The Occipital Lobe

The brain's visual centre at the back of the head: primary visual cortex, the maps of the visual field, the 'what' and 'where' streams, and what happens when it is damaged.

The occipital lobe is the smallest lobe, at the back of the brain. It is almost entirely devoted to vision.

Primary visual cortex

Along the calcarine sulcus on the inner surface lies the primary visual cortex (V1), also called the striate cortex because of a visible white stripe in its layers. It receives input from the eyes via the thalamus. Each side of V1 handles the opposite half of the visual field: the left occipital lobe sees everything to your right, from both eyes. The central part of vision, the fovea, gets a disproportionately large share of cortex.

V1 neurons respond to simple features — edges at particular orientations, contrast, movement direction, colour — as David Hubel and Torsten Wiesel discovered.

Beyond V1

Surrounding visual areas (V2, V3, V4, V5/MT and more) extract increasingly complex features: V4 is important for colour and shape, and V5/MT for motion — damage to it can cause akinetopsia, where the world looks like a series of still frames. From here information flows forward along two streams: the ventral stream into the temporal lobe ("what is it?") and the dorsal stream into the parietal lobe ("where is it, and how do I act on it?").

Damage and blindsight

Damage to V1 on one side causes blindness in the opposite half of the visual field (hemianopia), even though the eyes work perfectly. Remarkably, some such patients can still point to or avoid objects in their "blind" field at above-chance levels while insisting they see nothing — a phenomenon called blindsight, studied by Lawrence Weiskrantz, which shows that some visual processing bypasses conscious awareness.