Unit Ten: The Nervous System: B. Special Senses

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 thehypothalamus, then into the midbrain, superiorcolliculus, 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 adjacentvisual columns and are activated specifically bycolor signals; primary areas for decipheringcolor

• Interaction of Visual Signals from the TwoSeparate Eyes

Organization and Function of the Visual Cortex

• Two Major Pathways for the Analysis of Visual Information

a. Analysis of three dimensional position, grossform, 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 cortexis concerned mainly with contrasts in the visualscene

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 linesand borders but also the direction of orientation ofeach line or border

b. For each orientation of a line specific neuronal cellsare 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. Voluntary fixationb. Involuntary fixationc. Saccadic movement (successive fixation)d. Fixation on moving objects (pursuit movement

Eye Movements and Their Control

Fig. 51.9 Movement of a spot of light on the fovea

• Fixation Movements of the Eye

Eye Movements and Their Control

• Fusion of the Visual Image from the Two Eyes

a. The visual images in the two eyes normallyfuse 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 registerwhich allows stereopsis (mechanism for judgingdistances 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- parasympatheticnerves decrease the pupillary aperture and sympathetic nerves increase the aperture