5.3

Learning Objectives

By the end of this section, you will be able to:

  1. Describe the basic anatomy of the visual system
  2. Discuss how rods and cones contribute to different aspects of vision
  3. Describe how monocular and binocular cues are used in the perception of depth

The visual system constructs a mental representation of the world around us (Figure 5.10). This representation is crucial for successful navigation through physical spaces and interactions with significant individuals and objects in our environments. This section provides an overview of the basic anatomy and function of the visual system, including the ability to perceive color and depth.

Anatomy of the Visual System

Structure of the Eye

The eye is the primary sensory organ involved in vision (Figure 5.11). Light waves enter the eye through the following structures:

  • Cornea: The transparent covering over the eye, which serves as a barrier between the inner eye and the outside world, also involved in focusing light waves entering the eye.
  • Pupil: The small opening through which light passes. The pupil's size adjusts based on light levels and emotional arousal:
    • Dilation occurs under low light levels to allow more light in.
    • Constriction occurs at high light levels to reduce light intake.

The pupil's size is controlled by muscles connected to the iris, the colored part of the eye.

Lens and Retina

After light passes through the pupil, it crosses the lens, a curved, transparent structure that provides additional focus. The lens is attached to muscles that change its shape to focus light from near or distant objects.

In individuals with normal vision, the lens focuses images on a small indentation at the back of the eye called the fovea, which is part of the retina, the light-sensitive lining of the eye.

Photoreceptor Cells

  • Cones:

    • Specialized photoreceptors functioning best in bright light conditions.
    • Provide high spatial resolution and are directly involved in color perception.
    • Concentrated in the fovea.

  • Rods:

    • Photoreceptors that work well in low light conditions, providing vision in dimly lit environments.
    • Lack the spatial resolution and color function of cones but are involved in peripheral vision and motion detection.
Summary of Photoreceptors

The two types of photoreceptors are shown in Figure 5.12: cones are represented in green and rods in blue.

Adaptation to Darkness

This transition from bright to dim light is marked by a delay as the rods are less efficient in transforming light into nerve impulses, leading to night blindness if rods do not function properly.

Rods and cones communicate with retinal ganglion cells through several interneurons. Axons from these retinal ganglion cells converge and exit through the back of the eye to form the optic nerve, which carries visual information to the brain.

Blind Spots and the Optic Chiasm

A blind spot exists in the visual field: light focused here is not perceived. This phenomenon occurs for two reasons:

  1. Each eye has a slightly different view, so the blind spots do not overlap.
  2. The visual system fills in the gap, making us unaware of the missing information.

The optic nerves from each eye converge below the brain at the optic chiasm, an X-shaped structure where information from the right visual field (received by both eyes) is sent to the left hemisphere and vice versa. (Refer to Figure 5.13)

Processing Visual Information

Once visual information enters the brain, it is sent to various structures for processing, primarily to the occipital lobe at the back of the brain. This process occurs via two parallel pathways:

  • “What pathway”: Involved in object recognition and identification.
  • “Where/how pathway”: Involved in spatial location and interaction with visual stimuli.

Example: Seeing a ball rolling down the street involves the “what pathway” recognizing the ball and the “where/how pathway” identifying its motion in space.

Ethical Considerations in Visual System Research

David Hubel and Torsten Wiesel were awarded the Nobel Prize in Medicine in 1981 for their extensive research on the visual system, primarily using animal models.

  • They developed techniques like single-unit recordings to monitor brain activity in response to visual stimuli.
  • Key discoveries:
    • Certain brain cells respond to specific line orientations, a phenomenon termed ocular dominance.
    • Mapping of these cells has shown arrangement in visual cortex areas called columns and hypercolumns.
Critical Period in Visual Development

Hubel and Wiesel sutured one eye of newborn kittens to study visual development, revealing a critical period where lack of input could lead to loss of neural connections that usually respond to that eye.

Ethical implications: The necessity and ethics of animal research are debated, with considerations of potential benefits against animal suffering.

Color and Depth Perception

Color Vision

Normal sighted individuals have three types of cones sensitive to different wavelengths of light:

  1. Cone Type 1: Sensitive to red wavelengths.
  2. Cone Type 2: Sensitive to green wavelengths.
  3. Cone Type 3: Sensitive to blue wavelengths.
Theories of Color Vision
Trichromatic Theory
  • Proposes that all colors in the spectrum can be created by combining red, green, and blue light.
  • The respective cone types correspond to their sensitivity.
Opponent-Process Theory
  • Asserts that color is coded in opponent pairs: black-white, yellow-blue, and green-red.
  • Cells are excited by one color and inhibited by its opponent.
  • Negative afterimages (e.g., seeing an afterimage of colors after staring at them) provide empirical support for this theory.
Real-world Example of Color Vision

A personal anecdote illustrates discovering colorblindness. The story reflects a common experience of color perception and the challenges faced by those with color deficiencies:

  • Approximately 8% of males of White European descent experience red-green color blindness.
  • The trichromatic and opponent-process theories apply to different visual processing levels (retina vs. brain).

Depth Perception

Understanding Depth Perception

Depth perception is our ability to understand spatial relationships in three-dimensional space. We perceive depth using numerous cues, which include:

  • Binocular cues: Require both eyes and include binocular disparity, the slightly different views each eye receives.
  • Monocular cues: Can be utilized with one eye, more prevalent than binocular cues, including:
    • Linear perspective: Lines converge in the distance, suggesting depth (Figure 5.17).
    • Interposition: Partial overlap of objects signals depth.
    • Relative size: Smaller images appear further away.
Life Experiences of Stereoblindness

An extreme case of stereoblindness highlighted how depth perception can be strikingly affected. Bruce Bridgeman, born with lazy eye, experienced a significant change in his perception of depth after watching a 3-D movie at the age of 70, which activated depth perception abilities that had been previously dormant.