The world of perception is multisensory. Even a simple task such as judging the position of a light in a dark room depends not only on vision but also on sensory signals about the position of our body in space. Likewise, how we experience food depends on sensory signals originating from the mouth, but also from nose signals, and even vision and hearing.
As an introduction to multisensory perception, this book is essential reading for students in psychology, philosophy, and neuroscience at the advanced undergraduate to postgraduate levels. As the chapters address topics that are often left out of standard textbooks, this book will also serve as a useful reference for specialist perception scientists and clinicians. Finally, as a monograph understandable to the educated non-specialist this book will also be of interest to professionals who need to take into account multisensory processing in domains such as, for instance, physiotherapy, neurological rehabilitation, human-computer interfaces, marketing, or the design of products and services.
Nicola Bruno is Full Professor of General Psychology, University of Parma, Italy.
Francesco Pavani is Full Professor of General Psychology, University of Trento, Italy.
The five senses reconsidered (pp. 3 -5)
The idea that humans have five senses is so deeply rooted in our culture that one could justly consider it as a key component of the way we think about ourselves. But are the senses really five? Ask anybody and you will get a puzzled look: Of course they are. Yet a quick tour to the libraries, real and virtu brings about quite a few surprises. Aristotle, and after him all philosophers in the Aristotelian tradition, maintained that we have six senses, not five. In addition to sight, hearing, smell, taste, and touch, he believed that humans have a common sense (see Gregoric, 2007). We return to this intriguing idea in the final part of this book. According to the witty Book of General Ignorance (Lloyd & Mitchinson, 2006), the senses are nine. In addition to the usual five, humans have senses for temperature, balance, pain, and bodily awareness. If instead we chose to lend credence to the ‘sense’ entry on Wikipedia (last checked on May 1, 2017; interestingly, this one keeps changing), the senses are ten. To the usual five, we must add other five ‘non-traditional’ senses for pain, balance, body position and movement, temperature, and other internal senses. Who is right?
Let’s take a look at a handbook of neural science (Kandel & Schwartz, 1985). We learn that sensory channels are defined in relation to the concept of a receptor. A receptor is a cell in the nervous system whose job is to transform external energy into a neural sign a function that is called transduction. This is not a book about human physiology, but we can try to apply this criterion. First of all, there are only two known mechanisms of transduction in vertebrates: those based on ion channels, which in humans code mechanical and thermal stimuli as well as acid and salty tastants, and those based on G-protein coupling, which code light patterns, odorants, and sweet, bitter, and umami tastants (see chapter 5). However, a complete list of receptors includes five types in the retina, the light-sensitive layer of tissue lining the inner surface of the eye (the three cone types for day vision, the rods for night vision, and the less-known light-sensitive ganglion cells for the regulation of night/day cycles), two types of receptor in the organ of Corti within the inner ear (the outer hair cells for pre-amplification and the inner hair cells for converting sound to a neural signal); at least nine different types in the skin (Pacinian corpuscles, Ruffini’s organs, Meisnsner’s corpuscles, and Merkel’s disks for pressure and vibration, thermal receptors for cold and warm, receptors for pain, plus specialized mechanoreceptors for itch and affective touch); two types in the muscles and joints (muscle spindles for changes in muscle length and the Golgi tendon organs for muscle tension); additional balance mechanoreceptors in the semi-circular canals and in the otolithic organs also within the inner ear (hair cells respectively sensing head rotation and linear acceleration); additional internal mechanoreceptors and nociceptors within internal organs; and a large number of chemoreceptors in the tongue, palate, cheek, epiglottis, and upper oesophagus (for gustation) and in the olfactory bulb in the nasal cavity (for detecting odorants). In short, if we were to use the receptor criterion, we would have to conclude that the number of human senses is either two or well above twenty, depending which criterion we choose for classifying.
But reading on through the handbook of neural science, we would also learn that sensory systems can be studied at the organ level. We certainly have sophisticated organs that play a fundamental role in perception. The eye, the hear, the tongue, and the nose immediately come to mind, as they are well localized and identifiable, and obviously relatable to four of the ‘senses’. But what about touch? That the skin can be considered the organ for touch is less obvious. The skin covers the whole body and coincides with it. It certainly is not as localized and identifiable as the previous four, and there are also sensory signals from the interior of the body, where there is no skin. If these are not part of touch, what sense should they be grouped with? But let us concede for the moment that we can consider the skin an organ. Do we now have a criterion for stating that there are five senses? Not really. There is at least another obvious organ that has a well-defined sensory function: it is the organ of balance within the inner ear. Do we have six senses then?
