The inferior optic radiations, which receive information from the inferior retina (superior visual field), form the loop of Meyer in the temporal lobe before travelling posteriorly to the visual cortex. This has clinical relevance as temporal lobe lesions eg tumours, can produce a homonymous superior quadrantinopia visual field defect. The superior optic radiations (inferior visual field) pass backwards through the parietal lobe.
Nerve fibres from corresponding areas on the retina of each eye become increasingly aligned and more organised as they travel further back in the visual pathway. Consequently disease processes affecting the posterior visual pathway (chiefly optic radiations or visual cortex) result in scotomas that are extremely congruous ie same shaped visual field defects in each eye.
The primary visual cortex (V1), also known as Brodmann’s area 17, occupies the walls of the deep calcarine sulcus in the occipital lobe. The cortex receives, via the optic radiations, fibres from the temporal half of the ipsilateral retina and the nasal half of the contralateral retina. The right half of the field of vision in either eye is therefore represented in the visual cortex situated in the left cerebral hemisphere and vice versa. Clinically a haemorrhage or infarct affecting the left occipital lobe, for example, is likely to result in a right homonymous hemianopia.
A significant proportion of the primary visual cortex, situated posteriorly at the occipital tip, is concerned with detailed macular vision. The peripheral visual field is served by the visual cortex that lies more anteriorly. The blood supply to the visual cortex is via the posterior cerebral artery although the middle cerebral artery may often contribute at the anterior aspect of the calcarine sulcus. Clinically anastomosis between these two vessels can result in a situation whereby the patient suffers from a homonymous hemianopia visual defect but with preservation of macular function (“macular sparing”).
Adjacent to the primary visual cortex are secondary visual areas 18 (V2) and 19 (V3). These areas are thought to receive fibres from area 17 and other cortical areas including the thalamus. These secondary areas are likely to be involved in recognition and appreciation of what is being seen, including relating current images to past visual experiences.
Visual cortex: The primary visual cortex (V1), shaded yellow, is situated along the superior and inferior borders of the calcarine fissure. A significant portion of the posterior occipital tip is concerned with detailed macular vision. The secondary visual areas (V2 and V3), shaded pink and blue respectively, are likely to be involved in the recognition and appreciation of visual images.