Classification of senses

Perception and illusions are not limited to our eyes. Figure 2.17 shows a classification of our basic senses. Recall that a sensor converts an energy source into signals in a circuit. In the case of our bodies, this means that a stimulus is converted into neural impulses. For each sense, Figure 2.17 indicates the type of energy for the stimulus and the receptor that converts the stimulus into neural impulses. Think of each receptor as a sensor that targets a particular kind of stimulus. This is referred to as sensory system selectivity. In each eye, over 100 million photoreceptors target electromagnetic energy precisely in the frequency range of visible light. Different kinds even target various colors and light levels; see Section 5.1. The auditory, touch, and balance senses involve motion, vibration, or gravitational force; these are sensed by mechanoreceptors. The physiology and perception of hearing are covered in Sections 11.2 and 11.3, respectively. The sense of touch additionally involves thermoreceptors to detect change in temperature. Touch is covered in Section 13.1. Our balance sense helps us to know which way our head is oriented, including sensing the direction of ``up''; this is covered in Section 8.2. Finally, our sense of taste and smell is grouped into one category, called the chemical senses, that relies on chemoreceptors; these provide signals based on chemical composition of matter appearing on our tongue or in our nasal passages; see Section 13.2.

Note that senses have engineering equivalents, most of which appear in VR systems. Imagine you a designing a humanoid telepresence robot, which you expect to interface with through a VR headset. You could then experience life through your surrogate robotic self. Digital cameras would serve as its eyes, and microphones would be the ears. Pressure sensors and thermometers could be installed to give a sense of touch. For balance, we can install an IMU. In fact, the human vestibular organs and modern IMUs bear a striking resemblance in terms of the signals they produce; see Section 8.2. We could even install chemical sensors, such as a pH meter, to measure aspects of chemical composition to provide taste and smell.