Session 3 - Visual Pathways Flashcards
What detects light?
Light is detected by the retina within the eye and impulses are passed via the optic nerves, then optic tracts and finally the optic radiations before reaching the primary visual cortex in the occipital lobe. The fibres divide and cross over in a complex way.
What are visual fields?
The areas of vision that we can see with each eye are known as the visual fields. Each visual field is divided into a temporal field and a nasal field. The temporal field is the lateral half of the visual field, and nasal field is the medial half. It is vital to understand that light entering from the lateral half of our visual field is received by the medial (nasal) part of the retina, and vice versa. So, the nasal retina provides our temporal visual field, and the temporal retina provides our nasal visual field.
What ad where is the optic chiasm?
Visual information from the retinas first travels along the right and left optic nerves to the optic chiasm. This structure is located immediately anterior to the midbrain and superior to the pituitary gland. At the optic chiasm, visual information from the temporal visual fields (nasal retinas) from each eye cross over, such that information about the temporal vision from the left eye now travels through the right side of the brain, and vice versa.
Where does VI travel after the OC?
After the optic chiasm, the visual information travels along the optic tracts. When the tracts reach the thalamus, the majority of fibres synapse in the lateral geniculate nucleus. After they synapse, the fibres carrying the visual information towards the primary visual cortex divide into a superior and inferior pathway on each side, known as optic radiations. The superior optic radiation travels in the parietal lobe, and is therefore known as the parietal radiation, and the inferior optic radiation travels in the temporal lobe, and is known as the temporal radiation, also known as ‘Meyer’s loop’.
What are the inferior fields of vision?
The visual information within the parietal radiations is that which is received in the superior aspects of the retinas, and therefore constitutes the inferior fields of vision. Conversely, the information in the temporal radiation is from the inferior retinas, which is therefore from the superior parts of our field of vision.
Where do the E-W nucleus and superior folliculi receive input from?
In earlier parts of the handbook, the Edinger-Westphal nucleus and superior colliculi were discussed and are both located in the midbrain. They both also receive input from the optic tracts at the point close to where the fibres synapse in the lateral geniculate nucleus of the thalamus.
What are visual field defects?
Because of how visual information is passed from the retinas to the primary visual cortex, damage to different parts of the pathway will cause a different area of the visual field to be lost. The diagram on the previous page will help to understand the terms described below.
• Monocular vision loss – this defect is caused by damage to an optic nerve, meaning all vision is lost from the ipsilateral eye.
What is bitermporal hemianopia?
Bitemporal hemianopia – this term means ‘both – temporal – half – vison loss’ and is caused by damage to the optic chiasm. The fibres that cross at the optic chiasm are carrying visual information from the nasal retinas, and therefore information about the temporal visual fields.
What si HH?
Homonymous hemianopia – this term means ‘same – half – vision loss’ and is caused by damage to an optic tract, between the optic chiasm and the lateral geniculate nucleus. If the left optic tract is damaged, information from the left temporal retina and right nasal retina is lost, meaning the left nasal visual field and the right temporal visual field are lost. This would mean the patient has lost the right side of their vision in both eyes, so would be termed a right homonymous hemianopia
What is HQ?
• Homonymous Quadrantanopia – this term means ‘same – quarter – vision loss’ and is caused by damage to either the parietal or temporal optic radiation between the lateral geniculate nucleus and the primary visual cortex. This is a more complex field loss than the homonymous hemianopia but follows similar principles. If the left parietal optic radiation is damaged, information from the left superior temporal retina and right superior nasal retina is lost, meaning the left inferior nasal visual field and the right inferior temporal visual field are lost. This would mean the patient has lost the bottom-right corner of their vision in both eyes, so would be termed a right inferior homonymous hemianopia
What is a pituitary tumour?
A tumour of the pituitary gland may cause it to enlarge. As it sits immediately beneath the optic chiasm, it can enlarge enough to compress the optic chiasm. This will cause the characteristic bitemporal hemianopia.
