Visual cortex

The visual cortex is located in the occipital lobe, with the primary visual cortex (V1, or striate cortex) at the posterior pole. V1 receives input from the lateral geniculate nucleus of the thalamus via the optic radiation. Neurons in V1 are arranged in ocular dominance columns and orientation columns, forming a retinotopic map of the visual field.¹

Beyond V1, the visual cortex comprises the extrastriate areas, including V2, V3, V4, and the middle temporal area (MT/V5). These areas are organized into two main streams: the ventral stream (object recognition) and the dorsal stream (spatial processing and motion).²

Each visual area specializes in different aspects of vision. V1 detects edges, orientation, and spatial frequency. V2 integrates contours and illusory boundaries. V4 is involved in color perception, while MT/V5 processes motion direction and speed.³

The ventral stream, often called the "what" pathway, includes the fusiform face area (FFA) and parahippocampal place area (PPA), which respond selectively to faces and scenes, respectively. The dorsal stream, the "where" pathway, includes the intraparietal sulcus, which supports spatial attention and visuomotor control.⁴

The visual cortex undergoes critical periods of development, during which sensory experience shapes neural circuits. Monocular deprivation during this period can lead to amblyopia, as demonstrated by Hubel and Wiesel's Nobel Prize-winning work.⁵

In adulthood, the visual cortex retains some plasticity, as shown by studies of visual perceptual learning and cross-modal plasticity in blind individuals, where the visual cortex responds to touch and sound.⁶

The visual cortex contains a region known as the visual word form area (VWFA), which is specialized for reading and is located in the left fusiform gyrus. Damage to this area causes pure alexia, the inability to read despite normal vision.⁷

Another obscure fact is that the visual cortex has a retinotopic map that is distorted: the fovea (central vision) occupies a disproportionately large area, a phenomenon called cortical magnification. Also, some individuals with damage to V1 experience blindsight, where they can respond to visual stimuli without conscious perception.⁸

Retinotopic map

A spatial map of the visual field on the cortex, preserving the layout of the retina.

Ocular dominance columns

Clusters of neurons in V1 that respond preferentially to input from one eye.

Blindsight

Residual visual ability without conscious awareness, often after V1 damage.