Sensory Processes 7

PSY_C07.qxd 2/2/05 2:43 pm Page 134 Sensory Processes 7 CHAPTER OUTLINE LEARNING OBJECTIVES INTRODUCTION HOW DO WE GATHER INFORMATION? Light Sound The chemical senses The somatosenses SENSE ORGANS How do we see? How do we hear? Tasting and smelling Invisible forces and phantom limbs FINAL THOUGHTS SUMMARY REVISION QUESTIONS FURTHER READING PSY_C07.qxd 2/2/05 2:43 pm Page 135 Learning Objectives By the end of this chapter you should appreciate that: n psychologists have developed rigorous, objective methods for investigating sensory processes; n processing of sensory information relies on transduction (or transformation) of energy by sensory receptors; n information conveyed by light differs from that conveyed by sound; n light travels very fast and in straight lines over large distances, gives highly precise directional information, and can tell us about properties of distant objects; n different types of light radiation are perceived differently by different animal species; n many tasks are made much easier for humans by the way in which we process colour information; n sound travels in lines that are not straight, so we cannot use it to determine locations in space as well as we can with light; n sound also travels less quickly than light, so its time-of-arrival information can tell us about the direction of the sound source and the locations of sound-reflecting surfaces around us; n the analysis of speech is one of the most important tasks of the auditory system for humans; n we have other important senses apart from vision and hearing; for example, the perception of pain has probably been very important for human survival. INTRODUCTION It seems clear that in order to survive and func- dog. Sensing those traces will tell it that there is tion well in the world, an animal needs to know an intruder who needs to be persuaded to leave. what is in its environment. What kinds of things it This chapter is about ‘sensation’ – the process needs to know will depend on the kind of animal of ‘sensing’ information about our environment. it is. We can see from the hawk and dog examples that For example, a hawk flying high above ground there are two major aspects to the process, and and looking for prey needs some system of therefore the study, of sensation: detecting that prey at a great distance; it seems equally obvious that it would be in the prey’s 1. Understanding what is ‘out there’ to be interests to be able to see the hawk and take eva- sensed: what types of information are avail- sive action. In another situation, a dog may need able that could, in principle, be used to tell to know whether another dog has been present us what lies around us? This first area in its territory, and therefore needs some way of involves thinking about the physical proper- detecting faint chemical traces left by the other ties of the world. PSY_C07.qxd 2/2/05 2:43 pm Page 136 136 Sensory Processes 2. How this information may be utilized by a the first in which psychological techniques were biological system. perfected, long before the word ‘psychology’ was coined. This chapter shows you how these tech- It is no good having information all around us niques allow us to study the function of mechan- if we have no means of sensing it. So a major isms without requiring a person to describe his or part of this chapter will be about the sense her sense experiences in detail. organs and the type of information they send to But first we will be moving within the realms of the brain. This process is often placed within the physical science, which might feel a little strange domain of physiology, which is the study of the to students of psychology. From here, we move detailed functioning of biological systems in through physiology towards psychology. terms of their ‘hardware’. We will make the link Hopefully, what will emerge is an appreciation into psychology by asking how psychological tech- of the amazing cleverness of the sense organs, niques can be used to study the process of sen- and the intricate nature of the information that sation in humans. Historically, this area is one of they send for further processing by the brain. of UV radiation around you. Sunburn is the first stage of the pro- HOW DO WE GATHER INFORMATION? cess of the skin dying as a result of damage. So we know that UV radiation is damaging to skin, and Our world is a complex place. Right now, I am sitting at a desk. presumably other biological tissue too. This is the most likely I see the computer screen, and beyond that a window through explanation for our eyes having an in-built filter to remove UV which I see a garden and then a pine forest. I hear the faint radiation. To put it simply, if we were able to see UV rays, they whirring noise of the fan in the computer and the buzz of cicadas would be likely to damage our eyes. outside. I can smell the jasmine and pine sap from the garden – Some animals do possess UV vision, especially insects and these smells become more potent if I concentrate on them. I can birds. It is thought that they are less vulnerable to this hazardous taste the coffee, which I recently sipped. My skin feels pleasantly radiation because they live a shorter timespan than humans. Our warm in the summer heat, but my knee hurts where I grazed it a eyes must function throughout a long lifetime. few days ago. I also feel the itching from some mosquito bites. Other forms of short-wavelength information, such as X-rays How does all this information reach me? Examining the above and gamma rays, are even more damaging to tissue, but these are description in more detail, especially the physical sources of infor- filtered out by the earth’s atmosphere. mation, will help to explain what is going on when we receive information from the world. Infra-red radiation Why are we unable to see infra-red (IR) radiation? Would it be LIGHT helpful if we could? The answer to the second question is cer- tainly ‘yes’. IR radiation is given off in proportion to an object’s Arguably our most important perceptual ability is vision. We temperature. This is why it is used in night-vision devices, which know that vision depends on light: when there is no light, we can- can locate a warm object, such as a living body, even in the not see. What are the important characteristics of light, and how absence of light. This information could be extremely useful to do these affect the kind of information it conveys to us? us. So why do we not see it? Light is a form of electromagnetic radiation. ‘Visible’ light Precisely because we are warm creatures ourselves. Imagine forms just a small part of the full spectrum of this radiation (see trying to see while holding a strong light just below your eyes. figure 7.1). The sun emits radiation over a much larger part of the The glare from the light prevents you from seeing other objects. spectrum than the chunk of it that we can see. Why might this In the same way, we would suffer from glare if we could see IR be so? radiation. It would be like having light-bulbs inside your own eyes. To answer this question, it may help to consider why we do not Again, some animals do see IR radiation, but these are cold- see the two parts of the spectrum that border on the visible part. blooded creatures, such as pit vipers, which do not suffer from this glare problem. The IR information is very useful in helping Ultra-violet radiation them to locate warm objects, such as the small mammals they hunt for food. There is plenty of ultra-violet (UV) radiation about, especially as Humans build devices that transform IR into visible light – use- you get nearer to the equator and at high altitude. You will have ful for armies (and the psychopath in the movie Silence of the heard about your skin being at risk of sunburn when there is a lot Lambs) needing to ‘see’ warm objects at night, such as vehicles PSY_C07.qxd 2/2/05 2:43 pm Page 137 How Do We Gather Information? 137 1 mm Wavelength 0.001 nm 1 nm 10 nm 1 mm 1 cm 1 km Ultra- Micro- Cosmic rays Gamma rays X-rays Infra-red TV Radio violet waves UHF FM AM Visible spectrum Ultra-violet Infra-red (danger) (confusion) 300 400 500 600 700 1600 Wavelength (nm) Figure 7.1 The range of electromagnetic radiation. Note the small range of this spectrum (wavelengths from about 400 nm to 700 nm, where 1 nm is 10−9 metres) which gives rise to visual sensation. with hot engines and living humans. More humane uses of this Visible light – speed and spatial precision technology include looking for living earthquake victims. Figure 7.2 shows an example of an IR terrain image. A Landrover is Light travels extremely quickly, at a rate of about 300,000 km clearly visible from its engine’s heat. A normal photo of this scene per second. In effect, this means that light transmission is instan- would simply look black. taneous. So we cannot determine where light is coming from by perceiving differences in arrival time. No biological system exists that could respond quickly enough to signal such tiny time intervals.

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