Cerebral cortex

The cerebral cortex is the wrinkled outer surface of the brain — a thin sheet of gray matter, only 1 to 4.5 millimetres thick, folded into ridges (gyri) and valleys (sulci) to fit a large surface area inside the skull.

It's where most of what we think of as "thinking" happens: vision, hearing, language, movement, memory, planning, decision-making, social cognition. Different regions of the cortex specialise in different functions.

How it's organised

The cortex has four main lobes per hemisphere:

• Frontal lobe — executive function, planning, voluntary movement, speech production
• Parietal lobe — touch, spatial awareness, attention
• Temporal lobe — hearing, language comprehension, memory formation
• Occipital lobe — vision

Each lobe contains many specialised sub-regions, each connected to specific subcortical and contralateral structures.

Why we measure it

The cortex changes with age and disease in characteristic ways:

• Cortical thickness declines with age, faster in some regions than others
• Specific thinning patterns signal specific conditions (for example, entorhinal cortex thinning in early Alzheimer's)
• Total cortical volume is a standard output of volumetric pipelines

Surface vs. volume

Modern neuroimaging measures the cortex in two ways:

• Volume — total amount of cortical tissue in cubic millimetres
• Surface metrics — thickness, surface area, and gyrification (how folded the surface is)

Surface metrics often pick up subtle changes that volume measurements miss.