Visual Modality

The Eye

• Lens - important for focus
• Cornea - refraction of light so it is focused onto the lens
• Ciliary Muscles - control the shape of the lens
• Vitreous Humor - fluid that fills the majority of eye
• Retina - responsible for the transmission of light to neural structures
• Fovea - part of the retina with highest visual acuity
• Iris - controls the size of the pupil (opening)
• Sclera - protective outer layer (white part)
• Optic Nerve - used for transmission of signals from the eye to the brain. Reason for blind spot.

Common Eye Problems

• Myopia (nearsightedness) - eyeball too long
• Hyperopia (farsightedness) - eyeball too short

The Retina

• It's a sheet-like structure made up of many layers that lines the back of the eye
• Absorb light wavelengths in the visible range
• Set up backwards - the receptors are furthest from light

Retinal Cell Types

• Photoreceptors
• Rods and cones respond to light
• Horizontal Cells
• Bipolar Cells
• Amacrine Cells
• Ganglion Cells
• Axons converge to form the optic nerve

• Involved in phototransduction
• Rods
• Black and white vision
• Relied on in the dark
• Found in peripheral vision
• High sensitivity: single photon causes many Na+ channels to close

• Cones
• Color vision (Cones are for Color)
• Trichromatic - three different types of cones that each respond to one of the primary colors
• Relied on during the day
• Found in high amounts in central vision (fovea)
• Low sensitivity

• When transitioning between dark and light environments, there's an adaptation period that corresponds to switching between rods or cones becoming active or inactive

Retinal Ganglion Cell Receptive Fields

• A retinal receptive field is the region of the visual space that makes a retinal ganglion cell change its firing under the presence of a certain visual stimulus.
• The receptive field of an individual sensory neuron is the particular region of sensory space (for example a piece of skin or a part of your visual field) in which a stimulus will trigger that neuron to fire.
• Think: a policeman patrolling a certain territory.
• Because the retinal ganglion cells are not directly doing the “sensing”, they are not interacting with light, they must use the photoreceptor cells to sense “for” them. So their receptive field is actually composed of input from all of the photoreceptors which synapse with it.

• There are two types:
• ON-center cells: fire when light is at the center
• OFF-center cells: fire when light is in surroundings

Visual Pathway - From Eye to Primary Visual Cortex

1. The eye (retina)
2. Optic nerve
3. Optic chiasm
4. Optic tract
5. Lateral geniculate nucleus
6. Primary visual cortex (V1)