22 Vision: Central Processing

Casey Henley

Visual information from the retina undergoes extensive processing in the brain to create our visual perception of the world. Central visual pathways and cortical areas analyze different aspects of vision, from basic features like orientation and motion to complex object recognition.

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Visual Fields

Before learning the pathway that visual information takes from the retina to the cortex, it is necessary to understand how the retina views the world around us. The full visual field includes everything we can see without moving our head or eyes.

An illustration showing how the retinas of both eyes together define the boundary of the full visual field. Link to detailed alternative text in caption.
Figure 22.1. The two eyes together can view the entire visual field, which is all the visual space we can see without moving our head or eyes. ‘Full Visual Field’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Visual fields can be divided in multiple ways, each dependent on different regions of the retina

The full visual field can be divided in a few ways. Each individual eye is capable of seeing a portion of, but not the entire, visual field.

An illustration comparing the portion of the full visual field visible to the left eye alone versus the right eye alone. Link to detailed alternative text in caption.
Figure 22.2. Each eye individually can view only a portion of the full visual field. The temporal retina of each eye views the larger, ipsilateral portion of that eye’s visual field. ‘Single Eye Fields’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

The full visual field can also be divided into the right and left hemifields. The hemifields range from the most peripheral point to the center point, splitting the full visual field into two equal regions. Both eyes are involved in viewing each hemifield. The fovea separates the retina into two sections: the nasal retina and the temporal retina. The nasal retina is the medial portion that is located toward the nose. The temporal retina is the lateral portion that is located toward the temple and temporal bone. The nasal retina from one eye and the temporal retina from the other eye are able to view an entire hemifield.

An illustration showing that each visual hemifield is viewed using the nasal retina of one eye combined with the temporal retina of the other eye. Link to detailed alternative text in caption.
Figure 22.3. The full visual field can be divided into left and right hemifields. Both eyes contribute to viewing these regions. The nasal retina of the left eye and the temporal retina of the right eye view the left hemifield. The nasal retina of the right eye and the temporal retina of the left eye view the right hemifield. ‘Visual Hemifields’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Finally, the full visual field can be separated into monocular and binocular regions. Each monocular field is visual space that can only be viewed by one eye. The binocular region is visual space that can be viewed by both eyes.

An illustration showing that the peripheral visual field is monocular, viewed by only one eye, while the central visual field is binocular, viewed by both eyes. Link to detailed alternative text in caption.
Figure 22.4. Monocular visual fields are viewed by only one eye and are located toward the periphery of the full visual field. The binocular visual field is viewed by both eyes and is located in the center of the full visual field. ‘Monocular and Binocular Visual Fields’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Pathway to Brain

The right visual field is processed by the left side of the brain; the left visual field is processed by the right side of the brain

Visual information from each eye leaves the retina via the ganglion cell axons at the optic disc, creating the optic nerve. Prior to entering the brain, axons from the nasal portion of each retina cross the midline at the optic chiasm. Since the axons from the nasal retina cross to the opposite side of the nervous system but the temporal retina axons do not, this leads to the brain processing input from the contralateral (opposite side) visual hemifield. Therefore, the right side of the brain receives visual information from the left hemifield and vice versa.

An illustration showing that axons from the nasal retina cross at the optic chiasm while temporal retina axons do not, so each optic tract carries information from the contralateral visual hemifield. Link to detailed alternative text in caption.
Figure 22.5. Information from each eye is carried away from the retina by the optic nerve. Information perceived by neurons in the nasal retina of each eye crosses the midline at the optic chiasm. Information from the contralateral visual hemifield then travels to the brain. ‘Pathway from Retina’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

View the optic nerve (cranial nerve II) using the BrainFacts.org 3D Brain

The optic tract enters the brain and ascends to synapse in the lateral geniculate nucleus of the thalamus. From there, axons project to the primary visual cortex, also called the striate cortex or V1, located in the occipital lobe.

A horizontal brain section showing the optic tract projecting to the lateral geniculate nucleus of the thalamus and then to the primary visual cortex in the occipital lobe. Link to detailed alternative text in caption.
Figure 22.6. A horizontal section of the brain. The optic tract enters the brain and synapses in the lateral geniculate nucleus of the thalamus. Information is then sent to the primary visual cortex in the occipital lobe. ‘CNS Visual Pathway’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.
A flowchart of the visual pathway from photoreceptor through bipolar and ganglion cells in the retina, via the optic nerve, chiasm, and tract, to the thalamus and primary visual cortex. Link to detailed alternative text in caption.
Figure 22.7. Visual information that is sent through the full visual pathway, therefore, moves from photoreceptor to bipolar cell to ganglion cell in the retina. It leaves the retina via the optic nerve, optic chiasm, and optic tract to the lateral geniculate nucleus of the thalamus and then travels to the primary visual cortex. ‘Visual Pathway in Text’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

View the thalamus using the BrainFacts.org 3D Brain

View the primary visual cortex using the BrainFacts.org 3D Brain

Receptive Fields

Receptive field complexity increases at each synapse along the visual pathway, so neurons respond to progressively more specific and abstract features the farther they sit from the retina

As information moves from the retina to the cortex, receptive fields become larger and more complex. Receptive fields in the thalamus continue to be circular in shape like the receptive fields of the retinal neurons. However, once information reaches the primary visual cortex, these circular receptive fields combine to create receptive fields that are activated by lines.

