Visual Processing in Retina-Visual Pathway

Visual Processing in the Retina-Visual Pathway

• Objectives: Explain phototransduction, describe the duplicity theory of vision, describe visual pathways, understand how visual field deficits indicate specific defects in pathways, describe visual cortex organization, and describe how the brain processes visual information, including color.

Receptor and Neural Function of the Retina

• The retina, the innermost layer of the eyeball, contains photoreceptors (rods and cones) and neurons (bipolar and ganglion cells).
• It's arranged into four layers from outside in:
• Pigmented Layer: The outermost layer, directly contacting the choroid. It contains melanin to prevent light reflection and stores vitamin A.
• Layer of Rods and Cones: These are the photoreceptors. Rods are for night vision; cones for day, acute, and color vision; each cell has an outer (photo-sensitive) and inner segment (nucleus and mitochondria).
• Layer of Bipolar Cells: The first-order neurons in the visual pathway. They transmit signals from the rods and cones to the ganglion cells.
• Layer of Ganglion Cells: Transmit visual signals to the brain.

Specialized Areas of the Retina

• Optic Disc (Blind Spot): The point where the optic nerve exits the eye. No photoreceptors are present here, so light falling on this area cannot be seen.
• Macula Lutea: Contains relatively few rods and mostly cones, located near the posterior pole.
• Fovea Centralis: The central part of the macula lutea; contains cones only, responsible for the highest visual acuity.

Criteria for Greatest Visual Acuity in the Fovea

• Light rays fall directly on cones, with no intervening layers.
• High cone density, with each cone connected to a single bipolar and ganglion cell for direct pathways to the brain.

Duplicity Theory of Retinal Function

• Input from two kinds of receptors (rods and cones).
• Each maximally functional under different lighting conditions.
• Day vision (photopic/central):
• Receptors: Cones
• Location: Central retina
• Sensitivity to low light: Low
• Light threshold: High
• Color: Present (3 types of cones)
• Visual acuity: Acute
• Details of objects: Well seen
• Adaptation time: Short
• Night vision (scotopic/peripheral):
• Receptors: Rods
• Location: Peripheral retina
• Sensitivity to low light: High
• Light threshold: Low
• Color: Absent (achromatic)
• Visual acuity: Poor
• Details of objects: Not well seen
• Adaptation time: Long

Photochemistry of Vision

• Rhodopsin (visual purple): The light-sensitive pigment in rods. Composed of opsin (protein) and retinal (carotenoid).
• Retinal (cis form): In the dark; all-trans retinal in the presence of light.

Phototransduction (Light into Electrical Signal)

• Dark Current: Constant sodium ion movement in the rod, keeping the membrane partially depolarized continuously releasing glutamate.
• Light Activation: Exposure to light causes rhodopsin to decompose, activating a cascade of proteins.
• Sodium Channel Closure: The activation of enzymes decrease cGMP levels, decreasing sodium permeability. The membrane hyperpolarizes and reduces the release of glutamate.
• Light Signal Transduction: Leads to changes in membrane ion permeability resulting in a modified pattern of action potentials in the optic nerve.

Termination of Events in Rods after Light Excitation

• Rhodopsin inactivation: Inactivation of the light-activated rhodopsin (metarhodopsin II).
• cGMP Resynthesis: cGMP levels increase, re-opening sodium channels to resume dark current.

Further Processing of Visual Information in Retina

• Bipolar Cells: Receive input from rods and cones; output is transmission to the ganglion cells; excitation in the presence of light.

• Ganglion Cells: Generate action potentials representing the processed signal.

• Cone Pigments: Similar chemical composition to rhodopsin, but different opsin, allowing for different sensitivities to wavelengths (red, green, blue).

Further Processing of Visua Information in the Retina cont’d

• Primary Visual Cortex (V1): Located on the medial sides of the occipital lobe; organization, including color blobs and interblob neurons. Signals from the fovea are near the occipital pole, others more toward the anterior part.
• Secondary Visual Areas: Regions adjacent to V1; processing of details, motion, color; includes V2,V3,V4, and V5 (MT).

Dorsal and Ventral Pathways for Visual Information Analysis

• Dorsal Pathway (“Where” Pathway): Processes motion, location, and spatial relationships of objects. Involves the MT area and medial superior temporal gyrus.
• Ventral Pathway (“What” Pathway): Processes shape, color, and object recognition. Involves the inferior temporal lobe (area IT).

Retinal Adaptation

• Dark Adaptation: The process of increasing retinal sensitivity in dim light. Rod function predominates, and pupils dilate.
• Light Adaptation: The process of reducing retinal sensitivity in bright light. Cone function predominates, and pupils constrict.