After reading this article you will learn about:- 1. Mechanism of Visual Sensation 2. The Retina 3. The Duplicity Theory of Vision 4. Characteristics of Visual Sensations 5. Complementary and Non-Complementary Colours 6. Colour Vision 7. Evolutionary Theory of Ladd Franklin 8. Purkinje Phenomenon 9. Sensory Adaptation 10. Apparent Movement 11. Visual Constancies.
Mechanism of Visual Sensation:
The sensory experience which brings into awareness objects in the environment through the act of seeing is referred to as vision or visual sensation. The sense organ concerned with this is the human eye. The stimuli for visual sensation are the light rays.
In brief, the mechanism of visual sensation is as follows:
Light rays from external objects impinge on the human eye. The human eye is more or less similar to a camera. Just as the camera has a lens, the human eye also has a lens. The light rays pass through the lens and strike the inner layer of the eye known as the retina. The retina is comparable to the film in the camera.
Just as in the camera, the area of exposure is controlled by varying the aperture in the lens; the lens in the human eye can also expand or contract through the actions of a set of muscles known as the ciliary muscles. The ciliary muscles act depending on the intensity of the light rays and thereby regulate the functioning of the lens.
The retina is the vitally sensitive part of the eye and receives light stimuli. The retina is made up of two types of neural structures known as rods and cones. The rods are elongated structures and are found in greater number in the peripheral or outer region of the retina, whereas the cones are rather conical in shape and are in large numbers in the central part of the retina. The retina actually is a continuation of a very important nerve, the optic nerve.
The optic nerve which opens out as the retina at the rear end of the human eye, carries the visual stimulations from the retina to the occipital lobe which is situated at the hind-side of the brain. The visual system, therefore, consists of the occipital lobe, the optic nerve and the rods and cones in the retina.
At the spot where the optic nerve enters the eye and opens out as the retina there are no rods or cones. This part is known as the blind spot. Any light stimulus reaching this part is not sensed. There is another point in the retina, the central spot known as the fovea centralis. This is the spot of maximum sensitivity and light stimuli striking this point enjoy the clearest vision.
The Duplicity Theory of Vision:
The retina consists of two types of structures, rods and cones. These are sensitive receptors to light stimuli. It was von Kries who showed that the rods and cones perform different functions. The rods are sensitive to achromatic (black and white) light rays while the cones are sensitive to chromatic (colour) light rays.
Since, the rods are more predominant in the outer regions of the retina, this region of the retina is primarily involved in our seeing of form, outline and black and white elements. The central region dominated by cones is responsible for colour vision. This point is important in understanding the phenomenon of colour blindness. Some individuals are not able to see colour. This is because of the underdevelopment or damage to the cones.
Characteristics of Visual Sensations:
Our visual sensations vary in three dimensions. The first dimension is known as hue. The dimension of the hue represents the variations from a neural gray sensation, a white and black sensation to the different colours. Variations in hue are related to a characteristic of the light rays known as the wavelength.
Different light rays reaching our eyes have different wavelengths and the colour we experience depends on the wavelength of the particular ray. The next characteristic of the visual sensation is brightness. The visual sensations experienced by us differ in brightness, some being less bright and the others more.
Variations in brightness depend on the variations in intensity of the light waves. The intensity depends on the amplitude or the height of the light wave. The third dimension of visual experience refers to what is called saturation or purity. We experience either pure colours or mixed colours. This depends on the different kinds of- light rays (different wavelengths) reaching the retina.
We have referred to three characteristics of the visual sensation – hue, brightness and saturation. The relationship among these three can be represented schematically in the form of a colour pyramid or a colour solid (See Figure 6.3).
In the diagram below variations in hue are represented by points on the circumference. Variations in brightness are represented by points on the vertical axis and saturation by points on the radius as one moves from the centre to the circumference.
The colour pyramid gives us an idea of the enormous number of visual sensations the human eye is capable of experiencing. This number is estimated at about seven million visual sensations. Fortunately we do not have names for all of them.
