Visual field . Methods of examination .Pathologic changes Topic 11
We have two visual field one of the left eye and one of the right but due to binocular vision they appear as one but if you close one eye you can still see but not the same range . During binocular vision the periphery is eradicated to a certain extent
The refractive power of our eye inverts the images but the chiasm and the crossing of nerve fibres ensures that we see a real image.
Due to refraction the nasal fibres obtain information of their respective lateral fields while the lateral fibres obtain information from the middle field
The fibres collected from the temporal side, don’t cross and go to the ipsilateral side of the brain but the nasal fibres do cross, thus the nasal fibres from the right cross over to the left side and eventually enter the left side of the brain and the nasal fibres of the left eye cross and eventually reach the right side of the brain.
Overall the pathways is as follows
Fibres from the right temporal go directly via the optic nerve bypassing the chiasm to the lateral geniculate body of the eye while the right nasal fibres cross over to form the left optic tract with the temporal nerve fibres from the left eye . The nasal fibres from the right eye join the aforementioned right temporal fibres to form the right optic tract which then connect to the lateral geniculate bodies. From there you have multiple radiations reaching the visual cortex located in the occipital lobe.
If you put both information together you can understand disorder of the visual field and also know the location of the lesion.
- 1. If lesions of the optic nerve occurs, before crossing, there will be complete blindness in affected side hence no light reflex be it direct or indirect of the affected eye and no indirect reflex of the normal eye as the lesion prevents conduction. Cause: optic atrophy, indirect neuropathy ´ neuritis
- 2. Proximal part of the left optic nerve is affected (hence include fibres from the right nasal fibres which is responsible for right half of the visual field of the right eye thus the outcome is : blindness of the left eye +contralateral hemianopia (right half vision loss of the right visual field (contralateral) due to right nasal fibres not carrying information)