The world that we are aware of is a construction of our central nervous system. The building blocks are nerve impulses, travelling into the CNS along sensory pathways, produced by the various stimuli which surround us. Therefore the only stimuli which we can be aware of are those which produce neural activity - action potentials or nerve impulses - in those sensory pathways.The stimuli in the environm ent around us exist in various forms. Vision depends upon electro-magnetic radiation from the object, hearing relies upon mechanical pressure waves transm itted via the molecules in the air, touch is a direct m echanical deformation of the skin, while taste and smell represent information coded as chemical structure. The crucial stage leading to our being able to sense and perceive these stimuli is the conversion of the various forms of energy into a neural code; i.e. whatever the stimulus, it is eventually coded as trains of nerve impulses travelling along sensory nerve fibres.The conversion of stimulus energy into a neural code is known as transduction, and involves sensory receptors. These range in complexity from the highly specialized visual and auditory receptor cells found in the eye and ear, to the free nerve endings found in the skin which, even though barely modified from straightforward axons, are sensitive to painful stimuli and to heat and cold. W hatever their nature, the purpose of sensory receptors is to react to an appropriate stimulus by triggering action potentials in sensory afferent neurons. 86
The absolute dependence of our experiential world upon the nature and distribution of sensory receptors can be easily dem onstrated. The first two chapters described various intentional and accidental m anipulations of the brain in the conscious hum an subject. Electrodes can be implanted to stim ulate or record sites within the brain, with the patient able to provide a running commentary. Devastating damage can occur to whole regions such as the frontal lobes (p.53), with the victim aware that something has happened, but unaware of the extent of the injury.The brain can therefore suffer physical insult and yet the patient feels little or no pain. This is because brain tissue itself does not contain sensory receptors which respond to painful stimuli. In a pure sense, damage to the brain does not ‘exist’ as a painful stimulus for the brain, because it cannot be transduced into a neural code that the brain can operate upon. Obviously any other effects of the damage - on sensory, cognitive, affective, or motor systems - will be noticed, and there are regions of the brain devoted to the perception of painful stimuli encoded in pain pathways by sensory receptors in the body (see later). Stimulation in these latter areas can produce the experience of pain, just as stimulation of the visual cortex can produce the experience of vision. Even the ‘headache’ represents changes in fluid volumes encoded by sensory pressure receptors in cranial blood vessels, and only then passed to the pain centres of the brain for interpretation.W ith each of our senses, or modalities, we respond to (or are aware of) a range of stimuli; we see bright and dark objects, hear high-pitched and low-pitched sounds, taste sweet and sour foods. The range we respond to represents only a proportion of the available range, and for every sensory modality we could find another species with a different battery of sensory receptors and which therefore responded to a different range of stimuli. So, to return to my opening point, each species constructs its own view of the world, based upon the surrounding stimuli it can transduce into the sensory information which serves as the basis of perception; but although each species occupies a unique perceptual niche, related species such as the higher mammals usually show substantial similarity in the
structure and organization of sensory systems. O ur sensory systems may be readily divided into the five traditional modalities - vision, hearing, touch, smell, taste - plus pain and proprioception, which concerns the awareness of bodily movement and position in space. They may be further classified into functional groups: e.g. exteroceptive senses, covering those modalities dealing with the outside world such as vision, hearing, touch, taste, and smell; proprioceptive senses (as above); and interoceptive senses, covering those sensory receptors handling information from internal structures, such as pressure receptors and glucose receptors in the walls of blood vessels. However, the most straightforward approach is to consider the various sensory modalities one by one. I shall outline the organization of sensory systems dealing with touch, taste, and smell, cover pain and hearing in more detail, and then discuss the visual system at length. Although this matches relative im portance as assessed by the am ount of research devoted to each, it should be emphasized that the hearing or auditory system in particular is as relevant to the psychologist as the visual system.