List of top nine psychological experiments on sensation!
Experiment # 1. Laws of Colour Mixture:
The objects that we see in our daily life can be broadly grouped as coloured or chromatic and colourless or achromatic. The presence or absence of colour is due to property of the light waves which emerge from the object and stimulate our eyes. This property is called the wavelength. Newton while experimenting on these colours, showed that the various colours, like red, green, blue and yellow actually result from a breakdown of white or colourless sensation.
That is to say, by combining various colours it is possible to produce a colourless or a grey light. He found that two colours when mixed in appropriate intensities produce the sensation of white. Such pairs of colours are called complementary colours.
If however, they are mixed otherwise what results is a colour which predominantly has the appearance of one of the two combined colours, depending on the respective proportions, Newton also found that when all the primary colours are mixed in proper proportions it gives white light. These findings were formulated as laws of colour mixture.
To verify the three laws of the colour mixture:
(1) Complementary colours mixed in proper portions produce a neutral grey sensation.
(2) Non-complementary colours when combined do not produce a coloured sensation, but tend to produce a colour in between the two combined colours, depending on the relative proportions of the two.
(3) The primary colours, yellow, green, blue and red when mixed in proper proportions produce a colourless sensation.
A colour-mixing apparatus; this consists of a circular metallic disc graduated into different degrees and operated by a motor in a concealed box-like arrangement. The speed of the motor is adjustable.
Colour discs of red, green, blue and yellow. The discs are cut in such a way along the sector that they can all be slipped into each other.
1. Select a suitable subject and seat him on a chair of convenient height in front of the table on which the colour-mixing apparatus is kept. The apparatus should be kept in such a way that the light falling on the disc must be uniform on all parts of the disc. Further, care should also be taken to see that the subject is not colour blind.
2. Slip the red disc and the green disc into each other and fix them up on the colour-mixing apparatus so that half the area is red and the remaining half is green.
3. Give the following instructions to the subject:
“Observe the apparatus carefully. This disc will rotate. When it rotates tell me in which parts you see a gray colour.” Switch on the colour mixing apparatus and note down the subject’s observation.
4. If he does not report a sensation of grey colour, stop the colour apparatus and slightly vary the proportions of green and red. Now switch on the apparatus again and ask the subject what his observations are now.
Proceed in this manner until the subject reports the coloured sensation. (Gray colour)
5. When he reports sighting a gray colour stop the experiment and measure the proportions of green and red necessary to produce a gray colour.
6. Repeat the experiment with the blue and the yellow discs.
Record your results as follows:
Observe the proportions in which the two colours must be mixed. Ordinarily they will be equal. If, however, this is not the case then it may be due to lack of purity in the colour discs used or uneven lighting conditions or variations in perception.
1. Instruct the subjects as follows- “Once again observe the apparatus when it rotates. Tell me which colour you see, whether it is reddish, greenish, bluish, etc.”
2. Now fix the yellow and green discs and switch on the apparatus. Note down the subject’s response. Repeat the experiment varying the proportions of yellow and green, with larger proportions of green on some trials and larger proportions of yellow on other occasions. Note down the subject’s response. Repeat the experiment with the combination of yellow and red, red and blue.
Tabulate the results as follows:
Find out from the subject’s responses the different colours seen by him under different combinations using different proportions. See whether the results confirm the second law.
This time fix all the four colour discs, blue, yellow, red and green, and instruct the subject as follows:
“Observe the wheel when it rotates and tell me when you see a grey colour.” Rotate the wheel with different proportions of different colours. Note down the subject’s response. If he does not see a grey colour repeat the experiment altering the proportions of different colours, until the gray sensation is produced. Note the final proportions of different colours.
The results of such an experiment can be useful to painters, both of walls and of canvases, in combining colours to produce different shades.
Most of us are able to see and respond to different colours. However there are some people who cannot respond to some colours or sometimes to all the colours. They are colour blind. Colour blindness can be total or partial. The former group is blind to all colours, while the latter group is responsive to some colours and not to others.
