Vision: Eye Structure and Function

• Objects emit energy that stimulates sensory receptors in sense organs, transmitting information to the brain.
• The brain codes this information, which is then interpreted by the cortex.

Eye Structure and Function

• The pupil, an opening in the iris, allows light to enter the eye.
• The lens (adjustable) and cornea (not adjustable) focus light onto the retina.
• The retina, at the rear of the eye, contains visual receptor neurons.
• Visual receptor neurons transduce light into a neural code, sent to the brain.

Visual Receptors

• Rods and cones contain photopigments that release energy when struck by light.
• The ratio of rods to cones is approximately 20:1.
• Rods are for peripheral and night vision and are located in the periphery of the retina.
• Cones are for visual acuity, detail, and color vision and are located in the fovea.

Fovea vs. Periphery of Retina

• Vision is dominated by the fovea, packed with cones for detailed vision.
• Fovea vision is least impeded, and each cone connects to a single bipolar cell, then to a midget ganglion cell.
• Each cone in the fovea has a direct line to the brain, registering precise light location.
• Peripheral regions have more receptors per bipolar cell, reducing detail but increasing dim light sensitivity.

Color Vision

• Color vision compares responses of different cone types.
• Shortest wavelengths (~350 nm) are violet, and longest (~700 nm) are red.

Theories of Color Vision

• The trichromatic theory states there are 3 cone types, each sensitive to different light wavelengths (short, medium, long).
• The opponent-process theory perceives color in opposing pairs: red-green, yellow-blue, white-black.
• Negative color afterimages are evidence for the opponent-process theory.
• The retinex theory explains color constancy by comparing retinal inputs in the visual cortex to determine brightness and color.

Neural Pathways in the Eye

• Rods and cones transduce light into neural signals, synapsing with horizontal and bipolar cells.
• Horizontal cells use lateral inhibition to enhance contrast by reducing bipolar cell activity near edges.
• Bipolar cells synapse with amacrine cells and ganglion cells.
• Ganglion cell axons form the optic nerve.

Optic Nerve and Chiasm

• Optic nerves from each eye meet at the optic chiasm, where half the axons cross to the opposite brain side.

Brain Processing of Visual Information

• The optic nerve projects to the lateral geniculate nucleus (LGN) of the thalamus.
• Ganglion cells synapse onto the LGN, with parvocellular neurons (small, near fovea) and magnocellular neurons (large, throughout retina).
• LGN axons project to the primary visual cortex (V1) in the occipital lobe, responsible for the first stage of visual processing.
• V1 damage results in a loss of conscious vision and imagery.

Visual Cortex

• V1 sends information to the secondary visual cortex (V2) for further processing.
• Connections between V1 and V2 are reciprocal.
• Information becomes more complex from V1 to V2, with cells responding to specific shapes or properties.

Ventral Stream

• The ventral stream is for object and shape recognition known as the “what pathway”.
• The pathway goes from V1 through V2 and V4 to the inferior temporal cortex (IT cortex).
• Damage to the ventral stream results in the ability to see where objects are but not identify them.

Dorsal Stream

• The dorsal stream guides actions in the motor system and locates objects in space, known as the “where or how pathway”.
• The dorsal stream goes from V1 to the posterior parietal cortex.
• Damage to the dorsal stream results in the ability to recognize what objects are but not where they are.

Shape Perception: Inferior Temporal Cortex

• Cells in the inferior temporal cortex respond to identifiable objects.
• The inferior temporal cortex is important for shape constancy.
• Damage to the inferior temporal cortex leads to visual agnosia (inability to recognize objects).

Recognizing Faces

• Face recognition involves the occipital face area, amygdala, and fusiform gyrus in the temporal cortex.
• The fusiform gyrus is specialized for face recognition and is activated when identifying object categories.
• Prosopagnosia is the impaired ability to recognize faces.
• Newborns are predisposed to pay more attention to faces more so than other objects.

Motion Perception: Middle Temporal Cortex

• Area MT (V5) and area MST are important for motion detection.
• Damage to these areas results in motion blindness (inability to perceive motion).