The telencephalon is the largest division of the human brain, forming the two cerebral hemispheres that dominate the skull and house nearly every function we associate with being human: language, reasoning, memory, emotion, voluntary movement, and sensory perception. Its outer layer of gray matter, the cerebral cortex, ranges from about 1.5 to 4.5 millimeters thick and folds into ridges and grooves that pack roughly 70 percent of the cortex out of sight within those creases.1Elsevier (Anaesthesia & Intensive Care Medicine). Physiology The brain: functional divisions But the telencephalon is far more than just the wrinkled surface. Beneath the cortex sit clusters of neurons that drive habit formation, emotional processing, and smell, and the entire structure emerges from an embryonic process so precisely choreographed that even small disruptions can produce severe malformations.
What the Telencephalon Actually Includes
People often use “telencephalon” and “cerebral cortex” interchangeably, but the cortex is only the outermost shell. The telencephalon encompasses two broad divisions. The pallium, which becomes the cortex, handles higher-order processing: sensation, movement planning, thought, and language. The subpallium sits beneath it and gives rise to the basal ganglia, parts of the limbic system, and structures involved in smell. Developmental studies across vertebrates show that this basic pallium-subpallium organization is remarkably conserved, with comparable progenitor zones and migratory patterns appearing in fish, amphibians, reptiles, birds, and mammals alike.2PubMed. Development and evolution of the subpallium What differs dramatically across species is how much the pallium expands and how it arranges itself internally, a theme that runs through everything from cortical folding to intelligence.
How the Telencephalon Builds Itself Before Birth
The telencephalon begins as a hollow swelling at the front end of the embryonic neural tube. Within weeks, chemical signals called morphogens start carving this swelling into distinct zones. Sonic hedgehog pushes ventral tissue toward becoming the subpallium, while bone morphogenetic proteins and WNT proteins push dorsal tissue toward becoming the cortex.3PubMed Central. Two-Phase Lineage Specification of Telencephalon Progenitors Generated From Mouse Embryonic Stem Cells The balance between these opposing gradients determines where cortex ends and basal ganglia begin, and disrupting that balance is one route to major birth defects.
Once regional identities are set, the cortex has to populate itself with neurons in the right order. This happens through a process called radial migration. Progenitor cells lining the inner surface of the developing brain give rise to neurons that climb outward along elongated guide cells called radial glia. Critically, newly born neurons leapfrog past the ones that arrived earlier, settling at the top of the growing cortical plate. The result is an “inside-out” layering pattern: the deepest cortical layers form first, and the most superficial layers form last.4PubMed Central. Inside-Out Radial Migration Facilitates Lineage-Dependent Neocortical Microcircuit Assembly A signaling molecule called Reelin appears to orchestrate this by telling migrating neurons to detach from their radial glia guide and lock into position once their leading edge reaches the brain’s outer margin.5PubMed. A mechanism for inside-out lamination in the neocortex
Not all cortical neurons travel this radial path. Inhibitory interneurons, which use the neurotransmitter GABA, are born in the ventral forebrain and migrate sideways (tangentially) over long distances to reach the cortex.6PubMed Central. Interneuron odyssey: molecular mechanisms of tangential migration They originate in structures called the ganglionic eminences, cross the boundary between the developing striatum and cortex, and then switch to radial movement to settle into their final layer.7PubMed Central. Timing of cortical interneuron migration is influenced by the cortical hem Some of these tangentially migrating cells end up not just in the neocortex but also in the hippocampus and dentate gyrus, where they become the local inhibitory neurons those memory-critical regions need to function properly.8Neuron. Subcortical Origin of GABAergic Interneurons in the Dentate Gyrus and Hippocampus
Why Some Brains Fold More Than Others
The degree of cortical folding varies enormously across mammals. A mouse brain is nearly smooth; a human brain is deeply creased. The difference comes down largely to what happens during the progenitor cell stage. The human cortex contains a specialized type of progenitor, called outer radial glia, that sits in an expanded zone away from the inner ventricle surface. These cells are enormously proliferative, capable of generating many more neurons per division cycle than the progenitors found in animals with smaller, smoother brains.9PubMed Central. Molecular Identity of Human Outer Radial Glia During Cortical Development The expansion in cortical surface area that results from this extra proliferation is what forces the tissue to fold, much the way cramming a large sheet into a small container creates wrinkles. This link between progenitor abundance and cortical expansion has been confirmed across species: brains with more of these secondary progenitors consistently have more folded cortices.10PubMed. Neural progenitor cells and their role in the development and evolutionary expansion of the neocortex
Subcortical Structures of the Telencephalon
Beneath the cortex, the telencephalon houses several clusters of neurons with distinct and essential functions. The most prominent is the striatum, the input hub of the basal ganglia. Striatal circuits are central to motor control, habit learning, action selection, and the ability to switch between competing goals.11PubMed Central. Interpreting the role of the striatum during multiple phases of motor learning The basal ganglia do not initiate movement on their own. Instead, they act as a filtering system: they become active when a stimulus is being attended to or when a voluntary movement is being planned, using inhibitory opponent mechanisms to green-light some actions while suppressing others.12PubMed. Selection of actions in the basal ganglia-thalamocortical circuits: review and model