Complementary and Non-Complementary Colours:
The reader has already heard about the colour pyramid. In this pyramid the various wavelengths or hues are arranged in the form of a circle. Thus, for every hue in the circle there is a hue diametrically opposite. An interesting phenomenon happens if hues which are diametrically opposite to each other are mixed. If two such hues are mixed, the resulting colour is grey.
Those which when mixed give a grey sensation are called complementary colours. Among the common colours, red and green are complementary and similarly yellow and blue are also complementary. On the other hand, colours which are “not complementary when mixed produce an experience which is a mixture of the two colours. Such colours are called non-complementary colours.
The reader must have come across another interesting experience. Light rays when allowed to strike a prism and pass through the same, split into the seven colours commonly seen in a rainbow. This shows that white light (sunlight) is a combination of these seven colours. When all the basic colours are mixed the result is a neutral grey.
The above phenomenon illustrates the properties of visual sensation. This is of interest in explaining some visual experiences like afterimages and visual contrast. If we look at a red patch of light intensely for some time, then even when the light is switched off we continue to see the red patch for some time.
This is called a positive after-image. Positive after-images have the same properties as the original experience. In the same example, if we continue to look at the place where the red patch of light was seen, the positive after-image (red image) vanishes and in its place we experience a green patch of light.
This experience is called a negative after-image. The green sensation appears even though we have not looked at a green patch of light at all. The reader will find that in the colour pyramid, green is situated diametrically opposite to red and is, therefore, complementary to red.
Negative after-images take on a hue which is complementary to red. The reader will now understand the relationship between complementary colours and after-images. Similarly, we can see that if we continue to stare at a white patch of light, after some time we begin to see black.
Our ability to see colours is possible because of the functioning of the cones. It is estimated that there are six million cones and that they are found concentrated in the central part of the retina. The colour or hue that we see, depends on the wavelength of the light rays. Each cone transmits an impulse to the brain, thus, producing a high degree of precision.
The human eye is able to differentiate between different colours. These colours, however, are variations of certain basic colours. One of the main questions regarding colour vision is: What are the basic or primary colours? There have been different theories but, in general, it has been shown that from a psychological point of view there are four primary colours, red, green, blue and yellow. The human being is able to see these colours because there are three different types of cones sensitive to wavelengths of light rays corresponding to these colours.
An important experience in human visual sensations is the inability of some people to respond to colours. There are some people who are not able to differentiate any type of colour. There are also others who cannot respond to red and green but can see other colours. There are still others who cannot see blue and yellow, though this is rare.
How can this happen? The answer to this question has come in the form of several theories on colour vision. The earliest theory formulated by Young and Helmholtz held that the retina contains three types of cones; one receptive to red, a second receptive to green and the third receptive to blue.
This theory tries to explain different aspects of colour vision on the basis of these three different types of receptors. But this theory is not in a position to explain certain aspects of colour vision. According to this theory, our ability to experience yellow depends on the combined activity of the receptors meant for red and green. But there are individuals who cannot see red and green but are able to see yellow.
Similarly, it is found that among colour blind people who cannot see “red”, there is also an inability to see “green” but these people are able to see “blue”. Thus, there appears to be a relationship between red and green but no relationship between blue and either of the two colours mentioned above. Young and Helmholtz’s theory does not explain these facts.
Another theory propounded by Herring also assumes that there are three types of cones. One type is responsive to the white-black range. A second type is responsive to the red-green range and a third type to the yellow-blue range. This theory, while explaining some facts relating to colour blindness and after-images is, however, not in a position to explain all other aspects of colour vision.
Evolutionary Theory of Ladd Franklin:
This theory proposed by Christina Ladd Franklin is evolutionary in its approach. According to this theory, at the earliest stage of development, the human individual does not respond to colour but responds only to form and outline. This is because only the rods are fully developed at this stage.
At the next stage the child is able to respond to yellow and blue and at a still later stage to red and green. Thus, the ability to experience different colours is evolutionary in nature. This theory is able to explain different facts of colour vision and also the phenomenon of colour blindness. It has been found that colour blind people who are unable to see yellow are also unable to see blue. A similar combination has been found in the case of red and green. The Ladd Franklin theory thus appears to be in a position to explain many facts of colour vision.
