How does the world become an experience?

From physical forces to sensory experience

We see light falling across the table, hear someone speak and feel the warmth of a cup of coffee. To us these are colours, voices and warmth. To understand how such experiences become possible, we also need to examine what is happening in our surroundings and how it affects our sense organs.

Four forces behind the world we meet

Physics describes four fundamental interactions – four basic ways in which matter and energy influence one another. They operate in very different settings:

• GravityGravity is tied to mass and energy. It keeps us on the Earth and governs the motions of the planets.
• ElectromagnetismElectromagnetism acts between electrically charged particles. Light is electromagnetic radiation. The same interaction plays a decisive role in chemical bonds and in the forces that arise when objects come into contact.
• Strong interactionThe strong interaction binds quarks together inside protons and neutrons. A residual part of it holds protons and neutrons together in atomic nuclei.
• Weak interactionThe weak interaction is involved in, among other things, certain forms of radioactive decay.

Different routes into the sense organs

Electromagnetism is therefore central to the physical and chemical conditions on which the senses depend. But what reaches a sense organ may be light, mechanical force or molecules. That difference is needed to understand how each sense works.

Light can be absorbed by molecules in the photoreceptors of the retina. Sound, by contrast, consists of mechanical pressure waves that set structures in the ear in motion. That electromagnetic forces contribute to the properties of materials does not make sound electromagnetic radiation. In touch the skin is deformed, whereas smell and taste begin when substances act on sensitive proteins and ion channels in sensory cells. The cup’s warmth is registered through temperature-sensitive mechanisms in the skin.

How do we sense gravity?

In two small sacs of the inner ear, the utricle and the saccule, hair cells extend their bundles into a gel-like layer carrying tiny crystals of calcium carbonate. When the head tilts, this layer can shift relative to the hair cells. The bundles bend, and the cells' electrical activity changes.

These organs register the effects of both gravity and linear acceleration. Their signals alone are therefore not always enough to tell a tilt from a movement. The nervous system combines them with information from the semicircular canals and from vision, among other sources, to judge how the head is oriented and moving.

From stimulation to neural activity

When light, movement or substances change the electrical activity of a sensory cell, this is called sensory transduction. In the retina, absorbed light triggers a chain of chemical reactions. In the ear, bending of hair-cell bundles can change which ions pass through the cell membrane. Different mechanisms thus turn influences from the surroundings into signals that the nervous system can process.

Being electrical does not make the signals all alike. Which cells respond, when they respond and how activity is distributed among them carry information about what is happening.

How does this become an experienced world?

A description of the wavelengths of light is not the same as a description of the colour we see. Nor are the pressure variations in the air the experience of a voice. To explain perception, we need to follow how the nervous system processes the signals and how that processing relates to what we perceive. The terms sensation and perception emphasise different aspects of it.

This processing takes place in many parallel streams with feedback. Context, experience and attention can influence how the information is interpreted. At the same time, experience is bound to the information that is available; we cannot freely decide how something will look or sound.

That the brain uses expectations to interpret incoming signals is an important research idea, supported among other things by studies of the visual cortex. It offers a way to investigate perception, but it is not a finished explanation of everything we experience.

Perceiving and acting belong together

When you turn your head, shift your gaze or feel the surface of the cup, you change the information that reaches your senses. What you perceive in turn helps you choose your next movement. Perception and motor control are therefore part of recurring loops in which the information is constantly changing. The chapter returns to this in how the brain controls movement and prediction in movement.

The light from the cup

We begin with the light that reaches the eye from the cup and the table.