In response, a psychophysicist might argue that human senses respond to information carried only by three forms of physical energy: electromagnetic (light), mechanical (pressure, stretch, vibration including sound), and chemical (tastants and odorants). Therefore, sensory receptors could also be grouped into three categories only: photoreceptors, mechanoreceptors, and chemoreceptors, suggesting that in reality humans have only three senses. An evolutionary biologist might however respond that humans senses evolved to serve only two basic functions: obtaining information about the external world, and knowing about the internal state of one’s body. Perhaps we have only two senses after all? Perhaps. But we might then stumble into the work of English neurophysiologist Charles Scott Sherrington (1906), who argued that we should distinguish between two types of internal sense, proprioception that codes the position and movement of our body in space, and interoception that codes internal signals related to states such as hunger, drowsiness, or the need to go to the toilet. Maybe three is a more correct count of the number of senses?
If all this sounds confusing, perhaps we should change the angle of attack completely. Let us try a philosopher. An old and respected philosophical approach is that of phenomenology, the study of the qualitative character of percepts (phenomena are, indeed, things as they appear to us in our perceptual conscious experience). Incidentally, phenomenology also plays an important role in perception science, as mentioned in Tutorial 1.1, so the idea that we might find there useful criteria for ‘sensehood’ is not so far fetched. We could therefore ask our philosopher if all phenomena are of the same kind, or if there are distinguishable perceptual modes that have unique, specific phenomenal characters that belong to each mode and to no others. Presumably, he will respond that percepts are certainly not all of the same kind. For instance, there is something about the character of visual phenomena that is intrinsically different from that of auditory phenomena. These two kinds of perceptions have different qualia, the term invented by philosopher of mind Lewis (1929) to define ‘what it is like’ to experience a given percept (Nagel, 1974). Our analysis might eventually lead to the conclusion that there are at least seven different such modes of human perception: the visual, auditory, tactile, gustatory, olfactory, noxious, and thermal. In addition, and importantly, there is an eighth mode that is different from the previous seven because it is defined not by the presence of a certain perceptual character, but by its absence. In this mode, something is actually perceived, not merely imagined or thought-of, but the way the percept is given to our consciousness does not have a modality, i.e., it is a-modal. The notion of amodal perception is somewhat counterintuitive, and may seem puzzling. But consider Figure B1.1.2. It is quite clear here that recognizable grey letter figures are perceived as if they were behind the grey occluding blobs. Yet about 50 percent of those figures is not given in the visual modality, because the black occluders prevent you from actually seeing the corresponding parts. We return to the issue of amodal perception in section 1.6 and elsewhere, for it has several important implications. For the purpose of the present box, however, it will suffice to stress that phenomenological analysis also fails to provide a clear justification for the notion that we have five senses.
So how many senses do humans have? Having tried different angles of attack, we can appreciate that providing a scientifically accurate answer is far from trivial. Paradoxically, the only firm conclusion that seems to emerge from our list of potential answers is that by no criterion the senses are five. This leads to a different, and at least as interesting question: why then are the five senses such a deeply ingrained idea in our language and in our naïve psychology? The answer lies, in our opinion, in the tight coupling between perceiving and acting that characterizes the human mind (and, we suspect, the minds of most non-human animals). Perception is a complicated business that must be studied at several different levels of analysis. Our criteria for sensehood focused either on the initial levels (the ‘input’ to the mind, so to speak), or on the final product (the ‘output’). Thus we discussed external stimuli, receptors, and consciously perceived qualities of objects in the world. However, the path from external stimuli to percepts is not a linear one. Sensory channels do not act as passive sensors recording levels of external energy, but are part of systems that actively seek information in flows of ever-changing energy patterns. Perception is a form of exploratory activity, and there are exactly five ways that humans can actively and consciously explore the environment: by looking, listening, feeling, tasting, and sniffing. Presumably, it is this fundamental feature of the way we relate to our environment that forms the basis for our impression that there are five senses.
Courtesy by Oxford University Press.