6- Visual Acuity & Visual Fields (9.8.25)

Visual acuity & visual fields

9.8.25

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Agenda

Announcements - Exit Ticket Recap - Visual Acuity - Visual Field - Intro to Brain Vision Pathways

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• Ch. 2 Quiz posted in Quizzes

Announcements

• Syllabus clarification - cheat sheet is a 3" x 5" index card

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Recap on retina cell types

• Photoreceptors are the sensory neurons (rods and cones); at the back of the retina

• Bipolar cells receive input from photoreceptors and send output to ganglion cells

• Ganglion cells are the output neurons; axons form the optic nerve to the brain

Two other cell types process information:

• Horizontal cells contact photoreceptors and bipolar cells.
• Amacrine cells contact bipolar and ganglion cells.

All cell types except ganglion cells generate only graded, local potentials, affecting each other through the graded release of neurotransmitters.

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Photoreceptors: rods & cones

Cones (color, center)

• Photopic system (light)
• Require more light
• High acuity
• Sensitive to different wavelengths (color)
• More numerous in the fovea of the retina (center)

Rods

• Scotopic system (darkness)
• High sensitivity to dim light
• Low acuity
• Insensitive to color
• More numerous in the periphery of retina

Photoreceptors: rods & cones

Light particles (photons) strike discs in the outer segment of rods/cones and are captured by light-sensitive photopigments. Photoreceptors in the dark continually release neurotransmitter. Light triggers a graded hyperpolarization of the cell, causing it to release less neurotransmitter.

Photoreceptors: rods & cones

Rods use rhodopsin as their photopigment. Rhodopsin is synthesized from vitamin A (retinol). Vitamin A deficiency causes night blindness (reduced vision from rods).

Light Adaptation

We can detect light across a 10 billion-fold range of intensities, but photoreceptors can only detect over a 100-fold range. Adaptation changes sensitivity to light.

1. Pupil size: adjustment of the size of the pupil (the opening in the iris) changes the amount of light hitting the retina.
2. Range fractionation: different photoreceptors to handle different intensities: rods vs. cones.
3. Photoreceptor adaptation (*primary mechanism): each photoreceptor adjusts its level of sensitivity to match the average ambient level of light.

Light Adaptation

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Why did so many pirates wear eye patches?

• Pirates needed to go between above-deck (bright light) and below-deck (very dark).
• Photoreceptors adapt quickly going from darkness to light, but can take up to 25 minutes to adapt from bright light to darkness.
• Each eye can undergo adaptation independently.

Visual Acuity Demo

1st, focus on the green circle & read the 4 letters. Next, focus on the red square & try to read the 4 letters.

Visual acuity (ability to see fine detail) is greatest where your eyes are pointed.

Visual Acuity

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Visual acuity (sharpness) is best at the fovea (center of the retina) because:

1. High density of smaller, tightly-packed cones

Visual Acuity

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Visual acuity (sharpness) is best at the fovea (center of the retina) because:2. Direct light input that does not pass through other cells or blood vessels

Visual Acuity

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In peripheral retina, more rods means increased sensitivity but reduced acuity. When looking at a dim star or the Andromeda galaxy (right), it is better to slightly divert your eyes and use peripheral vision (higher rod density).

Visual Acuity

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Better night vision in some animals due to tapetum lucidum, a reflective structure at the back of the eye that mirrors light back through retina.

Back of cow’s eyeball

Red eye in photos is due to light hitting retina’s rich blood supply.

Visual Field - Surrounding space that can be seen for a given eye position

Visual Field

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The whole area that is visible without movement is the visual field. The retina represents a two-dimensional map of the visual field. Ganglion cells conduct action potentials along axons that make up the optic nerve (cranial nerve II). In vertebrates, some or all of each optic nerve crosses the midline at the optic chiasm; after crossing, it is known as the optic tract.

Visual Field

• Prey animals: wider field of view
• Predators: better depth perception (by comparing overlapping visual fields)
• These differences are seen in birds that are prey vs. predator.
• Other facts about bird vision: Some birds have a double fovea to enhance binocular vision.
• Most birds cannot move their eyes.

Field-of-View: Project Ideas

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Next Class: The Visual Brain

Remember to complete your Exit Ticket

Exit Ticket

Next class: Vision in the Brain