Psychology


An example of some of the fascinating study undertaken in the field of A2 Psychology. Psychology has many applications today, and is a useful choice when carefully combined with other subjects.

Discuss the structure and function of the visual system

The structure of the visual system is designed for the purpose of conscious visual experience. A key to the ‘setting up’ of such an encounter is binocular disparity, whereby three-dimensional images are projected with different information onto opposing ‘screens’ of the retina. The light that is encoded into electrical signals and projected onto the retina later pursues two separate pathways to the lateral geniculate nucleus (LGN) of the thalamus, and to the superior colliculus. From these two points it moves towards the visual cortex. One might distinguish between the basic sensory nature of the retina stage of processing to the higher level perceptual processing in the visual cortex, but these combine in one experience of vision. Vision has the obvious purpose of clearly distinguishing the surrounding environment, but a complex largely unconscious process takes place to produce the effect.

First we might consider the first stage of the visual system – from the eye to the retina. The radiant energy of light enters the eye through the cornea having encoded the image in photons. The eye is adjusted to the amount of light through a process of accommodation controlled by the ciliary muscles that adjust the iris and thereby the dilation of the pupil. The image is then transduced at the retina, inverted and projected onto the retina. The retina is divided into central and peripheral vision. The former is located in the centre at the fovea and is characterised by greater visual acuity. The optic chiasm produces a blind spot in our vision towards the edge of the retina, but this is ‘filled in’ so that we are not conscious of it. The retina consists of five rows of cells through which visual information is processed back and forth. The most important of these are the bipolar and the ganglion cells. At the rear of the retina lie a number of rods and cones. Rods are involved in adapting the light code as a contrast to darkness, whereas the rods are sensitive to colour. Having passed through these five cell layers the neural message leaves the retina through the optic nerve and is sent contralaterally to the optic chiasm.

The retina collects 136 million points of light, many of which are repetitive. It does not therefore process them all but rather summarises them. This summary process of the retina is the beginning of the process of what we actually see. But the structure of the visual system does not function without the second stage, which involves the two pathways mentioned in the introduction. The LGN is the main pathway to the visual cortex in the occipital lobe. It is organised in a similar way to the retina such that the two are adjacent to one another. Dobelle et al (1974) have demonstrated the significance of the primary visual cortex by stimulating the area of blind patients. The patients reported ‘seeing’ the shape stimulated by the electrodes. The normal pattern of vision can be observed by the stimulation of neurons in the retina, which produces a corresponding stimulation in the primary cortex; the excitement of other neurons has a similar effect in other cortical areas (Carlson).

How does this structure relate more closely to the function of the visual system? One way is through sensory adaptation. Rods and cones in the retina contain photopigments, biochemical substances that react with light photons. One example of a photopigment is rhodopsin which adapts the retina to light and darkness. A great deal of light causes rhodopsin to break down into elements: vitamin A and a protein molecule. This bleaches the photopigment and obscures vision, although the rhodopsin quickly reassembles itself. A similar need for rhodopsin occurs in situations of darkness. The combination of light and biochemical reactions reveals the responsive nature of the visual system indicating an inherent purpose. The second aspect of the visual system indicating its function more closely is colour processing. Colour processing enables us to detect and to discriminate. The former enables us to distinguish between an object and it’s background, and the latter makes it easier to see fine distinctions e.g. ripe and unripe fruit. This is described by the component process model on the basis of the strength of the excitatory signals of the component model as, compared with the inhibitory signals of the opponent model. This process determines the prevailing colour in perception. From this model we can see the beginnings of the way that the visual system may select visual information on the basis of both innate ability and some accumulation of perceptual experience.

The distinct nature of the structure and function of the visual system have been discussed in this essay enabling us to gain some insight into the rich nature of conscious visual experience and the very necessary functions it performs.