Occipital Lobe

The occipital lobe is the region of the brain responsible for perception of our visual world, including color, form, and motion. The human brain has two occipital lobes, one in each hemisphere of the brain, that are divided by the central cerebral fissure. Each lobe is further divided by the calcarine sulcus into the superior occipital lobe, or the cuneus gyrus, and the inferior occipital lobe, or the lingual gyrus. 

The occipital lobe is the smallest of the four lobes of the brain. The other lobes include the frontal lobe, parietal lobe, and temporal lobe. The occipital lobe has two functional units: the primary visual cortex (V1, also known as Brodmann area 17) and the secondary visual cortex, also known as the visual association cortex (V2, V3, V4). The brain receives visual information from the visual pathway, which consists of the retina, optic nerve, and a portion of the brain known as the lateral geniculate nucleus (LGN). After passing through the LGN, the messages within visual stimuli ultimately coalesce in the visual cortex of the occipital lobe. 

The occipital lobe is located at the posterior-most part of the brain: posterior and inferior to the parietal lobe and posterior and superior to the temporal lobe. The parieto-occipital sulcus separates the occipital from the parietal lobe. The occipital lobe sits on top of the tentorium cerebelli, which is a thick membrane of tissue separating the cerebrum and cerebellum.

The function of the occipital lobe is to process visual stimuli from the external world while also assigning meaning to visual perceptions. The occipital lobe functions in  spatial reasoning (e.g. assess distance, size, and depth of an object) and determines color properties of an object. It further contributes to visual memory, which allows for identification of familiar objects, faces, and environments. In addition to transmitting information, the occipital lobe receives information from and sends information to other regions of the brain. For example, a brain region called the dorsal stream, or the “where” pathway, is responsible for integrating information from the parietal lobe to facilitate spatial awareness and guidance of actions (e.g. reaching to grab an object). In addition, the ventral stream, or the "what” pathway, connects the temporal lobe to the occipital lobe to allow for object identification and recognition. The ventral pathway is responsible for long-term stored representations and functions slower than the dorsal stream. 

The most common symptom to occur if the occipital lobe is damaged is cortical blindness. The individual will have difficulty locating objects in their environment, seeing colors (i.e. color agnosia), and may experience hallucinations (i.e. seeing things that are not truly there) or illusions (i.e. inaccurately seeing objects). Additionally, the person may have difficulty recognizing words or drawn objects, seeing movement of an object, reading, and writing. If the person is still able to see, they may have changes in depth perception, thereby leading to inappropriate movements and difficulty navigating the visual field. This can affect fine and gross motor skills as well as balance. 

Specifically, damage to the dorsal stream may cause simultanagnosia, defined as the  inability to perceive and describe more than one object and not in their context (e.g. seeing individual trees and not the forest). Other symptoms include optic ataxia (i.e. inability to use visuospatial information to guide arm movements), hemispatial neglect (i.e. inability to see and comprehend half of the world), akinetopsia (i.e. inability to perceive motion), and apraxia (i.e. inability to carry out voluntary movement). In contrast, damage to the ventral stream may cause inability to recognize faces, known as prosopagnosia, and difficulty interpreting facial expression due to the connection between the ventral stream and the limbic system which controls emotions.

As in any case of traumatic brain injury, the occipital lobe can be damaged as a result of motor vehicle accidents, falls, and firearms. Diseases affecting the brain can be localized to the occipital lobe, such as vascular conditions (stroke, high blood pressure, cerebral artery aneurysms); infections (meningitis and encephalitis); tumors; and autoimmune diseases, such as lupus and multiple sclerosis. The occipital lobe can be damaged as a result of seizures, which can subsequently increase vulnerability for subsequent seizures.

Treating occipital lobe injury typically involves occupational therapy, and depends on the visual disturbance or loss. Those with word blindness can learn to utilize text-to-speech programs, or braille, or engage in eye exercises that can increase the brain’s neuroplasticity to help improve vision. Scanning therapy can help those with visual field loss by encouraging those with neglect to actively pay attention to stimuli on the affected side of the body. A therapy known as prismatic adaptation is used for those with hemispatial neglect. In prismatic adaptation, the individual wears prismatic glasses that shift the visual field to one side, forcing the person to look towards the affected side in order to reach an object.

The occipital lobe is imperative for visual processing in the brain. It is the smallest of the four lobes and is located in the posterior-most region. It allows humans to see color, movement, facial expressions, and words in the environment while also integrating this information from the other lobes of the brain for memory and contextualization. The dorsal, or “where,” pathway connects the occipital lobe to the parietal lobe to facilitate spatial awareness. The ventral, or “what,” pathway connects the occipital lobe and temporal lobe to allow for object identification. The occipital lobe can be damaged as a result of trauma, tumor, vascular injury, autoimmune disease, or seizure. This can lead to partial or complete blindness or to other specific symptoms, such as optic ataxia, hemispatial neglect, akinetopsia, and apraxia. Treatment includes occupational therapy and eye exercises, such as scanning therapy and prismatic adaptation.