Visual Pathways Flashcards
What is the visual field?
The visual field of an eye is all we can see with this eye. The centre of the visual field is the point of fixation. Around the centre, we differentiate between the four quadrants of the visual field.
Where is the blind spot?
Blind Spot is in Temporal Visual Hemifield Since Photo-insensitive Optic Disc
is in Nasal Hemiretina
What is the pathway of light, from the visual field to the retina?
The Pathway of Light, from the Visual Field to the Retina
The four quadrants of the visual field are projected onto the retina.
The superior half of the visual field is projected to the inferior half of the retina, and vice versa.
The left half of the visual field is projected on the right half of the retina, and vice versa.
As an example, for one of the quadrants of the visual field, light originating in the left superior quadrant of the visual field, after passing through the pupil, ends up in the inferior right quadrant of the retina.
It becomes a bit more difficult when we talk about the orientation of both the visual field and the retina, relative to the anatomy of the head. The nasal visual hemifield of the right eye, for example, ends up in the temporal hemiretina of the right eye. The temporal visual hemifield of the right eye, on the other hand, ends up on the nasal hemiretina of the right eye.
This often causes confusion when we talk about visual field defects, which we will discuss later, in the clinical correlations. It should be clear that visual field defects refer to deficits in the visual fields, which is the outside world, the visual environment.
So, when we describe a visual field defect as “bitemporal hemianopia,” for example, this means that the patient cannot see the temporal hemifields, i.e., cannot see the right visual hemifield with the right eye (temporal hemifield of the right eye) and cannot see the left visual hemifield with the left eye (temporal hemifield of the left eye)
What are the projections of retinal ganglion cells?
The schematic diagram below gives you an overview of the projections of retinal ganglion cells, which are the output neurons of the retina. Please note that this does not necessarily imply that every single retinal ganglion cell has a branch into all of the four target areas shown in this schematic diagram.
Fibers of retinal ganglion cells enter the optic nerve at the optic disk and then pass through the optic nerve. About half of them cross at the optic chiasm (details later). The pathway follows the optic tract to the different synaptic terminals of retinal ganglion cells.
Most of the axons of retinal ganglion cells or their collaterals terminate in the lateral geniculate nucleus (LGN) of the thalamus, which is the relay station between the retina and the primary visual cortex.
Where do fibers end from fibers of the retinal ganglion?
Some fibers also terminate in the suprachiasmatic nucleus of the hypothalamus, where they trigger the circadian clock (which will be presented in more detail later in the module), in the pretectal nucleus, forming part of the afferent limb of the pupillary light reflex, or in the superior colliculus of the midbrain, where they contribute to eye movements.
During this lecture, we will discuss the main visual pathway, via the LGN to the primary visual cortex (V1). We will follow the representation of each of the four quadrants of the visual field along the visual pathway, up to V1.
Visual field deficits based on lesions of the visual pathway will be discussed in the clinical correlations
What happens from the retina onwards?
Lateral geniculate nucleus (thalamus) (required for visual experience)
Suprachiasmatic nucleus (hypothalamus)
Pretectal nucleus(midbrain)
Superior colliculus (midbrain)
Describe the elements of the visual pathway
We will follow the representation of the four quadrants of the visual field (superior / inferior and left / right) from the visual fields of both eyes.
The circle in the center of the visual field diagram represents the macular
region of vision, the outer circle defines the outer boundaries of the visual field, divided into the four quadrants. We have chosen the display of the visual fields of the left and the right eye as separate entities.
The reason is that lesions along the visual pathways often affect only one eye, or the other, and, consequently, visual field testing in a clinical scenario is performed separately for each eye. Note, however, that there is a substantial overlap between the visual fields of both eyes, which allows binocular vision and includes all four quadrants of the visual fields. Only a small region in the periphery of vision for each eye is monocular (known as the ‘monocular crescent’)
What are the 2 rules of thumb from the retina?
- Information from left visual field is “viewed” by right cerebral hemisphere and vice versa
- Information from superior portions of visual field is “viewed” by inferior portions of the visual pathway and vice versa
Describe the left superior quadrant
In class we will work through an example, starting our pathway in the left superior quadrant of the visual field.
Light originating in the left superior quadrant of the visual field strikes the retina of the left eye in the inferior portion of the nasal hemiretina. The retina of the right eye receives this sensory information in the inferior portion of the temporal hemiretina. Fibers originating in nasal hemiretina cross at the optic chiasm, while fibers originating in the temporal hemiretina do not.
