Transcript Visual Pathways

Chapter 51: The Eye: III. Central
Neurophysiology of Vision
Guyton and Hall, Textbook of Medical Physiology, 12th edition
Visual Pathways
• Visual Pathways
a. New System- the optic tracts synapse in the
thalamus and then pass to the visual cortex
b. Old System- the optic tracts synapse in the
hypothalamus, then into the midbrain, superior
colliculus, and then the thalamus
c. In humans the new system is responsible for
perception of virtually all aspects of visual form,
colors, and other conscious vision
Visual Pathways (cont.)
•
Function of the Dorsal Lateral Geniculate
Nucleus of the Thalamus
a. Relays visual information from the optic tract
to the visual cortex
b. Controls how much of the signal is allowed to
pass to the cortex
Organization and Function of the Visual Cortex
Fig. 51.2 Visual cortex in the calcarine fissure
of the occipital cortex
Fig. 51.3 Transmission of visual signals from the
primary visual areas into the secondary
visual areas
Organization and Function of the Visual Cortex
• Primary Visual Cortex- 6 major layers
• Secondary Visual Cortex
Fig. 51.4
Organization and Function of the Visual Cortex
• Color “Blobs”- receive lateral signals from adjacent
visual columns and are activated specifically by
color signals; primary areas for deciphering
color
• Interaction of Visual Signals from the Two
Separate Eyes
Organization and Function of the Visual Cortex
• Two Major Pathways for the Analysis of Visual
Information
a. Analysis of three dimensional position, gross
form, and motion of objects
b. Analysis of visual detail and color
Neuronal Patterns of Stimulation During
Analysis of the Visual Image
• Analysis of Contrasts
a. Areas if maximum excitation occur along the
sharp borders of the visual pattern
b. The visual signal in the primary visual cortex
is concerned mainly with contrasts in the visual
scene
c. Intensity of stimulation is proportional to the
gradient of contrast
Neuronal Patterns of Stimulation During
Analysis of the Visual Image
• “Simple Cells”-Orientation of Lines and Borders
a. Visual cortex not only detects the existence of lines
and borders but also the direction of orientation of
each line or border
b. For each orientation of a line specific neuronal cells
are stimulated
c. Cells are referred to as Simple Cells and are in
layer IV of the primary visual cortex
Neuronal Patterns of Stimulation During
Analysis of the Visual Image
• “Complex Cells”-Orientation When Displaced
Laterally or Vertically
a. As the visual signal progresses away from level IV,
complex cell neurons respond to orientation in
the same direction but are not position specific
Neuronal Patterns of Stimulation During
Analysis of the Visual Image
• Detection of Lines of Specific Lengths, Angles,
and Other Shapes
• Detection of Color- detected by color contrast;
contrasting colors (opponent colors) excite
specific neuronal cells
Eye Movements and Their Control
• Muscular Control of Eye Movements
Fig. 51.7 Extraocular muscles of the eye and their innervation
Eye Movements and Their Control
• Neural Pathways for Control of Eye Movements
Fig. 51.8 Extraocular muscles of the eye and their innervation
Eye Movements and Their Control
• Fixation Movements of the Eye
a.
b.
c.
d.
Voluntary fixation
Involuntary fixation
Saccadic movement (successive fixation)
Fixation on moving objects (pursuit movement
Eye Movements and Their Control
• Fixation Movements of the Eye
Fig. 51.9 Movement of a spot of light on the fovea
Eye Movements and Their Control
• Fusion of the Visual Image from the Two Eyes
a. The visual images in the two eyes normally
fuse with each other on “corresponding points”
of the two retinas
b. If not precisely fused, they are not in register
c. The nearer the object, the less the degree of register
which allows stereopsis (mechanism for judging
distances of visual objects 200 feet away
d. Is the phenomenon of depth perception
Autonomic Control of Accommodation
• Autonomic Nerves to the Eyes
Fig. 51.11 Autonomic innervation of the eye
Autonomic Control of Accommodation
• Control of Accommodation (Focusing the Eyes)ability to focus on near and far objects
• Control of Pupillary Diameter- parasympathetic
nerves decrease the pupillary aperture and
sympathetic nerves increase the aperture