Sensory Systems and Vision

The Law Of Specific Nerve Energies: "Whatever excites a particular nerve establishes a special kind of energy unique to that nerve."
Sensory Pathway: Stimuli (Light, sound, touch, chemicals) → Sensory organ → Thalamus (except for olfaction) → Primary/Secondary/Association sensory areas.
Visible Light: Occupies a specific wavelength range within the electromagnetic spectrum between ultraviolet and infrared ( $450\text{ nm}$ to $700\text{ nm}$ ).

Pupil: Initial entrance for light.
Cornea and lens: Responsible for focusing light.
Retina: Site of transduction containing light-sensitive receptors.
Optic nerve: Transmits visual data to the brain.
Fovea: Area responsible for sharp central vision (foveal vision) essential for details like reading and driving.
Blind Spot: Retinal area where the optic nerve leaves the eye; it contains no receptors.

RODS: Approximately $120 \times 10^6$ receptors; responsible for dim light response and peripheral vision.
CONES: Approximately $6 \times 10^6$ receptors; responsible for bright light and detailed vision.
Distribution: The fovea contains only cones, while the peripheral retina contains mostly rods.
Local Processing: Rods and cones synapse with horizontal and bipolar cells, which transmit signals to amacrine and ganglion cells using Glutamate.

Convergence Ratio: On average, $120$ rods pool to one ganglion cell, compared to only $6$ cones per ganglion cell.
Sensitivity vs. Acuity: Rods provide higher sensitivity due to greater convergence; cones provide better visual acuity (detail vision) due to lower convergence.

Hyperopia (Farsightedness): Eyeball is too short; near objects are blurry; corrected with a plus lens.
Myopia: Light focuses in front of the retina due to a long eyeball or excessive cornea curvature.

Optic Chiasm: Point where optic neurons contralateralize.
Primary Projection: Lateral Geniculate Nucleus (LGN) of the Thalamus → Striate Cortex (Visual Cortex).
Superior Colliculi: Manages eye movements and reflexes.
Hypothalamus: Regulates circadian rhythm.
Functional Pathways:
- M pathway (Magnocellular): Responds to motion and depth.
- P pathway (Parvocellular): Responds to details, colour, and shape.
- K pathway (Koniocellular): Responds to color.

V1: Primary visual cortex.
V2: Primary association area with hippocampal connections.
V5: Specialized for motion processing.
Ventral Stream (Occipito-Temporal): The “What” pathway (form and color).
Dorsal Stream (Occipito-Parietal): The “Where/How” pathway (spatial location and motion).
Segmented Processing: Pathologies like color blindness and motion blindness demonstrate that vision is not a single process but distributed across different brain regions.