Guyton and Hall Physiology Chapter 52 - The Eye III. (Central Neurophysiology of Vision)

The Visual Pathways

• Visual nerve signals travel from the retinas to the visual cortex.
• The optic nerves carry these signals.
• At the optic chiasm, fibers from the nasal halves of the retinas cross to the opposite side, joining fibers from the temporal retinas to form the optic tracts.
• The optic tracts then synapse in the dorsal lateral geniculate nucleus of the thalamus.
• From the thalamus, geniculocalcarine fibers pass to the primary visual cortex in the calcarine fissure of the medial occipital lobe through the optic radiation.
• Aside from the pathway to the visual cortex, visual fibers pass to several older brain regions:
  • Suprachiasmatic nucleus: controls circadian rhythms
  • Superior colliculus: controls rapid eye movements
  • Ventral lateral geniculate nucleus: controls behavioral functions

Two Visual Systems

• Old System: Projects to the midbrain and base of the forebrain.
• New System: Directly transmits visual signals to the visual cortex.
• In humans, the new system is responsible for conscious visual perception (form, color, etc.).
• In primitive animals, the older system is more dominant, relying on the superior colliculus for visual form detection.

Visual Cortex Organization

• The visual cortex is organized into six layers:
  • Layers I-III receive input from layer IV.
  • Layer IV is where most input from the thalamus terminates.
    • Layer IVcα: receives signals from M (magnocellular) retinal ganglion cells responsible for motion and position.
    • Layer IVcβ: receives signals from P (parvocellular) ganglion cells responsible for color and fine detail.
    • Layer IVa: receives signals from P ganglion cells.
• Each layer contains a specialized population of neurons, contributing to the analysis of visual information.

Vertical Neuronal Columns

• The visual cortex is further organized structurally by vertical neuronal columns, each about 30-50 micrometers in diameter.
• Each column functions as a unit for processing visual information.
• This columnar organization is seen in other brain areas responsible for different senses, motor control, and analytical functions.

Major Visual Pathways

• Path 1: Position, Form, and Motion (Fast, Coarse Detection):

  • This pathway analyzes the position of objects, their gross form, and movement directly in space around the body.
  • It is responsible for understanding "where" things are and if they are moving.
  • Signals travel from the primary visual cortex into the posterior midtemporal area, and then upward into the occipitoparietal cortex.
  • This pathway is linked to the superior colliculi and older visual system.

• Path 2: Color, Detail (Accurate, Point-to-Point):

  • Analyzes accurate details and color information.
  • This pathway is primarily responsible for detailed form perception and color.
  • Signal flow is thought to be more complex and less well-defined than the "position" pathway.

Lateral Geniculate Nucleus (LGN)

• The LGN acts as a "gate" for visual signal transmission to the visual cortex.
• It controls the amount of signal that reaches the cortex.
• Control signals originate from two sources:
  • Corticofugal fibers (from the visual cortex to the LGN)
  • Reticular areas of the mesencephalon (midbrain)

Organization & Processing in the LGN

• Magnocellular Layers (I & II): Contain larger neurons and receive input from M ganglion cells.
• Parvocellular Layers (III-VI): Contain smaller neurons and receive input from P ganglion cells.
• These layers are separated by koniocellular layers, which receive signals from the retina as well.

The Importance of Eye Movements

• The visual field is the area that the eyes are currently perceiving.
• The nasal field is the area seen towards the inside of the nose.
• The temporal field is the area seen towards the side of the head.
• Perimetry maps the visual field to diagnose retinal problems.
• The blind spot is a region in the field where no signal is perceived due to the absence of photoreceptor cells.

Muscles of Eye Movement

• Three pairs of muscles control eye movements:
  • Medial and lateral rectus: move eyes from side to side
  • Superior and inferior rectus: move eyes up and down.
  • Superior and inferior obliques: rotate the eyeballs to maintain an upright field of vision.

Neural Control of Eye Movements

• These muscles are controlled by cranial nerves III, IV, and VI.
• Connections between the cranial nerve nuclei in the brain stem and the ocular muscles.
• The lateral portion of the ipsilateral retina (same side as the eye) connects to the ventral layers of the LGN.
• The medial portion of the contralateral retina (opposite side of the eye) connects with the dorsal layers of the LGN.

The Visual Cortex

• The visual cortex is located in the calcarine fissure of the medial occipital lobe.
• It receives visual information from the LGN via the optic radiation.
• The visual cortex is organized by a map of visual space with a foveal representation.
• The visual cortex is sensitive to different aspects of vision, such as orientation, movement, and color.
• It also plays a vital role in integrating sensory information from different sources.

Central Neurophysiology of Vision

• The visual cortex is organized into vertical columns of neurons, each column representing a functional unit
• The visual cortex is mainly concerned with analyzing contrasts in visual scenes, rather than non-contrasting areas
• Neurons in layer IV of the visual cortex receive information from the lateral geniculate nucleus (LGN)
• Color is detected in much the same way as lines—by means of color contrast
• The superior colliculi play a global role in orienting the eyes, head and body with respect to external disturbances
• The visual images in the two eyes normally fuse with each other on "corresponding points" of the two retinas
• Strabismus, also called squint or cross-eye, is a condition in which the eyes do not align properly
• Strabsismus is caused by an abnormal "set" in the neuronal control pathways of the visual system
• Suppression of the visual image from one eye can occur in patients with strabismus, leading to reduced visual acuity
• Depth perception, or stereopsis, is determined by which sets of pathways in the visual system are excited

Voluntary and Involuntary Fixation

• Destruction of the fixation area in an animal results in difficulty with or complete inability to direct the eyes towards a fixation point.
• Voluntary fixation is controlled by the frontal cortex and pretectal nuclei.
• Involuntary fixation is controlled by the superior colliculi.

Saccadic Movements

• Eyes fixate on a visual target by jumping from one highlight to another, resulting in saccades.
• Opticokinetic movements describe saccades during continuous visual stimulation.
• During saccades, the eyes spend 90% of the time fixated and 10% of the time moving.
• Superior colliculi receive topographic projections of the retina and map the direction of light flashes, triggering eye movements.
• Superior colliculi are involved in head and body movements, including turning towards disturbances.

Depth Perception

• Depth perception, also known as stereopsis, is achieved based on the relative convergence of optic nerve fibers from both eyes.
• The brain registers different pathways for objects at different distances.

Strabismus

• Strabismus (squint, cross-eye) is a lack of eye fusion in one or more visual coordinates: horizontal, vertical, or rotational.
• Accommodation of the lens to new focal points relies on mechanisms like chromatic aberration, convergence, and foveal depth perception.
• Chromatic aberration utilizes the lens's different refractive indices for red and blue light to adjust focus.
• Convergence of the eyes triggers simultaneous lens strengthening.
• Foveal depth perception relies on the differing focus between the fovea and the edges of the retina.

Pupillary Light Reflex

• Pupillary constriction in response to light is called the pupillary light reflex.
• The light reflex pathway starts in the retina and involves the pretectal nuclei, Edinger-Westphal nuclei, and parasympathetic nerves.
• The pupillary light reflex adapts the eye to changing light conditions.

Horner Syndrome

• Horner syndrome is a clinical condition resulting from interruption of sympathetic nerve fibers.
• Signs of Horner syndrome include constricted pupil, drooping eyelid, dilated blood vessels on the face and head.