulley attached to this square.

A meter scale to measure the distance between the subjects and the standard square.

The experiment should be conducted in long corridors which are relatively free of furniture and other objects. The subject is comfortably seated and the standard stimulus (a fixed square which is also known as a test square) is first kept at a distance of one metre directly in front of the subject. The variable stimulus(the adjustable square which is also known as the Reference square) is kept at a distance of one metre, towards the side of the subjects so that he has to turn his head about 45 degrees in order to see it.

The experimenter then repeats the experiment in both ascending and descending orders. In the ascending series, the initial size of the reference square should be perceptibly smaller than the test square and in that of the descending series, the initial size of the reference square should be perceptibly larger than that of the test square.

The subjects are given the following instructions:

“(Showing the test square), this is a square and its size is fixed, (showing the reference square) and the size of this square is not fixed. That is, its size can be changed. In some trials I will keep the initial size of the reference square very small compared to that of the fixed square. I will keep increasing the size of the squares. In some trials I will adjust the initial size of this square such that it appears larger than the fixed square, then I keep decreasing the size of the square. You must ask me to stop when you perceive the sizes of the two squares as equal. I will repeat this experiment by placing this square (showing the reference square) towards your right and left always at a distance of one metre and I keep changing the distance of this fixed square (pointing towards the test square) from your seat. Whatever may be the alterations I make, your job is simply to ask me to stop whenever you perceive the sizes of the two squares as equal.”

The test square is thus successively moved to a distance of 2, 4 & 8 meters. At each of these distances, the sizes to the reference which are perceived as equal to test square are obtained as before. Besides, increasing the distance of the test square from the subject, it is desirable to decrease it successively from a distance of 8 meters to 1 metre. Thus, there is a receding and approaching series.