The limbic structures of the telencephalon handle emotion, memory, and motivated behavior. The hippocampus is essential for forming new episodic memories, while the amygdala processes the emotional weight of experiences, particularly threat and fear.13PubMed Central. The limbic system Also tucked into the telencephalon’s base is the olfactory system. The piriform cortex, which processes smell, sends feedback projections all the way back to the olfactory bulb, creating a loop that lets higher-level processing shape how raw smell signals are interpreted before they even fully arrive.14Frontiers in Neuroanatomy. Centrifugal telencephalic afferent connections to the main and accessory olfactory bulbs
Connecting the Two Halves
The two cerebral hemispheres are not isolated. They communicate through a massive bundle of nerve fibers called the corpus callosum, the largest white-matter structure in the brain. Building this bridge is a multistep process: a new neuron first has to grow an axon, migrate to its correct cortical position, send its axon toward the midline, cross into the opposite hemisphere, and then find the right target area to form a synapse.15PubMed Central. Molecular mechanisms of corpus callosum development: a four-step journey Recent work in mice suggests that these crossing axons find their correct partners through a kind of molecular handshake: axons from matching regions on both sides carry similar levels of adhesion molecules that make them stick together, producing a topographic map within the callosum that mirrors the layout of the cortex above it.16PubMed Central. Symmetry in levels of axon-axon homophilic adhesion establishes topography in the corpus callosum and development of connectivity between brain hemispheres
Prefrontal Cortex and Cognitive Control
The prefrontal cortex, sitting at the very front of the telencephalon, is where the brain does its most sophisticated executive work. Different subregions handle different aspects of this. The dorsolateral prefrontal cortex contributes to working memory, goal-directed attention, planning, and task switching. The ventrolateral region handles response inhibition and monitoring. The medial prefrontal cortex is involved in self-knowledge, motivation, and updating goals. And the orbitofrontal cortex contributes to personality, emotional reasoning, and social decision-making.17PubMed. Executive Dysfunction and the Prefrontal Cortex One nuance that has emerged from research is that the prefrontal cortex may not store working memories directly. Instead, its activity during memory tasks may function more as a top-down control signal that keeps sensory areas in the back of the brain maintaining the relevant information.18PubMed Central. The Role of Prefrontal Cortex in Working Memory: A Mini Review
These cognitive functions depend not just on cortical circuits but on the chemical environment that modulates them. Neuromodulatory systems using dopamine, serotonin, noradrenaline, and acetylcholine track environmental signals like rewards, risks, and novelty. The interplay between these chemical systems and brain areas such as the frontal cortex, amygdala, and hippocampus is what gives rise to attention, emotion, goal-directed behavior, and decision-making.19PubMed Central. Neuromodulatory Systems and Their Interactions: A Review of Models, Theories, and Experiments
Language and Hemispheric Asymmetry
One of the telencephalon’s most striking features is that the two hemispheres are not perfectly symmetrical in function. Language processing is the classic example. Studies combining brain imaging with fiber-tract tracing in right-handed individuals have shown that the white-matter connections between frontal and temporal language regions (traditionally called Broca’s and Wernicke’s areas) are more extensive on the left side than the right, and people with more strongly left-lateralized brain activation during language tasks also show greater structural asymmetry in those fiber tracts.20PubMed. Hemispheric asymmetries in language-related pathways: a combined functional MRI and tractography study This lateralization appears to have physiological roots that predate language itself: left auditory, motor, and parietal cortices show speech-related oscillatory properties even in the absence of language processing, suggesting that the hardware was biased before language fully evolved.21PubMed Central. Neurophysiological origin of human brain asymmetry for speech and language
The picture is not as clean-cut as the textbook version. Resting-state connectivity studies have found that while Broca’s area (in the frontal lobe) reliably shows left-lateralized connectivity, Wernicke’s area (in the temporal lobe) can actually show predominantly right-lateralized functional connectivity at rest.22PLoS ONE. Temporal Reliability and Lateralization of the Resting-State Language Network The traditional “left brain equals language” story captures the dominant pattern but paints over significant complexity.
Postnatal Maturation Through Adolescence
The telencephalon is not finished at birth, not by a long stretch. Two major remodeling processes continue well into early adulthood: synaptic pruning and myelination. During adolescence, pruning is highly region-specific and can eliminate close to half of the synaptic connections in some cortical areas while barely touching others. This trimming is thought to rewire the brain into more efficient adult-typical patterns. Simultaneously, the fatty insulating sheath called myelin wraps around axons connecting distant brain regions, dramatically increasing the speed and energy efficiency of electrical signaling. Both processes contribute to the cortical “thinning” observed on brain imaging during the teenage years, which, despite sounding alarming, is actually a sign of maturation rather than deterioration.23PubMed Central. Adolescent Neurodevelopment
Puberty initiates many of these changes, amplifying the brain’s responsiveness to its environment and facilitating neural adaptation.24PubMed Central. The connecting brain in context: How adolescent plasticity supports learning and development The prefrontal cortex, which handles impulse control and long-term planning, is among the last regions to fully myelinate, leaving adolescent behavior influenced by brain circuitry that has not yet reached its mature configuration.25PubMed Central. Maturation of the adolescent brain This mismatch between a mature emotional system and an immature control system is one neurobiological reason risk-taking peaks in the teenage years.
