You
A human being with intent
Everything starts here. Your brain's motor cortex fires, nerve signals race down your arm at 120 m/s, and your finger strikes a key.
How It Works
Every interaction with a computer begins with a biological decision. Your prefrontal cortex forms an intention — "I want to search for something" — and your motor cortex translates that into a precise motor plan. Nerve signals travel from your brain through your spinal cord and peripheral nerves to your finger muscles at roughly 80–120 meters per second.
The human input system is analog, noisy, and imprecise. Your brain compensates with a continuous feedback loop: eyes track the cursor, proprioception feels the key depress, and auditory feedback confirms the click. This perception-action loop runs at the speed of your display's refresh rate. Average human reaction time is about 200 ms for visual stimuli and 150 ms for auditory — setting a hard floor for how fast any interactive system can feel.
The Signal Flow
Key Concepts
The brain region (precentral gyrus) that plans and executes voluntary movements. Different body parts are mapped to different areas — fingers get a disproportionately large region.
Myelinated nerve fibers conduct at 80–120 m/s. From brain to fingertip is about 1 meter — so the signal takes roughly 10 ms to arrive.
~200 ms for visual stimuli, ~150 ms for auditory. This is the minimum round-trip: perceive → decide → act. It sets the floor for perceived interactivity.
The continuous cycle of seeing, deciding, and acting. Your brain integrates visual information in ~100 ms windows — responses within that window feel instant.
Deep Dive
Why 200 ms feels instant
The human perceptual system integrates visual information in ~100 ms windows. When a computer responds within 100–200 ms, it feels instantaneous because the response arrives within the next perceptual frame. Above 300 ms, users perceive a distinct delay. Above 1000 ms, they lose flow state and attention wanders. This is why every web performance metric ultimately traces back to human neuroscience.
Fitts's Law
The time to acquire a target is a function of the distance to the target divided by its size. This fundamental law of human motor control directly shapes UI design: buttons need minimum tap targets (44px on mobile), menus exploit screen edges as infinitely large targets, and frequently-used actions should be close and large. Every pixel of your interface is constrained by the biomechanics of the hand that uses it.