Читать книгу Bird Senses - Graham R. Martin - Страница 17

We very rarely think about what our senses are doing for us. Always working in the background, our senses are extracting a continuous flow of diverse information from the environment surrounding us. This informational flow is essential for guiding our every action. Only if we perform carefully controlled tests can we gain insights into the information that constantly flows into us. Even when we apparently ‘switch off’ by falling asleep, our senses are providing information vital for our wellbeing. Most of the time this information is accurate and reliable, but it can sometimes be erroneous or unreliable. Not only do our senses have limits, but uncertainties can arise from the senses themselves, and from the way the brain interprets the information that they provide. Simple tricks, such as sound and visual illusions, demonstrate that the information extracted from our senses can be fallible (Box 2.1).

Box 2.1 Illusions and reality

Visual illusions are often used to amuse. Simple drawings that can be ‘seen in two ways’, but never both ways at once, always bring a smile. Lines that are the same length but look to be of different lengths can bring delight and consternation. Illusions, however, are far more than tricks. Properly used, they reveal a great deal about how sensory information is interpreted by the brain. Illusions, using either vision or sounds, demonstrate that the same information provided by the senses can be interpreted in different ways by the brain. Illusions also show how the brain constantly seeks to bring order when the senses provide ambiguous or scant information.

Illusions raise the rather uncomfortable question of ‘where does reality lie?’ In two of the illusions shown here (Ponzo and Müller-Lyer) the horizontal lines are of the same length. In the Ebbinghaus–Titchener illusion the two central black circles are of the same diameter. The information coming through our eyes is the same, but the brain interprets it differently. In the Illusory Contour, there is no vertical line, but we see one. So can we ever really be sure of what is out there in the world beyond ourselves? What actually happens when information received at the eyes is constantly changing? Are all interpretations correct? What happens when senses are working at their limits, when information starts to be unreliable?

The illusions presented here are particularly pertinent. They either make us see something that’s not there or they give us false information. Fascinating as these illusions are, it has been shown that these same illusions have similar effects in birds. Doves too see illusory contours and make incorrect size comparisons when faced with these kinds of illusion patterns.

Showing that birds respond to illusions has involved elaborate training and testing using behavioural techniques. For convenience, this work has usually been done with doves, but other species including finches and owls have also been shown to experience visual illusions. With some illusions the birds seem to experience the illusion in a slightly different way to how we do. This illustrates that the processing of visual information in bird brains is not the same as in ours. However, the important point is that under certain circumstances birds’ brains can give them rather tenuous understanding of the reality of the world about them, that their brains work hard to resolve ambiguity, and that their senses can be as unreliable as ours.

That we are usually unaware of what our senses are doing might also encourage us to believe that senses are cheap to run, that the information we receive is cost-free. Certainly, compared with the energy needed to maintain our body temperature or move our bodies about, the information that we use to control our movements seems to be gained without effort. However, while our muscles and internal organs burn up calories in a tangible way, our brains are also always working hard, burning lots of calories. And most of that brain work is in the extraction and integration of information from our senses.

Every day our brains use about 20% of the energy that we consume. Nearly all of this energy is used just to keep information flowing in from the senses and for directing our actions based upon it. We like to think that ‘thinking’ is hard work. However, it uses very little energy compared with the effort needed to provide us with the flow of well-integrated information from our senses upon which our thoughts are based.

That the activity of our senses is metabolically expensive strongly suggests that the information they provide must indeed be very valuable. Nature’s arbiter of utility and efficiency, natural selection, would have long ago weeded out most inefficiencies in information capture, and would have also weeded out any information that is superfluous. Natural selection will have honed very exactly what information our senses need to provide our brains in order to ensure our survival. The relatively high cost of running the senses, and of integrating the information that they provide, will have been a constant theme throughout all animal evolution. As animals have radiated from ancestral forms to occupy different ecological conditions and exploit different resources, it should not be surprising that their senses have also diversified and become tuned to provide information for the efficient execution of different tasks in different environments.

Only very rarely are we aware of the information that we respond to at any instant. Furthermore, we are usually unaware of how information is changing continuously from moment to moment. The brain integrates and smooths things out to provide continuity of experience. Conscious reflection on what we are seeing, hearing, smelling, or feeling is slow, and the world about us changes so rapidly that we are unable to reflect on what we are responding to.

Humans are very proud of their consciousness; we like to think of it as something that makes us special within the animal kingdom. Losing consciousness is always viewed as a catastrophe, something to be regained as soon as possible. Conscious thought is, however, a cumbersome process. So cumbersome that we are unable to keep up a conscious commentary on what we are responding to. Simply stating what you can see, hear, or smell at any instant skates lightly across the surface of our rich and varied sensory information. We need special procedures to probe our senses and to determine what information we potentially have available to guide our actions. Such probing is difficult to achieve in humans, but it is additionally difficult when we seek to find out what information is available to non-human animals.