Psychophysics is the scientific study of the relationship between physical stimuli and what we perceive. It was founded by Gustav Fechner in 1860 — and it's still one of the most powerful tools in neuroscience.

Modern definition: the analysis of perception by studying the effect on experience of systematically varying the properties of a stimulus. In practice: you change something in the physical world (brightness, weight, sound) and measure precisely what the human notices.

Why psychophysics matters for neuroscience:
To understand how the brain represents the world, you need to precisely measure both the stimulus (physics) and the perception (experience). Psychophysics does the second half. Without it, you'd have detailed neural recordings but no way to know what the brain was actually computing.

Example: Hermann von Helmholtz used psychophysics in the 19th century to deduce that humans have three types of color receptors — before anyone had looked at the retina with a microscope. The prediction was later confirmed exactly.

📐 Weber's Law: The Just Noticeable Difference (JND) is a constant fraction of the baseline stimulus — not a constant amount. If you can detect +1g from 20g, you'd need +50g from 1000g to notice the same difference.

Just Noticeable Difference (JND): the smallest change in a stimulus that you can reliably detect. Weber found this holds across many senses: vision, hearing, weight, touch. The JND is always ~5% of the baseline (the exact fraction varies by sense, but the proportional relationship holds).

The Weber-Fechner Law: if JND is proportional to baseline, then perception must be a logarithmic function of stimulus intensity.

Why? If you need 10× as much physical change to notice a difference at high intensities, your internal representation is compressing the stimulus — squishing the scale. A log function does exactly this: equal ratios feel like equal steps.

This is why music uses decibels (log scale), why the Richter scale is logarithmic, and why your phone's brightness slider feels linear even though it spans millions of photons.