36.5 Vision

Physics of Vision

Electromagnetic vs. Sound Waves

Vision depends on electromagnetic (EM) waves, specifically the narrow band we call visible light. EM waves travel through a vacuum at the speed of light ($c = 3 \times 10^8 \text{ m/s}$) and don't require a medium to propagate. Different wavelengths within the visible spectrum correspond to different colors, ranging from shorter wavelengths (blue/violet) to longer wavelengths (red).

Sound waves, by contrast, are mechanical waves that require a medium like air or water. They travel much slower than EM waves, and their speed depends on the medium. Sound is detected by the ear, not the eye. The key distinction for this unit: vision relies on EM radiation, hearing relies on mechanical vibration.

Anatomy and Physiology of the Eye

Light's Journey Through Eye Structures

Light passes through several structures before reaching the photoreceptors at the back of the eye. Each structure plays a specific role in directing and focusing that light.

• Cornea: the transparent outer layer that performs most of the eye's initial light refraction.
• Anterior chamber: the space between the cornea and the lens, filled with aqueous humor, a fluid that nourishes the cornea and lens and helps maintain intraocular pressure.
• Pupil: the opening at the center of the iris that regulates how much light enters the eye. The iris constricts the pupil in bright light and dilates it in dim light.
• Lens: a transparent, biconvex structure that fine-focuses light onto the retina. Through a process called accommodation, ciliary muscles change the shape of the lens so you can focus on objects at different distances. When the eye can't focus light properly on the retina, refractive errors result (e.g., myopia, hyperopia).
• Vitreous chamber: the large space behind the lens filled with vitreous humor, a gel-like substance that maintains the eye's spherical shape.
• Retina: the light-sensitive layer lining the back of the eye, packed with photoreceptors (rods and cones). The fovea is the small central pit of the retina that has the highest density of cones and provides the sharpest visual acuity.
• Optic nerve: a bundle of nerve fibers that carries visual information from the retina to the brain. At the optic chiasm, fibers from the nasal (inner) half of each retina cross to the opposite side of the brain. This partial crossing is what allows binocular vision and depth perception.

Photoreceptor Response to Light

The retina contains two main types of photoreceptors, each specialized for different visual tasks.

Rods handle low-light (scotopic) and peripheral vision. They contain the pigment rhodopsin, which makes them extremely sensitive to light. However, rods do not distinguish color. You rely heavily on rods when navigating a dark room.

Cones are responsible for color vision and high-acuity detail. Three types of cones exist, each most sensitive to a different wavelength range: short (blue), medium (green), and long (red). Cones need more light to function than rods, which is why colors are hard to distinguish in dim conditions. Color blindness occurs when one or more cone types are absent or dysfunctional.

Tonic Activity and the Light Response

Photoreceptors behave in a way that can seem counterintuitive: they are most active in the dark.

1. In darkness, photoreceptors continuously release the neurotransmitter glutamate. This glutamate inhibits the downstream bipolar cells, keeping them inactive.
2. When light strikes a photoreceptor, glutamate release decreases. This reduction lifts the inhibition on bipolar cells, effectively activating them and passing the visual signal forward.

So the signal to the brain isn't triggered by "turning on" a cell. It's triggered by reducing the activity of a cell that was already on.

Phototransduction

Phototransduction is the conversion of light energy into an electrical signal within photoreceptors. Here's the sequence:

1. A photon of light is absorbed by rhodopsin (in rods) or photopsins (in cones), causing the pigment molecule to change shape (a conformational change).
2. This conformational change activates a G-protein signaling cascade inside the cell.
3. The cascade ultimately closes ion channels in the photoreceptor membrane, causing the cell to hyperpolarize (become more negative inside).
4. Hyperpolarization reduces glutamate release, which signals the bipolar cells downstream.

Visual Processing

Once photoreceptors and bipolar cells have done their work, the signal passes to ganglion cells, whose axons form the optic nerve.

• The optic nerve carries visual information from each eye to the optic chiasm, where fibers partially cross.
• From the optic chiasm, signals travel to the lateral geniculate nucleus (LGN) of the thalamus, then on to the visual cortex in the occipital lobe.
• The visual cortex processes and interprets the information, including color, shape, motion, and depth.