The cones are usually very effective under intense illumination. But under conditions of darkness they are not active. When the intensity of illumination is low, cone vision gives place to rod vision. This phenomenon is known as the Purkinje phenomenon. It is a common experience that yellow is the brightest colour in daylight whereas blue is the brightest at night.
One can see that in a multi-coloured visual experience, yellow which is bright during daylight becomes less and less bright with the approach of darkness. Similarly, red also is brighter during day-time but becomes less bright in night vision. It is, therefore, seen that the brightness of colour changes from day vision to night vision.
This phenomenon of shift in brightness explains a number of experiences. Thus, the green leaves of a plant appear brighter after the red flowers have disappeared. Similarly, the increase in accident rates during twilight can again be explained in terms of the shifts in brightness.
A major characteristic of sensory experience is sensory adaptation. It was pointed out that as one continues to look at a red object, the object tends to appear less and less red. This is because there is a decrease in sensitivity of the receptors and consequently there is an increase in the threshold or limen.
A very interesting experience in this process of adaptation is the shift from day vision to night vision. For example, when we enter a cinema-hall which is dark inside we are unable to see anything for some time. Similarly, when we enter a brightly lit place, we experience some difficulty in seeing things properly.
This is because such experiences involve a shift from rod vision to cone vision or vice versa. This shift naturally takes some time. The phenomenon of sensory adaptation is a process of adaptation from one type of visual reaction to another type.
All of us have been to movies. In the movies we can see different types of action, like a person running. Actually the film consists of a number of shots with the person in different positions. But the person may appear to be running continuously because the picture frames are presented in quick succession.
We do not see any discontinuity or break in the film. Thus, we are able to see movements even though the pictures do not really show the movement as such. This experience is known as the illusion of movement or apparent movement. If two successive visual experiences are separated by an optimal time interval we do not see the break between the two, but see a continuous movement.
This phenomenon, highlighted for the first time by Wertheimer, was of great significance in the history of psychology and was known as the phi-phenomenon. Wertheimer argued that the human being tends to close gaps in perception between two successive perceptual experiences. It is this phenomenon which helps us to understand the active nature of perception.
This shows that our sensory experience can really go beyond the characteristics of the stimuli. It is probably not necessary to go into a further discussion of this phenomenon at this point. It has been mentioned here only to bring out the complexities involved in making a distinction between sensation and perception.
There are certain other phenomena which illustrate the complexity of the visual sensation. Such phenomena are of different types. Important among them is the phenomenon of constancy. For example, our visual experiences are often independent of stimulus or stimulating characteristics.
A one rupee coin appears to be of the same size whether you hold it near your eye or whether your friend holds it at a distance of 6 feet from you. This is called size constancy. Similarly, one rupee coin is perceived as round shaped, no matter at what angle it is held.
This is called shape constancy. Constancy phenomena illustrate the role of experience and knowledge in our sensory experiences. While the qualities and characteristics of stimuli are important, there are occasions when experience remains constant in spite of changes in the stimulus characteristics.
Phenomena like constancy and apparent movement indicate the complexity of sensory experiences and also the relative independence of sensory experience of stimuli and their characteristics. It is this characteristic of sensory experience that gives stability and consistency to visual experience.
Vision has been held to be the most important of all our sensory experience. In the above paragraphs, an attempt has been made to describe and explain different aspects of visual sensation and also to bring out the role of different factors in visual experience. Visual sensations as responses are, to a large extent, dependent on the characteristics of the stimulus.
Nevertheless, actual experience in many instances is not determined by stimulus characteristics alone but also by other factors. In fact, a very interesting aspect of visual experience is its occurrence when there is no stimulus at all. Some people see visions or ghosts when there are no stimuli.
Such experiences are known as hallucinations and are commonly found among psychologically abnormal individuals. Similarly, the intake of drugs and alcohol have also been found to cause hallucinatory experiences. Such experiences are explained on the basis of an activation of the brain. When the concerned parts of the brain are activated, sensory experiences occur even without actual stimulation.
Similarly, under certain conditions, like poor illumination, we mistake, for instance, a rope for a snake. Here, however, there is a stimulus but it is seen as different from its real form. Such experiences are called illusions. Hallucinations and illusions illustrate the complexity of sensory experience.