Summary
Highlights
The primary visual cortex (V1), located in the occipital lobes at the posterior-most region of the brain, is a highly studied area for understanding how the brain processes visual information. It is surrounded by higher-level visual cortex regions that extract different features.
V1's functional organization is based on the spatial location of stimuli within the visual field. The left visual field is represented in the right V1 and vice versa due to the partial fiber crossing at the optic chiasm. The visual field consists of regions visible to the right eye, left eye, and an overlapping central area visible to both.
Transecting the left optic nerve would obscure the portion of the left visual field visible only to the left eye. Transecting the left optic tract would result in the loss of the entire right visual field. Transecting the optic chiasm would lead to a loss of the peripheral visual field on both sides.
The primary visual cortex exhibits cortical magnification, where an outsized area of V1 is dedicated to representing stimuli at the point of fixation (fovea). Although the fovea comprises only about one percent of the retina, over 50 percent of V1 processes stimuli from this region, contributing to exceptional spatial resolution.
Mammalian cortex, including V1, is subdivided into multiple layers with unique functions. Projections from the lateral geniculate nucleus predominantly target neurons in layer 4 (the input layer), which then project to superficial layers that, in turn, project to other cortical areas.
Neurons in V1 extract basic stimulus features like location and orientation. They respond preferentially to light bars of particular angles at specific locations. This orientation coding arises from the convergence of multiple ascending projections onto single cortical cells, creating complex receptive fields and specific tuning curves for different orientations.
Simple cells in V1 extract single stimulus features, such as orientation, including edge detector and stripe detector cells. More complex cells can extract multiple stimulus features simultaneously, like preferred orientation and preferred direction of motion. These complex cells are less concerned with the exact spatial location of stimuli.
The primary visual cortex is organized into functional columns. Input from the two eyes remains discrete, forming ocular dominance columns, with separate columns for input from the left and right eyes. Within each ocular dominance column are smaller hypercolumns, which are preferentially tuned to stimuli of different orientations. Blobs within these columns are responsive to color.