The tools that enable the animal to gather and interpret incoming sensations and information involve a number of receptors (eyes, ears etc.) and a processing centre within the central nervous system. The receptors with which we, as humans, are most familiar are touch, vision, hearing, smell and taste. These convey information about the physical and chemical environment. The eyes record light, the ears record sound, the nose and mouth record smell and taste. This information is carried to the brain where it does not enter an empty room but mingles with the mental images already in store. When a visual image, a face, for example, is registered in the cortex it will be recognised as familiar if that image is already logged in the memory bank or identified as novel by virtue of subtle differences from the other images available for inspection. The same principle applies to sounds and smells. This is a simple expression of Gestalt theory, which argues that animals do not build up mental pictures de novo but interpret images in a holistic way; something that can only be done if the mind has some prior knowledge of what to expect (23, 68). A good demonstration of this phenomenon is that humans who have had their sight restored having been blind from birth or shortly after, do not comprehend, at first, what they see. Gestalt theory gets a mixed reception from neurophysiologists but, like all theories, it makes no claims to be more than a partial explanation based on the available evidence. When we talk of minds, as distinct from brains, I believe it makes for a good working hypothesis. The amount of information transduced by the special sense organs and sent to the brain is massive and would be overwhelming if the mind did not pay special attention to that which is important and reject the rest. Physiologists use the word attention to describe the ability of the mind to concentrate on what matters. In everyday speech, we are more likely to say we can ‘focus our minds’. I shall use this expression from now on, while fully aware that it does not mean the same thing as to focus the eye. I believe Gestalt theory offers a good, parsimonious explanation of the mind’s ability to prioritise the things that matter. The conscious mind can only give proper attention to the few details that are important if it is able to ignore the barrage of incoming information that does not matter at the time. I understand (from reading rather than experience) that some of the most alarming effects of psychotropic drugs such as LSD arise from the fact that this inhibitory process is impaired, resulting in brain overload where key signals are drowned out in the general noise.

The neurophysiological mechanisms involved in the reception and processing of incoming information by the special senses at the cellular level are essentially the same in all animals. However, there are large differences both within and between phyla (i.e. mammals, birds, fish, amphibians etc.) in the design and function of the special sense organs that pick up signals from the environment and the make‐up of the central nervous system wherein the information is processed and decisions are made. Our human concept of the brain is that it lies within our skull and our large cerebral cortex is the key to our consciousness. This is not necessarily how other species interpret the world. Fish do not possess what human anatomists would recognise as the pain centre in the brain yet experience pain. The octopus has a relatively large brain yet perceives and interprets much incoming information using nerve cells in its limbs. Jellyfish have no organ that could be defined as a brain yet recognise light signals and use them to migrate between feeding grounds in the open sea and shelter in mangrove swamps. This reinforces my argument that a strict neuroanatomical approach to the understanding of the brain may fail to recognise many of the workings of the mind. The most direct way to discover whether an animal, can experience pain, fear, pleasure (etc.) is to ask the animal, using a non‐verbal language that the animal can reasonably be expected to understand.

This chapter outlines the various ways in which animals identify and process information transduced by their special senses and how different species have, through evolution, given priority to the information most conducive to their own genetic success and downgraded or switched off others, not least to avoid information overload.

Vision

To open with a blinding flash of the obvious, the function of vision is to provide information at a distance in circumstances where and when there is enough light to see by. When we probe a little deeper, more profound questions emerge, such as: What is light? How much light? What sort of light? How does the brain triage the non‐stop flow of information from the eyes to sift the small amount of information that matters from the general noise?

Light is defined as electromagnetic radiation within the visual spectrum. At first sight, this definition sounds clear cut but on closer inspection it becomes both subjective and species‐specific. We humans define light in terms of the spectrum of electromagnetic radiation (EMR) that is sensed by the retina in our eyes, transduced into nervous impulses and interpreted in our brains in the form of images that we can describe as pictures. Birds can see EMR at high frequencies that we describe as ultraviolet since they are invisible to us. Humans sense low‐frequency EMR in the infrared through their skin and interpret it simply as heat. The development of infrared cameras has enabled us to experience night vision by transforming heat radiation into a visual signal. Snakes have built‐in infrared cameras that sense infrared radiation and process it into detailed information as to the size, position and direction of movement of potential prey. Frogs can catch passing flies with extreme skill by instant processing of visual information as to their direction and speed of movement but are apparently unable to recognise the fly when it is stationary. Both snake and frog have developed highly effective hunting skills on the basis of incoming EMR signals, but it is unlikely that either are seeing pictures.

There are two main types of photoreceptor in the retina, rods and cones. In most animals, the rods make up 95% or more of all receptors and are extremely sensitive to light (they can recognise a single photon). They are the major transducers of visual information, but they do not distinguish colour. This is the property of the cones, which are far less numerous (<5%) and require bright light to be fully effective. Humans and old‐world primates are trichromatic; they have three types of cone, so chimpanzees see colour the way we do. Most mammals have only two types of cone and, in consequence, a more limited range of colour discrimination. Lest this be seen as evidence of higher development in primates, I would add that birds, and the reptiles from which they evolved, have four cone types, as do most insects, while some species of butterfly have five. The greater the number of cone types, the greater the power of colour discrimination. This is a nice example of the principle that species develop the senses and skills that matter most. Colour perception matters a lot to birds and insects that depend largely on vision in sexual selection; not only selection of the favoured male within their species but also the correct species with which to mate. The eyes of nocturnal and marine mammals that operate at low light levels are dominated by rods, so they have less colour vision. Owls that hunt at night have very large eyes and a high density of rods but also depend for their hunting skills on an exquisite sensitivity to sound.

Many cephalopods, e.g. the octopus, communicate emotions such as threat and sexual attraction, by changing colour. They recognise colour changes in another individual and respond accordingly, using cells sensitive to light and colour situated all over the skin that react and drive responses to colour changes without necessarily involving the brain. Seasonal patterns of breeding in birds are driven by hormonal responses to changes in photoperiod, sensed by photoreceptors located not in the retina of the eye, but within the brain itself (encephalic photoreceptors). These must sense changes in light transmitted through the bones of the skull.

The ability to focus the mind on signals that matter and ignore the rest is particularly important in the context of visual information since the potential ‘noise’ level is so great. An elegant example of this is seen in the foraging behaviour of chickens. Hens foraging under trees in the autumn among a thick blanket of leaves and other unimportant vegetable material have an uncanny ability to spot and accurately pick up seeds and insects. These are the only bits of visual information that their mind bothers to register. The ability to focus the mind on what matters is an essential property of sentience.