Adult Neurogenesis in the Telencephalon
For most of the twentieth century, the dogma was that the adult brain does not produce new neurons. That turned out to be wrong, at least in two telencephalic locations. The dentate gyrus of the hippocampus and the subventricular zone lining the lateral ventricles both harbor neural stem cells that continue generating new neurons throughout life in mammals. Neurons born in the subventricular zone migrate along a dedicated route to the olfactory bulb, while new neurons in the dentate gyrus integrate into hippocampal circuits involved in memory.26PubMed Central. Adult neurogenesis in the mammalian brain: significant answers and significant questions The functional significance of adult-born neurons is still debated, but their presence means the telencephalon retains at least some capacity for structural plasticity well beyond the developmental window.
When Development Goes Wrong
Because telencephalon formation involves so many precisely timed steps, there are many points where things can derail. Three of the most studied developmental malformations illustrate this vulnerability.
- Holoprosencephaly: The telencephalon fails to split into two hemispheres. This is linked to disruptions in the sonic hedgehog signaling pathway, and recent genetic work suggests that multiple simultaneous “hits” in that pathway are often required for the malformation to develop fully.27PubMed. Recent advances in the genetic etiology of brain malformations
- Lissencephaly: The cortex fails to fold, remaining smooth. This reflects failures in the neuronal migration process. A recently identified gene variant, BAIAP2, demonstrates the link: loss of this gene’s function causes neurons to migrate abnormally and the cortex to develop without its characteristic ridges.28PubMed. A lissencephaly-associated BAIAP2 variant causes defects in neuronal migration during brain development
- Agenesis of the corpus callosum: The fiber bundle connecting the two hemispheres partially or completely fails to form, disrupting interhemispheric communication.29PubMed. Molecular genetics in fetal neurology
Neurodegenerative diseases also target the telencephalon with striking selectivity. Alzheimer’s disease attacks the hippocampus and association cortex. Parkinson’s disease hits the basal ganglia. Frontotemporal dementia ravages the prefrontal and temporal cortex. Each disease affects particular neurons first, then spreads in a stereotypical and predictable fashion, though the reasons certain neurons are more vulnerable than others remain under active investigation.30PubMed Central. Selective vulnerability in neurodegenerative diseases
Blood Supply and the Blood-Brain Barrier
The telencephalon is metabolically demanding, consuming a disproportionate share of the body’s glucose and oxygen. Its blood supply must be tightly regulated, which is why the blood-brain barrier, a highly selective lining of the brain’s blood vessels, develops early. In mice, vessels in the telencephalon become impermeable to large molecules around embryonic day 16.31Developmental Biology. Differentiation-dependent expression of proteins in brain endothelium during development of the blood-brain barrier The gene Foxc1 plays an early role in getting telencephalic blood vessels to form correctly. When this gene is disrupted in mouse embryos, the telencephalon enlarges abnormally, blood vessels dilate and clump, and hemorrhaging occurs in the ventral telencephalon by embryonic day 11.32PubMed. Foxc1 is required for early stage telencephalic vascular development This underscores that normal brain development depends not just on neurons finding the right places but on blood vessels building a properly sealed supply network around them.
The Telencephalon Across the Animal Kingdom
Humans are hardly the only animals with a telencephalon. Every vertebrate has one, but the structural variety is enormous. Mammals have a layered neocortex. Birds have a nuclear (cluster-based) pallium that looks nothing like a cortex under the microscope. Fish, amphibians, reptiles, and even jawless vertebrates like lampreys each have their own distinct telencephalic architecture, and the evolutionary relationships among these structures remain genuinely difficult to untangle.33PubMed Central. Evolution of the Chordate Telencephalon
What makes this fascinating is that some birds, particularly crows and parrots, display cognitive abilities that rival those of great apes: tool use, problem solving, social reasoning, and even something resembling planning. They accomplish this with a brain that is organized very differently from a mammalian one. An international consortium of neuroscientists formally revised the century-old terminology for the avian brain to reflect the fact that the bird pallium performs neocortex-like cognitive functions, even though its cells are arranged in clusters rather than layers.34PubMed Central. Avian brains and a new understanding of vertebrate brain evolution The mammalian neocortex has a characteristic layered architecture with a “canonical” microcircuit governing information flow among those layers, and remarkably, functional analogs of that circuit have been identified in the avian pallium.35PubMed Central. Coding principles of the canonical cortical microcircuit in the avian brain High intelligence in the animal kingdom, whether in insects, octopuses, birds, or primates, is consistently tied to highly ordered associative neuronal networks in multimodal brain centers, suggesting that natural selection converges on similar computational solutions even when starting from radically different anatomies.36PubMed Central. Convergent evolution of complex brains and high intelligence