Lower illustration photographic view

This is a higher magnification of the medial aspect of the brain (shown in Figure 17). The interthalamic adhesion, fibers joining the thalamus of each side across the midline, has been cut (see Figure 6, not labeled). The optic chiasm is seen anteriorly; posteriorly, the tip of the pulv-inar can be seen. The midbrain includes areas where fibers of the visual system synapse.

Fibers emerge from the pulvinar, the visually related association nucleus of the thalamus (see Figure 12 and Figure 63) and travel in the optic radiations to areas 18 and 19, the visual association areas of the cortex (shown in the previous diagram, alongside area 17). Some optic fibers terminate in the superior colliculi (see also Figure 9A and Figure 10), which are involved with coordinating eye movements (discussed with the next illustration). Visual fibers also end in the pretectal "nucleus," an area in front of the superior colliculus, for the pupillary light reflex (reviewed with the next illustration). Some other fibers terminate in the suprachiasmatic nucleus of the hypothalamus (located above the optic chiasm), which is involved in the control of diurnal (day-night) rhythms.

The additional structures labeled in this illustration have been noted previously (see Figure 17 in Section A), except the superior medullary velum, located in the upper part of the roof of the fourth ventricle (see Figure 10); this band of white matter is associated with the superior cer-ebellar peduncles (discussed with the cerebellum, see Figure 57).

Clinical Aspect

It is very important for the learner to know the visual system. The system traverses the whole brain and cranial fossa, from front to back, and testing the complete visual pathway from retina to cortex is an opportunity to sample the intactness of the brain from frontal pole to occipital pole.

Diseases of CNS myelin, such as multiple sclerosis (MS), affect the optic nerve or optic tract, causing visual loss. Sometimes this is the first manifestation of MS.

Visual loss can occur for many reasons, one of which is the loss of blood supply to the cortical areas. The visual cortex is supplied by the posterior cerebral artery (from the vertebro-basilar system, discussed with Figure 61). Part of the occipital pole, with the representation of the macular area of vision, may be supplied by the middle cerebral artery (from the internal carotid system, see Figure 60). In some cases, macular sparing is found after occlusion of the posterior cerebral artery, presumably because the blood supply to this area was coming from the carotid vascular supply.

Additional Detail

The work on visual processing and its development has offered us remarkable insights into the formation of syn-aptic connections in the brain, critical periods in development, and the complex way in which sensory information is "processed" in the cerebral cortex. It is now thought that the primate brain has more than a dozen specialized visual association areas, including face recognition, color, and others. Neuroscience texts should be consulted for further details concerning the processing of visual information.

Primary Visual Cortex

Parieto-occipital fissure

Visual association cortex (areas 18 & 19)

Calcarine fissure

Primary visual cortex (area 17)

Parieto-occipital fissure

Visual association cortex (areas 18 & 19)

Calcarine fissure

Primary visual cortex (area 17)

Cingulate gyrus

Corpus callosum

Lateral ventricle

Septum pellucidum (cut)

Optic chiasm

Mammillary body

Cingulate gyrus

Corpus callosum

Lateral ventricle

Septum pellucidum (cut)

Amygdala Septum Pellucidum

Aqueduct of midbrain

Superior medullary velum

4th ventricle

Optic chiasm

Mammillary body

Roof of 3rd ventricle

Posterior commissure

Splenium of corpus callosum

Pulvinar

Superior and inferior colliculi

Aqueduct of midbrain

Superior medullary velum

4th ventricle

F = Frontal lobe P = Parietal lobe T = Temporal lobe O = Occipital lobe

Th = Thalamus Md = Midbrain Po = Pons

M = Medulla SC = Spinal cord

FIGURE 41B: Visual System 2 — Visual Pathway 2 and Visual Cortex (photograph)

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