Question

One of the objects in your visual field is a ball that is round, red, and moving. Describe the steps involved in being able to see the ball and identify it as a ball that is round, red, and moving. In other words, describe the pathway through the visual system (the eye, the optic nerve, and the parts of the brain) that the information travels before you can identify the object as a ball that is round, red, and moving.

Answer 1

Figure 15.1 The visual pathway with the course of information flow from the right (green) and left (blue) hemifields of the two eye's visual fields.

The axons of the 3° visual afferents (the retinal ganglion cells) form the optic nerve fiber layer of the retina on their course to the optic disc. At the optic disc, the 3° visual afferents exit the eye and form the optic nerve. The fibers of the optic nerve that originate from ganglion cells in the nasal half of the retina (i.e., the nasal hemiretina) decussate in the optic chiasm to the opposite optic tract (Figure 15.1). Consequently, each optic tract contains retinal ganglion cell axons that originate in the nasal half of the contralateral retina and the temporal half of the ipsilateral retina. Recall that the ipsilateral temporal hemiretina and the contralateral nasal hemiretina have projected on them the images of corresponding halves of their visual fields. For example, the temporal (left) hemiretina of left eye and the nasal (left) hemiretina of right eye both have projected on them the right halves of their respective visual fields. Consequently, each optic tract has within it axons representing the contralateral half of the visual field.

The axons in the optic tract terminate in four nuclei within the brain :

• the lateral geniculate nucleus of the thalamus - for visual perception;
• the superior colliculus of the midbrain - for control of eye movements;
• the pretectum of the midbrain - for control of the pupillary light reflex; and
• the suprachiasmatic nucleus of the hypothalamus - for control of diurnal rhythms and hormonal changes.