Sensory information flows through parallel streams with distinct functions. The visual system has a ventral 'what' stream (inferior temporal cortex) processing object identity and a dorsal 'where' stream (parietal cortex) processing spatial location and motor control. Similar functional segregation occurs in auditory and somatosensory cortex.
Compare receptive field properties across streams. Use lesion studies to isolate stream functions.
Streams are segregated—they interact extensively. All visual information goes to V1 first—some routes bypass V1.
From your study of cortical organization, you know that the cerebral cortex is parceled into functionally distinct areas and that sensory information is processed through hierarchical stages. From color vision and perception, you know that the visual system extracts features like wavelength and contrast early in processing. The concept of cortical processing streams explains what happens next: rather than a single pipeline that progressively builds a complete picture of the world, sensory cortex splits information into parallel channels that emphasize different aspects of a stimulus.
The visual system provides the clearest example. After initial processing in the primary visual cortex (V1), visual information diverges into two major pathways. The ventral stream flows from V1 through areas V2 and V4 into the inferior temporal cortex. This pathway is specialized for recognizing what something is — its shape, color, texture, and identity. Neurons along the ventral stream have progressively larger receptive fields and respond to increasingly complex features: simple edges in V1, contours and surface properties in V4, and whole objects and faces in inferotemporal cortex. Damage to the ventral stream produces visual agnosia — patients can see objects clearly, describe their features, and navigate around them, but cannot recognize or name what they are looking at.
The dorsal stream flows from V1 through area V5/MT into the posterior parietal cortex. This pathway processes where something is and how to interact with it — spatial location, motion, and the visual guidance of actions like reaching and grasping. Neurons in area MT are highly sensitive to the direction and speed of motion, while parietal areas integrate visual information with motor planning. Damage to the dorsal stream produces optic ataxia — patients can identify objects but cannot accurately reach for them — and akinetopsia, the inability to perceive motion (the world appears as a series of frozen snapshots).
The ventral/dorsal distinction is sometimes oversimplified as "what vs. where," but the reality is richer. The dorsal stream is better described as a "how" pathway — it computes the spatial transformations needed for action, not just an abstract sense of location. Moreover, the two streams are not sealed off from each other. Extensive cross-connections allow information to flow between them, and many perceptual tasks require both streams working in concert. A similar principle of parallel functional streams has been identified in the auditory cortex (a "what" stream for sound identity and a "where" stream for sound localization) and in somatosensory processing. The general principle is that the cortex decomposes complex sensory input into parallel channels optimized for different behavioral demands, then recombines them as needed for unified perception and action.