Primary sensory cortices (V1 for vision, A1 for audition, S1 for somatosensation) preserve the spatial organization of their sensory inputs in a somatotopic map. These cortices receive thalamically-relayed sensory information and perform initial feature extraction. Plasticity in these maps can occur through experience, injury, or learning, allowing behavioral adaptation and skill development.
You already know that the thalamus acts as a relay and gating station for sensory information before it reaches cortex. The primary sensory cortices are the first cortical destinations of those relayed signals, and their defining property is that they maintain the topographic organization of the sensory surface. In S1, neighboring neurons respond to neighboring body parts; in V1, neighboring neurons respond to neighboring points in the visual field; in A1, neighboring neurons respond to neighboring sound frequencies (tonotopy). This preservation of spatial or feature-space organization is called a cortical map.
The most famous example is the sensory homunculus of S1 — a distorted body map drawn on the postcentral gyrus. It looks grotesque because the map is not proportional to body size; it is proportional to innervation density. Your lips and fingertips occupy far more cortical real estate than your back or thigh, because fine tactile discrimination requires many closely-spaced receptors and therefore many dedicated cortical neurons. This is a general principle: cortical area reflects the computational demand of that sensory region, not its physical dimensions.
What makes these maps especially interesting is that they are not fixed. Cortical plasticity — which you know from the neuroplasticity prerequisite — means that maps can be reorganized by experience, injury, or training. When musicians practice an instrument intensively, the cortical territory dedicated to their active fingers expands. When a finger is amputated, the cortical region representing that finger is gradually invaded by representations from neighboring fingers. This experience-dependent plasticity is most robust in early development but persists in attenuated form throughout life. The maps are competitive: neurons that receive more input, more frequently, claim more cortical space.
The key conceptual move is connecting thalamic relay to cortical organization: the thalamus preserves the topographic structure of sensory input during transmission, and the primary cortex inherits and elaborates this organization. V1 receives retinotopically organized input from the lateral geniculate nucleus; S1 receives somatotopically organized input from the ventral posterior nucleus. This chain of orderly projections — from sensory surface to thalamus to primary cortex — is what enables the cortex to perform spatially precise feature extraction as the first stage of perceptual processing.