The Eye (Chapter 9)

Vision importance: Over 1/3 of the human cortex is involved in visual perception.
Light definition: electromagnetic radiation visible to the eye; we are surrounded by electromagnetic radiation, but only a small portion of wavelengths are visible to the human eye.

Light is electromagnetic radiation visible to the eye.
Wavelength: distance from peak to peak or trough to trough; units: \text{nm}.
Frequency: number of peaks per unit time (waves/sec); determines perceived color.
Amplitude: measurement from trough to peak in a single wave; determines intensity or brightness.

Frequency of electromagnetic energy dictates color of visible light (e.g., gamma radiation and cool colors like blue-green).
Short wavelengths (high energy) purportedly associated with hot colors (orange-red) in the slide; note: there appears to be a misprint in the source—short wavelengths are higher energy and correspond to violet/blue end of the spectrum, while long wavelengths are lower energy (red end).
Long wavelengths correspond to lower energy.

Reflection: bouncing of light rays off a surface.
Absorption: transfer of light energy to a particle or surface.
Refraction: bending of light rays when moving from one medium to another; bending toward the perpendicular to the surface of the new medium; amount of bend depends on the difference in light speed between the two media.

Light passes through the cornea → enters the pupil → is focused by the lens on the retina.
This forms the optic disk, also known as the “blind spot.”
The image on the retina is initially inverted and upside down.

Pupil size is like a camera aperture; the pupil dictates how much light reaches the retina.
The eye lens (and cornea) focus light on the retina, similar to a camera lens.
The digital image produced by the retina is inverted; perception requires processing to interpret orientation.

Retina: photoreceptors convert light energy into neural activity and detect differences in light intensity.
Lateral Geniculate Nucleus (LGN) of the thalamus: first synaptic relay in the primary visual pathway.
Primary Visual Cortex (V1) and beyond: further processing of detected light; ultimately integration into vision and memory via the thalamus.

The visual field is the amount of space viewed by the retina when the eye is fixated straight ahead.
Question raised: Why isn’t the visual field for the right eye a full 180°?

Panels indicate differences in vision: Dog Vision, Horse Vision, Bird Vision, and general human vision.
These illustrate that peripheral visibility varies by species and by whether the field is visible to one or both eyes.

Refraction by the cornea is the first focusing step; the cornea bends incoming light to form images on the retina.
Focal distance is largely determined by the refractive power of the cornea; the cornea does about 3× as much refraction as the lens.

Accommodation is the process of changing the shape of the lens to adjust focus.
For distant objects: relaxation of ciliary muscles flattens the lens; little additional refraction is needed.
For near objects: contraction of ciliary muscles relaxes the lens, making it thicker for greater refraction.

In normal vision, the lens focuses light directly on the retina.
Myopia (nearsightedness): focal point falls short of the retina; typically due to an elongated eyeball or excessive corneal curvature; concave lens corrects by shortening the eye.
Hyperopia (farsightedness): focal point falls beyond the retina; eyeball may be too short or lens too flat; convex lens corrects by lengthening the eye.
Lens shapes:
- Concave: curves inward; middle thinner than edges.
- Convex: curves outward; middle thicker than edges.

Brainstem neurons control the pupillary muscles (iris muscles).
Muscles continuously adjust pupil diameter in response to ambient light (pupillary constriction).
The pupil functions like a camera aperture, allowing greater focus under higher light conditions.
It is consensual: a light change in one eye causes changes in both pupils.
Abnormalities in this reflex can indicate serious neurological problems.

Ophthalmoscopic imaging can detect retinal vascular diseases and conditions like glaucoma.
Macula: area with relatively few blood vessels.
Fovea: area within the macula that marks the center of the retina and the region of highest visual acuity.

The retina is organized from the inside-out for light, meaning light passes through inner layers before reaching photoreceptors.
The light message (detection) travels in a direct vertical pathway that is opposite to the direction of light travel: 3. Ganglion cells; 2. Bipolar cells; 1. Photoreceptors.
Lateral connections begin the initial cross-talk in retinal processing.

Horizontal cells: receive input from photoreceptors and project to other photoreceptors and bipolar cells.
Amacrine cells: receive input from bipolar cells and project to ganglion cells, bipolar cells, and other amacrine cells.

Photoreceptors convert electromagnetic radiation to neural signals (not yet an action potential).
Types:
- Rods
- Cones
Three main regions:
- Outer segment: contains disks with photopigment; rods have more disks, contributing to higher sensitivity to light.
- Inner segment: cell body location.
- Synaptic terminal.

Fovea cross-section: a pit where inner layers are pushed aside to leave only cones; maximizes visual acuity by minimizing light scatter.
Central retina (fovea): all cones; 1:1 ratio of cones to ganglion cells; highest visual acuity.
Peripheral retina: higher ratio of rods to cones; higher ratio of photoreceptors to ganglion cells; more sensitive to light; reference to stars at night.

Visual Acuity: ability to discriminate between two nearby points of light; depends on photoreceptor density and precision of refraction.
Visual Angle: distances across the retina described in degrees.
20/20 vision: defined by ability to recognize a letter at a standard visual angle; linked to a reference chart.

Acuity varies with retinal location; acuity is highest at the fovea and decreases toward the periphery (relative to fovea).
The material includes a representation of different letters across the field, illustrating acuity variation.

Light sensitivity across the visual field is lowest at the fovea (only cones, no rods) and greater in the periphery where rods are more prevalent.