Our visual system detects particles of light called photons. These photons may be directly emited from an object (such as a light bulb) or reflected from an object. Photons pass through the lens where they may be subject to refraction before passing through the pupil and eventually reaching the retina.
Special photosensitive neurons in the retina detect specific patterns of light in specific locations of the visual field. When the correct pattern is detected by a retinal neuron and action potential is fired. Action potentials travel down the optic nerve and tract to synapse on cells of the lateral geniculate nucleus (LGN) of the thalamus. The layout of cells in the retina is not random - cells that are next to each other report on adjacent areas of the visual field. This relationship between the position of retinal neurons and the corresponding visual field is called the retinotropic map. The retina and visual field can each be divided into four quadrants with superior and inferior defining the vertical axis and nasal and temporal defining the horizontal axis. - The nasal portion of the retina recieves information from the temporal portion of the visual field. - The temporal portion of the retina recieves information from the nasal portion of the visual field. - The superior portion of the retina recieves information from the inferior portion of the visual field. - The inferior portion of the retina recieves information from the superior portion of the visual field.
Axons from the neurons of the retina run in the optic nerve until the optic chiasm, at which point there is a rearrangement of axons. Axons from cells within the nasal portion of the retina decussate at the optic chiasm. This means that after the optic chiasm information from the right side of the visual field is sent to the left side of the brain and vice versa. Post-chiasm, this bundle of axons is referred to as the optic tract and proceeds towards the LGN of the thalamus.
Neurons in the LGN then pass information on to the primary visual cortex - known as V1 through a series of white mater pathways called the optic radiations. Information from the upper parts of the visual field runs throught the parietal lobe whereas information in the lower parts of the visual field runs through the temporal lobe until axons synapse upon targets within V1. From the primary visual cortex, information is passed to additional visual processing areas called V2 and V3. These areas then link to other areas of the brain that allow us to make sense of what we are seeing and where we are seeing it. This process is obviously exceptionally complex but it can be clinically useful when dealing with patients with higher cortical visual disorders to consider two streams of information: 1) A dorsal stream of information which is primarily concerned with where with connections to the parietal lobe. 2) A ventral stream of information which is primarily concerned with what with connections to the temporal lobe.
We will now look at wach of these steps in more detail but even the information above can provide great value in the localisation of neurological deficits.
Light emited or reflected for an object in enters the eye via the pupil and strikes the retina where it is detected by specialist neurons. The light-sensitive neurons of the retina have receptive fields which consist of a specific location in the visual field and a specific visual feature that results in a response. This encoding of a 'what' and a 'where' is common to the visual pathways from the retina to around V3.