Sensation

As explained in Chapter 9, attention is a major factor in the processing of information, as it helps us to direct our sensory systems towards relevant information, select appropriate information, allocate energy to process the information and regulate our attention to allow us to complete a task. Sensory information that is not attended to is poorly remembered, and, therefore, attention is a major component in the process of learning.

When we talk about sensation, what we are really referring to is the process of information coming in from the environment (internal or external) through the sense organs and being relayed to the central nervous system. The various sense organs (eyes, ears, nose, tongue, skin and proprioceptors) possess dedicated mechanisms that allow them to receive a specific form of energy from the environment (e.g. electromagnetic wavelengths of a specific range) and transduce this into nerve impulses. Transduction is the key component of sensation, as it is only after this has taken place that the nervous system can begin to process information in the form of action potentials. It is important to remember that sensory information, when it is first encountered, is in its rawest form and inherently meaningless. Assigning meaning to information requires the process of perception, which will be outlined in the next section. To be able to appreciate the complexity of sensation, we will take one example by going through the processes involved in vision. Although readers are referred to other texts (e.g. Kindlen 2003) for a description of the anatomy of the various other sense organs, and the sensory processes involved in each of these, what you should aim to remember from the section on vision are the generic principles involved in information processing.

Vision

Sight is the sense through which we are able to perceive electromagnetic radiation in the form of light waves. The light waves pass into the eye through the cornea (see Fig. 10.1), which starts to curve the light waves before they pass through the lens of the eye, where further curvature results in the light being focused onto the surface of the retina.

Fig. 10.1 Structure of the eyeball.

From Kindlen 2003, with permission

Retina

The retina can be considered to be the major structure within the eye since it is the location of the key process of transduction. Transduction is the process by which light energy gets converted into electrical energy (in the form of action potentials), thereby allowing the signal to be processed up through the various levels of the brain.

The retina (see Fig. 10.2) is a thin film of tissue lining most of the inside of the eyeball and contains blood vessels, nerve cells, photoreceptors and pigment cells. It is orangey-red in colour when viewed through an ophthalmoscope because of its blood supply and pigments. In one part of the retina a whitish patch (optic disc) can be seen, which consists of the axons of the optic nerve, i.e. the site where these axons leave the eyeball. The disc appears white because the axons here are myelinated.

Fig. 10.2 • Structure of the retina.

From Kindlen 2003, with permission

Photoreceptors

Two types of photoreceptors can be found in the retina – the rods and the cones – both of which are responsible for the conversion of wave energy into electrical energy. There are approximately one hundred million rods per retina and they are responsible for ‘shades of grey’ vision. The rods exhibit high sensitivity and low acuity, are more numerous in the periphery and are utilised primarily for night vision.

The cones on the other hand, of which there are only three million per retina, are responsible for colour vision and exhibit a low sensitivity and high acuity. The cones are utilised for day vision and can be found concentrated in the fovea.

The photopigments found in the pigment cells of the retina consist of retinal, a vitamin-A derivative, coupled to one of several lipoproteins (opsins). In the human eye there are four different lipoproteins creating four different photopigments: rhodopsin in the rods, and erythrolabe, chlorolabe, and cyanolabe in the cones. Only one type of photopigment is present in each cone and each pigment is best at absorbing light rays of a particular range of wavelengths. These ranges overlap and, as a result, the rays of one wavelength are absorbed by more than one photopigment, but to differing extents, e.g. 580 nm rays are absorbed by both erythrolabe and chlorolabe, but not by cyanolabe. The sensation of colour is created by the blend of signals generated by the three different types of cone. If one or more types of cone are absent from the retina, or lack photopigment, then our perception of colour changes (colour blindness), e.g. some colours may not be seen at all, while others are confused. Stacked, flattened membrane discs contain large amounts of the photopigment molecules. Each rod cell contains about 2 000 stacked discs, studded with 100 million molecules of the photopigment rhodopsin. Rhodopsin consists of an opsin protein and retinal, a light-absorbing vitamin A derivative existing as isomers.

If we take rhodopsin as an example, we can look in some detail at the biochemical cascade of events that takes place when light strikes the retina and stimulates a photoreceptor (see Fig. 10.3).

Fig. 10.3 • The visual transduction process.

From Kindlen 2003, with permission

The whole process occurs within 0.2 s of the photon reaching the rod cell and similar processes occur within the cones. In the cones, the variation in pigment is produced by different forms of opsin each with their own specific interaction with 11-cis-retinal. As a result of this there are different absorption sensitivities in the cone system:

• B cones at 420 nm (blue)

• G cones at 531 nm (green)

• R cones at 558 nm (red).

Visual pathways

The optic nerves from each eye meet up at the optic chiasma (see Fig. 10.4) where the nerve fibres carrying signals from the nasal half of the retina cross over to the other side so that information about objects on the right-hand side of the scene we are looking at (right visual field) is carried over to the left side of the brain and vice versa. If injury occurs to the visual pathway on one side of the brain after the chiasma, there will be loss of vision in one half of the visual field of both eyes, which is known as a homonymous hemianopia (see Chapter 5).