Some partially colour blind people are unable to see red and green (complementary colours) while some others fail to respond to yellow and blue. Colour blindness is usually hereditary. There have been several theories to explain the occurrence of colour blindness. Some of the well-known are those of Young-Helmholtz, Hering and Ladd and Franklin.
Several methods have been devised to identify colour blind people. Of these, the best known are the Ishiharar plates. Colour vision is possible because of the activity of the cone cells in the retina. The other cells in the retina called rods are sensitive to form and outline. The cones are predominately present in the centre of the retina and the rods towards the periphery.
To test whether a subject is colour blind.
Ishihara test, Manual of Instructions.
The Ishihara test consists of 38 colour plates. Out of them, the first 25 bear some numerals which the subject have to read and the other 13 plates have certain outlines which have to be traced by the subject with his hand. The latter are used only in cases where the subject cannot read numbers.
Seat the subject in a comfortable chair in front of a suitable stand on which the plates can be displayed. The plates have to be arranged in such a way that the light illuminates all of them evenly.
Give the subject the following instructions:
“I will be presenting a number of plates one after the other. After each plate is presented, tell me what is the number printed on it.”
Tabulate the subject’s responses as given below:
Go through the responses and correct them with the help of the key given in the manual. With the help of the manual find out the number of responses that are indicative of red-green blindness and of yellow-blue blindness. Now conclude whether your subject suffers from colour- blindness.
Applications of the Experiment:
Tests of colour blindness are very useful in selecting motor drivers, engine drivers, pilots, railway guards, etc. Such jobs require a capacity to respond to colour stimuli. In such job environments colour blindness can lead to serious accidents.
Our ability to see an object depends on the stimulation of some part of the retina. Thus an object, if it must be seen, must be located within a particular range or in other words, our eyes can see an object only if it is located at a suitable distance and angle so that its image falls on the retina. This area beyond which stimuli cannot stimulate the retina is called the visual field. The two eyes since they are differently located, have two visual fields which however overlap.
Within the same visual field the clarity of vision depends on the area or the zone of the retina which is stimulated. For example, a red object is perceived as clear red when its image falls on one particular zone or region of the retina. In other words, the different parts of the retina vary in sensitivity to lights of different colours or wavelengths. Some zones or regions of the retina are more sensitive to red while others are more sensitive to yellow, blue, etc. Thus it is also possible to map out the different colour zones of the retina.
(a) To map the visual field of the two eyes,
(b) To map the colour zones.
An eye perimeter, coloured squares with a holder, recording discs, a blindfold. The eye perimeter consists of a wooden board to which is fixed a circular disc graduated in degrees. The graduated disc is fixed with a metallic pointer, and a graduated metallic are which can be adjusted metallic to be any position.
The angular position of the metallic arc to the eye can be read with the help of a pointer on the graduated disc. In the middle of the disc is a point which is at the same level as the pupil of the eye. The metallic arc in its rotation passes through all the radii along a spherical orbit corresponding to the surface of the retina.
Any object moving in this orbit thus falls within the visual field. On the back of the graduated disc is an arrangement by which circular recording paper discs can be fixed. This disc is useful for marking the positions of the stimuli. The stimuli are in the form of small squares of stimuli. The stimuli are in the form of small squares of white, red, black or any other colour, which can be slid along the length of the arc with the help of the stimulus holder.
(a) Mapping the Visual Field:
The subject is seated comfortably with one of this eyes blindfolded. He is asked to rest his chin on the chin rest and keep it steady and look straight at the central point of the graduated disc.
“Here is a small object. I will keep on moving this object. Every time I ask you, tell me whether you see the object or not”. The stimulus is moved slowly along the metallic arc and the reading is noted when the subject spots the stimulus in the inward direction and when the subject loses sight of the stimulus in the outward direction.
The position of the metallic arc is shifted 45 degrees from its original position and the experiment is repeated. Continue thus until the entire 360 degrees is covered in 8 positions of 45 degrees intervals. Repeat the experiment now with the other eye blindfolded.
