Electrical potential is generated when light reaches the photoreceptors on the retina. This potential is then transmitted to the ganglion cells either via the bipolar and/or the horizontal and amacrine cells. Axons from the ganglion cells converge at the blind spot of the optic disc to form the optic nerve. The axons coming from the nasal half of the retina decussate at the optic chiasm while those situated on the temporal half remain on the ipsilateral side (Figure NE.10). These then synapse in the lateral geniculate nuclei. From here, synaptic connections are made via the optic radiation to the primary visual cortex giving rise to a topographical projection of the visual field around the calcarine fissure. Some fibres of the optic tracts relay to the superior colliculi, which are involved in the control of eye movements or posture. Lesions in the visual pathway will give visual field defects according to their position. Thus as shown in Figure NE.10, the following lesions will give rise to their corresponding defects:
• Lesion 2—bi-temporal hemianopia
• Lesion 3—right homonymous hemianopia
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