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 New System- the optic tracts synapse in the thalamus and then pass to the visual cortex Old System- the optic tracts synapse in the hypothalamus, then into the midbrain, superior colliculus, and then the thalamus 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 Relays visual information from the optic tract to the visual cortex 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 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 Areas if maximum excitation occur along the sharp borders of the visual pattern The visual signal in the primary visual cortex is concerned mainly with contrasts in the visual scene 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 Visual cortex not only detects the existence of lines and borders but also the direction of orientation of each line or border For each orientation of a line specific neuronal cells are stimulated 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 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 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 The visual images in the two eyes normally fuse with each other on “corresponding points” of the two retinas If not precisely fused, they are not in register 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