(b) Mapping the Colour Zone:
The procedure is similar to that of the mapping of visual field excepting for the difference that here the subject is asked to name the colour as soon as it comes into view in the inward movement, and say when it disappears in the outward movement.
Record the results as shown in Tables 1 and 2.
Compare the acuity for colour and vision for each eye.
Compare the acuity for different colours for the same eye.
Collect the group data and determine whether individual differences exist.
Experiment # 4. After Images:
One of the properties of sensations in general is that the sensory experience continues for some time even after the actual stimulus is removed. This is the case with all sensations, though most characteristic of visual and auditory sensations. Sensory experience occurring after the removal of the stimuli is sometimes called “after sensation” or “after-image”.
The term after image is considered more apt because the experience occurs in the absence of actual stimuli. Several psychologists have worked on the phenomenon of after-images, and their findings have revealed interesting facts about the nature of such experiences.
After-images are of two types, ‘positive’ and ‘negative’. If for example, you stare at a red patch of light for some time and then shift your gaze to a white wall, you continue to see the red patch. This is called a positive after-image. Positive after-images occur in continuation with the actual sensations and have the same properties, e.g., colour.
If, however, you continue to look at the white wall, the positive red image disappears and in its place, a bluish-green patch appears. This is called a negative after-image. Negative after-images occur after the positive after-images and take on complementary hue of the original sensation.
Another phenomenon of visual experience is that of visual contrast. Visual contrast as a phenomenon is closely related to the phenomenon of after-image. Contrast is of two types, ‘simultaneous contrast’ and ‘successive contrast’.
In simultaneous contrast also one can demonstrate the phenomenon of complementary colours. For example, if you concentrate on a red patch on a grey background for a long time, then you will find the surrounding key background taking on a green colour.
Here, because of the phenomenon of simultaneous contrast, the background has taken on a hue complementary to red. The phenomenon of successive contrast is identical with the phenomenon of negative after-image. Contrast phenomena are compensatory functions of the brain.
To demonstrate the phenomenon of positive and negative afterimages.
Small one inch squares of red, green, blue and yellow. A large square of gray and also larger squares of red, green, blue and yellow.
1. Place the small red square on the larger grey square and instruct the subject to concentrate on the red square. Suddenly after about two minutes ask him to shift his gaze to a sheet of white paper. Note down his/her experience.
2. Now cover the red square kept on the gray background with white tissue paper. Ask the subject to concentrate on it again, and shift his gaze as before. Note down his/her experience.
3. This time, keeping the tissue paper again, draw an outline of the red square on the tissue paper and repeat the experiment. Note down the subject’s experience.
4. Repeat the experiment with the other squares.
5. Place the red square against the green, blue, yellow and red background one after the other. Ask the subject to concentrate on each of these and shift her/his gaze as before to a gray background or a white sheet of paper. Note down the experience and analyse the responses.
1. Analyse the Experiences:
(i) What is the response of the subject immediately after shifting her/his gaze?
(ii) If he has an image what are the characteristics?
2. Does the image change? If so, how?
We experience visual sensations because the estimations of the retinal cells are carried to the brain through the optic nerve. If light stimulation falls on that part where the optic nerve enters the eye we do not experience any sensation. This particular region is called the Blind Spot.
To determine the area of the blind spot of an individual.
A wooden screen, a sheet of paper, a blindfold, a head rest.
Draw two parallel lines separated by about an inch in the middle of a sheet of paper. On the left edge, between these two lines draw a thin cross (this is for the left eye,) for the right eye, and draw the cross on the right edge. Place wooden screen at a distance of about one foot from the subject and pin the sheet of paper on the screen.
Place the screen in such a position that the cross mark on the sheet of paper is right in front of the right or left eye, whichever is being studied. Adjust the level of the screen to the level of the eye of the subject after his head has been fixed on the head rest. The other eye which is not being tested should be blindfolded.
The stimulus object which may be a small black square piece of card board fixed to a rod, is held near the middle of the cross.
