The human brain weighs about 1.4 kg, contains roughly 86 billion neurons, and is organized into anatomically distinct regions whose functions can be mapped with surprising specificity. The mapping has taken centuries — postmortem dissection, lesion studies (deducing function from what is lost when a region is damaged), electrical stimulation (Penfield's 1930s open-skull mapping of the somatosensory and motor cortices), neuroimaging (PET, fMRI, DTI), and now single-cell transcriptomics and connectomics. The brain is not a homogeneous information-processing organ but a layered assembly of evolutionarily distinct structures, each specialized for a class of computation.
The brain's gross divisions step inward and upward in evolutionary age. The brainstem (medulla, pons, midbrain) regulates breathing, heart rate, sleep-wake, and basic reflexes (damage here is rapidly fatal). The cerebellum, containing about 80% of the brain's neurons in 10% of its volume, handles timing and motor coordination. The diencephalon houses the thalamus (routing virtually all sensory information except smell to the cortex) and the hypothalamus (regulating homeostasis and the endocrine system). The limbic system (hippocampus, amygdala, cingulate cortex) handles memory and emotion: the hippocampus is essential for episodic memory (bilateral damage produced patient H.M.'s profound anterograde amnesia), the amygdala essential for fear conditioning. The basal ganglia handle action selection, habit learning, and reward, with Parkinson's the degeneration of substantia nigra dopamine neurons and Huntington's the degeneration of the striatum. The cerebral cortex — the outer 2–5 mm sheet folded into gyri and sulci — divides into four lobes: frontal (motor cortex, planning, working memory, language production in Broca's area, prefrontal control), parietal (somatosensory cortex, spatial attention), temporal (auditory cortex, language comprehension in Wernicke's area, face recognition), and occipital (vision — V1 and the dorsal/ventral streams). The cortex has a stereotyped six-layer cytoarchitecture, is lateralized (language left, spatial attention right), and is highly connected — most areas are reachable in 2–3 synaptic steps; resting-state functional connectivity reveals consistent networks (the default-mode, salience, dorsal-attention, and fronto-parietal control networks).
Modern neuroimaging (fMRI ~3 mm spatial / ~2 s temporal, MEG/EEG ~1 mm spatial / ~1 ms temporal, DTI for white-matter tracts) has produced cortical and subcortical maps with increasing detail, and the Human Connectome Project (2010–2015) and the UK Biobank (~50,000 subjects) have generated population-scale brain-imaging data. Brain atlases (Allen Brain Atlas, BigBrain, Human Brain Project) provide gene-expression, cell-type, and connectivity maps at increasing resolution, while brain-computer interfaces (Neuralink, BrainGate, Synchron) implant electrodes in specific cortical areas (typically motor, recently speech). Neurosurgery uses awake brain mapping to localize critical functions before resection, and psychiatric and neurological diseases are increasingly understood as circuit-level dysfunctions affecting specific networks. The map is more detailed than ever and still has vast areas marked here be dragons.