Bird Senses. Graham R. Martin

      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.

      The origins of investigating senses

      The origins of thinking about the problem of understanding human and animal senses go back more than 2000 years in western thought. However, it is only during the last 200 years that we have gained real insight into the diversity of sensory information across species. The Darwin–Wallace ideas of natural selection, and Alexander von Humboldt’s ideas about the interrelationships of organisms with their environment, provided two big ideas which now provide broad frameworks for thinking about the senses of animals. They give us many reasons for expecting that senses, and the information that they provide, will differ between species. Importantly they provide ways to account for these differences in terms of their evolution and their ecological functions. We can now appreciate why sensory information might differ between species, and how it is linked to specific behaviours and ecologies.

       Epicurus and Sextus

      Ideas about the challenges of understanding the sensory world of humans were first thought through and described by the Greek philosopher Epicurus (341–270 BCE). The writings of Epicurus, and the elaboration of his ideas by later philosophers, have given rise to the substantial body of thought known today as Epicureanism. Many prominent thinkers through to the present day have characterised themselves as Epicureans. Epicureanism has a number of key ideas, and prominent among them is being prepared to accept the limitations on the information that is available to guide actions.

      Epicurus was among the first to write explicitly about the way that our senses place very real constraints on our overall understanding of the world. He was the first western philosopher–scientist to recognise that the information we receive changes from moment to moment. He also argued that information, even though it is concerned with the same object, is radically different depending upon the sense involved. What becomes recognised by us as a particular object is constructed from many different, and ever-changing, sources of information received from different sensory systems.

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