In our example, the axons from the left eye carrying sensory information of the left visual hemifield cross over at the optic chiasm, run in the contralateral (right) optic tract and synapse in the right lateral geniculate nucleus (LGN). Axons of the retinal ganglion cells involved in this pathway in the right eye, which are located in the temporal hemiretina, do not cross over at the optic chiasm but stay ipsilateral (on the right side). They also run in the right optic tract and synapse in the right LGN
Fibers originating in the LGN form the optic radiation, which consists of two portions, the temporal radiation and the parietal radiation, indicating in the lobe through which the fibers pass enroute to the primary visual cortex.
LGN fibers carrying visual information of the superior half of the visual field follow the temporal radiation; fibers carrying visual information from the inferior half of the visual field follow the parietal radiation.
In our example, the axons originating in the right LGN which are carrying sensory information from the superior left quadrant of the visual field, use the right temporal radiation and synapse in the inferior portion (below the calcarine sulcus) of the right primary visual cortex (V1).
Give examples of the left superior visual quadrant
- Viewed by right cerebral hemisphere
2. Viewed by inferior portions of visual pathway
What is the significance of the left inferior quadrant?
The second example we will work through in class will deal with the left inferior quadrant of the visual field.
Axons originating in the superior nasal quadrant of the retina of the left eye cross at the optic chiasm, run in the contralateral (right) optic tract and synapse in the right lateral geniculate nucleus (LGN). Fibers originating in the superior temporal quadrant of the retina of the right eye stay ipsilateral (which is on the right side). They travel in the right optic tract and synapse in the right LGN, as in the previous example.
The LGN fibers carry sensory information from the inferior half of the visual field. They follow the parietal radiation pathway and synapse in the superior portion of the primary visual cortex (V1), above the calcarine sulcus.
In these two examples (left superior and left inferior quadrants) we have covered the pathway for the left visual hemifield of both eyes. The left visual hemifield also represents the temporal visual hemifield of the left eye and the nasal visual hemifield of the right eye. The projection of the left visual hemifield of both eyes had been on the nasal hemiretina of the left eye and on the temporal hemiretina of the right eye. The axons of the retinal ganglion cells travel along the optic nerves to the optic chiasm.
At the chiasm, fibers originating in the nasal hemiretina (of the left eye, in this case) cross over to the contralateral (right, in this case) side, whereas fibers originating in the temporal hemiretina (of the right eye, in this case) stay ipsilateral (i.e., on the right side, in this case).
These fibers synapse in the lateral geniculate nucleus and from there onwards a separation between superior and inferior quadrants takes place (temporal and parietal radiation, respectively), until the optic radiation reaches its primary destination in V1.
Fibers carrying visual information of the superior portion of the visual field terminate in the inferior portion of V1 (below the calcarine sulcus), whereas fibers carrying visual information of the inferior portion of the visual field terminate in the superior portion of V1 (above the calcarine sulcus).
Give examples of the right inferior quadrant
- Viewed by left cerebral hemisphere
2. Viewed by superior portions of visual pathway
Describe the visual pathways 3 and 4
Visual Pathways 3 and 4: The Superior and Inferior Quadrants of the Right Visual Hemifield
After the detailed exercise above which we will complete together in class, you should be able to complete the two remaining diagrams on visual pathways on your own, following the same principles.
Whats the importance of the primary visual cortex is localized in brodmann area 17?
The Primary Visual Cortex is localized in Brodmann Area 17
The primary visual cortex (V1) is localized in area 17 of Brodmann’s areas. This area is located in the occipital lobe. It occupies a small portion on the lateral aspect of the brain around the occipital pole.
The major portion of V1 is represented on the medial aspect of the brain.It is located along the two banks (gyri) of the calcarine sulcus, one portion superior (cuneus) and the other portion inferior (lingula) to the calcarine sulcus.
The primary visual cortex receives its major blood supply from calcarine branches originating from the posterior cerebral artery. Only a small portion of the primary visual cortex located around the occipital pole, may also receive blood supply from the middle cerebral artery (especially after an occlusion of the posterior cerebral artery)
Explain the reginopic organization of V1
The topographical organization of the primary visual cortex is called “retinotopic”, which means it refers to the positions of stimuli on the retina, although in most cases we would consider the representation of the visual fields of both eyes in the primary visual cortex.
From a practical perspective, it is much more interesting for a physician to relate visual field defects to certain damaged areas of the primary visual cortex, than to areas of the retina (although they are closely correlated, as shown above). We therefore show the representation of the visual field in the “retinotopic” map of the primary visual cortex in the following slide.
The left half of the visual field is represented only in the primary visual cortex on the right. Inferior portions of the visual field are represented superior to the calcarine sulcus, superior portions of the visual field inferior to the calcarine sulcus. The macular region of vision, which is the center of the visual field is represented close to the occipital pole, the periphery of the visual field closer to the parieto-occipital sulcus.
Please note that the macular region occupies much more space in the primary visual cortex, compared to the proportion of this area relative to the total size of the visual field.
