Opening the book…
The first synapse of the optic nerve is formed in the thalamus at a subregion called the lateral geniculate nucleus, or LGN (originating from the Latin genu-, meaning “knee”). The LGN is divided into six layers, named 1 through 6, going from ventral to dorsal. Histologically, the LGN is made up of three different populations of neurons. The magnocellular cells (magn- means large, as in “magnify” or “magnanimous”), or M cells, are large-diameter neurons (~20 µm) that are found in layers 1 and 2. The parvocellular cells (parv- means small), or P cells, are smalldiameter (~10 µm) neurons found in layers 3 through 6. Ventral to each layer is a row of very small neurons, called konio- cellular cells (koniomeans dust), or K cells.
We know that the visual information from the temporal retina synapses ipsilaterally, while information from the nasal retina synapses contralaterally. The specific synaptic inputs into the LGN follow a specific organization depending on the origin of the retinal ganglion neurons. Temporal retinal ganglion neurons synapse only onto layers 2, 3, and 5 of the ipsilateral LGN, while nasal retinal ganglion neurons form synapses exclusively onto contralateral layers 1, 4, and 6.
The outputs of the LGN are a series of axonal bundles called the optic radiations. These optic radiations are divided into two main bundles, the upper and lower divisions. The upper divisions carry information from the lower visual field, whereas the lower divisions carry information from the upper visual field.
From the LGN of the thalamus, the optic radiations project to the occipital lobe at the caudal (posterior) end of the brain. Once visual information travels into the cortex, the process is less about sensation and mostly about perception. For example, many of the post-optic radiation signaling pathways are concerned with questions like attention, object orientation, and recognition. Many visual perception functions are performed by multiple brain regions, and no single area is responsible for any one compartmentalized function. In other words, there is still much work to be done on understanding the precise function of the areas of the visual cortex.
The outputs of the LGN are axons which form synapses in the primary visual cortex, which is also called V1. Alternatively, V1 is also called the striate cortex because it has a large white stripe that can be seen in surgical dissection. This white stripe is the bundle of incoming optic tract axons, which are heavily myelinated.
Each neuron in V1 receives visual information from a specific patch of retinal cells. This organizational pattern, where a section of retinal inputs map onto neurons of a specific section of V1, is called retinotopic organization. For example, the dorsal part of V1 receives information from the bottom half of the visual field. Visual information from the fovea, despite being only 1% of the total visual field, takes up about half of all neurons in V1.
Health care professionals can diagnose the location of an injury based on the deficits in a patient’s visual field. To record the sites of vision loss, they may use a pair of circles which show which areas of the visual field are lost. For example, if a person is missing their right eye, all information from the right eye will be missing and the right circle will be colored in black.
If the visual information-conveying pathway is damaged, a person may suddenly lose half of their visual field resulting in a condition called hemianopsia (hemi- meaning half, a- meaning none, and -opsia referring to vision). There are a few forms of hemianopsia that correlate to specific injuries of the optic nerve. For example, injury to one branch of the nerve after the optic chiasm results in homonymous hemianopsia, the loss of either the left or right visual field of both eyes.
Injuries may damage some optic radiations while sparing the others. This results in a condition called quadrantanopsia, resulting in a loss of a fourth, or one quadrant, of a person’s visual field. If the upper division of the right optic radiation is injured, the person will lose the upper left visual field of both of their eyes: left superior quadrantanopsia.
After processing in V1, visual information is passed along to the secondary visual cortex, or V2. V2 represents the beginning of the visual association cortices. Following V2 are other visual cortical areas that contribute to various aspects of visual perception. For example, V3 and V5 help us comprehend motion, V4 contributes to color perception, and V6 helps with understanding our position in our surroundings.
Dual stream hypothesis
After V2, visual information passes through two streams of communication: the dorsal stream and the ventral stream.
The dorsal stream is described as the “where” pathway because these structures help us identify where objects are located in the space around us. The dorsal stream includes area V5, which contributes to perception of motion. These structures also guide us when we move through our environments, contributing to our sense of spatial awareness. For example, a task such as reaching out to grab an object in front of you uses a combination of these features, so this task is guided largely by dorsal stream structures.
The ventral stream is the “what” pathway, and helps in the identification of objects that we see. Ventral stream structures are important for visual memory. The fusiform face area (FFA) and the parahippocampal place area (PPA) are especially important for the storage of facial stimuli and scenery or buildings, respectively. When the visual system senses these complex stimuli, those signals get processed through these ventral stream pathways. These incoming stimuli are compared with the memories stored in the ventral stream, and this comparison contributes to our capacity for perception and identification.
The two streams are not independent of each other. Rather, successful organisms require the melding of both components of visual perception. For example, imagine you are a prehistoric organism living in a food-scarce environment. Approaching a small berry tree, you would use ventral stream structures to correlate the berries with memories: Did these berries taste good and give me the calories I need to survive? Or did these berries make me violently ill, and are therefore probably poisonous? If they are the delicious berries that I want, I will use the dorsal stream structures to take note of their precise location so I can reach out for them and pick the berries . In this example, proper interaction of the dual streams contributes to goal-driven actions.
Deficits in the circuitry of visual processing up until this point in the chapter result in largely predictable outcomes. However, once that information enters the cortex of brain, the circuitry involved becomes much more complex, and unusual visual perception disorders may result from atypical neural communication in the brain.
Akinetopsia, or motion blindness, is a very rare visual condition resulting in the inability to perceive objects in motion. Instead of seeing movement, these people see the world as a series of “freeze frames” that refresh occasionally, or only whenever the object stops moving. People with akinetopsia have severe difficulty crossing the street or pouring a cup of tea. Usually, akinetopsia results from brain injury like trauma or a stroke.
Prosopagnosia, or face blindness, is another unusual visual condition that results in people who have tremendous difficulty with identifying faces. People with prosopagnosia recognize that they are looking at a face. They see the eyes, nose, and mouth, but they often cannot identify who that face belongs to. Some cases are so severe, that they cannot even recognize themselves in a mirror. Prosopagnosia is far more common than akinetopsia; an estimated 1% of people have some degree of face blindness.
Saccadic eye movements
Take a look at the objects around you right now. You don’t notice it, but as you shifted your focus from one object to the next, your eyes made tiny, rapid, jerking motions over the different objects instead of moving smoothly across your field of vision. These rapid movements, driven by the actions of the extraocular muscles, are called saccades (pronounced “suh-KAHD”). Saccades are among the fastest muscle movements in the body.
These saccades help you accurately capture visual information from your surroundings. The fovea, which has the highest acuity vision, only collects information from a tiny portion of your visual field. By performing these saccades, you can scan over several objects with the highest acuity, thus allowing your brain to build a representation of the visual field.
This page titled 7.5: Visual perception in the brain was last modified on Wed, 08 Jul 2026 03:46:43 GMT and is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Austin Lim via source content that was edited to the style and standards of the LibreTexts platform.
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