An illustration showing that circular thalamic receptive fields arranged in a row or column converge onto a single visual cortex neuron to create a receptive field oriented as a line. Link to detailed alternative text in caption.
Figure 22.8. Circular receptive fields located in the thalamus combine to form straight receptive fields in the visual cortex. The orientation of the line in the visual cortex depends on the location of the thalamic receptive fields. ‘CNS Visual Line Receptive Field’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

These receptive fields cause neurons in the primary visual cortex to respond best to a line in a specific orientation. The firing rate of the neuron will increase as the line rotates toward the “preferred” orientation. The firing rate will be highest when the line is in the exact preferred orientation. Different orientations are preferred by different neurons.

A diagram showing that a visual cortex neuron fires most when a line matches its preferred orientation and less as the line rotates away from that orientation. Link to detailed alternative text in caption.
Figure 22.9. Neurons in the primary visual cortex show the highest firing rate in response to a preferred line orientation. Firing rate decreases as the line rotates away from that preferred orientation. ‘CNS Visual Line Orientation Responses’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Higher-Level Processing of Sensory Information

Sensory system processing of input does not end upon reaching the primary sensory cortex in any sensory system. Information typically gets sent from the primary sensory cortex to other sensory association regions throughout the brain. The characteristics of sensory information becomes more complex as this higher-level processing occurs.

Beyond the Primary Visual Cortex

The dorsal stream processes spatial location and movement; the ventral stream recognizes objects

In the visual system, there are two broad streams of information that leave the primary visual cortex. Information that travels from the primary visual cortex down through the inferior temporal lobe is responsible for determining object recognition, or what an object is. Differentiating between an apple and a person occurs in this stream. Information that travels from the striate cortex up through the parietal lobe is responsible for motion or spatial components of vision.

A brain diagram showing the dorsal stream extending from the primary visual cortex to the parietal lobe and the ventral stream extending to the temporal lobe. Link to detailed alternative text in caption.
Figure 22.10. Information continues to be processed after reaching the primary visual cortex. The dorsal stream travels to the parietal cortex and is important for spatial components of vision. The ventral stream travels to the temporal lobe and is important for object recognition. ‘Visual Processing Streams’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Dorsal Stream – “Where is it?”

One of the most important regions in the dorsal pathway is region MT, also called V5. In this region, neurons are preferentially activated by a specific direction of movement by an object – for example, left to right or up to down. As an example, remember the receptive fields in the primary visual cortex were activated by lines at a specific orientation. Like that, in V5, the neurons would be activated by lines moving in a specific direction.

As information continues to be processed through the dorsal stream, the neurons become selective for more complex motions. The dorsal stream is also important for processing our actions in response to visual stimulation, for example, reaching for an object in the visual field or navigating around objects while walking.

A brain diagram marking the location of area MT (V5) at the boundary of the primary visual cortex and the parietal lobe, along the dorsal stream. Link to detailed alternative text in caption.
Figure 22.11. Area MT, also called V5, is an early processing region of the dorsal stream through the parietal lobe. Neurons in the region are activated by movement of an object in a specific direction. ‘Area MT’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Ventral Stream – “What is it?”

Object identification is a key function of our visual system. The ventral visual stream is responsible for this process. Like the more complex activation characteristics of region MT in the dorsal stream, neurons in Area V4 show more complex receptive fields with sensitivity to shape and color. As visual information continues to be processed through the inferior temporal lobe, differentiation of objects occurs. For example, in a region called the fusiform face area, located in the fusiform gyrus, which lies on the ventral aspect of the temporal lobe, neurons are activated by faces and can be specialized for categories of faces, such as familiar people versus strangers.

A brain diagram showing the progression of the ventral stream from area V4 through the inferior temporal lobe to the fusiform gyrus. Link to detailed alternative text in caption.
Figure 22.12. The ventral stream is first processed by area V4, which recognizes shapes and color. Information then continues through the inferior temporal lobe and sends information to regions like the fusiform gyrus, which is an area responsible for the recognition of faces. ‘Ventral Stream’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

The inferior temporal lobe also makes reciprocal connections with the structures in the limbic system. The limbic system plays an important role in processing emotions and memory, both of which are significant components of visual perception. The amygdala ties visual stimuli with emotions and provides value to objects. A family member will have emotional ties that a stranger will not. The hippocampus is responsible for learning and memory and helps establish memories of visual stimuli.

