Senses and thresholds

We know the world through three linked processes. Right now you see these words (sensation), you keep your mind on them rather than on the noise around you (attention), and you understand what they mean (perception). Without all three, we could not experience the world at all.

Sensation is the initial experience of a stimulus, the first awareness we get when a sense organ registers something: the warmth of a cup of tea in your hands, the sound of a bell.

We have seven sense organs:

• Five external senses: eyes (vision), ears (hearing), nose (smell), tongue (taste) and skin (touch, pressure, warmth, cold, pain).
• Two internal senses: the kinesthetic system tells us the position and movement of our body parts (you can touch your nose with your eyes closed), and the vestibular system, in the inner ear, tells us about balance and the head's position against gravity.

Each sense is a sense modality: it is specialised for one kind of stimulus. Your ears cannot see and your eyes cannot hear. Information travels from the sense organ along nerves to the brain, so damage to that pathway can cause a loss of sensation even if the organ is healthy.

Our senses also have limits:

• Absolute threshold (absolute limen, AL): the minimum strength a stimulus must have to be noticed at all. A very faint whisper from across a large hall may not be heard; a little louder and it is. Since no one notices at exactly the same point every time, the AL is the value noticed on 50 per cent of trials.
• Difference threshold (difference limen, DL): the smallest change in a stimulus needed to notice that two stimuli are different, again judged on 50 per cent of trials. Turn the music up by one tiny step and you may not hear a change; a few steps more and you do.

Thresholds are not fixed: they differ between people, and for the same person under different conditions (tiredness, practice, attention). The study of how stimuli relate to the sensations they produce is called psychophysics.