An Immense World: How Animal Senses Reveal the Hidden Realms Around Us

Exafference refers to sensory signals produced by the outside world. For example, leaves rusting in a tree have nothing to do with the perception of those leaves or that tree. They exist on their own. These leaves would still rustle, and the tree would still exist, even if they were not seen, heard, smelled, or otherwise experienced by the senses of other beings. Exafference is external to the perceiving self.

Nociception is the sensory detection of damage to the body; this is distinct from pain, which is the suffering that often follows nociception. Nociception does not require consciousness and is instantaneous. A familiar example of nociception is the jerk of the hand off a hot burner; this happens before any pain is felt and is immediate. The immediate reaction of nociception does not always result in pain, though it often does.

Proprioception is the perception of one’s own body and its movements. This is one of the senses that is generally taken for granted. Proprioception allows a person to feel the body’s movements as well as its outlines and place in space.

Reafference is a signal produced by an animal’s actions and is internal to that animal; it is often paired with exafference, which refers to the world outside an animal. A tree’s leaves rustling in the wind is an example of exafference. Seeing the leaves rustling in the wind, however, is the result of looking at the tree and light waves hitting the retina within the sensory organ of the eye, where receptors then send signals to the brain.

Receptors are the cells that detect stimuli. These receptors are specific to the stimuli. For example, there are receptors that detect light (photoreceptors) and receptors that detect pressure or other movement (mechanoreceptors). Each species has receptors only for stimuli that are useful and efficient for it, which means that animals do not have receptors for every stimulus that exists in the outside world.

Sense organs, such as eyes and ears, contain receptors. The sense organs provide the infrastructure through which these receptors communicate their detections to the brain.

A sensory system is made up of the sense organs, which contain receptors, and the neurons that communicate the sense organs’ detections. For example, the visual sensory system for humans includes the sense organ (eye) and the photoreceptors housed within it, as well as the optic nerve and the visual cortex of the brain. The sensory system makes sense of stimuli as it processes them, translating them into information that the subject can then act on. Thus, molecules become smell, and light waves become vision. This information connects animals to the environment and to each other.

Sentience is the ability to distinguish between the self and the outside world. This is not necessarily conscious. Sentience is necessary for sensory systems to function, as they must receive stimuli from outside the self and recognize it as other than the self. All animals are sentient.

A sensescape is the sensory environment. A sensescape may be diminished or polluted. For example, artificial lights aid human vision, but they deprive nocturnal animals of the darkness that enables them to navigate and survive. Similarly, human shipping has made the oceans much noisier than they used to be, and this harms many animals.

Stimuli are detected by receptors, converted into electrical signals, and sent to the brain. Examples of stimuli include light, sound, smell, electricity, surface waves, and magnetic fields. Stimuli may already be in the outside world, as in the case of light. They may also be produced by the animal subject; this is the case for the sonic waves that bats and some marine mammals send out, which echo information.