Cortex

The cortex is the outermost surface of the brain — a thin, densely packed sheet of neurons a few millimetres thick that, if unfolded, would cover roughly the area of a desktop. In most contexts on these pages it means the cerebral cortex: the heavily folded grey mantle draped over the two cerebral hemispheres.

It is divided into four lobes, each handling different work. The frontal lobe manages planning, decision-making, and voluntary movement; the parietal lobe integrates touch and spatial information; the temporal lobe processes sound and plays a central role in memory; the occipital lobe handles vision. Deeper structures handle emotion and drives, but most of what we call thought — and much of what makes human experience distinctive — originates in cortical circuits.

How it works · its role

The neocortex — the evolutionarily newest and largest subdivision — is organised into six horizontal layers, each with characteristic cell types and connection patterns. Excitatory neurons using glutamate form long-range connections to other cortical regions and to subcortical structures; inhibitory interneurons using GABA shape the timing and intensity of local signalling.

Processing is broadly hierarchical. Primary sensory areas receive incoming signals — raw visual data in the occipital cortex, sound in the temporal cortex — and pass them to adjacent association areas that build richer representations. The prefrontal cortex sits at the top of this hierarchy, integrating information from across the brain to guide decisions and regulate impulses. Because the layers and regions are heavily cross-connected, a change in signalling chemistry at one point can ripple widely across function.

Relevance to substances & effects

Classic psychedelics such as LSD and psilocin bind selectively to 5-HT₂A receptors concentrated in the prefrontal and sensory cortex. This disrupts the brain's normal predictive filtering, producing perceptual distortions and the characteristic sense that perception is unmoored from its usual constraints.

Dissociatives such as ketamine block NMDA receptors throughout cortical circuits, suppressing the excitatory glutamate signalling that underpins sensory integration — the origin of the detached or hole experience. Stimulants raise dopamine and noradrenaline in the prefrontal cortex, sharpening attention and reducing impulsivity. Alcohol and benzodiazepines enhance inhibitory GABA-A signalling broadly, slowing cognition and loosening behavioural restraint.

Cannabis acts through CB1 receptors distributed across cortical regions, disrupting working memory and bending sensory perception. Because the cortex is a patchwork of specialised areas rather than a single organ, the character of a substance's effects often reflects which parts of that patchwork it engages most strongly.