The subject is now given the following instruction:
“Fix your vision on the cross. You will see black-square. After some time you will not be able to see. Tell me then that happens. After some time you will again see the square. Indicate when you see it again”.
The stimulus point is gradually moved from the left to the right along the top line. The points of disappearance and re-appearance of the stimuli are marked out. This is done for three trials. Then a similar procedure is followed on the bottom line and the points are marked.
The mid-points between these transition points on the upper and lower lines are noted and a perpendicular line is drawn connecting these points. The stimulus is now moved on this perpendicular line upwards and downwards to mark out the transition points on the vertical plane.
The various transition points are connected to form a hexagonal figure.
The experiment is repeated with the screen and stimulus placed at a distance of two feet.
Study the sketch and measure the area of the Blind Spot.
Experiment # 6. Properties of Auditory Sensation:
We hear sounds because sound waves reach the hearing apparatus in our ears and are conveyed to the brain by the auditory nerve. The various sounds we hear differ in certain fundamental properties.
Just as hue, brightness and saturation are the properties of visual sensation, the auditory sense also has some fundamental properties. The three basic properties of auditory sensations are pitch, loudness and timbre or quality. The pitch of a sound depends on the frequency of the sound waves.
The higher the frequency of sound waves, the higher the pitch. Loudness or intensity depends on the amplitude or the extent of compression and vibration of the sound waves. Timbre or quality of the sound refers to the homogeneity of the waves reaching the ear at the same time.
If at any time the waves reaching our ear have identical frequencies and amplitudes, then the sound has a good timbre. If, however, in the same mass of waves there are variations in frequency and amplitude, then the resulting sound is considered as ‘noise’.
To demonstrate the properties of auditory sensation:
(b) Intensity or volume;
(c) Timbre or saturation, and
Tuning forks of different frequencies, a rubber hammer, and a stopwatch.
This experiment is a purely qualitative experiment and the analysis of the results is based on the subjective experiences of the subject.
(a) Demonstration of Pitch Difference:
Strike a tuning fork with a frequency of 256 and ask the subject to make a mental note of the sound. Then strike another which has a frequency of 384. Ask the subject to write down the difference in experience, if any. Similarly, compare the experiences by pairing tuning forks of different frequencies.
(b) Intensity or Volume:
Take a particular tuning fork and strike it two or three times with the hammer with varying force. Ask the subject to note the differences in the auditory experience. Repeat the experiment with some other tuning fork.
(c) Timbre or Saturation:
Strike two different tuning forks of varying frequencies individually. Ask the subject to make a note of the two experiences. Now strike both of them at the same time. Ask the subject to compare this experience with the two earlier experiences.
Take four tuning forks with frequencies of 250,285,260 and 166. Set them into vibration and demonstrate beats. (Explains to the subject what a beat is.) Then the subject is instructed to count the number of beats using the stopwatch. The duration of the beats is also noted.
Analyse the introspective reports of the subject.
To study the distribution of heat, cold, pain and touch spots on a limited area of the skin.
A rubber stamp, a razor blade, a number of pointed metallic needles, a jar of ice, a jar of hot water about 50 degrees centigrade in temperature, and a bristle mounted on a thin rod.
A. Cold Spots:
Ask the subject to place his forearm on the table and choose a small portion of the forearm where there is little hair Mark this area. Two or three metallic rods are kept in the jar of ice for a few minutes. The subject is then blind folded.
The experimenter with the help of one of the rods taken from the ice jar touches one after the other the small points indicated in the rubber stamp impression on the subjects forearm. Whenever the subject reports experiencing cold sensation, the point touched is noted down in the impression on the record sheet. The experimenter explores the marked out area fully. The rods are changed frequently to retain the cold stimulus.
B. Heat Spots:
The procedure here is identical but instead of rods dipped in the ice jar, rods dipped in hot water are used. The points where the subject reports having experienced heat are noted down in the record sheet.
