Appendix 2 Overview
- Introduction
- Pathways mediating visual sensation
- Visual field defects
- Neural control of pupil size
Introduction
The primary purpose of this appendix is to provide you an overview of how the visual field is projected
onto the retina, how it is represented in central visual structures, and how damage to these central
structures can give rise to specific visual field deficits. You will also review the pathways responsible for
the pupillary light reflex, since they are of significant diagnostic value to clinicians. Because you have
already studied the orbit and eye movements in Normal Body, this topic will not be covered in this
appendix. However, understanding the neural control of eye movements is also of importance for
clinical diagnosis.
Pathways mediating visual sensation
The visual pathways from the retina to the cortex are shown in a unique dissection of the ventral
forebrain in Figure A2-1; Figure A2-2 shows an illustration from your textbook (Neuroscience, 5th Ed.)
that should help you interpret that anatomical structures that are exposed in the first figure. Figures A2-3 and A2-4 illustrate the ways in which the visual fields are projected onto the retina and the other
components of the visual pathways. There are several key points to be taken from these figures.
- Because light rays entering the eye are bent by the cornea and lens, the visual field is
represented on the retina upside down and reversed from left to right. It is important to keep
the relation between the visual field and the retinal representation straight: objects in the
superior part of the visual field are represented on the inferior retina, and objects in the right
half of the visual field are represented on the left half of the retina. (Diagrams of visual field
deficits—examples of which are illustrated later in this appendix—are always of the visual field,
not of the retina.)
- Both eyes see most, but not all, of the visual field. There are small “monocular crescents” in the
far periphery of the visual fields seen by only one or the other eye. (You can demonstrate the
extent of this crescent to yourself by observing what happens to your visual field when you close
one eye.)
- Information carried out of the retina by the optic nerve fibers is split. This is to accommodate
the frontal position of our eyes (point no. 2 above) and the principal of contralateral
representation in the forebrain—in this case, contralateral representation of half of the visual
scene. The fibers originating from the nasal part of each retina (the part next to the nose) cross
in the optic chiasm and enter the lateral geniculate nucleus (a thalamic nucleus) on the
opposite side. The fibers originating from the temporal part of the retina do not cross; they
enter the lateral geniculate nucleus on the same side. The result of this partial crossing is that
the left visual field is represented only on the right side of the brain and the right visual field is
represented only on the left side of the brain. Thus, unlike each eye which sees both sides of the
visual field, each half of the brain sees only the contralateral half of the visual field.
Visual field defecits
A deficit in the visual fields (a region in the visual field of one or both eyes in which there is a loss of
sight) is referred to as a scotoma. You might think that a scotoma would be very obvious to an individual
who has one, and many times they are. But sometimes, especially when it occurs in the periphery, a
scotoma may go unrecognized until the individual has an accident that all too vividly reveals her/his
sensory loss. (Traffic accidents are a common way to uncover such visual field deficits.)
On the right side of Figure A2-5, you will see examples of different types of visual field deficits. In each
case, the regions of the visual field of each eye that are affected are shown in black. That is, the part of
the visual field that is not visible in that eye is blacked out. Visual field diagrams are always done for
each eye individually. Some deficits would not be easy to demonstrate if both eyes were open during
the testing. On the left of the figure, lines are drawn through parts of the visual pathways to indicate
locations that could be damaged to give each of the illustrated patterns of deficits.
Some rather cumbersome names are used to refer to particular patterns of visual field deficits. Anopsia (also spelled anopia) simply means loss of sight in one or both eyes. Hemianopsia indicates loss of sight
in one half of the visual field. Quadrantanopsia is a loss of sight in one quadrant of the visual field.
Bitemporal hemianopsia is a loss of sight in the right visual field of the right eye and the left visual field
of the left eye. It is also called heteronymous hemianopsia because the affected regions of the visual
fields in the two eyes are not congruent. When the affected regions of the visual fields of both eyes
overlap (i.e., loss of vision in the left or right visual field of both eyes), the deficit is called homonymous.
A patient could be described as having a homonymous hemianopsia or a homonymous
quadrantanopsia, etc.
Widespread loss of vision without damage to the most central part of the visual field representation is
called macular sparing. Macular sparing is a phenomenon often associated with lesions in the visual
cortex but it can be found with lesions along the length of the visual pathways.
Challenge A2.1—name that deficit
Consider the visual field deficits shown in Figure A2-5 and identify each by the proper clinical term (or combination of terms) highlighted on this page in bold font. You should be able to relate this
figure back to the previous four figures and explain why each visual field deficit is associated with
damage to the particular structure along the visual pathway from retina to visual cortex. |
Neural control of pupil size
Consider the following three cases in Challenge A2-2. Refer to Figures A2-6 and A2-7 on the next two
pages to answer the questions associated with each case.
Challenge A2.2—be a pupil of the pupil
Patient 1. During a physical exam, you shine a light into a patient’s left eye and you note that the
pupils react only sluggishly to light. You stimulate the right eye and you find a brisk constriction of
both pupils. From this information alone, what sort of neurological problem might you suspect? How
would you test this idea?
Patient 2. A patient has come to you complaining of double vision. His left eye fails to adduct when
he makes eye movements to the right. His left eyelid droops (ptosis) and the pupil in his left eye is
larger than the pupil in the right. The pupil in the left eye does not react to light nor does it respond
when light is shown in the right eye. How would you explain this deficit in the pupillary light reflex?
Patient 3. A third patient comes to see you with ptosis of his left eye. You examine his eye
movements and these seem normal. Then you notice that the pupil in his left eye is measurably smaller than that of his right eye. How do you account for the symptoms in this case? |
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