Level 3 Unit 2 Part 15: Visual perception

The area that a person sees is called the visual field. The visual field can be divided several ways, such as into central versus peripheral vision, or into halves (called hemifields) or quarters (called quadrants) based on superior, inferior, medial, and lateral directions. For each eye, the medial hemifield is also called nasal, because it is on the side of the nose, and the lateral hemifield is also called temporal, because it is on the side of the temple (the part of the head lateral to the eyes). These terms are also used for the same parts of the retina.

Light passes through several parts of the globe (eyeball) to reach a layer of neural tissue called the retina, which contains photoreceptors that can detect light. The cornea (clear tissue anterior to the pupil) and the lens (clear tissue posterior to the pupil) bend light in such a way that they invert (flip) the visual image that hits the retina both horizontally and vertically, so that light from the superior visual field strikes the inferior retina, and vice versa, and light from the temporal visual field strikes the nasal retina, and vice versa. It can be helpful to think of this retinal image orientation as the way this information will travel from the retina through the rest of the visual pathways. The photoreceptors are mostly a type called rods in the periphery of the retina, which are sensitive to low intensity light, but not color, and mostly a type called cones in the center of the retina, which are sensitive to color, but only detect higher intensity light. The central retina, which receives light from the center of the visual field, is called the macula. Central, or macular, vision is tested with an acuity chart. Peripheral vision is tested by having the patient count fingers or indicate the location of moving fingers in the four quadrants of the visual field of each eye.

In the retina, photoreceptors synapse on neurons called retinal bipolar cells, which then synapse on neurons called retinal ganglion cells. These neurons, and several other types in the retina, perform some initial processing of visual information. Retinal ganglion cell axons travel from the retina to the thalamus. First, these axons travel through the retina to a spot called the optic disc, where the axons exit the posterior globe to form the optic nerve, which then passes posteriorly through the orbit (eye socket) and through the skull to enter the intracranial space. The optic nerves then fuse into a midline structure called the optic chiasm, from which bilateral structures called optic tracts pass posteriorly to the thalami. The optic nerves are called cranial nerves because they pass through the skull, but they are not true nerves because all of these visual neural structures outside the brain (retina, optic nerves, optic chiasm, and optic tracts) are central nervous system tissue, with oligodendrocytes instead of Schwann cells.

At the optic chiasm, axons from the nasal retina, which carry information from the temporal visual hemifield, decussate to enter the contralateral optic tract, while axons from the temporal retina, which carry information from the nasal visual hemifield, enter the ipsilateral optic tract. Therefore, the optic nerves have information from the entire visual field of the ipsilateral eye, while the optic tracts and the rest of the visual pathway carry information from the contralateral visual hemifield of both eyes. Most of the retinal ganglion cell axons in the optic tract synapse in the thalamus, but small number go instead to several other areas of the cerebrum and brainstem for unconscious functions that use visual information.

Thalamic neuron axons project posteriorly, in a tract called the optic radiation, to the primary visual cortex on the occipital lobe. These axons spread out superiorly and inferiorly in the white matter of the parietal and temporal lobes. Information from the contralateral superior visual quadrant of both eyes travels through the inferior optic radiation in the temporal lobe. Information from the contralateral inferior visual quadrant of both eyes travels through the superior optic radiation in the parietal lobe. The primary visual cortex is mostly on the medial cortex of the occipital lobe, and a small amount extends onto the occipital pole, which is the most posterior part of the cerebrum. The superior half of the primary visual cortex receives axons from the superior optic radiation, and the inferior half receives axons from the inferior optic radiation. Peripheral visual field information projects to the anterior part of the primary visual cortex, located deep in the fissure separating the cerebral hemispheres, and macular visual information projects to the posterior part of the primary visual cortex, located on or near the occipital pole. The primary visual cortex projects to association visual cortices, which make up the rest of the occipital cortex, as well as posterior parts of parietal and temporal cortex.

Visual loss can be monocular (of one eye) or binocular (of both eyes). Hemianopsia refers to loss of vision in a hemifield, and quadrantanopsia refers to loss of vision in a quadrant. The term homonymous means that there is loss of vision in the same part of the visual field in both eyes; for example, a left homonymous hemianopsia is loss of vision in the left hemifield of both eyes.


Level 3 Unit 2 Part 16: Auditory perception

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