The secondary visual cortex, or V2, is a processing station in the brain’s visual system. Located in the occipital lobe, this area takes in visual information that has already undergone initial processing and begins a more sophisticated analysis. It acts as an intermediary, preparing sensory data for more specialized regions of the brain. The function of V2 is not to see the world from scratch, but to build a richer, more detailed visual perception from simpler elements.
Anatomy and Connection to the Visual Pathway
The secondary visual cortex, also identified as Brodmann area 18, is positioned in the occipital lobe. It physically wraps around the primary visual cortex (V1), also known as Brodmann area 17. This anatomical arrangement reflects a hierarchical flow of information.
V2 receives most of its input directly from V1, the first cortical area to receive visual signals from the eyes. V1 detects the most basic components of a visual scene, such as lines, orientations, and edges. This raw data is then transmitted to V2 for the next stage of processing.
Within V2, these simple features are integrated and assembled into more meaningful forms. This process positions V2 as a bridge between initial sensation and complex perception.
Complex Feature Detection
Neurons in V2 respond to more intricate attributes than those in V1, processing complex patterns, object orientations, and differences in color and spatial frequency. This allows the brain to define surfaces, textures, and the relationships between different parts of a visual scene. It is here that the simple lines detected by V1 are combined to form angles and curves.
V2 plays a role in perceiving depth through binocular disparity. Because our eyes are set apart, they receive slightly different images of the same object. V2 neurons are tuned to these subtle differences, allowing the brain to calculate distance and create a three-dimensional representation of the world. This area also contributes to color constancy, the ability to perceive an object’s color as consistent under different lighting conditions.
V2 is also involved in processing “illusory contours.” These are the edges of a shape that the brain perceives without any physical line or color change in the image. An example is the Kanizsa triangle, where three shapes are arranged to suggest a triangle in the center. Even though the triangle is not drawn, neurons in V2 fire as if detecting its borders, showing this region constructs shapes from incomplete information.
Divergence into Visual Streams
The secondary visual cortex directs visual information into two distinct processing pathways that extend to other lobes of the brain. Information leaving V2 is segregated into a ventral stream and a dorsal stream, each specializing in different aspects of visual analysis. This division ensures that different types of visual questions can be answered simultaneously.
The ventral stream, often called the “what” pathway, travels from V2 into the inferior temporal cortex. This pathway is dedicated to object recognition and representation. It allows us to identify forms, faces, and objects by connecting what we see to our long-term memory. For instance, when you recognize a coffee mug, the ventral stream matches the visual input to your stored knowledge of what a mug is.
In contrast, the dorsal stream, or the “where/how” pathway, projects from V2 into the parietal lobe. This pathway is concerned with spatial information and visual-motor skills. It processes an object’s location and movement, guiding your physical interactions with the world. The dorsal stream calculates the mug’s position and guides your hand to pick it up, while the ventral stream identifies it as a mug.
Consequences of Damage
Injury to the secondary visual cortex from stroke, trauma, or disease can lead to specific visual deficits without causing complete blindness. Because V2 integrates basic features from V1, damage can disrupt the ability to assemble a coherent visual percept. A person with a V2 lesion might still see lines and colors but struggle to combine these elements into recognizable objects.
This condition can manifest as a form of visual agnosia, where object recognition is impaired. For example, a patient might describe the components of a pair of glasses—two circles and a connecting line—but be unable to identify the object as glasses. They see the parts but cannot perceive the whole, making their visual world seem fragmented.
Damage can also affect the perception of complex patterns, textures, and depth. The ability to interpret illusory contours may be lost, making it difficult to “fill in the blanks” in a visual scene. Depending on whether the damage affects the origins of the dorsal or ventral streams, deficits could be more related to spatial awareness or to object identification.