C. Touch Spots:
The procedure here is identical but instead of the metallic rods, the bristles mounted on the rod are used. The points where subject reports a sense of touch are noted down.
D. Pain Spots:
Here, a needle is employed instead of the bristles mounted on the rods. By mildly touching the various points without tearing the skin, make note of the points which give rise to pain sensation.
Compare the distribution of these different types of skin receptors.
To determine the absolute threshold for touch sensation. (Absolute threshold is the minimum stimulus intensity which elicits the response on 50% of the trials.)
A Hair Aesthesiometer:
It is a small instrument originally designed by Von Frey. It consists of a small tubular arrangement to which a strand of hair is attached. By means of a nut and screw arrangement, it is possible to vary the length of the hair and thereby the pressure on the skin. When an individual is touched with this hair, the smaller the length of the hair the greater is the pressure on the skin. There is a scale attached to the instrument which gives the length of the protruding hair at any time.
The experiment is done in two series:
i. Ascending and
i. Ascending Series:
Blindfold the subject and ask him to place his forearm on a table. Keeping the hair on the instrument fairly long, touch the subject with it on a particular point on the forearm, holding the instrument in your hand straight up. Ask the subject whether he felt any touch or not. Most probably he will say ‘no’. If however, he says ‘yes’ make the hair still longer. You should start with a length which gives a definite ‘no’ response.
After each trial go on decreasing the length of the hair by one millimeter; at each step touch the subject as before and ask for his responses. When he answers ‘yes’ three times consecutively stop the experiment. This is called the ‘ascending series’, because the pressure imparted through touch is being gradually increased. Note the subject’s response when each point is touched.
ii. Descending Series:
In the descending series, start with very short hair so that the subject definitely feels the touch, and go on increasing the length of the hair in steps of one millimeter until on three successive trials the subject gives negative response. As before note down the length of the hair and response.
Give ten trials on the whole, five in the ascending and five in the descending orders.
For each trial, locate the absolute threshold. This is done as follows- The absolute threshold is the length of the hair corresponding to the first of the three consecutive ‘yes’ judgements in the ascending series, For the descending series it corresponds to the length of the hair for the last ‘yes’ judgement before three consecutive ‘no’ judgements. Ex- In the table given below for the first trial, the absolute threshold is 12 mm and for the second descending trial it is 13 mm.
Tabulate the results as follows:
a. Calculate the average thresholds for all the ten trials.
b. Calculate the average for the ascending and descending series separately and test whether they differ significantly.
c. Tabulate the group results as follows:
d. Discuss the variations among the members. See whether there is any significant difference between the two series.
Experiment # 9. Auditory Localisation:
To study the subject’s ability to locate the direction of auditory stimuli in the absence of visual cues.
A sound perimeter or a cage, a blindfold and calling bell. A stool which can be placed inside the sound perimeter.
The sound perimeter is a cage like structure of about 4 to 5 feet in height. The diameter is about 3 feet. On the top of the cage, there are four cross-wires dividing the top portion of the cage into eight divisions. The stool can be kept inside the cage and the subject can sit comfortably on it.
The subject is blindfolded, made to sit on the stool and instructed to keep his head steady with the help of a head-rest. The subject’s head is kept in line with one of the central diameters of the roof touching the head-rest.
He is given the following instructions:
“You will now hear a series of bell ringing sounds. Of course, before each sound I will call, ‘now’. Listen carefully and as soon as you hear the sound of the bell point out the direction from which it comes.” The experimenter places the bell at each of the eight points marked by the endings of the four cross-wires on the top of the cage. These points can be respectively called back-left, back-right, front-left, front-right, middle-left, middle- right, front-middle and back-middle.
From each of these points he rings the bell and notes the direction reported by the subject. The bell must be sounded from these points in a random order. The whole series is to be repeated thrice.
1. Tabulate the results as follows:
4. Find out which points are easily located.
5. Compare the results of the points in the medial plane with the results of the points in the lateral plane.
6. Repeat the experiment first with right ear plugged and then with the left ear plugged. Discuss the advantages of having two ears.