Vision

THE VISUAL SYSTEM

Overview

• Visual pathways

• Visual reflexes

• Visual processing

VISUAL PATHWAY

Quadrants of the Visual Field

• Each eye's visual field is segmented into four quadrants: upper, lower, right, and left (nasal and temporal).

Retina Functionality

• The retina is responsible for receiving and projecting the visual field.

• Images are inverted and crossed in the retina.

RETINA PROJECTION

• Visual Field Projections:

  • Temporal half of the visual field projects to the nasal half of the retina.

  • Nasal half of the visual field projects to the temporal half of the retina.

  • Upper half of the visual field projects to the lower half of the retina.

  • Lower half of the visual field projects to the upper half of the retina.

OPTIC CHIASMA AND TRACT

Fiber Crossing

• At the optic chiasma:

  • Fibers partially cross.

  • Left half of the visual fields enters the right optic tract.

  • Right half of visual fields enters the left optic tract.

Synapse and Radiation

• Optic tracts synapse in the lateral geniculate nucleus.

• Lateral geniculate nuclei fibers form optic radiation:

  • Upper half of visual field → Loop of Meyer (temporal lobe optic radiation).

  • Lower half of visual field → Parietal lobe optic radiation.

TERMINATION OF OPTIC RADIATION

Locations

• Lingual Gyrus:

  • Termination of optic radiation fibers from the Loop of Meyer (upper visual field).

• Cuneus:

  • Termination of optic radiation fibers from the parietal lobe (lower visual field).

VISUAL REFLEXES

Light Reflex

• Involves:

  • Retina

  • Optic nerve and tract

  • Superior brachium

  • Pretectal nucleus (bilaterally)

  • Edinger-Westphal nuclei

  • Oculomotor nerve

  • Ciliary ganglion

  • Short ciliary nerve

  • Sphincter pupillae muscles (pupil constriction)

Accommodation Reflex

• Function: convergence, thickening of lens, and pupillary constriction.

• Involves:

  • Retina

  • Optic nerve and tract

  • Lateral geniculate nucleus

  • Optic radiation

  • Visual cortex

  • Frontal eye field (Area 8)

  • Corticobulbar tract

  • Pretectal nucleus bilaterally

  • Edinger-Westphal nuclei

  • Oculomotor nerve

  • Ciliary ganglion

  • Short ciliary nerves

  • Ciliaris (lens) and sphincter pupillae (constriction) muscles

SACCADIC MOVEMENTS

• Description: High-velocity eye movements that orient the eyes towards stimuli.

• Coordination: Superior colliculus and frontal eye field adjust head movements to stimuli.

• Superior Colliculus:

  • Receives motion information from the visual field.

  • Responsible for visual attentiveness and object outline identification.

• Frontal Eye Field:

  • Receives input from primary visual cortex.

  • Responsible for fine visual discrimination and saccadic movements to complex stimuli.

ANATOMY OF THE EYE

Key Components

• Sclera: Tough outer wall of the eye.

• Conjunctiva: Thin lining over sclera and inside eyelids.

• Cornea: Clear structure continued from sclera over the iris and pupil.

• Iris: Contains muscles that control pupil constriction/dilation.

Detailed Eye Anatomy

• Macula: Sensitive area of the retina for central vision.

• Fovea: 2mm pit in macula for minimal distortion of images received by photoreceptors.

• Pupil: Hole in iris controlled by iris muscles.

• Optic Disc: Blind spot with no photoreceptors; passageway for optic nerve and vessels.

MICROSCOPIC ANATOMY OF THE RETINA

Neuronal Structure

• Retina consists of three neuron sets and two interneuron sets:

  • Photoreceptors: Deepest layer.

  • Bipolar Cells: Intermediate layer.

  • Ganglion Cells: Superficial layer, forming the optic nerve.

• Horizontal and Amacrine Cells: Facilitate communication between photoreceptors, bipolar cells, and ganglion cells for convergence and lateral inhibition.

PHOTORECEPTORS

Types of Photoreceptors

• Rods:

  • High sensitivity to light (night vision), saturate in daylight, low visual acuity, no presence in fovea, single type of photopigment.

• Cones:

  • Lower sensitivity to light, responsible for color vision, high visual acuity, concentrated in fovea, three types of photopigments (blue, red, green).

PHOTOTRANSDUCTION

Process Overview

• Process where light waves convert to photoreceptor potential.

• Membrane potential in the dark is -40mV.

• Cyclic GMP keeps sodium channels open.

• Light absorbed by pigment epithelium triggers phototransduction events leading to hyperpolarization of photoreceptor membranes.

Phototransduction Mechanism

• Rhodopsin: Photopigment in rods activated by light; leads to enzymatic breakdown of cGMP, can hyperpolarize membrane to -70mV, changing neurotransmitter release (glutamate).

DARK ADAPTATION

• Transition from light to darkness significantly increases photoreceptor light sensitivity (1,000,000x increase).

• Pupils dilate; unbleached rhodopsin regenerates in rods, requiring calcium.

BIPOLAR CELLS

Functionality

• Receptive field defined by light stimulation area affecting membrane potential:

  • Center: Direct photoreceptor synapses.

  • Surround: Indirect photoreceptor synapses through horizontal cells.

• Antagonistic receptive fields: ON-center/OFF-surround or OFF-center/ON-surround configurations.

GANGLION CELLS

Characteristics

• Antagonistic receptive fields; fight for differences in illumination.

• High action potential response rates to stimulation in the center of receptive fields.

Types of Ganglion Cells

• M (Magnocellular):

  • Large receptive fields, responsive to movement and low contrast, enhancing low-resolution vision.

• P (Parvocellular):

  • Small receptive fields, color-opponent cells sensitive to wavelength, underpinning color vision and fine detail discrimination.

LATERAL GENICULATE NUCLEUS (LGN)

Structure

• Six layers from posterior to anterior:

  • Layers 1,2 (M type cells) receive synapses from M ganglion cells.

  • Layers 3-6 (P type cells) receive synapses from P ganglion cells.

  • Contralateral and ipsilateral retina fiber connections are distributed across layers.

PRIMARY VISUAL CORTEX (AREA 17)

Organization

• Six histological layers receiving information primarily from the LGN.

• Layers I and II collect information from the LGN.

• Layer III communicates P type input; layers IV separate M type inputs.

• Layer IVC: Alternating ocular dominance columns, integrating ipsilateral and contralateral LGB fibers.

Visual Cortex Functionality

• Receives and processes M-type cell information processing through V2 (Areas 18 and 19) for motion analysis.

• Separates P-type inputs into shape (interblobs) and color (blobs) for further processing in temporal cortex.