Eye Anatomy and Function

• Vision starts by the stimulus of light energy
• Transduction is the conversion of one type of energy into another
• Light energy is converted into patterns of neural activity

Protecting the Eye

• Lacrimal ducts protect the eyes
• Blinking produces tears
• Tears contain water and salt
• Tears are antibacterial
• Emotional tears relieve stress and have painkilling hormones
• The orbit and fat cushion also assist in protecting the eye

Major Eye Structures

• The sclera is the white of the eye and has protein fibers
• The cornea is transparent and bends light
• The cornea is rich in pain receptors
• The anterior chamber contains aqueous humor as well as Glucose and Amino Acids
• Glaucoma can occur in the anterior chamber

Iris

• The iris is a circular muscle that adjusts the pupil opening
• Sensitivity is the ability to detect dimly lit objects
• In conditions of low illumination the pupil dilates
• Acuity is the ability to see fine detail
• In conditions of high illumination the pupil constricts

Melanin

• Melanin is a pigment that determines the color of skin and iris muscle
• Albinism occurs when there is very little melanin

Lens

• The lens focuses light on the retina
• Accommodation occurs when the lens changes shape to focus on objects at different distances
• To focus on close objects the lens is cylindrical
• To focus on far objects the lens is flat

Retina

• The retina is light sensitive surface
• The vitreous chamber contains vitreous humor and floaters

How the Image is Formed

• Cells translate varying brightness and color into patterns of neural activity
• The image on the retina is upside-down and reversed left to right

Inside Out Problems

• Photoreceptors are light sensitive cells

• The photoreceptors are located away from the center of the eye

• Light has to pass through other cells before it hits the photoreceptors

• Photoreceptors are sensitive to change

• Cells in front of the photoreceptors never change position

• Cells in front of the photoreceptors stop responding when input is constant

Saccades

• Saccades are rapid, involuntary jumps of the eye
• Saccades occur about 3 times per second
• Saccades allow the eye to see stationary objects in visual field across the retina

Cones vs Rods

• Macaques rely on cones, while meerkats rely on rods alone
• Cones - 6 million, in the foveal region (central), bright light, detail and color
• Rods - 120 million, peripheral, dim light, motion sensitive

Fovea

• Fovea - Indentation on retina; is the area of highest visual acuity
• Fovea has a high concentration of photoreceptors (cones)
• The location of the fovea differs across species
• Predatory birds location vs prey location

Light

• Light travels in waves of electromagnetic energy
• Wavelength is the perception of color
• Intensity is the perception of brightness

Flow of Information

• Information flows from the retina to the brain
• 1st Photoreceptors
• 2nd Bipolar cells
• 3rd Ganglion cells
• Then to the superior colliculi, thalamus, and finally the visual cortex

Optic Chiasm

• Optic nerves partly cross before entering brain

Lateral Geniculate Nucleus (Thalamus)

• Has P & M channels

P & M Cells in the LGN

• Parvocellular cells respond to color, fine details, and form, and receive input from cones
• Magnocellular cells respond to movement and receive input from rods

Primary Visual Cortex (aka Striate Cortex)

• The input is P & M pathways
• Blobs (dark) are responsible for color & detail perception
• Interblobs (light) are responsible for shape & motion perception
• The dorsal stream (parietal)
• The ventral stream (temporal)

Right and Left Visual Fields

• The brains visual fields are split with each side corresponding to the opposite visual field
• The left visual field is processed in the right cortex
• The right visual field is processed in the left cortex

Ganglion Cell Receptive Field

• The ganglion cell receptive field is the region in the visual field and on the retina where stimulation influences firing rate
• On-Center/Off-Center nature of receptive fields plays a key role