Vision Notes

VISION

General Principles of Perception

• Ibn al-Haytham (965-1040):
  • Observed that we see distant stars immediately when opening our eyes at night.
  • Reasoned that sight is not sent out via rays, but rather light rays bounce off objects and strike our retina.
• Perception is in the brain, not in the object itself.
• We see an object when it emits or reflects light that stimulates receptors, which then transmit information to the brain.
• René Descartes:
  • Believed nerves send the brain a pattern of impulses like a picture of the object.
  • However, the brain encodes information in a way that doesn't resemble what we see.
• Johannes Müller (1838):
  • Proposed the law of specific nerve energies.
  • Whatever excites a particular nerve establishes a special energy unique to that nerve.
• Demonstration:
  • Rubbing eyes can cause you to see spots or flashes of light, even in a dark room, due to mechanical pressure exciting visual receptors.

The Eye and Its Connections to the Brain

• Pupil:
  • Opening in the center of the iris where light enters.
• Lens:
  • Adjustable, focuses light.
• Cornea:
  • Not adjustable, also focuses light.
• Retina:
  • Rear surface of the eye lined with visual receptors.
  • Light from the left side of the world strikes the right half of the retina, and vice versa. Light from above strikes the bottom half, and light from below strikes the top half.
• The inversion of the image is not a problem for the nervous system; it codes information through neuronal activity.

Route Within The Retina

• Receptors
• Bipolar Cells:
  • Retinal interneurons.
  • Receive messages from receptors at the back of the eye.
  • Located closer to the center of the eye.
  • Sends messages to the ganglion cells.
• Ganglion Cells:
  • Conveys the messages from the Bipolar Cells.
  • Located still closers to the center of the eye.
  • The Ganglion cells’ axon join together and travel back to the brain
• Amacrine Cells:
  • Additional cells
  • Obtains information from Bipolar Cells and send it to other Bipolar which are Amacrine and Ganglion Cells.
  • Refines the input to Ganglion Cells, enabling certain ones to respond mainly to particular shapes, directions of movement, changes in lighting, color, and other visual feature.
• Light passes through ganglion, amacrine, and bipolar cells en route to the receptors; these cells are transparent.
• Optic Nerve:
  • Formed by the axons of ganglion cells.
  • Exits through the back of the eye.
  • Transmits electrical impulses from the eyes to the brain.
  • The point where the optic nerve leaves is a blind spot because it has no receptors and is where blood vessels enter and leave..
• Why we do not notice our blind spot?
  • The brain fills in the gap.
  • Anything in the blind spot of one eye is visible to the other eye.

Fovea and Periphery of the Retina

• Fovea:
  • Tiny area specialized for acute, detailed vision.
  • Blood vessels and ganglion cell axons are almost absent, providing nearly unimpeded vision.
  • The tight packing detail of receptors aids perception of detail.
  • Each receptor connects to a single bipolar cell, which connects to a single ganglion cell that has an axon to the brain.
• Midget Ganglion Cells:
  • Ganglion cells in the fovea of humans and other primates.
  • Each is small and responds to just a single cone, with each cone having a direct route to the brain.
  • Provides 70% of the input to the brain; vision is dominated by what an eye sees in and near the fovea.
• Many bird species have foveas per eye, one pointing ahead and one pointing to the side, enabling perception of detail in the periphery.
• Hawks and other predatory birds have a greater density of visual receptors on the top of their retinas (looking down).
• Many prey species such as rats have most of their receptors on the bottom half of the retina enabling them to see up better than they see down.
• Toward the periphery of the retina, more and more receptors converge onto bipolar and ganglion cells.
• The summation enables perception of fainter light in the periphery.
• Foveal vision has better acuity, and peripheral vision has better sensitivity to dim light.
• In the periphery, the ability to detect detail is limited by interference from other nearby objects.

Visual Receptors: Rods and Cones

• Rods:
  • Abundant in the periphery of the human retina.
  • Respond to faint light but are not useful in daylight because bright light bleaches them.
  • 120 million rods
• Cones:
  • Abundant in and near the fovea.
  • Less active in dim light, more useful in bright light, and essential for color vision.
  • 6 million cones
  • Good color vision in the fovea but not in the periphery.
• Cones provide about 90 percent of the brain’s input.
• Both rods and cones contain photopigments, chemicals that release energy when struck by light.

Summary of the differences between fovea and peripheral vision

Color Vision

• Visible light consists of electromagnetic radiation within the range from less than $400$ nm (nanometer, or $10^{-9}$ m) to more than $700$ nm.
• We perceive the shortest visible wavelengths as violet. Progressively longer wavelengths are perceived as blue, green, yellow, orange, and red.

The Trichromatic Theory

• We perceive color through the relative rates of response by three kinds of cones, each one maximally sensitive to a different set of wavelengths.
• Short-wavelength / Blue cones
• Medium-wavelength / Green cones
• Long-wavelength / Red cones
• People could match any color by mixing appropriate amounts of just three wavelengths, therefore, he concluded that three kinds of receptors are sufficient to account for human color vision.

The Opponent-Process Theory

• We perceive color in terms of opposites.
• The brain has a mechanism that perceives color on a continuum from red to green, another from yellow to blue, and another from white to black.
• After you stare at one color in one location long enough, you fatigue that response and swing to the opposite.

The Retinex Theory

• The cortex compares information from various parts of the retina to determine the brightness and color for each area.

Color Vision Deficiency

• Inability or decreased ability to see color, or perceive color differences
• Occurs when one or more of the cone types are defected.
• Some women can make somewhat finer than average distinctions from one color to another.

Summary

1. You see because light strikes your retina, causing it to send a message to your brain. You send no sight rays out to the object.
2. According to the law of specific nerve energies, the brain interprets any activity of a given sensory neuron as representing a particular type of sensory information.
3. Sensory information is coded so that the brain can process it. The coded information bears no physical similarity to the stimuli it describes.
4. Light passes through the pupil of a vertebrate eye and stimulates the receptors lining the retina at the back of the eye.
5. The axons from the retina loop around to form the optic nerve, which exits from the eye at a point called the blind spot.
6. Visual acuity is greatest in the fovea, the central area of the retina. Because so many receptors in the periphery converge their messages to their bipolar cells, our peripheral vision is highly sensitive to faint light but poorly sensitive to detail.
7. The retina has two kinds of receptors: rods and cones. Rods, more numerous in the periphery of the retina, are more sensitive to faint light. Cones, more numerous in fovea, are more useful in bright light.
8. People vary in their number of axons from the retina to the brain. Those with more axons show a greater ability to detect brief, faint, or rapid changing stimuli.
9. According to the trichromatic (or Young-Helmholtz) theory of color vision, color perception begins with a given wavelength of light stimulating a distinctive ratio of responses by three types of cones.
10. According to the opponent-process theory of color vision, visual system neurons beyond the receptors respond with an increase in activity to indicate one color of light and a decrease to indicate the opposite color: red-green, yellow-blue, and black-white.
11. According to the retinex theory, the cortex compares the responses across the retina to determine the brightness and color of each object.
12. For genetic reasons, certain people are unable to distinguish one color from another. Red-green color deficiency is the most common type.