Two coronal brain sections showing the amygdala and hippocampus located deep within the medial temporal lobe. Link to detailed alternative text in caption.
Figure 22.13. The limbic system structures, the amygdala and the hippocampus, also play important roles in visual processing. Both regions are located deep in the temporal lobe and have reciprocal connections with the ventral stream as it moves through the temporal lobe. ‘Deep Temporal Lobe’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

View the amygdala using the BrainFacts.org 3D Brain

View the hippocampus using the BrainFacts.org 3D Brain

Pathways through the Amygdala

The amygdala receives visual information through multiple pathways

In addition to the projections to the amygdala via the ventral stream, the amygdala also appears to rely on input from the thalamus that is independent of the ventral stream pathway. A shorter pathway travels from the retina to the amygdala via the thalamus. It is believed that this pathway allows for a rapid responses to threats and allows visual stimuli to activate the amygdala quicker than processing through the visual cortex. In fact, studies have shown that pictures of angry or fearful faces can cause amygdala activation without conscious awareness of seeing the image, a result of images being shown for only milliseconds to the subject.

A mid-sagittal brain section showing a direct pathway from the retina to the amygdala via the thalamus, separate from the longer pathway to the visual cortex. Link to detailed alternative text in caption.
Figure 22.14. Information from the retina travels to the thalamus. From the thalamus, there is a direct pathway to the amygdala (solid red arrows) in addition to the pathway to the occipital lobe and ventral stream (dashed red arrows). ‘Amygdala Visual Pathway’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

Non-Thalamic Pathways

The retina sends projections to more regions than just the thalamus

Although most retinal output projects to the lateral geniculate nucleus of the thalamus and then to the primary visual cortex, there are some axons that project to other areas of the brain. A subset of specialized retinal ganglion cells project to the suprachiasmatic nucleus in the hypothalamus. This region is critical for circadian rhythms and the sleep/wake cycle. Other retinal neurons send axons to the pretectum, a midbrain region that communicates with motor nuclei and is responsible for pupillary control. Finally, other ganglion cells project to the superior colliculus, another midbrain region. This pathway is responsible for movements that will orient the head and eyes toward an object to focus the object in the center of the visual field, the region of highest visual acuity.

A horizontal brain section showing retinal projections to the suprachiasmatic nucleus, pretectum, and superior colliculus, in addition to the lateral geniculate nucleus of the thalamus. Link to detailed alternative text in caption.
Figure 22.15. In addition to the thalamus, the retinal neurons send projections to other regions of the brain. The suprachiasmatic nucleus (pink) is located in the hypothalamus and is important for biological rhythms. The pretectum (light blue) is a midbrain structure that plays a role in muscle control of the pupil. Finally, the retina projects to the superior colliculus (blue), another midbrain region important in eye and head movements. The lateral geniculate nucleus of the thalamus (green) is also shown. ‘Non-Thalamic Retinal Pathways’ by Casey L. Henley (CC-BY-NC-SA). View detailed alternative text.

View the hypothalamus using the BrainFacts.org 3D Brain

View the midbrain using the BrainFacts.org 3D Brain

Conclusion

Central visual processing transforms simple retinal signals into increasingly complex representations through parallel processing streams and hierarchical organization, from receptive fields tuned to lines and edges in the primary visual cortex to specialized regions for motion and object recognition downstream. Beyond this cortical hierarchy, retinal output also reaches limbic structures like the amygdala and hippocampus, and projects directly to non-thalamic targets that control circadian rhythms, pupillary reflexes, and orienting movements.

Key Takeaways

  • The nasal and temporal retinal regions are responsible for viewing specific regions of the visual field
  • Some retinal projections cross the midline at the optic chiasm, causing the left side of the brain to process the right visual hemifield and vice versa
  • The retinal axons synapse in the lateral geniculate nucleus of the thalamus. Information then travels to the primary visual cortex
  • Receptive fields and the preferred visual stimuli for neuron activation become more complex as information moves through the visual pathway
    • Retinal cells and thalamic neurons have circular receptive fields with inhibitory surround
    • Primary visual cortex neurons have linear receptive fields and are activated by a line in a specific orientation
    • Area MT / V5 is activated by motion in a specific direction
    • Area V4 is activated by specific shapes and colors
    • The fusiform face area, located in the fusiform gyrus, is activated by faces.
  • The retina also projects to midbrain regions

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Vision: Central Processing Copyright © 2021 by Casey Henley is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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